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APOD: June 16, 1995 - Neutron Star Earth
If the Earth could somehow be transformed to the ultra-high density of a neutron star, it might appear as it does in the above computer generated figure. Due to the very strong gravitational field, the neutron star distorts light from the background sky greatly. If you look closely, two images of the constellation Orion are visible. The gravity of this particular neutron star is so great that no part of the neutron star is blocked from view - light is pulled around by gravity even from the back of the neutron star.

APOD: June 20, 1995 - Pleiades Star Cluster
The Pleiades star cluster, M45, is one of the brightest star clusters visible in the northern hemisphere. It consists of many bright, hot stars that were all formed at the same time within a large cloud of interstellar dust and gas. The blue haze that accompanies them is due to very fine dust which still remains and preferentially reflects the blue light from the stars.

APOD: June 21, 1995 - Supernova 1987a Aftermath
In 1987 a star in one of the Milky Way's satellite galaxies exploded. In 1994 the Hubble Space Telescope, in orbit around the earth, took a very detailed picture of the remnants of this explosion. This picture, above, showed unusual and unexpected rings, and astronomers are not sure how they formed.

APOD: June 22, 1995 - Earth from Apollo 17
In 1972 Astronauts on the United States's last lunar mission, Apollo 17, took this picture looking back at the Earth on their way to the moon. The continents of Antarctica and Africa are visible below the delicate wisps of white clouds.

APOD: June 23, 1995 - Gamma Ray Sky Map
What if you could "see" gamma rays? This computer processed image represents a map of the entire sky at photon energies above 100 million electron Volts. These gamma-ray photons are more than 10,000 times more energetic than visible light photons and are blocked from the Earth's surface by the atmosphere. In the early 1990s NASA's Compton Gamma Ray Observatory, in orbit around the Earth, scanned the entire sky to produce this picture. A diffuse gamma-ray glow from the plane of our Milky Way Galaxy is clearly seen across the middle. The nature and even distance to some of the fainter sources remain unknown.

APOD: June 25, 1995 - Jupiter from Voyager
Imagine a hurricane that lasted for 300 years! This picture of the planet Jupiter was taken by the Voyager 1 spacecraft as it passed the planet in 1979. Jupiter, a gas giant planet with no solid surface, is the largest planet in the solar system and is made mostly of the hydrogen and helium. Clearly visible in the photo is the Great Red Spot, a giant, hurricane-like storm system that rotates with the clouds of Jupiter. It is so large three complete Earths could fit inside it. Astronomers have observed this giant storm on Jupiter for over 300 years.

APOD: June 26, 1995 - Spiral Galaxy M100
The M100 galaxy is a large spiral galaxy similar to our own Milky Way, containing over 100 billion stars. It is over 150 million light years away, so the light we see left when dinosaurs roamed the Earth. The picture was taken in 1993 with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope.

APOD: June 27, 1995 - An Ultraviolet Image of M101
This giant spiral galaxy, Messier 101 (M101), was photographed by the Ultraviolet Imaging Telescope onboard the Space Shuttle Endeavour during the Astro-2 mission (March 2 - 18, 1995). The image has been computer processed so that the colors represent the intensity of ultraviolet light. Pictures of galaxies like this one show mainly clouds of gas containing newly formed stars many times more massive than the sun, which glow strongly in ultraviolet light. In contrast, visible light pictures of galaxies tend to be dominated by the yellow and red light of older stars. Ultraviolet light, invisible to the human eye, is blocked by ozone in the atmosphere so ultraviolet pictures of celestial objects must be taken from space.

APOD: June 28, 1995 - The Cat's Eye Nebula
Three thousand light years away, a dying star throws off shells of glowing gas. This Hubble Space Telescope image reveals "The Cat's Eye Nebula" to be one of the most complex "planetary nebulae" known. In fact, the features seen in this image are so complex that astronomers suspect the visible central star may actually be a double star system. The term planetary nebula, used to describe this general class of objects, is misleading. Although these objects may appear round and planet-like in small telescopes, high resolution images reveal them to be stars surrounded by cocoons of gas blown off in the late stages of evolution.

APOD: June 29, 1995 - The Earth-Moon System
A double planet? From 4 million miles away on December 16, 1992, NASA's robot spacecraft Galileo took this picture of the Earth-moon system. The bright, sunlit half of the Earth contrasts strongly with the darker subdued colors of the moon. Our moon is one of the largest moons in the solar system. It is even larger than the planet Pluto. In this picture, the Earth-moon system actually appears to be a double planet.

APOD: June 30, 1995 - Ida and Dactyl: Asteroid and Moon
An asteroid with a moon! The robot spacecraft Galileo whose primary mission is to explore the Jupiter system, has encountered and photographed two asteroids during its long journey to Jupiter. The second asteroid it photographed, called Ida, was discovered to have a moon which appears as a small dot to the right of Ida in this picture. The tiny moon, named Dactyl, is about one mile across, while the potato shaped Ida measures about 36 miles long and 14 miles wide. Dactyl is the first moon of an asteroid ever discovered. The names Ida and Dactyl are based on characters in Greek mythology.

July 1995

APOD: July 1, 1995 - The Hooker Telescope on Mt. Wilson
In the 1920s, pictures from the Hooker Telescope on Mt. Wilson fundamentally changed our understanding of the cosmos. Astronomer Edwin Hubble, using photographs he took with this telescope, demonstrated that the objects his contemporaries called "spiral nebulae" were actually huge systems of stars - spiral galaxies, similar to our own Milky Way galaxy but incredibly distant. Prior to Hubble's work it was argued that the spiral nebulae were mere clouds of gas and that they, along with everything else in the universe, were contained in our own galaxy. The Hooker Telescope mirror is 100 inches in diameter which is nearly the size of the mirror of the orbiting Hubble Space Telescope named in Hubble's honor. The Mount Wilson Observatory offers a virtual walking tour of this historic telescope.

APOD: July 2, 1995 - The Cartwheel Galaxy
The Cartwheel Galaxy shows a ring that is the result of a collision between a small and a large galaxy. After a small galaxy has moved through a big galaxy - in this case one that probably resembled our own Milky Way - a star formation wave moves out from the impact point like ripples across the surface of a pond. When galaxies collide it is rare that any two stars actually collide. Gravity, however, causes density waves to move out through the galaxy which in turn triggers the formation of hot, bright young stars, producing the ring that we see in this picture.

APOD: July 3, 1995 - The Great Nebula in Orion
The Great Nebula in Orion, M42, can be found on the night sky just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion. This nebula is one of the closest stellar nurseries - where young stars are being formed even now. Clumps of gas (mostly hydrogen and helium) and dust in the nebula are squeezed together by their own gravity until they collapse and form stars. Some stars we can see here partially obscured by the nebula, are only about 100,000 years old - just babies compared to the 5 billion (5,000,000,000) years of our Sun.

APOD: July 4, 1995 - The Firework Nebula
The Firework Nebula, known to astronomers as "GK Per", is the result of a type of stellar explosion called a nova. In a nova, a very compact star called a white dwarf blasts away gas that had accumulated on its surface. In this case the nova occurred in the year 1901 and is called Nova Persei 1901. This nova became as bright as one of the brighter stars we see in the night sky, but then faded until only a telescope could see it. Soon astronomers could see an expanding shell of gas that eventually became this spectacular nebula. The unusual "fireworks" type feature of this nebula is still a matter of research and discussion.

APOD: July 5, 1995 - The Night Side of Saturn
This image of Saturn was made in November 1980 by the Voyager 1 spacecraft as it flew past the ringed gas giant planet. From a spectacular vantage point, looking back toward the inner solar system, the robot spacecraft recorded this view of the night side of Saturn casting a sharp shadow across the bright rings. No Earth based telescope could ever take a similar picture. Since Earth is closer to the sun than Saturn, only the day side of the planet is visible from the Earth.

APOD: July 6, 1995 - Saturn, Rings, and Two Moons
This image of Saturn was made by NASA's robot spacecraft Voyager 2 as it began to explore the Saturn system in 1981. Saturn's famous rings are visible along with two of its moons, Rhea and Dione which appear as faint dots in the right and lower right part of the picture. Astronomers believe that Saturn's moons play a fundamental role in sculpting it's elaborate ring system.

APOD: July 7, 1995 - Lunar Farside from Apollo 13
In April of 1970, after an explosion damaged their spacecraft, the Apollo 13 astronauts were forced to abandon their plans to make the third manned lunar landing. Still, while coasting around the moon in their desperate attempt to return to earth they were able to photograph the moon's far side. The large, dark, smooth looking feature on the left in this picture is known as the "Mare Moscoviense". It was created by a lava flow filling in a large impact crater on the lunar surface. As suggested by the name, the Mare Moscoviense was first photographed by an early Soviet lunar probe.

APOD: July 8, 1995 - Damage to Apollo 13
In April of 1970, after an oxygen tank exploded and damaged their service module, the Apollo 13 astronauts were forced to abandon their plans to make the third manned lunar landing. The extent of the damage is revealed in this photo, taken as the crippled module was drifting away - jettisoned prior to their reentry and eventual safe splashdown. An entire panel on the right side of the module is seen to have been blown away and damage to internal structures is apparent.

APOD: July 9, 1995 - A Meteoric View of Apollo 13
Meteors, also called shooting stars, normally begin as bits of dust from the tails of comets or even small pieces chipped off asteroids. Falling toward Earth, these particles enter the atmosphere at extremely high speeds. Friction with the air heats them up and makes them glow brightly. Their rapid motion across the sky causes them to show up as bright streaks in photographs. In this picture, however, the bright streaks which appear to be meteor trails are believed to be two large pieces of the Apollo 13 spacecraft, the service and lunar modules, reentering the atmosphere.

APOD: July 10, 1995 - Abell 2218: A Galaxy Cluster Lens
Sometimes one of the largest concentrations of mass known can act like a lens. Almost all of the bright objects in this image are galaxies in the cluster known as Abell 2218. The cluster is so massive and so compact that it bends light from galaxies that lie behind it, causing many of them to appear as stretched out arcs. Many dim, elongated arcs are visible on this photograph. This picture was taken with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope.

APOD: July 11, 1995 - Microlensing of the Einstein Cross
The famous "Einstein Cross" is a case where a single object is seen four times. Here a very distant QSO happened to be placed right behind a massive galaxy. The gravitational effect of the galaxy on the distant QSO was similar to the lens effect of a drinking glass on a distant street light - it created multiple images. But stars in the foreground galaxy have been found to act as gravitational lenses here too! These stars make the images change brightness relative to each other. These brightness changes are visible on these two photographs of the Einstein Cross, taken about 3 years apart.

APOD: July 12, 1995 - Eta Carinae Before Explosion
The star Eta Carinae, at the center of the photo, will likely destroy itself in a spectacular explosion in a few million years - or sooner! Currently it is one of the brightest, most massive, and least stable stars known. Much of the gas in this Hubble Space Telescope photograph was blown off the star itself. Some of these gas clouds are similar in size to our solar system. Astronomers cannot yet fully explain the motions of the surrounding nebula, and continue to study this system.

APOD: July 13, 1995 - A String Of Pearls
Comet Shoemaker-Levy 9 named after its co-discoverers, was often referred to as the "string of pearls" comet. It is famous for its unusual appearance as well as its collision with the planet Jupiter! The comet's original single nucleus was torn to pieces by Jupiter's strong gravity during a close encounter with the solar system's largest planet in 1992. The pieces are seen in this composite of Hubble Space Telescope images to be "pearls" strung out along the comet's orbital path. In July of 1994 these pieces collided with Jupiter in a unique and spectacular series of events.

APOD: July 14, 1995 - Comet Impacts on Jupiter
In July of 1994, pieces of Comet Shoemaker-Levy 9, also known as the "string of pearls" comet, collided with the planet Jupiter. As the comet fragments smashed in to Jupiter, the resulting explosions scattered large quantities of dusty cometary debris into the Jovian atmosphere. The clouds of debris created the multiple dark smudges visible in this picture. Jupiter's rotation causes the successive impact sites to be strung out along the cloud bands while the strong winds cause the appearance of the smudges to change with time. Jupiter's famous red spot is also visible to the left of center.

APOD: July 15, 1995 - The Crater Chain
NASA's robot spaceprobe Voyager 1, took this closeup image of the surface of Jupiter's crater scarred moon Callisto in 1979. A mysterious chain of craters is seen to extend diagonally across the image (upper left to lower right). What could cause the craters to line up in such a regular fashion? Scientists were at a loss to explain this crater chain along with several other chain like features observed on Callisto's surface. Fifteen years later, with the discovery of Comet Shoemaker-Levy 9, also known as the "string of pearls" comet, the mystery was solved. Comets whose orbits stray too close to Jupiter are torn apart by the strong gravity. When the individual pieces, strung out along the orbital path of the comet hit an object like Callisto, the sequence of impacts produces a crater chain.

APOD: July 16, 1995 - The Exploration of Mars
Thirty years ago NASA's exploration of Mars began. In July of 1965 the Mariner 4 spacecraft flew within 6,000 miles of Mars and returned 21 pictures of the mysterious red planet. NASA's continued exploration of Mars has produced detailed views of the red tinged Martian surface like the one shown above which is a composite of 102 images from the Viking missions to Mars. The composite was constructed by the US Geological Survey.

APOD: July 17, 1995 - Barsoom
"Yes, I have been to Barsoom again ..." begins John Carter in Edgar Rice Burroughs' 1913 science fiction classic "The Gods of Mars". In Burroughs' novels describing Carter's adventures on Mars, "Barsoom" is the local name for the red planet. Long after Burroughs' stories were published, Mars has continued to capture the imagination of science fiction writers as a popular location for extraterrestrial adventures. This dramatic picture of a crescent Mars was taken by NASA's Viking 2 spacecraft in 1976.

APOD: July 18, 1995 - Cygnus Loop Supernova Shockwave
15,000 years ago a star in the constellation of Cygnus exploded. This picture shows a portion of a shockwave from this supernova explosion still expanding past nearby stars. The collision of this gaseous shockwave with a stationary gas cloud has heated the gas causing it to glow in a spectacular array of colors, known as the Cygnus Loop. This picture was taken with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope.

APOD: July 19, 1995 - The Mountains of Mars
Volcanic activity on Mars has produced towering mountains. The largest one, Olympus Mons, is pictured here in this Viking Orbiter image. Olympus Mons is a shield volcano nearly 15 miles high and over 300 miles wide at its base. By comparison, Earth's largest volcano, Mauna Loa in Hawaii, is just over 5 miles high and about 12 miles wide.

APOD: July 20, 1995 - The Grand Canyon of Mars
The Mariner Valley, also known as the Valles Marineris canyon system, appears in this mosaic of images from NASA's Viking spacecraft as a huge gouge across the red planet. This "Grand Canyon" of Mars is about 2500 miles long and up to 4 miles deep. By comparison, the Earth's Grand Canyon is less than 500 miles long and 1 mile deep.

APOD: July 21, 1995 - The Search for Life on Mars
Although images of Mars taken from space revealed the planet to have a barren and cratered surface, scientists did not give up the search for martian life. In 1976 NASA's Viking project succeeded in landing two robot probes on the surface of Mars. These landers were able to carry out sophisticated chemical experiments to look for signs of microscopic life in the martian soil. However, the experiments failed to produce any convincing evidence for life on Mars. Cameras onboard the Viking Landers also returned spectacular photos of the rocky martian landscape, like the one above, which showed no sign of martian animal or plant life.

APOD: July 22, 1995 - The Face on Mars
This image, showing what looks to be a human face sculpted on the martian surface, was produced using data from NASA's Viking 1 orbiter in 1976. Described in a press release as a "face-like hill" it caused some to offer the sensational speculation that it was an artificial construct built by an intelligent civilization on Mars! As a result, this image was splashed across the covers of many grocery store tabloids at the time. A detailed analysis of multiple images of this feature reveals a natural looking martian hill whose illusory face-like appearance depends on viewing angle and angle of illumination.

APOD: July 23, 1995 - M20: The Trifid Nebula
The vivid blue and violet colors present in the Trifid Nebula result from the abundance of young stars there. The light from young massive stars is quite blue and has the ability to remove electrons from surrounding gas. When these electrons re-combine with the gas, radiation rich in blue and violet light is emitted. Some of the nebula's light also results from the reflection of star light off of extremely small carbon specks known as 'dust'. This object is known to astronomers as M20 - the twentieth object on Charles Messier's list of diffuse sky objects. This image was taken with a 6-inch refracting telescope.

APOD: July 24, 1995 - M31: The Andromeda Galaxy
Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about 2 million years for light to reach us from there.

APOD: July 25, 1995 - M1: The Crab Nebula
In the year 1054 a star in the constellation of Taurus exploded in a spectacular supernova so bright it appeared to dominate the sky except for the Sun and Moon for many days. It left behind one of the most brilliant nebulae, listed first in Charles Messier's list of nebulous sky objects. Today we know that the center of the nebula houses the remnant of the explosion: a spinning neutron star called a pulsar. The Crab pulsar is visible in almost every part of the electromagnetic spectrum, and has been a useful astronomical tool. It is still unclear how the the pulsar emits the light that we see.

APOD: July 26, 1995 - M15: A Great Globular Cluster
A globular cluster is a system of about one million stars that together orbit a galaxy. One of the brightest globular clusters in our Milky Way galaxy is the pictured M15, the fifteenth object on Messier's list of diffuse objects on our sky. Most stars in globular clusters are older and redder than our Sun, which is about 5 billion years old.

APOD: July 27, 1995 - M57: The Ring Nebula
A star with mass similar to that of our Sun will throw off its outer gasses after fusion has stopped in its core. Possibly the most visually spectacular of these planetary nebula is the pictured Ring Nebula. The appearance as a ring is really an illusion of projection - the nebula is actually a spherical shell. At the center a blue dot is visible which is the old core of the star, known as a white dwarf. It is still not known exactly how the star throws off the gas that becomes the nebula.

APOD: July 28, 1995 - M82: An Irregular Galaxy
Not all galaxies have spiral structure like our Milky Way. Many have smooth elliptical shapes, but also many have irregular shapes such as the bright sky object M82, the 82nd object on Messier's list. The strange structure of this galaxy is thought to be caused by young stars ejecting gas in energetic bubbles, and by lanes of absorbing dust.

APOD: July 29, 1995 - M27: The Dumbbell Nebula
The Dumbbell Nebula is a beautiful red and blue planetary nebula in the constellation of Vulpecula. It is the 27th object on Charles Messier's list of diffuse sky objects, and so is referred to as M27. Its high surface brightness makes it a good target for small telescopes. The term planetary nebula, used to describe this general class of objects, is misleading. Although these objects may appear round and planet-like in small telescopes, astronomers have determined them to be stars surrounded by cocoons of gas blown off in the late stages of evolution.

APOD: July 30, 1995 - The Rings of Saturn
Saturn's spectacular system of bright rings has been the subject of study and wonder since Galileo first turned his telescope on the ringed planet in 1610. To Galileo, the blurry image produced by his small telescope was confusing. Saturn appeared to him to have "ear-like" appendages which he thought might be large moons. Eventually, larger telescopes revealed the incredible truth - Saturn was surrounded by bright rings. The image above, made by NASA's Voyager 2 spacecraft, further reveals the intricate structure of the ring system. The image has been computer enhanced and color coded to bring out the subtle details.

APOD: July 31, 1995 - Exploring Saturn's Rings
By watching a star flicker and fade as it passed behind Saturn's rings, NASA's Voyager 2 spacecraft was able explore the ring system in amazing detail. Data produced by Voyager's instruments as the star Delta Scorpii was occulted by some of the outer rings was used to reconstruct this image which shows details almost 1000 times smaller than normally possible with Voyager's cameras.

August 1995

APOD: August 1, 1995 - Crossing The Ring Plane
"I do not know what to say in a case so surprising, so unlooked for and so novel." announced Galileo when Saturn's rings appeared to vanish in 1612. In fact, every 15 years Saturn's rings seem to almost disappear as viewed from the Earth. This happens just as the orbiting Earth crosses the plane of Saturn's rings. The edge on perspective temporarily robs astronomers of a spectacular sight, however, the ring plane crossing affords them the opportunity to measure the rings' thickness and search for undiscovered moons. In this image of Saturn, produced on May 22, 1995 by NASA's Hubble Space Telescope, two of Saturn's, known moons are visible as star like objects to the left of the planet.

APOD: August 2, 1995 - Jupiter's Rings
Astronomers using NASA's Voyager spacecraft to search for a ring system around Jupiter discovered these faint rings in 1979. Unlike Saturn's bright rings which are composed of chunks of rock and ice, Jupiter's rings appear to consist of fine particles of dust. One possibility is that the dust is produced by impacts with Jupiter's inner moons. This false color image has been computer enhanced. The gas giant planets, Jupiter, Saturn, Uranus, and Neptune are all known to have rings.

APOD: August 3, 1995 - Io: A Volcanic Moon
In 1610, Galileo turned his telescope to the heavens and discovered that the planet Jupiter had four bright moons. The innermost of these Galilean moons, Io, turned out to be one of the most exotic objects in the solar system. About the size of the earth's moon, Io is covered with volcanoes, many of which are currently active. The material expelled in the volcanic eruptions may contain compounds of Sulfur which take on a variety of colors and could account for its mottled, "pizza-like" appearance.

APOD: August 4, 1995 - Closeup of an Io Volcano
In 1979, one of NASA's Voyager spacecraft made a spectacular and unexpected discovery. Io, the innermost Galilean moon of Jupiter, was covered with volcanoes and some of them were erupting! In all, Voyager 1 observed nine volcanic eruptions during its encounter with the moon. When Voyager 2 flew past four months later it was able to confirm that at least six of them were still erupting. This Voyager image of Ra Patera, a large shield volcano, shows colorful flows up to about 200 miles long emanating from the dark central volcanic vent.

APOD: August 5, 1995 - Geysers on Triton
In August of 1989 NASA's Voyager 2 spacecraft passed by Neptune, the most distant of the solar system's gas giant planets. Its encounter with Neptune climaxed with its closest approach to Neptune's largest moon Triton. From a distance of about 24,000 miles the robot space probe surveyed Triton's surface, whose temperature averages nearly -400 degrees Fahrenheit, and discovered surprising evidence of a complex and active world. For example, the prominent dark streaks in this image seem to come from small volcanoes and may consist of nitrogen frost mixed with organic compounds ejected during geyser-like eruptions.

APOD: August 6, 1995 - Liftoff of Space Shuttle Columbia
On April 12, 1981, space flight entered a new era with the first launch of Space Shuttle Columbia, shown above. NASA's Space Shuttles land like a normal plane, carry a heavy cargo, carry a large crew, make use of cheap solid fuel, and are reusable. Previous to this flight, no manned orbiting space ship had ever landed on a runway. Space Shuttles now are the flagships and the workhorses of NASA's space going rockets.

APOD: August 7, 1995 - Night Launch of Endeavour
Space Shuttle Endeavour thunders off into orbit in a rare night launch. This March 1995 mission is most famous for operating the set of astronomical telescopes known as Astro-2. Astro-2's telescopes observed the universe in ultraviolet light - light so blue humans by themselves can't see it. Astro-2 measured the amount of ultraviolet light emitted by several interesting stars, galaxies, planets, and QSOs. Analysis of data from this NASA mission has yielded the first clear detection of helium created during the beginning of the universe.

APOD: August 8, 1995 - Columbia Waits, Discovery Launches
Space Shuttle Discovery launches while the Space Shuttle Columbia is readied for a future mission. Space shuttles are launched from the Kennedy Space Center in Florida. At this date there are four operating space shuttles: Atlantis, Columbia, Discovery, and Endeavour. This mission is most famous for carrying the Hubble Space Telescope into orbit. The Hubble Space Telescope is the largest public optical telescope ever to operate in orbit, and because it is above Earth's atmosphere it's photographs are more clear than any ground based telescope. Hubble continues to make fundamental discoveries about planets, stars, galaxies, comets, QSOs, and the universe we live in.

APOD: August 9, 1995 - Challenger Launches Spacelab 2
Space Shuttle Challenger jumps off the launch pad in 1985 to begin a successful mission. The main purpose of this mission was to test a new space based laboratory named Spacelab 2. Spacelab 2 takes advantage of the effectively weightless conditions in orbit to carry out experiments in areas such as astronomy, physics, life science, materials science, and atmospheric physics. Challenger and her crew were lost in 1986 when a booster failure resulted in the breakup of the vehicle. Previously, Challenger and her crews flew nine successful Space Shuttle missions and made significant contributions to America's scientific growth.

APOD: August 10, 1995 - The Orbiting Hubble Space Telescope
The Hubble Space Telescope (HST) is the largest orbiting public optical telescope in history. Its 2.4 meter diameter reflecting mirror and its perch above Earth's atmosphere allow it to create exceptionally sharp images. Originally launched in 1990, HST optics were repaired to their intended accuracy in 1993 by the first of several regular servicing missions. Astronomers using HST continue to make numerous monumental scientific discoveries, including new estimates of the age of our universe, previously unknown galaxies, evidence of massive black holes in the centers of galaxies, previously unknown moons, and a better understanding of physical processes in our universe.

APOD: August 11, 1995 - The Compton Gamma Ray Observatory
The Compton Gamma Ray Observatory (CGRO) was the most massive instrument ever launched by a NASA Space Shuttle and continues to revolutionize gamma-ray astronomy. This orbiting observatory sees the sky in gamma-ray photons - light so blue humans can't see it. These photons are blocked by the Earth's atmosphere from reaching the Earth's surface. Results from CGRO have shown the entire universe to be a violent and rapidly changing place - when viewed in gamma-rays. Astronomers using CGRO data continue to make monumental discoveries, including showing that mysterious flashes of gamma-rays are much more powerful than previously imagined, discovery of a whole new class of QSO, and discovery of objects so strange that astronomers can't yet figure out what they are.

APOD: August 12, 1995 - Atlantis Landing
Space Shuttle Atlantis lands at Edwards Air Force Base in California. The Space Shuttle is the first orbital space vehicle to land on a runway like an airplane. Space Shuttles sometimes have the option of landing in White Sands, New Mexico or at the Shuttle Landing Facility at the Kennedy Space Center in Florida. The runways the shuttles land on are among the longest in the world. Shuttles that do not land in Florida must generally be strapped to the back of a 747 airplane and flown back.

APOD: August 13, 1995 - The Sun Flares
The sun was captured in 1973 throwing one of the largest flares ever recorded. Sol, our sun, is a normal star. It formed about 5 billion years ago, and will last about another 5 billion years. The sun will never explode, and a solar flare will never destroy the earth. Eventually the sun will become a white dwarf star. The sun is made of mostly hydrogen and helium. The sun's center is so hot that when hydrogen nuclei collide, they stick together and release energy - a process called nuclear fusion No one knows why the center of the sun emits so few neutrinos.

APOD: August 14, 1995 - Mercury: Closet Planet to the Sun
This picture was compiled from images taken by the NASA spacecraft Mariner 10 which flew by the planet three times in 1974. Mercury is the closest planet to the Sun, the second hottest planet (Venus gets hotter), and the second smallest planet (Pluto is smaller). Mercury rotates so slowly that one day there - "day" meaning the normal time it takes from sunset to sunset - lasts 176 days on Earth. It is difficult to see Mercury not because it is dim but because it always appears near the Sun, and is therefore only visible for a short time just after sunset or just before sunrise. Mercury is made of rocky material like Earth. No one knows why Mercury has the magnetic field that it does.

APOD: August 15, 1995 - Venus: Earth's Sister Planet
This picture in visible light was taken by the Galileo spacecraft. Venus is very similar to Earth in size and mass - and so is sometimes referred to as Earth's sister planet - but Venus has a quite different climate. Venus' thick clouds and closeness to the Sun (only Mercury is closer) make it the hottest planet - much hotter than the Earth. Humans could not survive there, and no life of any sort has ever been found. When Venus is visible it is usually the brightest object in the sky after the Sun and the Moon. More than 20 spacecraft have visited Venus including Venera 9, which landed on the surface, and Magellan, which used radar to peer through the clouds and make a map of the surface. There are still many things about Venus's unusual atmosphere that astronomers don't understand.

APOD: August 16, 1995 - Uranus: The Tilted Planet
This picture was snapped by the Voyager 2 spacecraft in 1986 - the only spacecraft ever to visit Uranus. Uranus is the third largest planet after Jupiter and Saturn. Uranus has many moons and a ring system. Uranus is composed mostly of liquid water, methane and ammonia, surrounded by a thick gas atmosphere of mostly hydrogen and helium. Uranus is peculiar in that its rotation axis is greatly tilted and sometimes points near the sun. It remains an astronomical mystery why Uranus' axis is so tilted. Uranus and Neptune are very similar.

APOD: August 17, 1995 - Neptune: Big Blue Giant
This picture was taken by the Voyager 2 spacecraft in 1986 - the only spacecraft ever to visit Neptune. Neptune will be the furthest planet from the Sun until 1999, when the elliptical orbit of Pluto will cause it to once again resume this status. Neptune, like Uranus, is composed mostly of liquid water, methane and ammonia, is surrounded by a thick gas atmosphere of mostly hydrogen and helium, and has many moons and rings. Neptune's moon Triton is unlike any other and has active volcanoes. The nature of Triton's unusual orbit around Neptune is the focus of much discussion and speculation.

APOD: August 18, 1995 - Pluto: The Frozen Planet The Hubble Space Telescope imaged Pluto and its moon Charon in 1994. Pluto is usually the most distant planet from the Sun but because of its elliptic orbit Pluto crossed inside of Neptune's orbit in 1979 and will cross back out again in 1999. Compared to the other planets, very little is known about Pluto. Pluto is smaller than any other planet and even smaller than several other planet's moons. From Pluto, the Sun is just a tiny point of light. Pluto is probably composed of frozen rock and ice, much like Neptune's moon Triton. Pluto has not yet been visited by a spacecraft, but a mission is being planned for the next decade.

APOD: August 19, 1995 - Our Solar System from Voyager
After taking its spectacular pictures of the outer solar system planets, Voyager 1 looked back at six planets from the inner solar system. Here Venus, Earth, Jupiter, Saturn, Uranus, and Neptune, were all visible across the sky. Each, however, was now just a small speck of light, dimmer than many of the stars in the sky. Voyager 1 is only one of four human-made objects to leave our Solar System, the other three being Voyager 2, and Pioneer 10 and 11.

APOD: August 20, 1995 - Announcing Comet Hale-Bopp
The pictured fuzzy patch may become one of the most spectacular comets this century. Although it is very hard to predict how bright a comet will become, Comet Hale-Bopp, named for its discoverers, was spotted further from the Sun than any previous comet - a good sign that it could become very bright, easily visible to the naked eye. This picture was taken on July 25th 1995, only two days after its discovery. A comet bright enough to see without a telescope occurs only about once a decade. The large coma and long tail of bright comets are so unusual and impressive that they have been considered omens of change by many cultures. A comet does not streak by in few seconds - but it may change its position and structure noticeably from night to night.

APOD: August 21, 1995 - An Orbiting Iceberg
A comet nucleus, formed from the primordial stuff of the solar system, resembles a very dirty iceberg. Orbiting far from our Sun, it can remain frozen, preserved for billions of years. Occasionally, a chance gravitational encounter will alter this distant orbit and send the nucleus plummeting towards the inner solar system. In 1986, the European spacecraft Giotto visited the nucleus of Halley's comet as it approached the sun. Data from Giotto's cameras were used to produce this enhanced image which shows surface features on the dark nucleus against the bright background of clouds of gasses produced as the icy material was vaporized by the Sun's heat. The potato shaped nucleus measures about 10 miles across.

APOD: August 22, 1995 - Venus UnVeiled
The surface of Venus is perpetually covered by a veil of thick clouds and remains hidden from even the powerful telescopic eyes of earth-based astronomers. However, using an imaging radar technique, the Magellan spacecraft was able to lift the veil from the Face of Venus and produce spectacular high resolution images of the planet's surface. The bright area running across the middle of this picture represents the largest highland region of Venus known as Aphrodite Terra. The large highland regions are analogous to continents on Earth. Scientists are particularly interested in exploring the geology of Venus because of its similarity to Earth. For more information about Venus and this image see the Overview of Venus.

APOD: August 23, 1995 - A Venusian Tick
Data from the Magellan spacecraft has shown the Face of Venus to contain a host of volcanic features. This image shows an example of a fairly common type of venusian volcanic feature. Known as a "tick" it represents a volcano about 20 miles wide at the summit with ridges and valleys radiating down its sides lending it an insect like appearance. Scientists are particularly interested in exploring the geology of Venus. Because of its similarity with the Earth in size, density, and overall location in the solar system, Venus may offer key insights into the workings of our own planet. Do the familiar processes of volcanism and plate tectonics occur on our sister planet as well as our own? The detailed radar mapping of the planet performed by the Magellan probe has gone a long way toward answering this question.

APOD: August 24, 1995 - A Radar Image of Planet Earth
This image of Mt. Rainier, Washington USA, planet Earth, was produced by the Spaceborne Radar Laboratory which flew on the Space Shuttle Endeavour in 1994. Radar, short for RAdio Detection And Ranging, is a technique which coordinates the operation of a radio transmitter and receiver to measure the direction, strength, and timing of radio echos from the surface of distant objects. An actual image of an object can be constructed by recording and analyzing many echos. One advantage of using radar imaging in planetary studies is that images can be made regardless of cloud cover or lighting conditions. During the early 90s, NASA's Magellan spacecraft was able to use radar imaging to produce similar high resolution maps of the surface of Venus.

APOD: August 25, 1995 - A World Explorer
Ferdinand Magellan was a world explorer. Many consider him the greatest navigator of Europe's 16th century age of sea going exploration and credit his expedition with the first circumnavigation of planet Earth. NASA's Venus probe, the aptly named Magellan spacecraft shown above in an artist's conception, provided a global view of the poorly known surface of Venus - just as Magellan's expedition provided the beginnings of a global perspective of the Earth. Ferdinand Magellan's expedition of 5 ships and 265 men left Spain in 1519 in search of a western route to the Spice Islands of Indonesia. In 1522 one ship and 17 men returned. NASA launched the Magellan probe on May 4, 1989. Placed in a polar orbit, Magellan's many circumnavigations resulted in a detailed radar mapping of 98% of the Venusian surface. As pictured, the radar mapper's antenna resembles a large inverted bowl. Power for the radar was produced by the wing like solar panels. In October of 1994, the Magellan probe entered the Venusian atmosphere and ground controllers lost contact with the spacecraft.

APOD: August 26, 1995 - Two Tails of Comet West
Here Comet West is seen showing two enormous tails that wrap around the sky. The ion tale of a comet usually appears more blue and always points away from the Sun. The dust tail trailing the comet's nucleus is the most prominent. Comet West was a visually spectacular comet, reaching its most picturesque in March of 1976. A comet this bright occurs only about once a decade. Comets are really just large dirty snowballs that shed material when they reach the inner solar-system. Many astronomers are hopeful that Comet Hale-Bopp will look as spectacular as this in the spring of 1997.

APOD: August 27, 1995 - Gamma Ray Bursts from the Unknown
Gamma Ray Bursts (GRBs) pose one of the greatest mysteries of modern astronomy. About once a day, the gamma-ray sky lights up with a spectacular explosion. No one knows what causes these explosions or even how far away they are. The above map represents the entire sky in coordinates centered on our Galaxy, the Milky Way. It shows the positions of over 800 of these mysterious bursts of energy detected by the BATSE instrument on board NASA's Compton Gamma Ray Observatory. Before BATSE, most astronomers thought that most GRBs occurred in the disk of our Galaxy, but the above sky map shows little sign of this. The distance scale of GRBs was the topic of a historic debate in April 1995. The positions in the above map are currently being studied in great detail in an effort to uncover a clue about the nature of GRBs. In the above 3B map created by Robert Nemiroff, spot size is proportional to peak flux and spot color is indicative of hardness. Click here for a postscript version of BATSE's latest map of 1122 GRB locations (3B Catalog).

APOD: August 28, 1995 - Dusty Galaxy Centaurus A
One of the most unusual galaxies known, Centaurus A, is pictured above. Cen A is marked by dramatic dust lanes that run across the galaxy's center. These dust lanes are so thick they almost completely obscure the galaxy's center in visible light. Our Milky Way Galaxy contains dust, but not in the same proportion. Cen A is also unusual compared to a normal galaxy because it contains a higher proportion of young blue stars, is a very strong source of radio emission, and has a unique structure. Cen A is thought to be the result of the collision of two normal galaxies.

APOD: August 29, 1995 - Saturn V: NASA's Largest Rocket
Pictured, a NASA Saturn V rocket blasts off on July 16th, 1969 carrying the crew of Apollo 11 to the Moon. The Saturn V rocket was the largest rocket ever used by NASA, and the only one able to lift the large masses needed to land astronauts on the moon and returning them safely. Saturn V rockets launched all of the Apollo moon missions, and several to Earth orbit as well.

APOD: August 30, 1995 - Skylab Over Earth
Skylab was an orbiting laboratory launched by a Saturn V rocket in May 1973. Skylab was visited three times by NASA astronauts who sometimes stayed as long as two and a half months. Many scientific tests were preformed on Skylab, including astronomical observations in ultraviolet and X-ray light. Some of these observations yielded valuable information about Comet Kohoutek, our Sun and about the mysterious X-ray background - radiation that comes from all over the sky. Skylab fell back to earth on 11 July 1979.

APOD: August 31, 1995 - X-Raying the Moon
Above is a picture of the Moon taken in X-rays by the Roentgen Observatory Satellite ROSAT in 1990. This famous picture shows three distinct regions: a bright X-ray sky, a bright part of the Moon, and a relatively dark part of the Moon. The bright X-ray region is exemplary of the mysterious X-ray background that is seen everywhere on the sky. The bright lunar crescent shines because it reflects X-rays emitted by the Sun. The dark lunar face is surprising because it is not completely dark, and its slight emission is thought to result from energetic particles from the solar wind striking the Moon.

September 1995

APOD: September 1, 1995 - Subrahmanyan Chandrasekhar 1910-1995
On August 21, 1995 one of the greatest astrophysicists of modern times passed away. Subrahmanyan Chandrasekhar was a creative, prolific genius whose ability to combine mathematical precision with physical insight changed humanity's view of stellar physics. His most famous discovery was that not all stars end up as white dwarf stars, but those retaining mass above a certain limit - today known as "Chandrasekhar's limit," undergo further collapse. His detailed mathematical papers and books on a wide variety of astrophysical subjects, including, for example, black holes, are classic references for research at every level. Obituaries are available from the University of Chicago Press and Reuters News Service, and a WWW page has been set-up to record personal memories.

APOD: September 2, 1995 - Hot Gas and Dark Matter
Is the gravity of the above galaxies high enough to contain the glowing hot gas? Superposed on an optical picture of a group of galaxies is an image taken in X-ray light. The X-ray picture, taken by ROSAT, shows confined hot gas highlighted in false red color, and provides clear evidence that the gravity exerted in groups and clusters of galaxies exceeds all the individual component galaxies combined. The extra gravity is attributed to dark matter, the nature and abundance of which is the biggest mystery in astronomy today.

APOD: September 3, 1995 - Earth's Moon, A Familiar Face
The above mosaic of the Earth's Moon was compiled from photos taken by the spacecraft Clementine in 1994. This image represents the side of the Moon familiar to Earth dwellers. The Moon revolves around the Earth about once every 28 days. Since its rate of rotation about its axis is also once in 28 days, it always keeps the same face toward the Earth. As the Moon travels around its orbit, the Earth based view of the half of the Moon that faces the Sun changes causing the regular monthly progression of Lunar phases. Humans first crashed a spacecraft into the Moon in 1959, but the first humans to reach the Moon landed in 1969. There are now golf balls on the Moon.

APOD: September 4, 1995 - Ganymede: Moonquake World
Ganymede probably undergoes frequent ground shaking events not unlike terrestrial earthquakes. Ganymede, the largest moon of Jupiter and the Solar System, has a thick outer coating of water ice. Passing Voyager spacecraft found a large number of cracks and grooves in the ice so it is thought that Ganymede, like the Earth, has large shifting surface masses called tectonic plates. Ganymede was discovered by Galileo and Marius in 1610, and is larger than the planets Mercury and Pluto. The NASA spacecraft Galileo is scheduled to arrive at Jupiter is December of 1995.

APOD: September 5, 1995 - Europa: Ancient Water World
Beneath the cold icy surface of Jupiter's moon Europa are probably the only oceans of water in our Solar System outside of Earth. These oceans, possibly 50 kilometers deep, might also be the most likely local place to find extra-terrestrial life. Europa's smooth surface is unlike any other known planet or moon, giving evidence for relatively few craters or mountains. Europa was discovered by Galileo and Marius in 1610. The NASA spacecraft Galileo is scheduled to arrive at Jupiter in December of 1995.

APOD: September 6, 1995 - Callisto: Dark Smashed Iceball
Callisto is a dirty battered world, showing the most beaten surface of Jupiter's major moons. Made of a rocky core covered by fractured ice, Callisto's past collisions with large meteors are evident as large craters surrounded by concentric rings. The four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto were all discovered by Galileo and Marius in 1610 with early telescopes and are now known as the Galilean satellites. The NASA spacecraft Galileo is scheduled to arrive at Jupiter is December of 1995.

APOD: September 7, 1995 - Distant Galaxies
This Hubble Space Telescope image of a group of faint galaxies "far, far away" is a snap shot of the Universe when it was young. The bluish, irregularly shaped galaxies revealed in the image are up to eight billion light years away and seem to have commonly undergone galaxy collisions and bursts of star formation. Studying these objects is difficult because they are so faint, however they may provide clues to how our own Milky Way Galaxy formed.

APOD: September 8, 1995 - The Milky Way's Center
NASA's COBE satellite scanned the heavens at infrared wavelengths in 1990 and produced this premier view of the central region of our own Milky Way Galaxy. The Milky Way is a typical spiral galaxy with a central bulge and extended disk of stars. However, gas and dust within the disk obscure visible wavelengths of light effectively preventing clear observations of the center. Since infrared wavelengths, are less affected by the obscuring material, the Diffuse InfraRed Background Experiment (DIRBE) on board COBE was able to detected infrared light from stars surrounding the galactic center and produce this image. Of course, the edge on perspective represents the view from the vicinity of our Sun, a star located in the disk about 30,000 light years out from the center. The DIRBE experiment used equipment cooled by a tub of liquid helium to detect the infrared light which, composed of wavelengths longer than red light, is invisible to the human eye.

APOD: September 9, 1995 - The Last Moon Shot
In 1865 Jules Verne predicted the invention of a space capsule that could carry people. In his science fiction story "From the Earth to the Moon", he outlined his vision of constructing a cannon in Florida so powerful that it could shoot a "Projectile-Vehicle" carrying three adventurers to the Moon. Over 100 years later, NASA, guided by Wernher Von Braun's vision, produced the Saturn V rocket. This rocket turned Verne's fiction into fact, launching 9 Apollo Lunar missions and allowing 12 astronauts to walk on the Moon. Pictured above is the last moon shot, Apollo 17, awaiting a night launch in December of 1972. Spot lights play on the rocket and launch pad while the full Moon looms in the background. Humans have not walked on the lunar surface since. Should we return to the Moon?

APOD: September 10, 1995 - White Dwarfs Cool
The circled stars in the above picture are from a class that is hard to see in the cosmos: white dwarfs. The entire photo covers a small region near the center of a globular cluster known as M4. Researchers using the Hubble Space Telescope discovered a large concentration of white dwarfs in M4. This was expected - low mass stars, including the Sun, are known to evolved to the white dwarf stage. White dwarfs do not usually evolve further, they just gradually cool down from their high temperatures. It is hoped that studying how these stars cool could lead to a better understanding of their ages, of the age of their parent globular cluster, and hopefully even the age of our universe!

APOD: September 11, 1995 - Proplyds: Infant Solar Systems
The fuzzy blobs seen above may be some of the first ever images of entire solar systems forming right before our eyes. This close up of the Orion Nebulae taken by the Hubble Space Telescope shows only a few stars, but these stars are surrounded by proto-planetary disks known as "proplyds." As the stars have only just recently formed - in the past few million years - the disks around them are likely condensing to form planetary systems and may be similar to the disk that formed our own solar system 5 billion years ago. These HST results suggest that stars with planets may be relatively common place. Are there extra-terrestrial civilizations out there as well?

APOD: September 12, 1995 - Spiral Galaxy M83
Long winding spiral arms are clearly evident on this spectacular picture of the spiral galaxy M83. The blue color of the spiral arms is caused by the relatively large fraction of young blue stars there. Dark dust lanes are mixed in with the stars and trace the spiral structure of the galaxy. This galaxy contains many billions of stars, and its light took many millions of years to reach us. Our own Milky Way Galaxy would appear similar to this if viewed from M83! This picture is number eight on a publicly posted list of images from the Anglo-Australian Telescope (AAT).

APOD: September 13, 1995 - Elliptical Galaxy M87
Elliptical galaxy M87 is a type of galaxy that looks much different than our own Milky Way Galaxy. But even for an elliptical galaxy M87 is peculiar. M87 is much bigger than an average galaxy, appears at the center of a whole cluster of galaxies known as the Virgo Cluster, and shows a very high number of globular clusters. These globular clusters are visible as faint spots surrounding the bright center of M87. In general, elliptical galaxies contain similar numbers of stars as spiral galaxies, but are ellipsoidal in shape (spirals are mostly flat), have no spiral structure, and little gas and dust. This picture is number sixty on a publicly posted list of images from the Anglo-Australian Telescope (AAT).

APOD: September 14, 1995 - The Far Side
This historic picture was humanity's first glimpse of the far side of the Moon. It was taken by the Soviet spacecraft Luna 3 in October of 1959. Luna 3 followed closely on the heels of another Soviet probe, Luna 2, which had become the first spacecraft to impact the Moon on September 13th of that same year. Why does the Moon have a far side? Gravitational tidal forces within the Earth-Moon system have synchronized the Moon's period of rotation around its axis with its orbital period at about 28 days. So, as the Moon moves around its orbit its rotation exactly compensates, keeping the same face toward the Earth.

APOD: September 15, 1995 - Space Station Mir Over Earth
This picture of the Russian space station Mir over the Pacific Ocean was recorded by the Space Shuttle Discovery in February 1995. During this mission Discovery docked with Mir and many scientific experiments and astronomical observations were completed jointly by the American astronauts and the Russian cosmonauts. An IMAX camera took many pictures of this historic encounter. Some cosmonauts have spent more than a year on board Mir, the longest anyone has ever lived in space. Work on an International Space Station is in progress.

APOD: September 16, 1995 - Rockets and Robert Goddard
Robert H. Goddard, one of the founding fathers of modern rocketry, was born in Worcester Massachusetts in 1882. As a 16 year old, Goddard read H.G. Wells' science fiction classic "War Of The Worlds" and dreamed of spaceflight. By 1926 he had designed, built, and launched the world's first liquid fuel rocket. During his career he was ridiculed by the press for suggesting that rockets could be flown to the Moon, but he kept up his experiments in rocketry supported in part by the Smithsonian Institution and championed by Charles Lindbergh. Pictured above in 1937 in the desert near Roswell New Mexico, Goddard examines a nose cone and parachute from one of his test rockets. Widely recognized as a gifted experimenter and engineering genius, his rockets were many years ahead of their time. He died in 1945 holding over 200 patents in rocket technology. A liquid fuel rocket constructed on principles developed by Goddard landed humans on the Moon in 1969.

APOD: September 17, 1995 - Thousands of Coma Cluster Galaxies
Almost every object in the above photograph is a galaxy. The Coma Cluster of galaxies pictured is a dense cluster containing many thousands of galaxies. Many of these galaxies contain as many stars as our own Milky Way Galaxy. Although nearby when compared to most other clusters, light from the Coma Cluster still takes hundreds of millions of years to reach us. In fact, the Coma Cluster is so big it takes light millions of years just to go from one side to the other! This picture was created at the WWW site Skyview, a "virtual observatory" where it is possible to view any part of the sky in wavelengths from radio to gamma-ray.

APOD: September 18, 1995 - The Large Cloud of Magellan
Magellan and his crew had plenty of time to study the southern sky during their famous voyage around the world. As a result, two fuzzy cloud like objects, nestled among the southern constellations of Doradus and Tucana are now known as the Clouds of Magellan. The Magellanic Clouds are small irregular galaxies, satellites of our larger Milky Way spiral galaxy. The Large Magellanic Cloud (LMC) pictured above is the closest galaxy to our own Milky Way, at a distance of about 180,000 light years. The Magellanic Clouds are joined to the Milky Way by a stream of cold hydrogen gas whose origin is still controversial. An unusual effect called gravitational lensing has recently been detected in a few LMC stars, and there is hope this could tell us important information about the true composition of our universe.

APOD: September 19, 1995 - The Small Cloud of Magellan
Almost unknown to casual observers in the northern hemisphere, the southern sky contains two diffuse wonders known as the Magellanic Clouds. The Magellanic Clouds are small irregular galaxies orbiting our own larger Milky Way spiral galaxy. The Small Magellanic Cloud (SMC), pictured here, is about 250,000 light years away and contains a preponderance of young, hot, blue stars indicating it has undergone a recent period of star formation. There is evidence that the SMC is actually two galaxies superposed to appear as one. The bright blob near the right hand edge of the frame is a globular cluster near the outskirts of the Milky Way.

APOD: September 20, 1995 - GL 105C: The Coolest Star?
Is the dim star to the upper right of this false-color picture the coolest possible normal star? From this recent picture by the Hubble Space Telescope, astronomers have estimated its mass is just high enough for it to fuse hydrogen into helium in its core. In general, the higher an object's mass, the higher it's core density and temperature. Above a certain point, the intense core conditions cause hydrogen atoms to move so fast that many stick or "fuse" after collision, releasing enormous amounts of energy. Were this object less massive, the object would shine by gravitational contraction and so be termed a "brown dwarf" rather than a normal main-sequence "star." The star on the left is so much brighter than the "coolest star" that it creates the white streak and dramatic pattern visible in the image. More information is given by the Space Telescope Scientific Institute's press release.

APOD: September 21, 1995 - One Small Step
On July 20th, 1969, a human first set foot on the Moon. Pictured above is the first lunar footprint. The footprint and distinction of the first person to walk on the Moon belong to Neil Armstrong. It has been estimated that one billion people world-wide watched Armstrong's first step - making the live transmission from a camera mounted on the lunar lander the highest rated television show ever. Upon setting foot on the moon, Armstrong said: "That's one small step for (a) man, one giant leap for mankind." The Apollo missions to the Moon have been described as the result of the greatest technological mobilization the world has known.

APOD: September 22, 1995 - Standing on the Moon
Pictured, the second person to walk on the Moon: Edwin "Buzz" Aldrin. During this Apollo 11 mission, Neil Armstrong and Buzz Aldrin landed on the Moon while Michael Collins circled in the Command Module above. The lunar team erected a plaque on the surface that reads: HERE MEN FROM THE PLANET EARTH FIRST SET FOOT UPON THE MOON JULY 1969 A.D., WE CAME IN PEACE FOR ALL MANKIND. The Apollo missions demonstrated that it is possible to land humans on the Moon and return them safely.

APOD: September 23, 1995 - Titan: Saturn's Smog Moon
The largest moon of Saturn is a rare wonder. Titan is the only one of Saturn's moons with an atmosphere, and one of only two moons in the Solar System with this distinction (Neptune's Triton is the other). Titan's thick cloudy atmosphere is mostly nitrogen, like Earth's, but contains much higher percentages of "smog-like" chemicals such as methane and ethane. The smog may be so thick that it actually rains "gasoline-like" liquids. The organic nature of some of the chemicals found in Titan's atmosphere cause some to speculate that Titan may harbor life! Because of its thick cloud cover, however, Titan's actual surface properties remain mysterious. Voyager 1 flew by in 1980 taking the above picture, and recently much has been learned from Hubble Space Telescope observations. The Cassini mission currently scheduled for launch in 1997 will map Titan's surface, helping to solve some of its mysteries.

APOD: September 24, 1995 - Mimas: Small Moon with A Big Crater
Mimas is one of the smaller moons of Saturn but shows one of the largest impact craters! In fact, if the impact had been much greater, it would have disrupted the entire satellite. The large crater has been named Herschel after the 1789 discoverer of Mimas, Sir William Herschel. Mimas' low mass produces a surface gravity just strong enough to create a spherical body but weak enough to allow such relatively large surface features. Mimas is made of mostly water ice with a smattering of rock - so it is accurately described as a big dirty snowball. Voyager 1 flew by in 1980 and took the above picture.

APOD: September 25, 1995 - Orion's Horsehead Nebula
The black indentation to the red emission nebula seen just to the right of center of the above photograph is one of the most famous features in any nebulae on the sky. Because of its shape, it is known as the Horsehead Nebula. The bright star near the center is located in the belt of the familiar constellation of Orion. The horse head feature is dark because it is really a dense dust cloud which lies in front of the bright nebula and blocks the light. Like clouds in our sky, this cosmic cloud has chanced to assume a recognizable shape. After thousands of years, the internal motions of the cloud will alter its appearance. The emission nebula's red color is caused by electrons recombining with protons to form hydrogen atoms. Also visible in the picture are blue reflection nebulae. This type of nebula contains dust which preferentially reflects the blue light of nearby stars.

APOD: September 26, 1995 - Star Trails in Southern Skies
As the Earth spins on its axis, the sky seems to rotate around us. This motion produces the beautiful concentric arcs traced out by the stars in this time exposure of the night sky. In the foreground of the picture is the dome of the Anglo-Australian Telescope in central New South Wales Australia. In the middle of the picture is the South Celestial Pole, easily identified as the point in the sky at the center of all the star trail arcs.

APOD: September 27, 1995 - A Venus Landing
This image is part of the first color panoramic view from Venus. It was transmitted by a TV camera on the Soviet Venera 13 lander which parachuted to the surface on March 1, 1982. Venus' clouds are composed of sulfuric acid droplets while its surface temperature is about 900 degrees Fahrenheit (482 degrees C) at an atmospheric pressure of 92 times that of sea-level on Earth. Despite these harsh conditions, the Venera 13 lander survived long enough to send back a series of images and perform an analysis of the Venusian soil. Part of the lander itself is visible in the lower right portion of the image. An earlier Soviet Venus lander, Venera 7 (1970), was the first spacecraft to return data from the surface of another planet.

APOD: September 28, 1995 - A Venusian Landscape
This computer generated view of a Venusian volcano was created using data from NASA's Magellan spacecraft. Magellan used its onboard radar to map the surface of Venus which is hidden from telescopic observations by a perpetual cloud cover. Using this radar data to provide three dimensional information, a computer was then able to produce this view of Maat Mons, a 5 mile high volcano, from a dramatic perspective. The colors used to render the surface are based on earlier color images transmitted by TV cameras on the Soviet Venera 13 and 14 Venus landers.

APOD: September 29, 1995 - The International Ultraviolet Explorer
The International Ultraviolet Explorer (IUE) was launched by a NASA Delta rocket in 1978 to provide a space telescope for ultraviolet astronomy. A collaborative project among NASA, ESA and the British SRC (now PPARC) agencies, IUE's estimated lifetime was 3 to 5 years. Amazingly, 17 years and 8 months later, it continues to operate, having made over 100,000 observations of comets, planets, stars, novae, supernovae, galaxies, and quasars. The IUE story is a truly remarkable but little known success story which will continue. To reduce costs, on September 30, 1995, the IUE team at GSFC will turn over its science operations to the ESA ground station in Villafranca, Spain where the ESA/PPARC teams will continue to make astronomical observations. Congratulations to the GSFC team for outstanding service to astronomy. Good luck to IUE and best wishes for continued success!

APOD: September 30, 1995 - An Energetic Radio Galaxy
The radio and optical emission seen in the above superposed images of the radio galaxy 3C368 indicate that energetic processes are at work. A radio galaxy is a galaxy that is visible to radio telescopes. A large radio signal usually indicates very powerful phenomena. The optical light is shown above in red and the radio emission in blue contours. Currently, one can only guess what causes the unusual radio and optical features. Scientists working with the Hubble Space Telescope speculate that the radio emission might have resulted from jets of high-velocity material thrown off from the center of the galaxy, while the optical light comes from gas and dust that are remnants of a burst of star formation triggered by the radio jet.

October 1995

APOD: October 1, 1995 - Central Galactic Star Bursts
The pictured core of the galaxy NGC 253 shows an exceptionally high rate of star formation. In the center are some of the densest knots of stars ever found, surrounded by glowing gas and dust lanes. Galaxies where such high star formation rates are seen are called "starburst" galaxies. The unusual color is a combination of the light emitted from the young, hot, blue stars and their associated emission and reflection nebulae.

APOD: October 2, 1995 - Phobos: Doomed Moon of Mars
Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this 1977 Viking orbiter image, the largest moon, Phobos, is seen to be a heavily cratered asteroid-like object. It is about 17 miles across and zips through the Martian sky completing an orbit in less than 8 hours. Phobos is doomed. It orbits so close to Mars, (about 3,600 miles above the surface compared to 250,000 miles for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it could crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.

APOD: October 3, 1995 - Deimos: A Small Martian Moon
Mars has two tiny moons, Phobos and Deimos. Pictured above is Deimos, the smallest moon of Mars. In fact, Deimos is the smallest known moon in the Solar System measuring only 9 miles across. The diminutive Martian moons were discovered in 1877 by Asaph Hall, an American astronomer working at the US Naval Observatory in Washington D.C. The existence of two Martian moons was predicted around 1610 by Johannes Kepler, the astronomer who derived the laws of planetary motion. In this case, Kepler's prediction was not based on scientific principles, but his writings and ideas were so influential that the two Martian moons are discussed in works of fiction such as Jonathan Swift's "Gulliver's Travels", written in 1726, over 150 years before their actual discovery.

APOD: October 4, 1995 - The Sun Spews X-rays
Our Sun is really very hot. The Sun's outer atmosphere is so hot that it emits much light in the X-ray band, which was unexpected. X-rays are usually emitted from objects having a temperature in the millions of degrees, not the mere thousands of degrees of the Sun's surface. The above X-ray picture shows the Sun one particularly active day in August of 1992. Evident are hot spots on the solar surface, showing that areas above the Sun's surface really do reach millions of degrees. But possibly more puzzling is the broader X-ray glow visible surrounding the Sun. This glow is now attributed to the Sun's X-ray corona, the origin of which is currently a subject of much discussion and debate. The Sun is one of the most photographed objects, with frequently updated pictures available over the WWW. In fact, an X-ray picture from Yohkoh taken earlier today is usually available over the WWW. Compare it to the above picture!

APOD: October 5, 1995 - CG4: A Ruptured Cometary Globule
The odd looking "creature" to the right of center in the above photo is a gas cloud known as a cometary globule. This globule, however, has ruptured. Cometary globules are typically characterized by dusty heads and elongated tails. These features cause cometary globules to have visual similarities to comets, but in reality they are very much different. Cometary globules are frequently the birthplaces of stars, and many show very young stars in their heads. The reason for the rupture in the head of this object is not completely known. The galaxy to the left of center is very far in the distance and is only placed near CG4 by chance superposition.

APOD: October 6, 1995 - Dark Bok Globules in IC 2944
The dark spots in the above picture are not photographic defects but an unusual type of interstellar cloud known as a Bok globule. Bok globules, named after astronomer Bart Bok who studied them extensively, are small dark clouds made of gas and dust that are typically condensing to form a star or stars. These Bok globules are found in front of a glowing H II region known as IC 2499.

APOD: October 7, 1995 - Apollo 12's Lunar Module Descends
A few months after Apollo 11's historic Moon landing, Apollo 12 with commander Charles Conrad Jr., Command Module pilot Richard Gordan, and Lunar Module pilot Alan Bean returned for more geographic and scientific exploration. Gordan took the above picture from the Command Module, where he stayed during the mission, while Conrad and Bean in the Lunar Module descend to the lunar surface, where they landed, collected samples and conducted scientific experiments.

APOD: October 8, 1995 - Apollo 12 Visits Surveyor 3
Apollo 12 was the second mission to land humans on the Moon. The landing site was picked to be near the location of Surveyor 3, a robot spacecraft that had landed on the moon three years earlier. Pictured above, Apollo 12 astronauts Conrad and Bean retrieve parts from the Surveyor. The Lunar Module is visible in the distance. Apollo 12 brought back many photographs and moon rocks. Among the milestones made by Apollo 12 was the deployment of the Apollo Lunar Surface Experiments Package, which carried out many experiments including one that measured the solar wind.

APOD: October 9, 1995 - Saturn's Moon Dione
Dione, one of Saturn's larger moons, is remarkable for its bright surface streaks. These streaks run across some of Dione's many craters, which indicate that the process which created the streaks occurred later than the process which created the craters. Dione is made of mostly water ice but its relatively high density indicates that it contains much rock inside. Dione was discovered by Giovanni Cassini in 1684. NASA's Cassini mission to Saturn is currently scheduled for launch in October 1997. Dione's orbit is remarkable it that it also houses the much smaller moon Helene. This moon, once designated "Dione B", precedes Dione by about 1/6th of an orbit.

APOD: October 10, 1995 - Dione's Lagrange Moon Helene
Saturn's moon Helene is very unusual in that it circles Saturn near the orbit of a bigger moon: Dione. Helene is situated in what is called a "Lagrange point" of Dione - a place of stability created by Dione's gravity. Were Helene to stray slightly from its orbit 1/6 ahead of Dione, the larger moon's gravity would cause Helene to move back toward the Lagrange point. Many massive orbital bodies have stable Lagrange points, including the Earth and Moon. Helene was discovered from the ground by P. Laques & J. Lecacheux in 1980. The photograph above was taken by Voyager 2 as it passed Saturn in 1981. NASA's Cassini mission to Saturn is currently scheduled for launch in October 1997.

APOD: October 11, 1995 - LMC Star Clouds
Pictured above are clouds of young stars forming an arc in the nearby Large Magellanic Cloud, the nearest galaxy to the our Milky Way Galaxy. These stars are situated in a star forming region known as N 51. The stars are so young they shine mostly in blue and ultraviolet light, and so massive their lifetimes are only millions of years - much shorter than the billions of years of lower mass stars like our Sun. This picture was taken in ultraviolet light by NASA's Ultraviolet Imaging Telescope in March 1995. The reason the arc has the observed shape is currently unknown.

APOD: October 12, 1995 - HH-47 Star Jet
The star masked by a dust cloud at the left of the above photo is expelling an energetic beam of charged particles into interstellar space. This jet, moving from left to right, has burrowed through much interstellar material, and now expands out into the interstellar space. Although jet particles move at nearly three hundred kilometers per second , we still do not see any daily movement because of the enormous distances involved. In fact, the jet is trillions of kilometers long. This stellar jet occurs in a system called HH-47 which is located near the edge of the Gum Nebula.

APOD: October 13, 1995 - Jupiter, Io, and Ganymede's Shadow
Jupiter, the solar system's largest planet, is seen here next to Io, its closest Galilean moon. On the cloud tops of Jupiter near the left edge of the picture can be seen a dark circular spot which is caused by the shadow of Jupiter's largest moon Ganymede. Jupiter's cloud tops show light bands and dark belts. The clouds are primarily composed of hydrogen and helium, but their intense colors are probably caused by very small amounts of heavier elements such as sulfur or organic (carbon-containing) compounds.

APOD: October 14, 1995 - Rhea: Saturn's Second Largest Moon
Rhea is the second largest moon of Saturn, behind Titan, and the largest without an atmosphere. It is composed mostly of water ice, but has a small rocky core. Rhea's rotation and orbit are locked together (just like Earth's Moon) so that one side always faces Saturn. A consequence of this is that one side always leads the other. Rhea's leading surface is much more heavily cratered than its trailing surface. The above photograph was taken with the Voyager 1 spacecraft in 1980.

APOD: October 15, 1995 - Iapetus: Saturn's Disappearing Moon
Iapetus has an unusual surface, one half of which is very dark, the other half very light. This caused it's discoverer Cassini to remark that Iapetus could only be seen when on one side of Saturn but not the other. The reason for the difference between hemispheres is presently unknown. Iapetus is the third largest of Saturn's moon behind Titan and Rhea. Iapetus is composed predominantly of water ice.

APOD: October 16, 1995 - Starburst Galaxy M94
The spiral galaxy M94 is somewhat unusual in that it shows a great ring of bright young stars particularly apparent when observed in ultraviolet light, as shown above. Such a high abundance of these young blue stars may cause a galaxy to be designated a starburst galaxy. It is likely that many of the stars seen in the above ring were created when a density wave moving in the galaxy compressed existing galactic gas. The above picture was taken by the Ultraviolet Imaging Telescope during the Astro-2 mission in March 1995.

APOD: October 17, 1995 - Galaxy Dwingeloo 1 Emerges
If you look closely at the center of the above photograph, you will see a spiral galaxy behind the field of stars. Named Dwingeloo 1, this nearby galaxy was only discovered recently (1994) because much of its light was obscured by dust, gas and bright stars of our own Milky Way galaxy. In fact, all the individually discernible stars in the above photograph are in our Galaxy. Dwingeloo 1 turned out to be a large galaxy located only five times as distant as the closest major galaxy - M31.

APOD: October 18, 1995 - A Storm on Saturn
The white wisp shown on Saturn's cloud tops is actually a major storm system only discovered in December of 1994. Saturn's clouds are composed of primarily hydrogen and helium, but the storm's white clouds are actually ammonia ice crystals that have frozen upon upheaval to the top of the atmosphere. The storm may look small but has an extent similar to the diameter of the Earth. This picture is a color composite of several taken by the Hubble Space Telescope. Saturn is the second largest planet in the Solar System and has the most elaborate ring system.

APOD: October 19, 1995 - Globular Cluster M5
The globular cluster M5, pictured above, contains roughly 100,000 stars. These stars formed together and are gravitationally bound. Stars orbit the center of the cluster, and the cluster orbits the center of our Galaxy. So far, about 160 globular clusters are known to exist in a roughly spherical halo around the Galactic center. Globular clusters do not appear spherically distributed as viewed from the Earth, and this fact was a key point in the determination that our Sun is not at the center of our Galaxy. Globular clusters are very old. There is a straightforward method of determining their age, and this provides a very interesting lower limit on the age of our universe of about 14 billion years.

APOD: October 20, 1995 - Asteroid Gaspra's Best Face
Above is the best yet color image of the asteroid Gaspra based on data returned by NASA's Galileo spacecraft. Color variations have been added to high resolution images and enhanced to highlight changes in reflectivity, surface structure and composition. The illuminated portion of the asteroid is about 11 miles long. Galileo encountered Gaspra on October 29, 1991 during the cruise phase of its mission to study the Jovian system. When it arrives at Jupiter, in December 1995, the robot spacecraft's atmospheric probe will descend into Jupiter's atmosphere, becoming the first ever probe to enter the atmosphere of a gas giant planet. Updates on Galileo's progress can be found at "Online from Jupiter".

APOD: October 21, 1995 - A Glimpse of Titan's Surface
The surface of Titan, Saturn's largest moon, is normally hidden from view by its thick, hazy atmosphere. However, for the first time astronomers have been able to see surface features in images like the one above, made at near-infrared wavelengths with the Hubble Space Telescope. At these wavelengths (longer than visible light) Titan's smog like atmosphere begins to be transparent enough to allow glimpses of it's surface. The bright feature seen above is about 2,500 miles across, similar in size to Australia. Astronomers are still trying to work out what the bright and dark areas represent - oceans, continents, craters, or other features. The images represent important information for planning the Cassini mission, scheduled for launch in 1997. The Cassini spacecraft will explore the Saturn system and parachute a probe to Titan's surface.

APOD: October 22, 1995 - A Quasar - Galaxy Collision?
In 1963 astronomers were astounded to discover that certain faint, star-like objects have very large redshifts. The large redshifts imply that these objects, now known as quasars (QUASi-stellAR objects), lie near the edge of the observable Universe. To be visible at such extreme distances of billions of light years, they must emit tremendous amounts of energy. Where does the energy come from? In the most widely accepted model, a quasar is powered by a supermassive black hole in the center of a distant galaxy. This Hubble Space Telescope image shows a quasar known as PKS 2349 (the star-like object near the center) and a galaxy (surrounding fuzzy patch), but the quasar is not at the galaxy's center! In fact, the galaxy and quasar seem to be colliding or merging. This and other recent HST observations suggest that astronomers' standard ideas about quasars may be wrong.

APOD: October 23, 1995 - Gamma-Ray Quasars
Gamma rays are more than 10,000 times more energetic than visible light. If you could "see" gamma rays, the night sky would seem very different indeed. The bright object in the center of the false color gamma-ray image above is quasar 3C279, a nondescript, faint, starlike object in the visible sky. Yet, in June of 1991 a gamma-ray telescope onboard NASA's orbiting Compton Gamma Ray Observatory unexpectedly discovered that it was one of the brightest objects in the gamma-ray sky. Shortly after this image was recorded the quasar faded from view at gamma-ray energies. Astronomers are still trying to understand what causes these enigmatic objects to flare so violently. Another quasar, 3C273, is faintly visible above and to the right of center.

APOD: October 24, 1995 - A Total Solar Eclipse
Pictured above is a total solar eclipse, with the Sun's corona, clearly visible surrounding the Moon. This picture is a composite of differently exposed photos of an eclipse that occurred on July 11, 1991. Today there will be another total eclipse of the Sun, the last until March 1997. The path of today's total eclipse will begin in the Middle East, move though southern China, and end in the Pacific Ocean south of the Marshall Islands. Those only near the path will see a partial solar eclipse. NASA keeps detailed maps of the predicted path of the eclipse.

APOD: October 25, 1995 - Painting with Solar Neutrons
Solar flares are propelled high above the Sun's surface by powerful, twisted magnetic fields. The flares spew high energy atomic and subatomic particles into space. During an intense solar flare on June 15, 1991, a spray of solar neutrons was detected by the COMPTEL instrument onboard NASA's Compton Gamma Ray Observatory. Like paint from a spray can, the neutrons formed the above image (seen in false color) on the COMPTEL detectors. Such astronomical images made by high energy particles are unusual, astronomers typically use visible light and other forms of electromagnetic radiation to study the Universe. High energy particles from solar flares affect the Earth and near Earth environment so solar activity is constantly monitored.

APOD: October 26, 1995 - Aurora Astern
Sailing upside down, 115 nautical miles above Earth, the crew of the Space Shuttle Endeavour made this spectacular time exposure of the southern aurora (aurora australis) in October of 1994. The aurora, also known as the northern and southern lights, appear as luminous bands or streamers of light which can extend to altitudes of 200 miles. They are typically visible from the Earth's surface at high latitudes and are caused by high energy particles from the Sun. The delicate colors are caused by energetic electrons colliding with oxygen and nitrogen in the atmosphere. In this picture, the rear structure of the Space Shuttle is visible in the foreground with the vertical tail fin pointed toward Earth. Star trails are visible as small streaks above Earth's horizon.

APOD: October 27, 1995 - The Tarantula and the Supernova
In this close-up of the Large Magellanic Cloud, the spidery looking nebula on the left is fittingly known as as the Tarantula nebula. It is an emission nebula surrounding a cluster of hot, young stars called the 30 Doradus super cluster. This cluster may contain the most massive stars known (about 50 times the mass of the Sun). Such massive stars put out more than 100 times as much energy as our Sun. The bright "star" (lower right) is actually Supernova 1987a and is a harbinger of things to come for the stars within the Tarantula. Massive stars burn their nuclear fuel at drastically enhanced rates to support their high energy output. As a result their lives last only a few million years compared to the Sun's few billions of years. They end in a spectacular death explosion, a supernova, like the star which exploded in 1987 as seen above. Supernovae may leave behind imploded stellar cores which form neutron stars or black holes.

APOD: October 28, 1995 - The Delta Clipper
The Delta Clipper experimental rocket or DC-X is intended as a development vehicle to pave the way for a reusable single stage to orbit rocket. Shown here, in an artists conception, it has made several successful test flights since its maiden voyage in August of 1993 - taking off like a rocket, hovering and moving horizontally, and landing tail first on a designated landing pad. The DC-X is actually too heavy and underpowered to achieve orbit, but as part of NASA's Reusable Launch Vehicle Technology Program, lessons learned operating the DC-X may help provide science and industry cheaper access to space.

APOD: October 29, 1995 - Radioactive Clouds in the Milky Way
When massive stars explode they create large radioactive blast clouds which expand into interstellar space. As the radioactive elements decay, they produce gamma-rays. Possible locations of these stellar explosions known as supernovae, are indicated by the bright clumps in this map of the central regions of our Milky Way Galaxy. The map was made by a telescope onboard NASA's orbiting Compton Gamma Ray Observatory using detectors sensitive to gamma rays from the decay of radioactive aluminum.

APOD: October 30, 1995 - Comet Hale-Bopp Update
Will comet Hale-Bopp become the brightest comet of the Century in early 1997? Since its discovery in July this year, Hale-Bopp has caused much speculation. Even though it is still beyond the orbit of Jupiter it is astonishingly bright and expected to get much brighter as it plunges inward, toward the Sun. In this latest Hubble Space Telescope image a bright clump of material (above center) has apparently been ejected by evaporation and the rotation of the icy nucleus (below center). Astronomers are using this and other observations to try to figure out if Hale-Bopp is really a giant comet or a smaller object which will fizzle out sooner than expected as it approaches the Sun.

APOD: October 31, 1995 - A Halloween Invasion from Mars
Orson Welles became an instant legend on Halloween in 1938 for his radio dramatization of H.G. Wells' "War of the Worlds". Some listeners who did not realize it was a theatrical production were driven to near panic by this fictional account of invaders from Mars. In the story, as in the above Hubble Space Telescope image, Mars was at "opposition", its point of closest approach to the Earth, a distance of some 65 million miles. For the Martians, it was imagined that this was a good time to invade. For astronomers, opposition is a good time to study the red planet and this HST image, represents the clearest view of Mars ever for an Earth telescope. The icy north polar cap is visible at the top of the picture as well as a veil of white clouds along the planet's left edge. The dark markings represent areas where the reddish tinged dust characteristic of the surface has been blown away by the Martian winds.

November 1995

APOD: November 1, 1995 - M16: Dust and an Open Cluster
The photogenic M16 shown above is composed of a young star cluster and a spectacular emission nebulae lined with distinct regions of interstellar dust. Most of the stars in the cluster can be seen offset just above and to the right of the photograph's center. This type of star cluster is called an "open" or "galactic" cluster and typically has a few hundred young bright members. The redness of the surrounding emission nebula gas is caused by electrons recombining with hydrogen nuclei, while the dark regions are dust lanes that absorb much of the radiation that enters it. The dust absorbs so much light it allows astronomers to determine which stars are inside the nebula and which are in the foreground.

APOD: November 2, 1995 - The Red Rectangle
The unusual geometry of this stellar nebula creates somewhat of a mystery. At the nebula's center is a young binary star system that probably created the nebula, but how? This type of nebula shows a "bipolar flow" which carries a significant amount of mass away from the central stars. It has been speculated that the central stars create a pair of jets that precess like a spinning top. These jets might throw gas into a thick disk which we see here edge on - so that it appears to us as a rectangle. The nebula emission is also unusual in that some of the infrared light it emits might be associated with unusual carbon-containing molecules.

APOD: November 3, 1995 - Jupiter's Moon Amalthea
Amalthea is Jupiter's fifth largest moon, much smaller than the four Galilean satellites Io, Europa, Ganymede, and Callisto. The orbit of Amalthea is inside of these moons, and with its long axis always pointing toward Jupiter. It's dark surface color is probably due to sulfur being expelled from Io. Amalthea is not massive enough for its internal gravity to make it spherical. Amalthea was discovered in 1892 by Edward Barnard.

APOD: November 4, 1995 - Neptune's Moon Proteus
Proteus is the second largest moon of Neptune behind the mysterious Triton. Proteus was discovered only in 1982 by the Voyager 2 spacecraft. This is unusual since Neptune has a smaller moon - Nereid - which was discovered 33 years earlier from Earth. The reason Proteus was not discovered sooner is that its surface is very dark and it orbits much closer to Neptune. Proteus has an odd box-like shape and were it even slightly more massive, its own gravity would cause it to reform itself into a sphere.

APOD: November 5, 1995 - Vela Satellites: The Watchers
In October of 1963 the US Air Force launched the first in a series of satellites inspired by a recently signed nuclear test ban treaty. Signatories of this treaty agreed not to test nuclear devices in the atmosphere or in space. These "Vela" (from the Spanish verb velar, to watch) satellites were part of an unclassified program whose goal was to develop the technology to monitor nuclear tests from space. A Vela satellite is pictured above in an artist's conception, keeping watch over the Earth. The high energy radiation sensors onboard the Velas did not detect any clandestine nuclear explosions. Instead, in the most surprising discovery in the history of space based astronomy, they found bursts of gamma rays coming from deep space! The mysterious origin of these brief, intense flashes of gamma rays is one of the most hotly debated topics in modern astrophysics.

APOD: November 6, 1995 - M16: Stars Upon Pillars
How do stars form? This stunning picture taken recently by the Hubble Space Telescope gives us a first hand glimpse. Here evaporating gaseous globules (EGGs) are captured emerging from pillars of molecular hydrogen and dust in the Eagle Nebula (M16). These pillars, dubbed "elephant trunks," are light years in length and are so dense that interior gas gravitationally contracts to form stars. At each pillars' end, the intense radiation of bright young stars causes low density gas to boil away, leaving stellar nurseries of dense EGGs exposed.

APOD: November 7, 1995 - Eagle EGGs in M16
Star forming regions known as "EGGs" are uncovered at the end of this giant pillar of gas and dust in the Eagle Nebula (M16). EGGs, short for evaporating gaseous globules, are dense regions of mostly molecular hydrogen gas that fragment and gravitationally collapse to form stars. Light from the hottest and brightest of these new stars heats the end of the pillar and causes further evaporation of gas - revealing yet more EGGs and more young stars. This picture was taken by the Wide Field and Planetary Camera on board the Hubble Space Telescope.

APOD: November 8, 1995 - Simulating the Universe
The above cube represents a chunk of our universe as simulated by the Grand Challenge Cosmology Consortium (GC3). The cube is huge - it would take light 500 million years to cross it. Low density gas is shown as blue, and high density gas as red. In the beginning of the universe, matter and gas were spread quite uniformly. As this computer universe evolved, gravity caused high density areas to be created. This simulation hypothesized that 1/3 of the universe is composed of slow moving "cold dark matter", and 2/3 composed of fast moving "hot dark matter." The areas of high and low density resulting from this computer simulation are similar to those observed by astronomers in the real universe.

APOD: November 9, 1995 - M104: The Sombrero Galaxy
The famous Sombrero galaxy (M104) is a bright nearby spiral galaxy. The prominent dust lane and halo of stars and globular clusters give this galaxy its name. Something very energetic is going on in the Sombrero's center, as much X-ray light has been detected from it. This X-ray emission coupled with unusually high central stellar velocities cause many astronomers to speculate that a black hole lies at the Sombrero's center - a black hole a billion times the mass of our Sun. This image was taken in blue light by the 0.9 meter telescope at Kitt Peak National Observatory.

APOD: November 10, 1995 - Lightning and the Space Shuttle
There are many things about lightning that are not understood. Lightning has been seen in the atmospheres of Venus, Earth, Jupiter, and Saturn. A leading theory is that collisions of particles in clouds cause large areas of positive and negative charge. When large oppositely charged areas get close enough together, electrons and/or ions race between them and create a path where more charged particles can follow - lightning. On average, over 100 lightning strikes occur every second over the surface of the Earth. Here lightning strikes near a Space Shuttle before launch. Lightning can be extremely dangerous - stay out of open areas during thunderstorms.

APOD: November 11, 1995 - Red Sprite Lightning
Recently two new types of lightning have been verified: red sprites and blue jets. These atmospheric discharges occur very high in the Earth's atmosphere - much higher than the familiar form of lightning. Red sprites appear red in color and go from the tops of clouds to as high as the ionosphere - an ionized layer 90 kilometers above the Earth's surface. They last only a small fraction of a second. The existence of red sprites has been suggested previously, but only in 1994 were aircraft flown above massive thunderstorms with the high speed video equipment necessary to verify these spectacular events. Scientists are unsure of the cause and nature of red sprites.

APOD: November 12, 1995 - Blue Jet Lightning
Recently two new types of lightning have been verified: red sprites and blue jets. These atmospheric discharges occur very high in the Earth's atmosphere - much higher than the familiar form of lightning. Blue jets appear blue in color and go from the tops of clouds to a height of about 50 kilometers. Because blue jets typically last almost a full second, one can easily see the jet rise from the clouds with a fast video camera. Blue jets appear as narrow cones and travel about 100 kilometers per second. The existence of blue jets has been suggested previously, but only in 1994 were aircraft flown above massive thunderstorms with the high speed video equipment necessary to verify these spectacular events. The above black & white picture shows several blue jets rising from a thunderstorm simultaneously. Part of the aircraft is visible in silhouette on the right. Scientists are unsure of the cause and nature of blue jets.

APOD: November 13, 1995 - Virgo Cluster Galaxies
Pictured are several galaxies of the Virgo Cluster, the closest cluster of galaxies to the Milky Way. The Virgo Cluster spans more than 5 degrees on the sky - about 10 times the angle made by a full Moon. It contains over 100 galaxies of many types - including spirals, ellipticals, and irregular galaxies. The Virgo Cluster is so massive that it is noticeably pulling our Galaxy toward it. The above picture includes two galaxies that are also Messier objects: M84 and M86. M84 is the bright elliptical galaxy just above the center of the photograph, and M86 is the bright elliptical galaxy to its right.

APOD: November 14, 1995 - Aurora and Orion
Looking toward the south from low Earth orbit, the crew of the Space Shuttle Endeavor made this stunning time exposure of the Aurora Australis (southern lights) in April of 1994. The aurora are caused by high energy electrons from the Solar Wind which are funneled into the atmosphere by the Earth's magnetic field. The reddish colors occur at the highest altitudes (about 200 miles) where the air is least dense. At lower altitudes and greater densities green tends to dominate. At the lowest altitudes a pinkish glow is sometimes produced. The familiar constellation of Orion the Hunter is clearly visible above the dark horizon in the background. Because of the shuttle's orbital motion, the bright stars in Orion appear slightly elongated.

APOD: November 15, 1995 - A Quintet of Galaxies
Five closely grouped galaxies are visible in this image made using the Kitt Peak National Observatory 2.1 meter telescope. The grouping is commonly known as Stephan's Quintet. Four of the galaxies show essentially the same redshift suggesting that they are at the same distance from us. The large bluish spiral below and left of center actually has a smaller redshift than the others, indicating it is much closer. It is probably a foreground object which happens to lie along the line of sight to the more distant galaxies. Of the four distant galaxies, three seem to be colliding, showing serious distortions due to gravitational tidal forces. The fourth is a normal appearing elliptical galaxy (at the lower right edge of the field). Recent results suggest that collisions play an important role in the life cycles of galaxies.

APOD: November 16, 1995 - Repairing Hubble
The first Hubble Space Telescope (HST) servicing mission, STS-61, was one of the most complicated shuttle missions ever planned. Launched on December 2, 1993 in the Space Shuttle Endeavor, the astronauts were required to make 5 space walks to repair the HST. Astronaut F. Story Musgrave, holding one of the handrails on the HST, is seen here during what became the second longest space walk in NASA history at just under 8 hours. The highly successful mission demonstrated the ability of shuttle astronauts to repair satellites in orbit and allowed the HST to continue to explore the Universe. A future HST servicing mission is planned for 1997.

APOD: November 17, 1995 - The Sun Also Rises
Sunrise seen from low Earth orbit by the shuttle astronauts can be very dramatic indeed (and the authors apologize to Hemingway for using his title!). In this breathtaking view, the Sun is just visible peaking over towering anvil shaped storm clouds whose silhouetted tops mark the upper boundary of the troposphere, the lowest layer of planet Earth's atmosphere. Sunlight filtering through suspended dust causes this dense layer of air to appear red. In contrast, the blue stripe marks the stratosphere, the tenuous upper atmosphere, which preferentially scatters blue light.

APOD: November 18, 1995 - Water World
Water (Dihydrogen Oxide, H2O) is a truly remarkable chemical compound and is fundamental to life on Earth. Earth is the only planet in the Solar System where the surface temperature and pressure allow the three forms of water, solid (ice), liquid (ocean), and gas (cloud) to exist simultaneously on its surface. Water in one of these forms accounts for everything visible in this view of Earth from space looking north at the Bering Sea and the coast of Alaska, USA, around Bristol Bay.

APOD: November 19, 1995 - New York at Night
This tantalizingly clear photo of New York City at night was taken by the astronauts during the Space Radar Laboratory mission of the Space Shuttle Endeavor in March of 1990. In this view, oriented with East up and North to the left, a dense constellation of lights defines the major metropolitan areas. Manhattan, outlined by the dark fingers of the Hudson river below and the East river above, is clearly visible along with the well lit southwestern tip of Long Island. Central Park is visible as a dark streak in the center of Manhattan's orange glow.

APOD: November 20, 1995 - At the Core of M15
Densely packed stars in the core of the globular cluster M15 are shown in this Hubble Space Telescope (HST) image taken in April of 1994. The stars revealed are contained in an area 1.6 light years across and their colors roughly indicate their temperatures - hot stars appear blue, cooler stars look reddish-orange. M15 has long been recognized as one of the densest cluster of stars in our galaxy outside of the galactic center itself. Even the unprecedented resolving power of the HST cameras could not separate the individual stars in its innermost regions. However, this HST image reveals that the density of stars continues to rise toward the cluster's core, suggesting that a sudden, runaway collapse due to the gravitational attraction of many closely packed stars or a single central massive object, perhaps a black hole, could account for the core's extreme density.

APOD: November 21, 1995 - M42: Orion Nebula Mosaic
The Great Nebula in Orion is one of the most interesting of all astronomical nebulae known. Here fifteen pictures from the Hubble Space Telescope have been merged to show the great expanse and diverse nature of the nebula. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Much of the filamentary structure visible in this image are actually shock waves - fronts where fast moving material encounters slow moving gas. Some shock waves are visible near one of the bright stars in the lower left of the picture. The Orion Nebula is located in the same spiral arm of our Galaxy as is our Sun. It takes light about 1500 years to reach us from there.

APOD: November 22, 1995 - M1: The Exploding Crab Nebula
The Crab Nebula resulted from a star that exploded - a supernova. The outer layers of the star were thrown violently into space, while the inner core collapsed to form a neutron star. This neutron star is visible to us today as a pulsar - a rotating star at the center of the nebula that emits visible flashes of light. The Crab Pulsar flashes about 30 times every second. Although the stellar explosion that caused the Crab Nebula was seen over 900 years ago, the nebula itself still expands and shines. How the nebula obtains the energy needed to shine was a mystery eventually solved by noting that this energy could be released by the slowing of the pulsar's rotation.

APOD: November 23, 1995 - M1: Polarization of the Crab
The Crab Nebula resulted from a star that exploded - a supernova. Although the stellar explosion that caused the Crab Nebula was seen over 900 years ago, the nebula itself still expands and shines. Much of the emitted light has been found to be polarized. Light waves with the same polarization vibrate in the same plane. Light waves can be polarized by reflection from a surface, an effect familiar to sunglass wearing fishermen and skiers. Polarized light can also be emitted by regions that contain strong magnetic fields. Areas of different polarization above are highlighted by different colors. Mapping the polarization helps astronomers decipher which physical processes create the observed light.

APOD: November 24, 1995 - Saturn's Moon Tethys
Tethys is one of the larger and closer moons of Saturn. It was visited by both Voyager spacecraft - Voyager 1 in November 1980 and by Voyager 2 in August 1981. Tethys is now known to be composed almost completely of water ice. Tethys shows a large impact crater that nearly circles the planet. That the impact that caused this crater did not disrupt the moon is taken as evidence that Tethys was not completely frozen in its past. Tethys has two moons named Telesto and Calypso that orbit just ahead of it and behind it. Tethys was originally discovered in 1684 by Giovanni Cassini.

APOD: November 25, 1995 - Saturn's Cleanest Moon: Enceladus
Enceladus orbits Saturn between the smaller Mimas and the larger Tethys. Enceladus is composed mostly of water ice and has the cleanest and purest ice surface in the Solar System. It's surface therefore appears nearly white. The surface also has many unusual groves and relatively few craters, like Jupiter's moon Ganymede. This indicates that the surface is young and/or newly reformed. To explain this, some astronomers speculate that Enceladus is susceptible to some sort of volcanic activity. Enceladus was originally discovered in 1789 by William Herschel.

APOD: November 26, 1995 - A Star Where Photons Orbit
The above computer animated picture depicts how a very compact star would look to a nearby observer. The star pictured is actually more compact that any known except a black hole, so it is only hypothetical. The observer is situated at the photon sphere, where photons can orbit in a circle. To help the viewer better visualize the great distortions created by gravity, a map of the Earth was projected onto the star, and a map of the familiar night sky was projected above. From here one can either look down and see several duplicate images of the entire surface of the star, look up and see several duplicate images of the entire night sky, or look along the photon sphere and see the back of one's own head.

APOD: November 27, 1995 - Too Close to a Black Hole
What would you see if you went right up to a black hole? Above are two computer generated pictures highlighting how strange things would look. On the left is a normal star field containing the constellation Orion. Notice the three stars of nearly equal brightness that make up Orion's Belt. On the right is the same star field but this time with a black hole superposed in the center of the frame. The black hole has such strong gravity that light is noticeably bent towards it - causing some very unusual visual distortion. In the distorted frame, every star in the normal frame has at least two bright images - one on each side of the black hole. In fact, near the black hole, you can see the whole sky - light from every direction is bent around and comes back to you. Black holes are thought to be the densest state of matter, and there is indirect evidence for their presence in stellar binary systems and the centers of globular clusters, galaxies, and quasars.

APOD: November 28, 1995 - Shadow at the Lunar South Pole
In 1994, the space probe Clementine spent 70 days in lunar orbit mapping the Moon's surface. Shown above is a dramatically detailed composite view centered on the Moon's South Pole constructed from 1500 Clementine images. The top half shows the part of the Moon which faces the Earth while the bottom half is the lunar Farside. The images reveal a major depression very near the South Pole itself, probably caused by the impact of a comet or asteroid. The shadow region near the impact site is extensive and may be permanent - creating an area cold enough to trap water of cometary origin as ice.

APOD: November 29, 1995 - Releasing Compton
Named for Nobel laureate physicist Arthur Holly Compton, the Compton Gamma Ray Observatory (CGRO) Satellite was launched in April of 1991 aboard the Space Shuttle Atlantis. CGRO's mission is to explore the Universe at gamma-ray energies. The massive space based observatory is seen here held upright in the shuttle payload bay behind smiling astronaut Jerry Ross. Ross and his colleague Jay Apt have just finished a successful, unplanned spacewalk to free a jammed antenna prior to releasing CGRO into orbit. CGRO has been operating successfully since, providing the first all-sky survey at gamma-ray energies along with exciting new observations of the sun, quasars, pulsars, supernova, black holes, and gamma-ray bursts.

APOD: November 30, 1995 - NGC 2440 Nucleus: The Hottest Star?
In the center of the above photograph lies a star with the hottest surface yet confirmed. This bright white dwarf star's surface has been measured at greater than 100,000 degrees Celsius - more than 15 times hotter than that of our own Sun. The white dwarf's extreme heat makes it glow extraordinarily bright: intrinsically more than 250 times brighter than the Sun. The star is at the center of the planetary nebula titled NGC 2440, which lies inside our Milky Way Galaxy. The above computer sharpened image was taken by the Hubble Space Telescope.

December 1995

APOD: December 1, 1995 - 51 Pegasi: A New Planet Discovered
Are we alone in the universe? Do other stars have planets too? Humanity took one step closer to answering these questions in October 1995 when it was announced that the star 51 Pegasi harbors at least one planet. In the above picture of 51 Peg the planet is not visible - it can only be detected by noticing small changes in the star's motion. Claims of planets orbiting other stars are rare, with perhaps the most credible pertaining to a neutron star - a star much different than the Sun. But new ground was broken when the planetary detection claimed around the normal Sun-like star 51 Peg was confirmed. The planet, discovered by Michel Mayor and Didier Queloz, is thought to be like Jupiter - except orbiting so close to the parent star that it's year lasts only about 4 days! In the above picture the lines centered on 51 Peg are caused by the telescope itself and are not related to the star or planet.

APOD: December 2, 1995 - Lightning Below
In May of 1993, the Space Shuttle Columbia orbited the Earth carrying the Spacelab Deutsche 2 (SL-D2) science module and spacelab tunnel seen here riding in the shuttle's payload bay. Below, thunderstorms rage over the cloudy Earth. Several lightning flashes are visible in this high speed photograph as intense bright streaks within the clouds. The lights near the center of the picture, just above the the shuttle's tail, are believed to be Mexico City.

APOD: December 3, 1995 - An X-ray Hot Supernova in M81
In 1993, a star in the galaxy M81 exploded. Above is a picture of the hot material ejected by this supernova explosion. The picture was taken in X-rays with the Advanced Satellite for Cosmology and Astrophysics (ASCA). Since M81 is a relatively nearby galaxy, it can be examined in close detail by observatories on or near the Earth. Since the Earth's atmosphere protects the surface from interstellar X-radiation, the above photo was taken from space. Studying the nature and distribution of the X-rays has allowed astronomers to determine the composition and temperature of the expanding supernova gas.

APOD: December 4, 1995 - GL 229B: An Elusive Brown Dwarf?
What type of matter makes up most of the universe? This question is arguably the most perplexing astronomical mystery of our time. A leading candidate is a type of dim, low mass star called a "brown dwarf" star. Our universe could contain more brown dwarfs than any other type of star - but they are so dim they have so far escaped detection. The dramatic photograph above, taken in October 1994, sheds new light on this "dark matter" problem. The seemingly inconspicuous companion to the right of the overexposed image of a normal star is thought to be an elusive brown dwarf. Now that the existence of brown dwarfs has been demonstrated, a key remaining question is their abundance.

APOD: December 5, 1995 - The Swirling Center of NGC 4261
What evil lurks in the hearts of galaxies? The above picture by the Hubble Space Telescope of the center of the nearby galaxy NGC 4261 tells us one dramatic tale. Here gas and dust are seen swirling near this elliptical galaxy's center into what is almost certainly a massive black hole. The disk is probably what remains of a smaller galaxy that fell in hundreds of millions of years ago. Collisions like this may be a common way of creating such active galactic nuclei as quasars. Strangely, the center of this fiery whirlpool is offset from the exact center of the galaxy - for a reason that for now remains an astronomical mystery.

APOD: December 6, 1995 - 24 Hours from Jupiter
NASA's robot spacecraft Galileo began its long voyage to Jupiter more than six years and 2.3 billion miles ago. About 24 hours from now it will reach its destination. On arrival (December 7th, 1995), Galileo should become the first spacecraft to orbit around a gas giant planet and the first to drop a probe into a giant planet's dense atmosphere. In the above Hubble Space Telescope image, the predicted probe entry location is indicated by the arrow. If all goes well, the atmospheric probe will relay information about temperature, pressure, and composition for about an hour as it descends through Jupiter's clouds, while the orbiter will spend at least two years exploring the Jovian system. Along with advancing our knowledge of Jupiter and its environs, Galileo data is expected to reveal some of the secrets of the formation and evolution of the Solar System.

APOD: December 7, 1995 - Galileo's Jupiter Probe
Today, at about 5:00 pm EST, this 750 pound probe from NASA's robot spacecraft Galileo will plummet into Jupiter becoming the first probe to fly through the atmosphere of a gas giant planet. Released by the Galileo orbiter in July of this year, it has been coasting toward its rendezvous with the Solar System's largest planet. The probe will smack Jupiter's atmosphere at over 100,000 mph slowing to less than 1,000 mph in a matter of minutes, experiencing a deceleration of about 230 times the Earth's surface gravity. If all goes well, it will then deploy a parachute and descend, using sophisticated instruments to profile Jupiter's dense outer layers of hydrogen and helium gas. Pictured here before launch, the probe descent module (top) is suspended above its deceleration module aeroshell (bottom) prior to being joined. The aeroshell should protect the descent module from the initial shock and heat of entry, which will initially create an intense fireball, over twice as hot as the surface of the Sun.

APOD: December 8, 1995 - Descent To Jupiter
Hours ago, at about 5:00 pm EST (2200 GMT) December 7, Galileo's descent probe slammed into Jupiter's atmosphere. Above is an artist's vision of the probe's planned descent from a dramatic perspective. The protective aeroshell, still glowing from the fiery entry, is seen falling away, the 8 foot parachute has deployed, and the orbiter (upper left) is visible high above the cloud tops listening intently to the probe's data transmissions. As illustrated the probe may have encountered lightning, or at lower levels even water rain. Ultimately, the probe was expected to be crushed and vaporized by the intense heat and pressure deep below the clouds. NASA controllers have received telemetry signals from the orbiter indicating that it has recorded the probe's transmissions and has subsequently successfully fired its rocket engine entering orbit around Jupiter. The first playback of the recorded data to ground stations on Earth is scheduled for December 10-13. Congratulations to the Galileo Team!

APOD: December 9, 1995 - Apollo 14 on the Moon
The jewel-like glare from a brilliant sun reflects off the lunar lander of the Apollo 14 mission to the Moon as it rests on the lunar surface in February 1971. Astronauts Alan Shepard and Edgar Mitchell walked on the Moon's surface while astronaut Stuart Roosa piloted the orbiting command module. Coming only months after the abortive Apollo 13 mission, Apollo 14 was famous for long exploratory moon walks, collecting samples of lunar bedrock from Cone Crater, deploying the Apollo Lunar Surface Experiments Package, and hitting golf balls. The slope rising to the rim of Cone Crater is visible at the left edge of the photo.

APOD: December 10, 1995 - Apollo 14 Deploys ALSEP
Soon after the lunar lander of Apollo 14 set down on the Moon, Astronauts Alan Shepard and Edgar Mitchell deployed the Apollo Lunar Surface Experiments Package (ALSEP). The ALSEP scientific experiments included a seismometer sensitive to slight lunar surface movements, and charged particle detectors which measured the solar wind. The seismometer successfully measured surface tremors interpreted as moonquakes and meteoroids striking the Moon, while the solar wind experiment was sensitive enough to detect the element argon. These and other ALSEP experiments helped classify the internal structure and magnetic field of Earth's Moon.

APOD: December 11, 1995 - NGC 5189: A Strange Planetary Nebula
After a Sun-like star can no longer support fusion in its core, the center condenses into a white dwarf while the outer atmospheric layers are expelled into space and appear as a planetary nebula. This particular planetary nebula has a quite strange and chaotic structure. The inner part of this nebula contains an unusual expanding ring of gas that we see nearly edge-on. The exact mechanism that expels the planetary nebula gas is a current topic of astronomical speculation and research.

APOD: December 12, 1995 - Shapley 1: An Annular Planetary Nebula
This strange structure is what can result when a normal star runs out of nuclear fuel in its core. At that time, the center condenses into a white dwarf while the outer atmospheric layers are expelled into space and appear as a planetary nebula. This particular planetary nebula, designated Shapley 1 after the famous astronomer Harlow Shapley, has a very apparent annular ring like structure. Although some of these nebula appear like planets on the sky (hence their name), they actually surround stars far outside our solar system.

APOD: December 13, 1995 - A Delta Rocket Launches
A Delta rocket is seen being launched in 1988. In use since 1960, Delta rockets have been launched successfully over 200 times. Scientific satellites placed into orbit by a Delta rocket include IUE, COBE, LAGEOS-I, ROSAT, EUVE, GEOTAIL, and WIND. A Delta rocket is scheduled to launch the X-ray Timing Explorer Satellite (XTE) in the very near future. Commercial launches include INMARSAT. Many recent Delta launches have placed Navstar Global Positioning System satellites into orbit. Delta rockets are manufactured for the USAF and NASA by McDonnell Douglas Space Systems Co.

APOD: December 14, 1995 - An Atlas Centaur Rocket Launches
Atlas Centaur rockets have launched over 75 successful unmanned missions. These missions included the Surveyor series - the first vehicles to make soft landings on the Moon, Pioneer 10 and 11 - the first missions to fly by Jupiter and Saturn and the first man-made objects able to leave our Solar System, the Viking missions which landed on Mars, several satellites in the High Energy Astrophysics Observatory (HEAO) series, Pioneer Venus which circled and mapped the surface of Venus, and numerous Intelsat satellites. Of recent scientific interest was the Atlas launched SOHO mission which will continually observe the Sun. Atlas rockets are manufactured by Lockheed Martin Co.

APOD: December 15, 1995 - M64: The Sleeping Beauty Galaxy
The Sleeping Beauty galaxy may appear peaceful at first sight but it is actually tossing and turning. In an unexpected twist, recent observations have shown that the center of this photogenic galaxy is rotating in the opposite direction than the outer regions! Stranger still - there is a middle region where the stars rotate in the opposite direction from the surrounding dust and gas. The fascinating internal motions of M64, also cataloged as NGC 4826, are thought to be the result of a collision between a small galaxy and a large galaxy - where the resultant mix has not yet settled down.

APOD: December 16, 1995 - Hawaii
Aloha! With the graceful arc of the Earth's limb in the background, the entire Hawaiian Island archipelago is visible in this stunning photo taken by the astronauts onboard the shuttle Discovery in October of 1988. Along with popular beaches and tropical resorts, these volcanic islands offer extreme elevations with dark, dry, cloudless skies. Consequently they have also become popular sites for sophisticated ground based telescopes. The peak of Mauna Kea, on the Big Island (lower right), boasts an impressive array of astronomical instruments including the Keck Telescope, the Canada-France-Hawaii Telescope, the NASA IRTF, the JCMT and UKIRT, and the Gemini Telescope Project. The dormant volcanic cone of Haleakala on Maui (just above and to the left of the Big Island) is home to the Air Force Maui Optical Station and the Mees Solar Observatory. Mahalo nui loa!

APOD: December 17, 1995 - The Space Shuttle Docks with Mir
Hundreds of kilometers above the Earth's surface, the United States Space Shuttle Atlantis docks with the Russian Space Station Mir. The photograph was taken by Nikolai Budarin from a Russian Soyuz spacecraft on July 4, 1995. During this mission astronauts answered questions from school students over amateur radio, and performed life science experiments aboard the SpaceLab. The SpaceLab experiments were designed to understand how the Russian Space program combats the effects on the human body of the long-durations in space.

APOD: December 18, 1995 - M51: The Whirlpool Galaxy
The Whirlpool Galaxy is frequently given as an example of a galaxy with classic spiral design. At only 15 million light years distant, M51, also cataloged as NGC 5194, is also one of the brighter and more picturesque galaxies on the sky. The smaller galaxy appearing here above and to the right is also well behind M51, as can be inferred by the dust in M51's spiral arm blocking light from this smaller galaxy. Astronomers speculate that M51's spiral structure is primarily due to it's gravitational interaction with this smaller galaxy.

APOD: December 19, 1995 - Albert Einstein: 1879 - 1955
Albert Einstein is considered by many the greatest astrophysicist. He is pictured here in the Swiss Patent Office where he did much of his great work. Einstein's many visionary scientific contributions include the equivalence of mass and energy (E=mc^2), how the maximum speed limit of light affects measurements of time and space (special relativity), and a more accurate theory of gravity based on simple geometric concepts (general relativity). One reason Einstein was awarded the 1921 Nobel Prize in Physics was to make the prize more prestigious.

APOD: December 20, 1995 - A Galaxy Gravitational Lens
Sometimes it takes a keen eye to see a good mirage. Around the center of the red galaxy image in the above photograph lie four blue "smudges." Each smudge is actually a different image of the same background quasar. The central galaxy happens to fall directly in the light path of the quasar. Consequently, the huge mass of the galaxy is able to pull separate images of the quasar around it - an effect called gravitational lensing. Hence we see a gravitational mirage! Astronomers have hopes of using light differences between these quasar images to not only "weigh" the central galaxy but even provide clues about the expansion rate and composition of the universe.

APOD: December 21, 1995 - Hot Stars in the Trifid Nebula
In the center of the glowing red gas on the Trifid Nebula lies an open cluster of young hot stars. The energetic light from these stars strikes hydrogen atoms in the surrounding nebula causing them to lose their electrons. When an electron finds its way back to a hydrogen proton, it emits light at very specific colors - one of which is the red color of the nebula seen here. The red glow is thus indicative of an emission nebula. The dramatic dark sheets are made of interstellar dust grains, tiny needle shaped pellets which are thought to be created and expelled in the atmospheres of cooler stars.

APOD: December 22, 1995 - Summer at the South Pole
December 22 marks the Winter Solstice - the beginning of winter in the Earth's northern hemisphere, and the first day of summer in the south! This view of Earth's southern hemisphere near the beginning of its summer was created using images from the Galileo spacecraft taken during the December 1990 flyby. Dramatically centered on the South Pole, this mosaic was constructed by piecing together images made over a 24 hour period so that the surface appears to be entirely in sunlight. South America (middle left), Africa (upper right), and Australia (lower right), are visible as dark masses while Antarctica gleams brightly in the center. Swirling clouds marking the major weather systems are also prominent.

APOD: December 23, 1995 - Prometheus, Pandora and Saturn's F Ring
The third and fourth innermost moons of Saturn were unexpectedly discovered to be gravitational "shepards." The inner moon Prometheus and the outer moon Pandora use their gravitational attraction to define Saturn's outermost ring. Were any of the smaller chunks of ice and rock that compose Saturn's F Ring to stray, the ring particle would be gravitationally pulled back into place by one of these passing moons. This complex interaction creates a ring structure with two narrow braids and several unusual knots.

APOD: December 24, 1995 - Uranus' Moon Miranda
NASA's robot spacecraft Voyager 2 passed the planet Uranus and its moons in 1986. While the cloud tops of Uranus proved to be rather featureless, the surface of Miranda, the innermost of Uranus' large moons, showed several interesting features. Voyager 2 passed closer to Miranda than to any Solar System body and hence photographed it with the clearest resolution. Miranda's heavily cratered terrain shows grooves like Jupiter's moon Ganymede and several valleys and cliffs. Miranda is made of a roughly equal mix of ice and rock. Miranda was discovered by Gerard Kuiper in 1948.

APOD: December 25, 1995 - Earth Rise
During the 1968 Christmas season Frank Borman, James Lovell, and William Anders flew the Apollo 8 command module From the Earth to the Moon and back (launched Dec. 21, achieved 10 lunar orbits, landed Dec. 27). The Apollo 8 mission's impressive list of firsts includes; the first manned flight using the Saturn V rocket, the first humans to journey to the Earth's Moon, and the first to photograph the Earth from deep space. The famous picture above, showing the Earth rising above the Moon's limb as seen from lunar orbit, was a marvelous gift to the world. This was astronaut James Lovell's third mission. His last flight would be as commander of Apollo 13.

APOD: December 26, 1995 - Accretion Disk Binary System
Our Sun is unusual in that it is alone - most stars occur in multiple or binary systems. In a binary system, the higher mass star will evolve faster and will eventually become a compact object - either a white dwarf star, a neutron star, or black hole. When the lower mass star later evolves into an expansion phase, it may be so close to the compact star that its outer atmosphere actually falls onto the compact star. Such is the case diagrammed above. Here gas from a blue giant star is shown being stripped away into an accretion disk around its compact binary companion. Gas in the accretion disk swirls around, heats up, and eventually falls onto the compact star. Extreme conditions frequently occur on the surface of the compact star as gas falls in, many times causing detectable X-rays, gamma-rays, or even cataclysmic novae explosions. Studying the extreme conditions in these systems tells us about the inner properties of ordinary matter around us.

APOD: December 27, 1995 - Nova Cygni 1992
In 1992 a tremendous explosion occurred in the constellation of Cygnus. Dubbed Nova Cygni 1992, this event most probably occurred in an accretion disk binary system. Astronomers hypothesize that this system's white dwarf had so much gas dumped onto it's surface that conditions became ripe for nuclear fusion. The resulting thermonuclear detonation blasted much of the surrounding gas into an expanding shell. The Hubble Space Telescope photographed this expanding shell in 1994. Nova Cygni 1992 was the brightest nova in recent history - at its brightest it could be seen without a telescope. It was observed in every part of the electromagnetic spectrum.

APOD: December 28, 1995 - NGC 6240: When Galaxies Collide
Sometimes even galaxies can suffer a fatal attraction. Here gravity causes two galaxies to collide in a spectacular display of energetic gas, dust, and light. When galaxies collide it is rare that any stars in the galaxies themselves collide, or that any change will be seen in a human lifetime. Rather the structure of one or both galaxies gets slowly disrupted, while interior gas condenses to new star forming regions. Stellar motions in the center of the NGC 6240 frenetic mix are among the highest in any stellar system. Galaxy mergers may emit energetic radiations across the electromagnetic spectrum. This galactic jumble is, in fact, extremely bright in infrared light.

APOD: December 29, 1995 - NGC 4361: Galaxy Shaped Planetary Nebula
Glowing in the red light emitted by hydrogen atoms, the planetary nebula NGC 4361 is pictured above. The nebula itself is formed by the outer layers of gas shrugged off by the central star visible in the image. The star's nuclear fuel almost exhausted, it is cooling and shrinking - entering the white dwarf phase of its life. The curved tendrils of emission reaching out from the body of the nebula have a shape reminiscent of the arms of of a spiral galaxy.

APOD: December 30, 1995 - LMC X-1: A Black Hole Candidate
The strongest source of X-rays in the Large Magellanic Cloud originates from an unusually energetic binary star system. This strong source, dubbed LMC X-1, is thought to be a normal and compact star orbiting each other. Gas stripped of the normal star falls onto the compact star, heats up, and emits X-rays. The X-rays shining from the system knock electrons off atoms for light years around, causing some atoms to glow noticeably in X-rays when the electrons re-combine. Motion in the binary system indicates the compact star is probably a black hole, since its high mass - roughly five times that of our Sun - should be enough to cause even a neutron star to implode.

APOD: December 31, 1995 - The X-ray Sources of M31
Just like our own Milky Way Galaxy, the nearest major galaxy M31 has many star systems spewing high energy radiation. High energy X-radiation is visible to certain satellites in Earth orbit such as ROSAT - which took the above picture. The X-ray sources in M31 occur in globular clusters, the spiral arms, and near the galaxy's center. Probably most of these sources are accretion disk binary star systems. M31 has more X-ray sources near its center than our Galaxy, and the reason for this is currently unknown.

January 1996

APOD: January 1, 1996 - Shuttle Engine Blast
The Space Shuttle Discovery's orbital maneuvering system (OMS) engine firing produced this dramatic flare as it cruised "upside down" in low Earth orbit. Discovery was named for a ship commanded by Captain James Cook RN, the 18th Century English astronomer and navigator. Cook's voyages of discovery established new standards in scientific exploration and brought extensive knowledge of the unknown Pacific regions, including Australia, New Zealand, and the Hawaiian Island archipelago to Europeans. NASA's four-orbiter Space Shuttle fleet will begin a new year of operation on January 11, with the planned launch of the shuttle Endeavour (STS-72). Also named after one of Cook's ships, Endeavor is the latest addition to the shuttle fleet.

APOD: January 2, 1996 - The X-Ray Sky
What if you could see X-rays? If you could, the night sky would be a strange and unfamiliar place. X-rays are about 1,000 times more energetic than visible light photons and are produced in violent and high temperature astrophysical environments. Instead of the familiar steady stars, the sky would seem to be filled with exotic binary star systems composed of white dwarfs, neutron stars, and black holes, along with flare stars, X-ray bursters, pulsars, supernova remnants and active galaxies. This X-ray image of the entire sky was constructed with Skyview, using data from the first High Energy Astronomy Observatory (HEAO 1), and plotted in a coordinate system centered on the galactic center with the north galactic pole at the top. Sources near the galactic center are seen to dominate in this false color map which shows regions of highest X-ray intensity in yellow. Astronomers' ability to observe the sky at X-ray energies will be greatly enhanced by the recently launched X-ray Timing Explorer (XTE) satellite.

APOD: January 3, 1996 - The X-ray Timing Explorer
Launched Saturday on a Delta rocket, the X-ray Timing Explorer (XTE) will watch the sky for rapid changes in X-rays. XTE carries three separate X-ray telescopes. The Proportional Counter Array (PCA) and the High Energy X-ray Timing Experiment (HEXTE) will provide the best timing information in the widest X-ray energy range yet available. They will observe stellar systems that contain black holes, neutron stars, and white dwarfs as well as study the X-ray properties of the centers of active galaxies. XTE's All Sky Monitor (ASM) will scan the sky every 90 minutes to find new X-ray transients and track the variability of old ones. XTE has a planned life time of two years.

APOD: January 4, 1996 - Symbiotic Star System R Aquarii
Sometimes stars work together to create a spectacular display. Such is the case with the binary star system R Aquarii - a "symbiotic" star system. Here two stars, a variable giant and a white dwarf companion - create both a nebula and a jet. Light from the giant star and the unusually shaped nebula are visible in the above photograph. The two visible shells in the nebula likely resulted from two separate outburst events. Much about symbiotic star systems remains unknown.

APOD: January 5, 1996 - The Toby Jug Nebula
Is the only thing unusual about this nebula its shape? Pictured above is the IC 2220 - the Toby Jug Nebula - a reflection nebula surrounding a normal red giant star. Reflection nebulae shine by light reflected from the central star(s). This nebula appears to have been created from the current phase of mass-loss from the central star. Normal gas and dust compose the nebula. The nebula's peculiar but photogenic "bi-polar, bi-conical" shape, however, continue to make it a popular object of study.

APOD: January 6, 1996 - Dwarf Elliptical Galaxy M32
Being the largest galaxy around can sometimes make you popular. Pictured is M31's companion galaxy M32. M31, the Andromeda galaxy, is the largest galaxy in our Local Group of galaxies - even our tremendous Milky Way Galaxy is smaller. Little M32 is visible in most pictures of M31 - it is the small circular spot north of M31's center. M32 is a dwarf elliptical galaxy. Elliptical galaxies have little or no measurable gas or dust - they are composed completely of stars and typically appear more red than spiral galaxies. Elliptical galaxies do not have disks - they generally have oblong shapes and therefore show elliptical profiles on the sky.

APOD: January 7, 1996 - Mercury Astronauts and a Redstone
Space suited project Mercury astronauts John H. Glenn, Virgil I. Grissom, and Alan B. Shepard Jr. (left to right) are pictured here posing in front of a Redstone rocket in this 1961 NASA publicity photo. Project Mercury was the first U.S. program designed to put humans in space. It resulted in 6 manned flights using one-man capsules and Redstone and Atlas rockets. Shortly after the first U.S. manned flight on May 5, 1961, a suborbital flight piloted by Alan Shepard, President Kennedy announced the goal of a manned lunar landing by 1970. This goal was achieved by NASA's Apollo program and Shepard himself walked on the moon as a member of the Apollo 14 mission.

APOD: January 8, 1996 - Local Group Galaxy NGC 205
The Milky Way Galaxy is not alone. It is part of a gathering of about 25 galaxies known as the Local Group. Members include the Great Andromeda Galaxy (M31), M32, M33, the Large Magellanic Clouds, the Small Magellanic Clouds, Dwingeloo 1, several small irregular galaxies, and many dwarf elliptical galaxies. Pictured is one of the many dwarf ellipticals: NGC 205. Like M32, NGC 205 is a companion to the large M31, and can sometimes be seen to the south of M31's center in photographs. The above image shows this galaxy to be unusual for an elliptical galaxy in that it contains at least two dust clouds (at 7 and 11 o'clock - they are visible but hard to spot) and signs of recent star formation. This galaxy is sometimes known as M110, although it was actually not part of Messier's original catalog.

APOD: January 9, 1996 - M100 and the Expanding Universe
The distance to the swirling grand design spiral M100 is causing quite a stir among astronomers. Many believe that the Hubble Space Telescope's recent distance measurement to this galaxy accurately calibrates the expansion rate of the universe. Others believe this distance measurement is misleading. The universe's expansion rate is usually given as a quantity called "Hubble's constant", a factor dividing well-measured recession velocity of a galaxy to give actual distance. Scientific debate over the value of Hubble's constant has been ongoing since it was first measured by Edwin Hubble in 1929. A real live debate involving the value of Hubble's constant titled "The Scale of the Universe" will occur in April 1996 in Washington, DC.

APOD: January 10, 1996 - The Cepheids of M100
Can this blinking star tell us how fast the universe is expanding? Many astronomers also believe it may also tell us the age of the universe! The photographed "Cepheid variable" star in M100 brightens and dims over the course of days as its atmosphere expands and contracts. A longer blinking cycle means an intrinsically brighter star. Cepheids variable stars are therefore used as distance indicators. By noting exactly how long the blinking period is and exactly how bright the star appears to be, one can tell the distance to the star and hence the star's parent galaxy. This distance can then be used to match-up easily measured recessional velocity ("redshift") with distance. Once this "Hubble relation" is determined for M100, it should be the same for all galaxies - and hence tell us how fast the universe is expanding. The exact magnitude of this calibration is under dispute and so a real live debate involving the value of Hubble's constant titled "The Scale of the Universe" will occur in April 1996 in Washington, DC.

APOD: January 11, 1996 - Lasers in Eta Carinae
Have you heard about the great LASER light show in the sky? Well, nobody had until it was announced just yesterday by a team led by K. Davidson (U. Minn.) and S. Johansson (U. Lund). The research team discovered that the unusually variable star Eta Carinae emits ultraviolet light of such a specific color it is most probably LASER light! The artist's conception shown above depicts a possible model for the Hubble Space Telescope observations. In this model, Eta Carinae emits many LASER beams from its surrounding cloud of energized gas. Infrared LASERS and microwave MASERS are extremely rare astrophysical phenomena, but this ultraviolet LASER is the first of its kind to be discovered.

APOD: January 12, 1996 - Mare Orientale
Looking like a target ring bull's-eye, the Mare Orientale is one of the most striking large scale lunar features. Located on the Moon's extreme western edge, this impact basin is unfortunately difficult to see from an earthbound perspective. It is over 3 billion years old, about 600 miles across and was formed by the impact of an asteroid sized object. The collision caused ripples in the lunar crust resulting in the three concentric circular features visible in this 1967 photograph made by NASA's Lunar Orbiter 4. Molten lava from the Moon's interior flooded the impact site through the fractured crust creating a mare. Dark, smooth regions on the moon are called mare (Latin for sea), because early astronomers thought these areas might be oceans.

APOD: January 13, 1996 - Lunokhod 1: Moon Robot
On November 17, 1970 the Soviet Luna 17 spacecraft landed the first roving remote-controlled robot on the Moon. Known as Lunokhod 1, it weighed just under 2,000 pounds and was designed to operate for 90 days while guided by a 5-person team on planet Earth at the Deep Space Center near Moscow, USSR. Lunokhod 1 actually toured the lunar Mare Imbrium (Sea of Rains) for 11 months in one of the greatest successes of the Soviet lunar exploration program. The futuristic looking eight wheeler is pictured here in an artist's conception atop its landing module. Ramps extend from both sides of the spacecraft allowing the rover to choose an alternative route to the surface if one side is blocked by boulders.

APOD: January 14, 1996 - A Distant Cluster of Galaxies
Every bright object in this 1994 photograph by the Hubble Space Telescope is a galaxy. Oddly - most of the objects are spiral galaxies. This rich cluster of galaxies, named CL 0939+4713, is almost half way across the visible universe. Photos like this indicate that clusters in the past contained a higher fraction of spirals than do nearby clusters, which are usually dominated by elliptical galaxies.

APOD: January 15, 1996 - The Dawn of the Clusters
What did the universe look like near the beginning? This exciting photo by the Hubble Space Telescope is one of the deepest ever taken, and shows galaxies as far away as ever before photographed. The universe back then - when only one third of its present age - was a strange and violent place. Back then a large fraction of galaxies were colliding and interacting. In fact, the shapes of many galaxies in the above photo are more distorted than most nearby galaxies. At this early universe epoch many clusters of galaxies were just forming. The bright twisted group of galaxies below the photographs center contains the energetic radio galaxy 3C324.

APOD: January 16, 1996 - Wild Duck Open Cluster M11
Many stars like our Sun were formed in open clusters. The above open cluster, M11, contains thousands of stars and is just over three thousand light years distant. The stars in this cluster all formed together about 150 million years ago. The many bright stars in the cluster appear blue. Open clusters, also called galactic clusters, contain fewer and younger stars than globular clusters. Also unlike globular clusters, open clusters are generally confined to the plane of our Galaxy.

APOD: January 17, 1996 - NGC 7027: A Dying Star's Nebula
This pseudo-color composite of two recent Hubble Space Telescope images is a picture of a Sun-like star nearing the end of its lifetime. The exquisite details visible in this planetary nebula indicate that when the star passed through its Red Giant phase it initially shrugged off its outer atmosphere gently and evenly producing the outer faint spherical shells. As the process continued, material was apparently ejected in dense clumps producing dust clouds in the bright inner regions. The whole ejection process was amazingly rapid, taking only a few thousand years compared to a 10 billion year lifetime typical for Solar type stars. In the end the hot stellar core, now a white dwarf star, was left - seen here as a white dot at the center of the nebula. Our middle-aged Sun will experience a similar fate ... in about 5 billion years!

APOD: January 18, 1996 - MyCn18: An Hourglass Nebula
The sands of time are running out for the central star of this hourglass-shaped planetary nebula. With its nuclear fuel exhausted, this brief, spectacular, closing phase of a Sun-like star's life occurs as its outer layers are ejected - its core becoming a cooling, fading White Dwarf. Astronomers have recently used the Hubble Space Telescope (HST) to make a series of images of planetary nebulae, including the one above. Here, delicate rings of colorful glowing gas (nitrogen-red, hydrogen-green, and oxygen-blue) outline the tenuous walls of the "hourglass". The unprecedented sharpness of the HST images has revealed surprising details of the nebula ejection process and may help resolve the outstanding mystery of the variety of complex shapes and symmetries of planetary nebulae.

APOD: January 19, 1996 - The Dusty Disk of Beta Pic
This dusty disk, viewed edge on surrounding Beta Pictoris, a star only 50 lightyears distant, may signal the presence of an infant solar system. Beta Pictoris is a young Sun-like star just completing its formative stages. About 10 years ago it made the news when astronomers detected a disk around the star because planets are expected to form in such circumstellar disks. Did planets form around Beta Pic? The above recent Hubble Space Telescope image offers supporting evidence that they did. In this false color image, the overwhelming light from the star itself is masked out and features of the inner disk are revealed. The inner part (white area) appears to be slightly warped with respect to the line centered on the axis of the outer parts of the disk. This warp could be caused by a large planet orbiting within the inner clear zone, too faint to be seen directly against the stellar glare. If so, this is an indication that planetary systems are common in our galaxy. Is anybody out there?

APOD: January 20, 1996 - Mercury's Caloris Basin
Mercury, the closest planet to the Sun, has a surface with so many craters it resembles the Earth's Moon. The largest surface feature on Mercury is the Caloris Basin, which resulted from a collision with an asteroid. The basin, which is more that 1000 kilometers across, is visible as the large circular feature at the bottom of the above photograph. Similar features, such as the Mare Orientale, are seen on the Moon. The Caloris Basin gets very hot because it is near the "sub-solar point" - the point on Mercury's surface that is directly under the Sun when Mercury is closest to the Sun.

APOD: January 21, 1996 - Mercury's Faults
The surface of the planet Mercury is not without fault. In this case, however, "fault" refers to unusual surface features that are the topic of much speculation. The above fault line is called Santa Maria Rupes, and runs through many prominent craters. The meandering feature is thought to be the result of huge forces of compression on Mercury's surface. Such rupes probably originate from large impacts and a general shrinking of Mercury's crust, which in turn causes parts of the crust to push above other parts.

APOD: January 22, 1996 - Betelgeuse, Betelgeuse, Betelgeuse
Betelgeuse (sounds a lot like "beetle juice"), a red supergiant star about 600 lightyears distant, is shown here in this Hubble Space Telescope image which represents the first direct picture of the surface of a star other than the Sun. While Betelgeuse is cooler than the Sun, it is more massive and over 1000 times larger - if placed at the center of our Solar System, it would extend past the orbit of Jupiter. This image reveals a bright, as yet unexplained hotspot on its surface. Betelgeuse is also known as Alpha Orionis, one of the brightest stars in the familar constellation of Orion, the Hunter. Like many star names, Betelgeuse is Arabic in origin. It is derived from a phrase which refers to the hunter's shoulder or armpit, the general area occupied by this star in drawings of the figure in the constellation. As a massive red supergiant, it is nearing the end of its life and will soon become a supernova.

APOD: January 23, 1996 - Beneath Jupiter's Clouds
This near-infrared image of Jupiter was made using instrumentation at NASA's Infrared Telescope Facility, located on the summit of Mauna Kea, Hawaii, in support of the Galileo mission to Jupiter. The brightest spots indicated by the false red shading are relatively clear areas and represent glimpses beneath the outer layer of Jupiter's obscuring cloud tops. On December 7, 1995 a probe from the Galileo spacecraft parachuted through these clouds for 57 minutes before melting, all the while providing the first direct sampling of the conditions there. In a recent press release of the probe's findings scientists announced some surprising results. Discoveries based on probe data included a new radiation belt 31,000 miles above the cloud tops, relatively constant high velocity winds (up to 330 mph), no obvious water clouds, low abundances of Helium and Neon, lightning occurring only 1/10th as much as on Earth, and unexpectedly high temperatures. The Galileo orbiter continues its two-year mission to explore the Jovian system.

APOD: January 24, 1996 - The Deep Field
The image above is part of the Hubble Deep Field and represents humanity's most distant yet optical view of the Universe. Galaxies like colorful pieces of candy fill the field, some as faint as 30th magnitude (about four billion times fainter than stars visible to the unaided eye). The dimmest objects are the most distant galaxies and reveal what the Universe looked like in the extremely distant past, perhaps less than one billion years after the Big Bang. To make the Deep Field image, astronomers selected an uncluttered area of the sky in the constellation Ursa Major (the Big Dipper) and pointed the Hubble Space Telescope at a single spot for 10 days accumulating and combining many separate exposures. With each additional exposure, fainter objects were revealed. The final result can be used to explore the mysteries of galaxy evolution and the infant Universe.

APOD: January 25, 1996 - Catching Falling Stardust
This carrot shaped track is actually little more than 5 hundredths of an inch long. It is the trail of a meteroid through a gel exposed to space in low earth orbit by the shuttle launched EURECA (European Recoverable Carrier) spacecraft. The meteoroid itself, about a thousandth of an inch in diameter, is visible where it came to rest, just beyond the tip of the carrot (far right). Chemical analyses of interplanetary dust particles similar to this one suggest that some of them may be bits of comets and represent samples of material from the early stages of the formation of the Solar System. NASA's Stardust mission, planned for launch in 1999, will attempt to directly collect dust from the tail of a comet and return it to Earth.

APOD: January 26, 1996 - Quadrantids: Meteors in Perspective
Meteor showers are caused by streams of solid particles, dust size and larger, moving as a group through space. In many cases, the orbits of these meteor streams can be identified with the dust tails of comets. When the Earth passes through the streams, the particles leave brilliant trails through the night sky as they burn up in the atmosphere. Above is an image of a meteor shower known as the Quadrantids. It was made in January 1995 using MOVIE, a new system for making video meteor observations. To make the image, frames from a video tape were computer processed and superposed to show the relative paths of many meteors in the shower. The meteor paths are all parallel to each other, but the effect of perspective causes the trails to appear to originate from a distant radiant point in the sky. In contrast to the elongated meteor trails, the brighter stars of the familiar constellation Ursa Major (the Big Dipper) are visible as points in the lower half of the image.

APOD: January 27, 1996 - M8: An Open Cluster in the Lagoon
The large majestic Lagoon Nebula is home for many young stars and hot gas. The Lagoon Nebulae is so large and bright it can be seen without a telescope. Formed only several million years ago in the nebula is the open cluster known as NGC 6530, whose young stars show their high temperature by their blue glow. The nebula, also known as M8 and NGC 6523, is named "Lagoon" for the band of dust seen to the left of the open cluster's center. A bright knot of gas and dust in the nebula's center is known as the Hourglass Nebula. Star formation continues in the the Lagoon Nebula as witnessed by the many globules that exist there.

APOD: January 28, 1996 - Orbiting Repairmen
In December of 1993 astronauts Story Musgrave and Jeffrey Hoffman performed the orbiting repairmans' ballet 400 miles above the Earth. They are seen in this photo perched at the end of the Space Shuttle Endeavour's robotic arm making final repairs to the four story tall Hubble Space Telescope. The coast line of western Australia is visible below. The complex and highly successful repair mission allowed HST to see into the Universe with unprecedented clarity.

APOD: January 29, 1996 - Searchlight Beams from the Egg Nebula
The dramatic and mysterious looking object revealed in this Hubble Space Telescope image is known as the Egg Nebula. It is an aging star about 3,000 lightyears distant, entering its Planetary Nebula phase of evolution. Surrounded by an expanding cloud of gas and dust, a dense cocoon of dust (seen as the dark band running diagonally across the center) encloses the star itself and blocks it from direct view. The searchlight appearance is created as light from the star shines more easily through the thinner parts of the cocoon. Dust particles in the expanding cloud scatter and reflect the starlight making the beams visible. The sharpness of the HST image reveals a wealth of detail which will help to understand this complex and spectacular part of the stellar lifecycle.

APOD: January 30, 1996 - 70 Virginis b: A New Water Planet?
The star 70 Virginis has a planet. This recent discovery is the second known case of a planet orbiting a normal star other than our Sun itself. The first case involved 51 Pegasi and was announced last year. The star 70 Vir, shown in the center of the above false-color picture, is very much like the Sun. The planet is not visible above - the unusual structure surrounding the star is caused by the telescope. The planet, designated 70 Vir b for short, was discovered by very slight periodic shifts in its colors. Defining characteristics of this planet include that it is at least eight times the mass of Jupiter, it's orbit is much smaller than Jupiter's, and it's temperature allows water to exist in liquid form - like on the Earth. Life on Earth is based on liquid water - could life exist here too?

APOD: January 31, 1996 - Planets Around Sun-Like Stars
Do many Sun-like stars have planets? Speculation on this point has been ongoing since humanity's realization that other stars existed. Only in the past year, however, have answers and discoveries been realized. The above plot summarizes the four known cases of normal stars having planets. These cases are: our Solar System, 51 Pegasi, 70 Virgini, and 47 Ursae Majoris. The later two cases were discovered by astronomers led by Geoff Marcy and Paul Butler as part of a greater project inspecting 120 stars for orbiting planets. Interestingly enough, the planets around the later two stars have temperatures in the right range to allow liquid water - and hence may have conditions ripe for the development of life.

Authors & editors: Robert Nemiroff (GMU) & Jerry Bonnell (USRA).
NASA Technical Rep.: Sherri Calvo. Specific rights apply.
A service of: LHEA at NASA/ GSFC