Astronomy Picture of the Day Search Results for "SOHO"

APOD: 2024 October 10 - Five Bright Comets from SOHO
Explanation: Five bright comets are compared in these panels, recorded by a coronograph on board the long-lived, sun-staring SOHO spacecraft. Arranged chronologically all are recognizable by their tails streaming away from the Sun at the center of each field of view, where a direct view of the overwhelmingly bright Sun is blocked by the coronagraph's occulting disk. Each comet was memorable for earthbound skygazers, starting at top left with Comet McNaught, the 21st century's brightest comet (so far). C/2023 A3 Tsuchinshan-ATLAS, following its perihelion with the active Sun at bottom center, has most recently grabbed the attention of comet watchers around the globe. By the end of October 2024, the blank 6th panel may be filled with bright sungrazer comet C/2024 S1 (ATLAS). ... or not.

APOD: 2024 April 17 – Total Eclipse and Comets
Explanation: Not one, but two comets appeared near the Sun during last week's total solar eclipse. The expected comet was Comet 12P/Pons-Brooks, but it was disappointingly dimmer than many had hoped. However, relatively unknown Comet SOHO-5008 also appeared in long duration camera exposures. This comet was the 5008th comet identified on images taken by ESA & NASA's Sun-orbiting SOHO spacecraft. Likely much smaller, Comet SOHO-5008 was a sungrazer which disintegrated within hours as it passed too near the Sun. The featured image is not only unusual for capturing two comets during an eclipse, but one of the rare times that a sungrazing comet has been photographed from the Earth's surface. Also visible in the image is the sprawling corona of our Sun and the planets Mercury (left) and Venus (right). Of these planets and comets, only Venus was easily visible to millions of people in the dark shadow of the Moon that crossed North America on April 8.

APOD: 2022 January 30 - A Solar Prominence from SOHO
Explanation: How can gas float above the Sun? Twisted magnetic fields arching from the solar surface can trap ionized gas, suspending it in huge looping structures. These majestic plasma arches are seen as prominences above the solar limb. In 1999, this dramatic and detailed image was recorded by the Extreme ultraviolet Image Telescope (EIT) on board the space-based SOHO observatory in the light emitted by ionized Helium. It shows hot plasma escaping into space as a fiery prominence breaks free from magnetic confinement a hundred thousand kilometers above the Sun. These awesome events bear watching as they can affect communications and power systems over 100 million kilometers away on planet Earth. In late 2020 our Sun passed the solar minimum of its 11-year cycle and is now showing increased surface activity.

APOD: 2021 January 7 - Total Solar Eclipse 2020
Explanation: Along a narrow path crossing southern South America through Chile and Argentina, the final New Moon of 2020 moved in front of the Sun on December 14 in the year's only total solar eclipse. Within about 2 days of perigee, the closest point in its elliptical orbit, the New Moon's surface is faintly lit by earthshine in this dramatic composite view. The image is a processed composite of 55 calibrated exposures ranging from 1/640 to 3 seconds. Covering a large range in brightness during totality, it reveals the dim lunar surface and faint background stars, along with planet-sized prominences at the Sun's edge, an enormous coronal mass ejection, and sweeping coronal structures normally hidden in the Sun's glare. Look closely for an ill-fated sungrazing Kreutz family comet (C/2020 X3 SOHO) approaching from the lower left, at about the 7 o'clock position. In 2021 eclipse chasers will see an annular solar eclipse coming up on June 10. They'll have to wait until December 4 for the only total solar eclipse in 2021 though. That eclipse will be total along a narrow path crossing the southernmost continent of Antarctica.

APOD: 2020 May 8 - Long Tailed Comet SWAN
Explanation: Blowing in the solar wind the spectacular ion tail of Comet SWAN (C/2020 F8) extends far across this 10 degree wide telephoto field of view. Captured on May 2 its greenish coma was about 6 light-minutes from Earth. The pretty background starfield lies near the border of the constellations Cetus and Aquarius. This comet SWAN was discovered at home by Australian amateur Michael Mattiazzo by checking images from the Sun-staring SOHO spacecraft's SWAN (Solar Wind ANisotropies) camera. The comet has now become just visible to the naked-eye as it sweeps from southern to northern skies. Appearing in morning twilight near the eastern horizon, Comet SWAN will make its closest approach to planet Earth on May 12 and reach perihelion on May 27.

APOD: 2020 April 29 - The Ion Tail of New Comet SWAN
Explanation: Newly discovered Comet SWAN has already developed an impressive tail. The comet came in from the outer Solar System and has just passed inside the orbit of the Earth. Officially designated C/2020 F8 (SWAN), this outgassing interplanetary iceberg will pass its closest to the Earth on May 13, and closest to the Sun on May 27. The comet was first noticed in late March by an astronomy enthusiast looking through images taken by ESA's and NASA's Sun-orbiting SOHO spacecraft, and is named for this spacecraft's Solar Wind Anisotropies (SWAN) camera. The featured image, taken from the dark skies in Namibia in mid-April, captured Comet SWAN's green-glowing coma and unexpectedly long, detailed, and blue ion-tail. Although the brightness of comets are notoriously hard to predict, some models have Comet SWAN becoming bright enough to see with the unaided eye during June.

APOD: 2018 September 2 - A Powerful Solar Flare
Explanation: It was one of the most powerful solar flares in recorded history. Occurring in 2003 and seen across the electromagnetic spectrum, the Sun briefly became over 100 times brighter in X-rays than normal. The day after this tremendous X 17 solar flare -- and subsequent Coronal Mass Ejection (CME) -- energetic particles emitted from the explosions struck the Earth, creating auroras and affecting satellites. The spacecraft that took these frames -- SOHO -- was put in a turtle-like safe mode to avoid further damage from this and subsequent solar particle storms. The featured time-lapse movie condenses into 10 seconds events that occurred over 4 hours. The CME, visible around the central sun-shade, appears about three-quarters of the way through the video, while frames toward the very end are progressively noisier as protons from the explosions strike SOHO's LASCO detector. One this day in 1859, the effects of an even more powerful solar storm caused telegraphs on Earth to spark in what is known as the Carrington Event. Powerful solar storms such as these may create beautiful aurora-filled skies, but they also pose a real danger as they can damage satellites and even power grids across the Earth.

APOD: 2017 November 13 - Comet Machholz Approaches the Sun
Explanation: Why is Comet Maccholz so depleted of carbon-containing chemicals? Comet 96P/Machholz's original fame derives from its getting closer to the Sun than any other short period comet -- half as close as Mercury -- and doing so every five years. To better understand this unusual comet, NASA's Sun-monitoring SOHO spacecraft tracked the comet during its latest approach to the Sun in October. The featured image composite shows the tail-enhanced comet swooping past the Sun. The Sun's bright surface is hidden from view behind a dark occulter, although parts of the Sun's extended corona are visible. Neighboring stars dot the background. One hypothesis holds that these close solar approaches somehow cause Comet Machholz to shed its carbon, while another hypothesis posits that the comet formed with this composition far away -- possibly even in another star system.

APOD: 2017 September 27 - Layers of a Total Solar Eclipse
Explanation: Neither rain, nor snow, nor dark of night can keep a space-based spacecraft from watching the Sun. In fact, from its vantage point 1.5 million kilometers sunward of planet Earth, NASA's SOlar Heliospheric Observatory (SOHO) can always monitor the Sun's outer atmosphere, or corona. But only during a total solar eclipse can Earth-based observers also see the lovely coronal streamers and structures - when the Moon briefly blocks the overwhelmingly bright solar surface. Then, it becomes possible to follow detailed coronal activity all the way down to the Sun's surface. In the outside layer of this composite image, SOHO's uninterrupted view of the solar corona during last month's eclipse is shown in orange hues. The middle, donut-shaped region is the corona as recorded by the Williams College Eclipse Expedition to Salem, Oregon. Simultaneously, the inner view is from NASA's Earth-orbiting Solar Dynamics Observatory, which, being outside of totality, was able to image the face of the Sun in extreme ultraviolet light, shown in gold.

APOD: 2016 April 12 - Combined Solar Eclipse Corona from Earth and Space
Explanation: Sometimes, a total eclipse is a good time to eye the Sun. Taking advantage of an unusual juxtaposition of Earth, Moon and Sun, the featured image depicts the total solar eclipse that occurred last month as it appeared -- nearly simultaneously -- from both Earth and space. The innermost image shows the total eclipse from the ground, with the central pupil created by the bright Sun covered by a comparatively dark Moon. Surrounding the blocked solar disk is the tenuous corona of Sun imaged in white light, easily visible from the ground only during an eclipse. Normally, this corona is hard to track far from the Sun, but the featured montage matches it to false-colored observations of the Sun from NASA and ESA's space-based, Sun-orbiting, Solar and Heliospheric Observatory (SOHO). Observations like this allow the study of the constantly changing magnetic activity both near and far from the Sun, the same activity that ultimately drives Earth's auroras.

APOD: 2016 January 10 - Sun Storm: A Coronal Mass Ejection
Explanation: What's happening to our Sun? Another Coronal Mass Ejection (CME)! The Sun-orbiting SOHO spacecraft has imaged many erupting filaments lifting off the active solar surface and blasting enormous bubbles of magnetic plasma into space. Direct light from the sun is blocked in the inner part of the featured image, taken in 2002, and replaced by a simultaneous image of the Sun in ultraviolet light. The field of view extends over two million kilometers from the solar surface. While hints of these explosive events, called coronal mass ejections or CMEs, were discovered by spacecraft in the early 70s, this dramatic image is part of a detailed record of this CME's development from the presently operating SOHO spacecraft. Near the maximum of the solar activity cycle, CMEs now typically occur several times a week. Strong CMEs may profoundly influence space weather. Those directed toward our planet can have serious effects.

APOD: 2015 November 2 - Comet ISON Being Destroyed by the Sun
Explanation: Most comets don't survive a close encounter with the Sun. Two years ago this month, though, Comet ISON was thought by some to be big enough to withstand its perilous sungrazing dive. The featured video shows the drama as it was recorded by the Solar and Heliospheric Observatory (SOHO), a joint mission of ESA and NASA. As many Earthlings watched in fascination, a bright area did emerge from closest approach, but it soon faded and dispersed. It is now assumed that no large fragments of Comet C/2012 S1 (ISON) survived. Besides the comet, the active Sun is seen to eject puffs of plasma known as coronal mass ejections. Launched in 1995, sun-orbiting SOHO has become a historic device in the discovery and tracking of comets known as sungrazers. Two months ago, a comet designated SOHO 3000 was named in honor of the record 3000th comet that was discovered on SOHO images, a total that amounts to about half of all known comets.

APOD: 2015 January 25 - A Twisted Solar Eruptive Prominence
Explanation: Ten Earths could easily fit in the "claw" of this seemingly solar monster. The monster, actually a huge eruptive prominence, is seen moving out from our Sun in this condensed half-hour time-lapse sequence. This large prominence, though, is significant not only for its size, but its shape. The twisted figure eight shape indicates that a complex magnetic field threads through the emerging solar particles. Differential rotation of gas just inside the surface of the Sun might help account for the surface explosion. The five frame sequence was taken in early 2000 by the Sun-orbiting SOHO satellite. Although large prominences and energetic Coronal Mass Ejections (CMEs) are relatively rare, they are again occurring more frequently now that we are near the Solar Maximum, a time of peak sunspot and solar activity in the eleven-year solar cycle.

APOD: 2013 November 30 - Surprising Comet ISON
Explanation: After failing to appear for Sun staring spacecraft at perihelion, its harrowing closest approach to the Sun, sungrazing Comet ISON was presumed lost. But ISON surprised observers yesterday as material still traveling along the comet's trajectory became visible and even developed an extensive fan-shaped dust tail. Edited and processed to HD format, this video (vimeo, youtube) is composed of frames from the SOHO spacecraft's coronographs. It follows the comet in view of the wide (blue tint) and narrow (red tint) field cameras in the hours both before and after perihelion passage. In both fields, overwhelming sunlight is blocked by a central occulting disk. A white circle indicates the Sun's positon and scale. With questions to be answered and the tantalizing possibility that a small cometary nucleus has survived in whole or part, surprising comet ISON will be rising before dawn in planet Earth's skies in the coming days.

APOD: 2013 November 29 - Comet ISON Before and After
Explanation: Sungrazing Comet ISON reached perihelion, its closest approach to the Sun, yesterday, November 28, at 18:45 UT. The comet passed just over 1 million kilometers above the solar surface, a distance less than the diameter of the Sun. These two panels follow ISON before (right) and after its close approach, imaged by the LASCO instrument onboard the Sun staring SOHO spacecraft. Overwhelming sunlight is blocked by LASCO's central occulting disk with a white circle indicating the Sun's positon and scale. The bright comet is seen along its path at the bottom of the before panel, but something much fainter exits near the top of the after panel, potentially a dust tail reforming from the debris left from ISON's perihelion passage.

APOD: 2013 November 28 - NGC 1999: South of Orion
Explanation: South of the large star-forming region known as the Orion Nebula, lies bright blue reflection nebula NGC 1999. At the edge of the Orion molecular cloud complex some 1,500 light-years distant, NGC 1999's illumination is provided by the embedded variable star V380 Orionis. That nebula is marked with a dark sideways T-shape near center in this cosmic vista that spans about 10 light-years. The dark shape was once assumed to be an obscuring dust cloud seen in silhouette against the bright reflection nebula. But recent infrared images indicate the shape is likely a hole blown through the nebula itself by energetic young stars. In fact, this region abounds with energetic young stars producing jets and outflows with luminous shock waves. Cataloged as Herbig-Haro (HH) objects, named for astronomers George Herbig and Guillermo Haro, the shocks look like red gashes in this scene that includes HH1 and HH2 just below NGC 1999. The stellar jets push through the surrounding material at speeds of hundreds of kilometers per second.

APOD: 2013 November 27 - Comet ISON Rising
Explanation: Will Comet ISON survive tomorrow's close encounter with the Sun? Approaching to within a solar diameter of the Sun's surface, the fate of one of the most unusual comets of modern times will finally be determined. The comet could shed a great amount of ice and dust into a developing tail -- or break apart completely. Unfortunately, the closer Comet ISON gets to the Sun, the harder it has been for conventional telescopes to see the brightening comet in the glare of the morning Sun. Pictured in the above short time lapse video, Comet ISON was captured rising over the Canary Islands just above the morning Sun a few days ago. If the comet's nucleus survives, the coma and the tails it sheds might well be visible rising ahead of the Sun in the next few days or weeks. Alternatively, satellites watching the Sun might document one of the larger comet disintegrations yet recorded. Stay tuned!

APOD: 2013 November 23 - Comet ISON from STEREO
Explanation: Still intact, on November 21 Comet ISON (C/2012 S1) swept into this animated field of view (left) from the HI-1 camera on the STEREO-A spacecraft. The camera has also captured periodic Comet Encke, Mercury, and Earth, with the Sun cropped out of the frame at the right, the source of the billowing solar wind. From STEREO's perspective in interplanetary space, planet Earth is actually the most distant of the group, seen in its orbit beyond the Sun. Mercury is closest, but both planets are still so bright they create sharp vertical lines in the camera's detector. Both comets clearly sport substantial tails, but ISON is closer to the camera and will continue to move more rapidly through the field. Cameras on STEREO and SOHO spacecraft will be able to follow Comet ISON as it falls towards its close encounter with the Sun on November 28, even as ISON gets more difficult to see in the bright dawn skies of planet Earth.

APOD: 2013 November 11 - An Active Sun During a Total Eclipse
Explanation: Sometimes, a total eclipse of the Sun is an opportunity. Taking advantage of such, the above image shows the solar eclipse earlier this month as covered and uncovered by several different solar observatories. The innermost image shows the Sun in ultraviolet light as recorded over a few hours by the SWAP instrument aboard the PROBA2 mission in a sun-synchronous low Earth orbit. This image is surrounded by a ground-based eclipse image, reproduced in blue, taken from Gabon. Further out is a circularly blocked region used to artificially dim the central sun by the LASCO instrument aboard the Sun-orbiting SOHO spacecraft. The outermost image -- showing the outflowing solar corona -- was taken by LASCO ten minutes after the eclipse and shows an outflowing solar corona. Over the past few weeks, our Sun has been showing an unusually high amount of sunspots, CMEs, and flares -- activity that was generally expected as the Sun is currently going through Solar Maximum -- the busiest part of its 11 year solar cycle. The above resultant image is a picturesque montage of many solar layers at once that allows solar astronomers to better match up active areas on or near the Sun's surface with outflowing jets in the Sun's corona.

APOD: 2013 October 27 - Sungrazer
Explanation: Arcing toward a fiery fate, this Sungrazer comet was recorded by the SOHO spacecraft's Large Angle Spectrometric COronagraph(LASCO) on December 23, 1996. LASCO uses an occulting disk, partially visible at the lower right, to block out the otherwise overwhelming solar disk allowing it to image the inner 8 million kilometers of the relatively faint corona. The comet is seen as its coma enters the bright equatorial solar wind region (oriented vertically). Positioned in space to continuously observe the Sun, SOHO has now been used to discover over 1,500 comets, including numerous sungrazers. Based on their orbits, the vast majority of sungrazers are believed to belong to the Kreutz family of sungrazing comets created by successive break ups from a single large parent comet that passed very near the Sun in the twelfth century. The Great Comet of 1965, Ikeya-Seki, was also a member of the Kreutz family, coming within about 650,000 kilometers of the Sun's surface. Passing so close to the Sun, Sungrazers are subjected to destructive tidal forces along with intense solar heat. This small comet, known as the Christmas Comet SOHO 6, did not survive. Later this year, Comet ISON, potentially the brightest sungrazer in recorded history but not a Kreutz sungrazer, is expected to survive.

APOD: 2012 May 27 - Mercury Spotting
Explanation: Can you spot the planet? The diminutive disk of Mercury, the solar system's innermost planet, spent about five hours crossing in front of the enormous solar disk in 2003, as viewed from the general vicinity of planet Earth. The Sun was above the horizon during the entire transit for observers in Europe, Africa, Asia, or Australia, and the horizon was certainly no problem for the sun-staring SOHO spacecraft. Seen as a dark spot, Mercury progresses from left to right (top panel to bottom) in these four images from SOHO's extreme ultraviolet camera. The panels' false-colors correspond to different wavelengths in the extreme ultraviolet which highlight regions above the Sun's visible surface. This was the first of 14 transits of Mercury which will occur during the 21st century. Next week, however, an event much more rare but easier to spot will occur -- a transit of Venus across the Sun. Need help spotting Mercury? Just click on the picture.

APOD: 2011 December 17 - Comet Lovejoy: Sungrazing Survivor
Explanation: Like most other sungrazing comets, Comet Lovejoy (C/2011 W3) was not expected to survive its close encounter with the Sun. But it did. This image from a coronograph onboard the sun-staring SOHO spacecraft identifies the still inbound remnants of the tail, with the brilliant head or coma emerging from the solar glare on December 16. The Sun's position, behind an occulting disk to block the overwhelming glare, is indicated by the white circle. Separated from its tail, Comet Lovejoy's coma is so bright it saturates the camera's pixels creating the horizontal streaks. Based on their orbits, sungrazer comets are thought to belong to the Kreutz family of comets, created by successive break ups from a single large parent comet that passed very near the Sun in the twelfth century. Most have been discovered with SOHO's cameras, but unlike many sungrazers, this one was first spotted by Australian astronomer Terry Lovejoy from an earth-based observatory. Comet Lovejoy is estimated to have come within 120,000 kilometers of the Sun's surface and likely had a large cometary nucleus to have survived its intense perihelion passage. Remarkable videos of the encounter from the Solar Dynamics Observatory can be found here.

APOD: 2011 October 5 - Comet and CME on the Sun
Explanation: Did a sun-diving comet just cause a solar explosion? Probably not. This past weekend a comet dove toward the Sun and was followed very quickly by a Coronal Mass Ejection (CMEs) from the other side of the Sun. The first two sequences in the above video shows the spectacular unfolding of events as seen by the Sun-orbiting SOHO satellite, while the same events were also captured by both Sun-orbiting STEREO satellites. Now sungrazer comets that break up as they pass near the Sun are not all that rare -- hundreds have been cataloged over the past few years. CMEs are even more common, with perhaps three lesser events occurring even during the eight hours of the above time-lapse movie. Therefore, the best bet of solar scientists is that the two events were unrelated. Another basis for this judgment is that CMEs are caused by rapid changes in the Sun's magnetic field, changes that a small comet seem unlikely to make. Such coincidences are even more likely during periods of high activity on the Sun's surface -- like now.

APOD: 2011 August 1 - Shuttle Reentry Streak from Orbit
Explanation: What's that strange bright streak? It is the last image ever of a space shuttle from orbit. A week and a half ago, after decoupling from the International Space Station, the Space Shuttle Atlantis fired its rockets for the last time, lost its orbital speed, and plummeted back to Earth. Within the next hour, however, the sophisticated space machine dropped its landing gear and did what used to be unprecedented -- landed like an airplane on a runway. Although the future of human space flight from the USA will enter a temporary lull, many robotic spacecraft continue to explore our Solar System and peer into our universe, including Cassini, Chandra, Chang'e 2, Dawn, Fermi, Hubble, Kepler, LRO, Mars Express, Messenger, MRO, New Horizons, Opportunity, Planck, Rosetta, SDO, SOHO, Spitzer, STEREO, Swift, Venus-Express, and WISE.

APOD: 2010 October 18 - It Came from the Sun
Explanation: What's that coming over the edge of the Sun? What might appear at first glance to be some sort of Sun monster is actually a solar prominence. The above prominence, captured by the Sun-orbiting SOHO satellite earlier this year during an early stage of its eruption, rapidly became one of the largest ever on record. Even as pictured, the prominence is huge -- the Earth would easily fit inside. A solar prominence is a thin cloud of solar gas held just above the surface by the Sun's magnetic field. A quiescent prominence typically lasts about a month, while an eruptive prominence like the one developing above may erupt within hours into a Coronal Mass Ejection (CME), expelling hot gas into the Solar System. Although very hot, prominences typically appear dark when viewed against the Sun, since they are slightly cooler than the surface. As our Sun evolves toward Solar maximum over the next three years, more large eruptive prominences are expected.

APOD: 2010 January 16 - New Year Sun Grazer
Explanation: Intense and overwhelming, the direct glare of the Sun is blocked by the smooth occulting disk in this image from the sun-staring SOHO spacecraft. Taken on January 3rd, an extreme ultraviolet image of the Sun to scale, is superimposed at the center of the disk. Beyond the disk's outer boundary is a sungrazer comet, one of the brightest yet seen by SOHO. The comet was discovered (movie link) by Australian amateur astronomer Alan Watson, while examining earlier images from another sun-watching spacecraft, STEREO-A. Based on their orbits, sungrazers are believed to belong to the Kreutz family of comets, created by successive break ups from a single large parent comet that passed very near the Sun in the twelfth century. Subjected to strong tidal forces and intense solar heat, this sungrazer comet did not survive its close encounter.

APOD: 2009 May 31 - A Solar Prominence from SOHO
Explanation: How can gas float above the Sun? Twisted magnetic fields arching from the solar surface can trap ionized gas, suspending it in huge looping structures. These majestic plasma arches are seen as prominences above the solar limb. In 1999 September, this dramatic and detailed image was recorded by the EIT experiment on board the space-based SOHO observatory in the light emitted by ionized Helium. It shows hot plasma escaping into space as a fiery prominence breaks free from magnetic confinement a hundred thousand kilometers above the Sun. These awesome events bear watching as they can affect communications and power systems over 100 million kilometers away on Planet Earth. Recently, our Sun has been unusually quiet.

APOD: 2009 March 22 - Sungrazer
Explanation: The Sun destroyed this comet. Arcing toward a fiery fate, this Sungrazer comet was recorded by the SOHO spacecraft's Large Angle Spectrometric COronagraph(LASCO) on 1996 Dec. 23. LASCO uses an occulting disk, partially visible at the lower right, to block out the otherwise overwhelming solar disk allowing it to image the inner 5 million miles of the relatively faint corona. The comet is seen as its coma enters the bright equatorial solar wind region (oriented vertically). Spots and blemishes on the image are background stars and camera streaks caused by charged particles. Positioned in space to continuously observe the Sun, SOHO has now been used to discover over 1,500 comets, including numerous sungrazers. Based on their orbits, they are believed to belong to a family of comets created by successive break ups from a single large parent comet which passed very near the Sun in the twelfth century. The Great Comet of 1965, Ikeya-Seki, was also a member of the Sungrazer family, coming within about 650,000 kilometers of the Sun's surface. Passing so close to the Sun, Sungrazers are subjected to destructive tidal forces along with intense solar heat. This comet, known as SOHO 6, did not survive.

APOD: 2009 March 15 - A Prominent Solar Prominence from SOHO
Explanation: What's happened to our Sun? It was sporting a spectacular -- but not very unusual -- solar prominence. A solar prominence is a cloud of solar gas held above the Sun's surface by the Sun's magnetic field. In 2004, NASA's Sun-orbiting SOHO spacecraft imaged an impressively large prominence hovering over the surface, pictured above. The Earth would easily fit under the hovering curtain of hot gas. A quiescent prominence typically lasts about a month, and may erupt in a Coronal Mass Ejection (CME) expelling hot gas into the Solar System. Although somehow related to the Sun's changing magnetic field, the energy mechanism that creates and sustains a Solar prominence is still a topic of research.

APOD: 2008 September 24 - Active Region 1002 on an Unusually Quiet Sun
Explanation: Why has the Sun been so quiet recently? No one is sure. Our Sun has shown few active regions -- that house even fewer associated sunspots -- for over a year now, and such a period of relative calm is quite unusual. What is well known is that our Sun is in a transitional period between solar cycles called a Solar Minimum, where solar activity has historically been reduced. The stark lack of surface tumult is unusual even during a Solar Minimum, however, and activity this low has not been seen for many decades. A few days ago, however, a bona-fide active region -- complete with sunspots --appeared and continues to rotate across the Sun's face. Visible above, this region, dubbed Active Region 1002 (AR 1002), was imaged in ultraviolet light yesterday by the SOHO spacecraft, which co-orbits the Sun near the Earth. Besides the tranquility on the Sun's surface, recent data from the Ulysses spacecraft, across the Solar System, indicate that the intensity of the solar wind blowing out from the Sun is at a fifty year low. Predictions hold, however, that our Sun will show more and more active regions containing more and more sunspots and flares until Solar Maximum occurs in about four years.

APOD: 2008 June 1 - A Twisted Solar Eruptive Prominence
Explanation: Ten Earths could easily fit in the "claw" of this seemingly solar monster. The monster, though, visible on the lower left, is a huge eruptive prominence seen moving out from our Sun. The above dramatic image taken early in the year 2000 by the Sun-orbiting SOHO satellite. This large prominence, though, is significant not only for its size, but its shape. The twisted figure eight shape indicates that a complex magnetic field threads through the emerging solar particles. Differential rotation inside the Sun might help account for the surface explosion. Although large prominences and energetic Coronal Mass Ejections (CMEs) are relatively rare, they are occurred more frequently near Solar Maximum, the time of peak sunspot and solar activity in the eleven-year solar cycle.

APOD: 2007 December 3 - A Complete Solar Cycle from SOHO
Explanation: Every eleven years, our Sun goes through a solar cycle. A complete solar cycle has now been imaged by the sun-orbiting SOHO spacecraft, celebrating the 12th anniversary of its launch yesterday. A solar cycle is caused by the changing magnetic field of the Sun, and varies from solar maximum, when sunspot, coronal mass ejection, and flare phenomena are most frequent, to solar minimum, when such activity is relatively infrequent. Solar minimums occurred in 1996 and 2007, while the last solar maximum occurred in 2001. This picture is composed of a SOHO image of the Sun in extreme ultraviolet light for each year of the last solar cycle, with images picked to illustrate the relative activity of the Sun.

APOD: 2007 September 27 - Hole in the Sun
Explanation: The dark expanse below the equator of the Sun is a coronal hole -- a low density region extending above the surface where the solar magnetic field opens freely into interplanetary space. Shown in false color, the picture was recorded on September 19th in extreme ultraviolet light by the EIT instrument onboard the space-based SOHO observatory. Studied extensively from space since the 1960s in ultraviolet and x-ray light, coronal holes are known to be the source of the high-speed solar wind, atoms and electrons that flow outward along the open magnetic field lines. The solar wind streaming from this coronal hole triggered colorful auroral displays on planet Earth begining late last week, enjoyed by spaceweather watchers at high latitudes.

APOD: 2007 February 6 - Sun Storm: A Coronal Mass Ejection
Explanation: What's happening to our Sun? Another Coronal Mass Ejection (CME)! The Sun-orbiting SOHO spacecraft has imaged many erupting filaments lifting off the active solar surface and blasting enormous bubbles of magnetic plasma into space. Direct light from the sun is blocked in the inner part of the above image, taken in 2002, and replaced by a simultaneous image of the Sun in ultraviolet light. The field of view extends over two million kilometers from the solar surface. While hints of these explosive events, called coronal mass ejections or CMEs, were discovered by spacecraft in the early 70s, this dramatic image is part of a detailed record of this CME's development from the presently operating SOHO spacecraft. Near the minimum of the solar activity cycle CMEs occur about once a week, but near solar maximum rates of two or more per day are typical. Strong CMEs may profoundly influence space weather. Those directed toward our planet can have serious effects.

APOD: 2007 January 20 - SOHO: Comet McNaught Movie
Explanation: This frame from a spectacular time lapse movie shows Comet McNaught - the Great Comet of 2007 - sweeping through the inner solar system. The movie frames were recorded from January 12 through Jan 16 by a coronograph onboard the sun-staring SOHO spacecraft. Bright planet Mercury also glides dramatically through the field of view but the Sun itself remains fixed, hidden behind the coronograph's central occulting disk. The broad-tailed comet is so bright it almost overwhelms SOHO's sensitive camera designed to explore the fainter structures in the Sun's outer atmosphere. Comet McNaught's closest approach to the Sun (perihelion on January 12) was only 0.17 astronomical units, or about half the distance between the Sun and Mercury. (Note: To download the movie file, click on the picture.)

APOD: 2007 January 13 - Comet Over Krakow
Explanation: Bright Comet McNaught (C/2006 P1) graced the twilight this week, seen by many and often described with superlatives. Watching the skies over Krakow, Poland, Andrzej Sawow recorded this view on Wednesday - with an ordinary handheld digital camera. He notes that "... astronomy is really for everyone who loves to look at the night sky. And fortunately (sometimes) the sky generously rewards its observer". Now very close to the Sun, Comet McNaught (along with Mercury) is visible in realtime images from the SOHO spacecraft. Otherwise, skywatchers will find the comet hard to see this weekend. But southern hemisphere observers could be rewarded next week as Comet McNaught begins to climb higher in southern skies.

APOD: 2007 January 5 - Comet McNaught Heads for the Sun
Explanation: Early morning risers with a clear and unobstructed eastern horizon can enjoy the sight of Comet McNaught (C/2006 P1) in dawn skies over the next few days. Discovered in August by R. H. McNaught (Siding Spring Survey) the comet has grown bright enough to see with the unaided eye but will soon be lost in the glare of the Sun. Still, by January 11 sun-staring spacecraft SOHO should be able to offer web-based views as the comet heads toward a perihelion passage inside the orbit of Mercury. This image captures the new naked-eye comet at about 2nd magnitude in twilight skies near sunset on January 3rd. After rounding the Sun and emerging from the solar glare later this month, Comet McNaught could be even brighter.

APOD: 2006 November 16 - Children of the Sun
Explanation: For a moment, planets Jupiter, Venus, Mars, and Mercury all posed near their parent star in this Sun-centered view, recorded on November 11. The picture, from a coronograph onboard the space-based SOlar Heliospheric Observatory, spans 15 degrees with the Sun's size and position indicated by the white circle. Background stars are also visible as the otherwise overwhelming sunlight is blocked by the coronograph's occulting disk. But the planets themselves, in particular Jupiter and Venus, are still bright enough to cause significant horizontal streaks in the image. Mercury is actually moving most rapidly (left to right) through the field and days earlier was seen to cross in front of the solar disk. So what's that bright double star to the left of Mars? Zubenelgenubi, of course.

APOD: 2006 November 8 - Simulated Transit of Mercury
Explanation: Mercury, the solar system's innermost planet, will spend about five hours crossing in front of the Sun today - beginning at 1912 UT (2:12pm EST), November 8. Specially equipped telescopes are highly recommended to safely spot the planet's diminutive silhouette however, as Mercury should appear about 200 times smaller than the enormous solar disk. This simulated view is based on a filtered solar image recorded on November 3rd. It shows active regions and the Mercury transit across the Sun at six positions from lower left to middle right. Depending on your location, the Sun may not be above the horizon during the entire transit, but webcasts of the event are planned - including one using images from the sun-staring SOHO spacecraft. This is the second of 14 transits of Mercury during the 21st century. The next similar event will be a transit of Venus in June of 2012.

APOD: 2006 October 29 - The Sun Puffs
Explanation: Our Earth endures bursts of particles from the Sun. On 1997 April 7, at 10 am (EDT), ground monitors of the SOHO spacecraft, which continually monitors the Sun, noticed a weak spot in the solar corona was buckling again, this time letting loose a large, explosive Coronal Mass Ejection (CME). Almost simultaneously, NASA's WIND spacecraft began detecting bursts of radio waves from electrons involved in this magnetic storm. Supersonic waves rippled though the solar corona as a puff of high energy gas shot out into the Solar System. The above image shows two photographs of the Sun taken about 15 minutes apart and subtracted, highlighting the explosion. CME's are not unusual. The CME gas had little lasting effect on the Earth, but likely created fleeting, if not picturesque, Earth auroras.

APOD: 2006 October 19 - SWAN Meets Galaxy
Explanation: This cosmic portrait recorded October 9th features the lovely blue-green coma of Comet SWAN posing with spiral galaxy NGC 5005 in the northern constellation Canes Venatici. At the time the comet (center) was in the close foreground, a mere 9 light-minutes from planet Earth, with the galaxy a more substantial 60 million light-years distant. Not actually related to a bird, Comet SWAN (C/2006 M4) was so named as it was first spotted in image data from the SWAN (Solar Wind ANisotropies) camera aboard the sun-staring SOHO spacecraft. Having rounded the Sun, this comet is headed for interstellar space, but first it will make its closest approach to Earth on October 24. With binoculars, northern hemisphere observers can now spot the comet above the northwestern horizon, near the handle of the Big Dipper in the early evening sky.

APOD: 2006 October 4 - Comet SWAN Brightens
Explanation: A newly discovered comet has brightened enough to be visible this week with binoculars. The picturesque comet is already becoming a favored target for northern sky imagers. Pictured above just last week, Comet SWAN showed a bright blue-green coma and an impressive tail. Comet C/2006 M4 (SWAN) was discovered in June in public images from the Solar Wind Anisotropies (SWAN) instrument of NASA and ESA's Sun-orbiting SOHO spacecraft. Comet SWAN, near magnitude six, will be visible with binoculars in the northeastern sky not far from the Big Dipper over the next few days before dawn. The comet is expected to reach its peak brightness this week. Passing its closest to the Sun two days ago, Comet SWAN and will be at its closest to the Earth toward the end of this month. Comet SWAN's unusual orbit appears to be hyperbolic, meaning that it will likely go off into interstellar space, never to return.

APOD: 2006 August 30 - A Backward Sunspot and the New Solar Cycle
Explanation: Why is sunspot 905 backwards? Perhaps it is a key marker for the beginning of a new magnetic cycle on our Sun. Every 11 years, our Sun goes through a magnetic cycle, at the end of which its overall magnetic orientation is reversed. An 11-year solar cycle has been observed for hundreds of years by noting peaks and valleys in the average number of sunspots. Just now, the Sun is near Solar Minimum, and likely to start a long progression toward the most active time, called Solar Maximum, in about 5.5 years. An indicator that the sun's magnetic field is reversing is the appearance of sunspots with the reverse magnetic polarity than normal. A few weeks ago, one small candidate reverse sunspot was sighted but faded quickly. Now, however, a larger sunspot with negative polarity is being tracked. This sunspot, numbered 905, appears as the unusual white spot in the above magnetic image of the Sun taken with the SOHO spacecraft a few days ago. In the past few days, Sunspot 905 has actually begun to break apart and might also become the source of coronal mass ejections and explosive solar flares. Solar astronomers predict that the coming Solar Maximum will be unusually active.

APOD: 2006 August 7 - An Erupting Solar Prominence from SOHO
Explanation: Our Sun is still very active. In the year 2000, our Sun went though Solar Maximum, the time in its 11-year cycle where the most sunspots and explosive activities occur. Sunspots, the Solar Cycle, and solar prominences are all caused by the Sun's changing magnetic field. Pictured above is a solar prominence that erupted in 2002 July, throwing electrons and ions out into the Solar System. The above image was taken in the ultraviolet light emitted by a specific type of ionized helium, a common element on the Sun. Particularly hot areas appear in white, while relatively cool areas appear in red. Our Sun should gradually quiet down until Solar Minimum occurs, and the Sun is most quiet. No one can precisely predict when Solar Minimum will occur, although some signs indicate that it has started already!

APOD: 2006 April 16 - A Solar Prominence from SOHO
Explanation: How can gas float above the Sun? Twisted magnetic fields arching from the solar surface can trap ionized gas, suspending it in huge looping structures. These majestic plasma arches are seen as prominences above the solar limb. In September 1999, this dramatic and detailed image was recorded by the EIT experiment on board the space-based SOHO observatory in the light emitted by ionized Helium. It shows hot plasma escaping into space as a fiery prominence breaks free from magnetic confinement a hundred thousand kilometers above the Sun. These awesome events bear watching as they can affect communications and power systems over 100 million kilometers away on Planet Earth.

APOD: 2006 March 31 - Solar Eclipse and SOHO
Explanation: Neither rain, nor snow, nor dark of night can keep the space-based SOlar Heliospheric Observatory (SOHO) from watching the Sun. In fact, from its vantage point 1.5 million kilometers sunward of planet Earth, SOHO's cameras can always monitor the Sun's outer atmosphere, or corona. But only during a total solar eclipse can earth-based observers see the lovely coronal streamers and structures - when the Moon briefly blocks the overwhelmingly bright solar surface. In this composite view, SOHO's uninterrupted view of the solar corona above the solar photosphere (center) and corona far beyond the Sun's disk, are shown in orange hues. The middle, donut-shaped region is the corona as recorded by the Williams College Eclipse Expedition to Kastelorizo Island, Greece, headed by Jay Pasachoff, during the March 29th total solar eclipse. Merging ground and space-based views allows astronomers to trace features in the corona that reach from just above the Sun's surface into the solar wind.

APOD: 2005 December 1 - SOHO s Uninterrupted View of the Sun
Explanation: Launched ten years ago this week, SOHO (the SOlar and Heliospheric Observatory) still enjoys an uninterrupted view of the Sun. Twelve sungazing instruments on board the spacecraft have explored the Sun's internal structure, the extensive solar atmosphere and solar wind, and discovered over 1,000 comets from a remarkable orbit around a point about 1.5 million kilometers directly sunward of planet Earth itself. At that location, known as a Lagrange point, the gravitational influence of the Earth and Sun are equal. With scientific instrument teams distributed around the world, the SOHO operations center is located at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Mission operations are planned through March of 2007 to allow the study of a complete 11-year solar cycle. Contributions from SOHO's instruments are represented in the colorful montage image. Happy tenth anniversary SOHO!

APOD: 2005 November 9 - A Solar Prominence from SOHO
Explanation: What happened to the Sun? Nothing very unusual: the strange-looking solar appendage on the lower left is actually just a spectacular looking version of a common solar prominence. A solar prominence is a cloud of solar gas held above the Sun's surface by the Sun's magnetic field. Pictured above in 2002 October, NASA's Sun-orbiting SOHO spacecraft imaged an impressively large prominence hovering over the surface, informally dubbed a flame. Over 40 Earths could line up along the vast length of the fireless flame of hovering hot gas. A quiescent prominence typically lasts about a month, and may erupt in a Coronal Mass Ejection (CME) expelling hot gas into the Solar System. Although somehow related to the Sun's changing magnetic field, the energy mechanism that creates and sustains a Solar prominence is still a topic of research.

APOD: 2004 September 25 - The Iron Sun
Explanation: The ultraviolet light emitted by eleven times ionized iron at temperatures over 2 million degrees Farenheit was used to record the above picture of the Sun on September 22, 2001, the date of that year's autumnal equinox. The image was made by the EIT camera onboard the SOHO spacecraft, a space observatory which can continuously observe the Sun. Eleven times ionized iron is atomic iron with eleven of its electrons stripped away. Here the electrons are stripped by the frantic collisions with other atoms and electrons which occur at the extreme temperatures in the Solar Corona. Since electrons are negatively charged, the resulting ionized iron atom is highly positively charged. Astronomer's "shorthand" for eleven times ionized iron is written "Fe XII", the chemical symbol for iron followed by a Roman numeral 12 (Fe I is neutral iron).

APOD: 2004 June 20 - Solstice Celebration
Explanation: Season's greetings! Today or tomorrow, depending on your time zone, the Sun reaches its northernmost point in planet Earth's sky marking a season change and the first solstice of the year 2004. In celebration, consider this delightfully detailed, brightly colored image of the active Sun. From the EIT instrument onboard the space-based SOHO observatory, the tantalizing picture is a false-color composite of three images all made in extreme ultraviolet light. Each individual image highlights a different temperature regime in the upper solar atmosphere and was assigned a specific color; red at 2 million, green at 1.5 million, and blue at 1 million degrees C. The combined image shows bright active regions strewn across the solar disk, which would otherwise appear as dark groups of sunspots in visible light images, along with some magnificent plasma loops and an immense prominence at the right hand solar limb.

APOD: 2004 June 6 - Mercury Spotting
Explanation: Can you spot the planet? The diminutive disk of Mercury, the solar system's innermost planet, spent about five hours crossing in front of the enormous solar disk on 2003 May 7, as viewed from the general vicinity of planet Earth. The Sun was above the horizon during the entire transit for observers in Europe, Africa, Asia, or Australia, and the horizon was certainly no problem for the sun-staring SOHO spacecraft. Seen as a dark spot, Mercury progresses from left to right (top panel to bottom) in these four images from SOHO's extreme ultraviolet camera. The panels' false-colors correspond to different wavelengths in the extreme ultraviolet which highlight regions above the Sun's visible surface. This was the first of 14 transits of Mercury which will occur during the 21st century, but the next similar event will be a much more rare transit of Venus this coming Tuesday. Need help spotting Mercury? Just click on the picture.

APOD: 2004 April 27 - Comet Bradfield Rising
Explanation: Comet Bradfield has become quite a sight just before sunrise -- for those with binoculars or cameras. Although fading noticeably each day, a sky chart, a northern location, and some persistence will allow curious sky gazers to locate the cosmic snowball and its spectacular tail. One might call Bradfield a "camera" comet as its extended tail is too long for most telescopes but caught nicely by normal cameras capable of long exposures and set to rotate with the sky. Pictured above just yesterday, Comet C/2004 F4 (Bradfield) was caught as it rose on successive three-minute exposures above the Rocky Mountains near Yampa, Colorado, USA. Visible on the upper left as a bright fuzzy smudge is the Andromeda Galaxy (M31), far in the distance. Comet Bradfield was discovered only last month and was briefly visible to the unaided eye. It was imaged in spectacular fashion by the SOHO spacecraft as it rounded the Sun early last week.

APOD: 2004 April 19 - Comet Bradfield Passes the Sun
Explanation: Today, Comet Bradfield is passing the Sun. The above image, taken yesterday in the direction of the Sun by the SOHO LASCO instrument, shows the comet and its dust tail as the elongated white streak. The Sun would normally be seen in the very center but has been blocked from view. Comet C/2004 F4 (Bradfield) was discovered just one month ago and has brightened dramatically as it neared the Sun. Careful sky gazers can see Comet Bradfield with the unaided eye near the Sun, although NASA's sun-orbiting SOHO satellite has the best view. During the day, Comet Bradfield will continually shift inside the LASCO frame as it rounds the Sun. There is even the possibility that the comet will break up. If not, the bright comet's trajectory will carry it outside the field of LASCO sometime tomorrow. Along with T7 and Q4, Comet Bradfield is now the third comet that is currently visible on the sky with the unaided eye, the most ever of which we are aware and quite possibly the most in recorded history.

APOD: 2004 March 30 - A Prominent Solar Prominence from SOHO
Explanation: One of the most spectacular solar sights is a prominence. A solar prominence is a cloud of solar gas held above the Sun's surface by the Sun's magnetic field. Last month, NASA's Sun-orbiting SOHO spacecraft imaged an impressively large prominence hovering over the surface, pictured above. The Earth would easily fit under the hovering curtain of hot gas. A quiescent prominence typically lasts about a month, and may erupt in a Coronal Mass Ejection (CME) expelling hot gas into the Solar System. Although somehow related to the Sun's changing magnetic field, the energy mechanism that creates and sustains a Solar prominence is still a topic of research.

APOD: 2003 November 6 - Flare Well AR 10486
Explanation: Almost out of view from our fair planet, rotating around the Sun's western edge, giant sunspot region AR 10486 lashed out with another intense solar flare followed by a large coronal mass ejection (CME) on Tuesday, November 4th at about 1950 Universal Time. The flare itself is seen here at the lower right in an extreme ultraviolet image from the sun-staring SOHO spacecraft's EIT camera. Saturating the EIT camera pixels and detectors on other satellites, this giant X-class flare was among the most powerful ever recorded since the 1970s, the third such historic blast from AR 10486 within the last two weeks. While energetic particle radiation from the flare did cause substantial radio interference, the associated CME is not expected to trigger extremely widespread aurorae as it glances off the magnetosphere, unlike the direct hits from last week's CMEs. Say farewell to the mighty AR 10486, for now. For the next two weeks, the sunspot region will be on the Sun's far side.

APOD: 2003 October 29 - A Powerful Solar Flare
Explanation: Yesterday, our Sun produced one of the most powerful solar flares in recorded history. Seen across the electromagnetic spectrum, the Sun briefly became over 100 times brighter in X-rays than normal. Over the next few days, as energetic particles emitted from these regions strike the Earth, satellite communications might be affected and auroras might develop. The flare and resulting CME, emitted from giant sunspot group 10486, was captured above as it happened by the by the LASCO instrument aboard the Sun-orbiting SOHO satellite. The disk of the Sun is covered to accentuate surrounding areas. The time-lapse movie shows the tremendous explosion in frames separated in real time by about 30 minutes each. The frames appear progressively noisier as protons from the flare begin to strike the detector. The SOHO satellite has been put in a temporary safe mode to avoid damage from the solar particle storm.

APOD: 2003 May 8 - Mercury Spotting
Explanation: Can you spot the planet? The diminutive disk of Mercury, the solar system's innermost planet, spent about five hours crossing in front of the enormous solar disk yesterday (Wednesday, May 7th), as viewed from the general vicinity of planet Earth. The Sun was above the horizon during the entire transit for observers in Europe, Africa, Asia, or Australia, and the horizon was certainly no problem for the sun-staring SOHO spacecraft. Seen as a dark spot, Mercury progresses from left to right (top panel to bottom) in these four images from SOHO's extreme ultraviolet camera. The panels' false-colors correspond to different wavelengths in the extreme ultraviolet which highlight regions above the Sun's visible surface. This is the first of 14 transits of Mercury which will occur during the 21st century, but the next similar event will be a transit of Venus in June of 2004. Need help spotting Mercury? Just click on the picture.

APOD: 2003 April 18 - Double Eruptive Prominences
Explanation: Lofted over the Sun on looping magnetic fields, large solar prominences are composed of relatively cool, dense plasma. When seen against the brilliant solar disk they appear as dark filaments, but these enormous magnetic structures are bright themselves when viewed against the blackness of space as they arc above the Sun's edge. In a rare visual treat, these two solar prominences arising from the Sun's southern (lower) hemisphere were captured in extreme ultraviolet light by the EIT camera on board the space-based SOlar and Heliospheric Observatory (SOHO) on March 21. For scale, the pair of plasma loops stretch above the Sun to a height of about twenty times the diameter of planet Earth. In a matter of hours, these prominences apparently erupted away from the Sun's surface and may have been associated with a flare and coronal mass ejection.

APOD: 2003 March 18 - Coronal Holes on the Sun
Explanation: The ominous, dark shapes haunting the left side of the Sun are coronal holes -- low density regions extending above the surface where the solar magnetic field opens freely into interplanetary space. Studied extensively from space since the 1960s in ultraviolet and x-ray light, coronal holes are known to be the source of the high-speed solar wind, atoms and electrons which flow outward along the open magnetic field lines. During periods of low activity, coronal holes typically cover regions just above the Sun's poles. These coronal holes, however, have just moved into view near the Sun's equator, and particles escaping them have already caused notable aurora here on Earth. Coronal holes like this one may last for a few solar rotations before the magnetic fields shift and change configurations. Shown in false-color, this picture of the Sun on March 9 was made in extreme ultraviolet light by the EIT instrument on board the space-based SOHO observatory.

APOD: 2003 February 24 - Comet NEAT Passes an Erupting Sun
Explanation: As Comet NEAT flared last week, the Sun roared. Just as the comet swooped inside the orbit of Mercury and developed a long and flowing tail of gas and dust, the Sun emitted a huge Coronal Mass Ejection (CME). Neither the fortuitous hot ball of solar gas nor the intense glare of sunlight appeared to disrupt the comet's nucleus. The action was too close to the Sun to be easily visible by humans, but the orbiting Sun-pointing SOHO satellite had a clear view of the celestial daredevil show. The above image was taken on February 18 when the comet was so bright it created an artificial horizontal streak on the camera image. During the encounter, Comet NEAT, official designation (C/2002 V1), brightened to second magnitude. An opaque disk blocked the Sun's image. The now-outbound comet remains bright but will surely fade as it moves away from the Sun. Nevertheless, Comet NEAT will likely be visible with binoculars to southern hemisphere observers for the next month.

APOD: 2003 February 23 - A Twisted Solar Eruptive Prominence
Explanation: A huge eruptive prominence is seen moving out from our Sun in this condensed half-hour time-lapse sequence. Ten Earths could easily fit in the "claw" of this seemingly solar monster. This large prominence, though, is significant not only for its size, but its shape. The twisted figure eight shape indicates that a complex magnetic field threads through the emerging solar particles. Recent evidence of differential rotation inside the Sun might help account for the surface explosion. The sequence was taken early in the year 2000 by the Sun-orbiting SOHO satellite. Although large prominences and energetic Coronal Mass Ejections (CMEs) are relatively rare, they are occurred more frequently near Solar Maximum, the time of peak sunspot and solar activity in the eleven-year solar cycle.

APOD: 2003 February 10 - Comet NEAT Approaches the Sun
Explanation: A comet may likely become visible to the unaided eye over the next few days above the horizon where the Sun has just set. Comet NEAT (C/ 2002 V1), discovered last November, has brightened dramatically as it approached the Sun. Over the next few days, the quickly setting comet could appear as bright as second magnitude. On February 18 it will round the Sun well within the orbit of Mercury. During surrounding days, the Sun's glare will effectively hide the comet to human observers. It is quite probable, though, that Comet NEAT will standout prominently in images taken by the Sun-looking SOHO satellite. Pictured above, Comet NEAT's complex and developing tail was photographed on January 29 (top) and February 2. Sky enthusiasts should remember to never look directly at the Sun.

APOD: 2003 January 30 - Comet Kudo-Fujikawa: Days in the Sun
Explanation: Cruising through the inner Solar System, new Comet Kudo-Fujikawa reached perihelion, its closest approach to the Sun, yesterday, January 29. Passing within 28.4 million kilometers of the Sun, this comet came much closer than innermost planet Mercury basking only 57.9 million kilometers from our parent star. So close to the Sun, comet Kudo-Fujikawa was extremely bright but impossible for earthbound observers to see against the solar glare. Still, the space-based SOHO observatory captured these views of the comet as it neared perihelion by using a coronograph's occulting disk to block the overwhelming sunlight. In the series of images, the size and location of the blocked-out Sun is indicated by white circles, while arrows point to the traveling comet's bright coma and developing tail. Though fading on its outbound journey, Kudo-Fujikawa should soon be visible to southern hemisphere comet-watchers in February's evening skies.

APOD: 2002 December 21 - Solstice Celebration
Explanation: Aloha and Season's greetings! On December 22nd, at 01:14 Universal Time (December 21, 3:14pm Hawaii-Aleutian Standard Time), the Sun reaches its southernmost point in planet Earth's sky marking the final season change for the year 2002. In celebration, consider this delightfully detailed, brightly colored image of the active Sun. From the EIT instrument onboard the space-based SOHO observatory, the tantalizing picture is a false-color composite of three images all made in extreme ultraviolet light. Each individual image highlights a different temperature regime in the upper solar atmosphere and was assigned a specific color; red at 2 million, green at 1.5 million, and blue at 1 million degrees C. The combined image shows bright active regions strewn across the solar disk, which would otherwise appear as dark groups of sunspots in visible light images, along with some magnificent plasma loops and an immense prominence at the righthand solar limb.

APOD: 2002 July 18 - Sunspot Region 30
Explanation: The solar active region designated number 10030 (or simply region 30) is now appearing on the visible hemisphere of the closest star. Dwarfed by the Sun's disk, the group of sunspots which make up region 30 actually cover an enormous area -- nearly 10 times the size of Earth. The panels above were recorded July 15, 16, and 17 (top to bottom) by the MDI instrument on the space-based SOHO Observatory as the solar rotation slowly carried the large, dynamic sunspot group across the Sun's nearside. On July 15, a powerful solar flare erupted from this region followed by a coronal mass ejection. The energetic cloud of electrically charged particles swept past our fair planet yesterday, and as a result enhanced auroral activity is possible.

APOD: 2002 May 16 - Double Trouble Solar Bubbles
Explanation: During April and May, attention has been focused on the western evening sky, presenting its spectacle of bright planets and crescent moons shortly after sunset. Meanwhile, the the Sun itself has not been just sinking quietly below the horizon. For example on May 2nd, two enormous clouds of energetic particles blasted away from the solar surface in nearly simultaneous eruptions. Known as coronal mass ejections (CMEs), they appear as large "bubbles" oriented at about 2 o'clock and 8 o'clock in this composite image from cameras onboard the sun-staring SOHO spacecraft. At picture center, an extreme ultraviolet image of the Sun recorded near the time of these eruptions has been superimposed for scale. The blank region surrounding it corresponds to an occulting disk in one of SOHO's coronagraphic cameras. Speeding outward at millions of kilometers per hour, these two CMEs missed our fair planet. But those that do impact Earth's magnetosphere often trigger auroral displays and disruptions.

APOD: 2002 March 21 - S is for Sun
Explanation: Taken yesterday from the SOHO spacecraft, this false-color image shows the active Sun near the March Equinox, the beginning of Autumn in the south and Spring in the northern hemisphere. Recorded in a band of extreme ultraviolet light emitted by highly ionized iron atoms, the Sun's upper atmosphere or solar corona shines with an array of active regions and plasma loops suspended in magnetic fields. The bright coronal structures and loops seen here have temperatures of about 1.5 million kelvins. By chance, the Sun's earth-facing side also seems to be marked with a twisting complex of dark filament channels shaped like a giant "S". Filaments represent relatively (!) cool material in the corona which show up as prominences when seen at the Sun's edge. For planet Earth, recent solar activity has made auroral displays likely around this year's March Equinox.

APOD: 2002 February 7 - Coronal Hole
Explanation: This ominous, dark shape sprawling across the face of the active Sun is a coronal hole -- a low density region extending above the surface where the solar magnetic field opens freely into interplanetary space. Studied extensively from space since the 1960s in ultraviolet and x-ray light, coronal holes are known to be the source of the high-speed solar wind, atoms and electrons which flow outward along the open magnetic field lines. During periods of low activity, coronal holes typically cover regions just above the Sun's poles. But this coronal hole, one of the largest seen so far in the current solar activity cycle, extends from the south pole (bottom) well into northern hemisphere. Coronal holes like this one may last for a few solar rotations before the magnetic fields shift and change configurations. Shown in false-color, this picture of the Sun on January 8th was made in extreme ultraviolet light by the EIT instrument on board the space-based SOHO observatory.

APOD: 2002 January 11 - Sunbather
Explanation: Intense and overwhelming, the direct glare of the Sun is blocked by the smooth disk centered in this image from the sun-staring SOHO spacecraft. Taken on January 8, the picture shows streamers of solar wind billowing radially outward for millions of kilometers above the Sun's surface indicated by the white circle. Below and right is inner planet Venus, so bright that its image is marred by a sharp horizontal stripe, a digital imaging artifact. Also impressively bright is a periodic visitor to the inner Solar System, sunbathing comet 96/P Machholz 1 (above and left). This comet is definitely not a member of the more suicidal sungrazer comet family often spotted approaching the Sun by SOHO. Seen here only 18 million kilometers from the Sun (about one eighth the Earth-Sun distance) with a substantial coma and foreshortened tail, Machholz 1 has now passed perihelion and is outbound in its orbit, to return again in just over 5 years.

APOD: 2001 November 29 - Coronal Inflow
Explanation: The active Sun has thrown a lot our way lately, including storms of particles streaming outward in the solar wind and clouds of plasma which triggered awesome auroral displays. Still, a growing body of intriguing observations from the LASCO instrument on board the space-based SOlar and Heliospheric Observatory (SOHO) indicates material also flows back toward the Sun, starting from over 2 million kilometers above its visible surface. Relatively hard to detect against the outflowing solar corona, a dark inflowing cloud's relative motion is tracked above in two highly processed images recorded an hour apart. The solar surface, graphically shown by the yellow quarter circle at the lower right, is blocked from view by a smooth occulting disk. Fighting against a solar wind outflow of about 120 kilometers per second the cloud seems to be moving inward at 50-100 kilometers per second. Occasionally appearing as often as once per hour, the clouds, seem to be dragged in by collapsing magnetic field loops rather than gravity alone. Researchers are now working to relate this surprising inflow to the solar wind and magnetic environment of the Sun.

APOD: 2001 November 9 - SOHO Comet 367: Sungrazer
Explanation: The most prolific comet discovering instrument in history rides aboard the sun-staring SOHO spacecraft, 1.5 million kilometers sunward of planet Earth. Of course, most of these SOHO comets have been sungrazers - like the one illustrated in the dramatic montage above. Three frames taken hours apart on October 23rd, show bright SOHO comet number 367 plunging toward the fiery solar surface, its tail streaming away from the Sun located just beyond the left hand border. Each panel spans about one million kilometers at the distance of the Sun. From bottom to top, the comet's tail grows as the intensifying solar radiation heats the frozen comet material and increases the outflow of gas and dust. Because of their orbits, sungrazers are believed to belong to a family of comets produced by the breakup of a single much larger comet. Coincidentally, this sungrazer was discovered shortly after solar active regions blasted out clouds of energetic particles, like those that triggered the recent spectacular auroral storms. And like all SOHO sungrazers so far, comet number 367 was not seen to survive its close solar encounter.

APOD: 2001 November 8 - Under A Sunspot
Explanation: At the Sun's surface, sunspots are known to be dark, planet-sized regions of intense magnetic fields. But what lies below? Using observations from the Michelson Doppler Imager (MDI) instrument aboard the space-based SOHO observatory, astronomers have derived this premier picture of the flow of material just beneath a visible sunspot. The MDI data indicate that immediately under the sunspot a strong inflowing current exists, shown above by the dark arrows. This converging undertow pulls near-surface material toward the spot and prevents the concentrated magnetic fields from flying apart, like repelling poles of iron magnets. Such a configuration appears to divert the normal flow of plasma bubbling up from the solar interior, creating a self-sustaining sunspot. The MDI instrument can explore the properties of the solar interior by detecting motions produced by sound waves as they interact at the solar surface.

APOD: 2001 September 29 - The Iron Sun
Explanation: The ultraviolet light emitted by eleven times ionized iron at temperatures over 2 million degrees Farenheit was used to record the above picture of the Sun on September 22, the date of the autumnal equinox. The image was made by the EIT camera onboard the SOHO spacecraft, a space observatory which can continuously observe the Sun. Eleven times ionized iron is atomic iron with eleven of its electrons stripped away. Here the electrons are stripped by the frantic collisions with other atoms and electrons which occur at the extreme temperatures in the Solar Corona. Since electrons are negatively charged, the resulting ionized iron atom is highly positively charged. Astronomer's "shorthand" for eleven times ionized iron is written "Fe XII", the chemical symbol for iron followed by a Roman numeral 12 (Fe I is neutral iron).

APOD: 2001 September 24 - A Solar Prominence Erupts
Explanation: Our Sun is still very active. Last year, our Sun went though Solar Maximum, the time in its 11-year cycle where the most sunspots and explosive activities occur. Sunspots, the Solar Cycle, and solar prominences are all caused by the Sun's changing magnetic field. Pictured above is a solar prominence that erupted on May 15, throwing electrons and ions out into the Solar System. The image was taken in the ultraviolet light emitted by a specific type of ionized helium, a common element on the Sun. Particularly hot areas appear in white, while relatively cool areas appear in red. Our Sun should gradually quiet down until Solar Minimum occurs in 2007.

APOD: 2001 May 17 - Solar Neutrino Astronomy
Explanation: Neutrinos are subatomic particles generated by the nuclear reactions which power stars like our Sun. Flying outward from the Sun's core, they easily pass through the Sun (and almost anything else!) unimpeded and should be detectable by earth-based neutrino "telescopes". Still, to the long-standing consternation of astrophysicists, the observed flux of solar neutrinos is less than expected. In a new twist to this solar neutrino saga, an analysis of data from the GALLEX / GNO neutrino detector finds a solar neutrino flux that varies over about 27 days ... approximately matching the Sun's rotation period. In fact, since different parts of the Sun rotate at different rates, the neutrino flux variations match most exactly the rotation rates of the areas shown in red on this colorful cross-sectional map of the solar interior rotation. So how could solar rotation affect the neutrino flux? Some theoretical models say that neutrinos can change quantum properties when they interact with tangled solar magnetic fields and become particles that the neutrino experiments were not designed to detect. Then, as the Sun rotates, the neutrinos sometimes come to us unaffected and sometimes come through magnetic fields, diminishing the flux that can be measured.

APOD: 2001 May 3 - Far Side of the Sun
Explanation: You may think it's impossible to see through the Sun, but maps of the Sun's far side are now made routinely by instruments on board the sun-staring SOHO spacecraft. This is one such map from April 12. At right, is a map projection of calculated magnetic field strengths on the Earth-facing solar hemisphere with yellow and red indicating high magnetic fields characteristic of solar active regions. At left is a similar map of the solar hemisphere opposite planet Earth, which shows the large active region AR9393 as the 27-day solar rotation carried it across the far side. The largest sunspot group in a decade, AR9393 was easily seen as it tracked across the Sun's Earth-facing hemisphere in late March. When AR9393 swung around to the Sun's far side, SOHO's Michelson Doppler Interferometer (MDI) instrument continued to map its position by measuring changes in motions caused by solar sound waves - transmitted through the Sun and influenced by the active region's strong magnetic fields. Known as helioseismology, analyzing solar sound waves is like using seismological records of earthquakes to probe the interior of the Earth. On the Sun, sound waves are produced by turbulent convection cells seen on the surface as dynamic solar granules.

APOD: 2001 April 19 - Sunspot Stack
Explanation: Welcome to multiwavelength astronomy! From top to bottom, these stacked panels show the largest sunspot group in a decade in visible, extreme ultraviolet, and x-ray light. All were taken on March 29, around the time the famous solar active region, cataloged as AR 9393, was at its peak size -- over 10 times the size of planet Earth. The panels illustrate how the "appearance" of the active region changes, when imaged in electromagnetic radiation (light) of progressively shorter wavelengths. In the visible light panel, dark islands of sunspots stand out against the bright solar surface, but the situation seems to be reversed in the extreme ultraviolet panel with a bright active region seen against a darker background. Finally, the x-ray panel reveals majestic loops of glowing plasma arcing far above the sunspot group. Why do pictures of the same part of the Sun look so different? Made at different wavelengths, each panel actually records a different layer in the solar atmosphere. Top to bottom, the altitude of each layer (along with temperature) increases; starting with the Sun's visible surface or photosphere (about 5 thousand kelvins), then the chromosphere / transition region (ten to a hundred thousand kelvins), and finally the solar corona (millions of kelvins).

APOD: 2001 April 11 - Large Sunspot Group AR 9393
Explanation: The largest sunspot group of the past ten years crossed the surface of the Sun late last month and early this month. The group was designated Active Region 9393 as it was the 9393rd region identified since counting officially began in 1973. The number of active regions on the Sun is high recently because the Sun is reaching the maximum of its current 11-year cycle of magnetic activity. The above time-lapse sequence shows AR 9393 as it evolved from 27 March to April 2 to become over 10 times larger than our Earth. Just after the end of the movie, on April 2, AR 9393 unleashed the largest solar flare of the last 25 years. Luckily, the flare was not pointed toward the Earth, or flare particles might have damaged satellites or even caused local electrical blackouts. Yesterday morning, however, a less powerful flare was ejected from a different sunspot group (AR 9415) toward Earth that has already caused radio interference. This and solar activity from Monday should cause significant aurorae over the next two nights. Will the above sunspot group remain as its region rotates back into view in a few days, or will it break up on the far side of the Sun? Currently, no one knows for sure.

APOD: 2001 April 7 - Stereo Sun
Explanation: This week's stereo offering features the now famous Active Region 9393, the largest sunspot group in the last 10 years. Viewed with red/blue glasses, the stereo pair of images merges into one 3D representation of the Sun with AR9393 above and right of center. The images were recorded in extreme ultraviolet light and AR9393 is seen as an extensive array of bright patches laced with large graceful loops of arcing plasma. In the extreme ultraviolet, active regions outshine the solar surface, just the reverse of their appearance as dark sunspots against a bright photosphere when viewed in visible light. Recorded 96 minutes apart on March 30 by the space-based SOHO EIT camera, the pair produces an exaggerated but pleasing stereo effect due to solar rotation -- the Sun's surface moving slightly between the two exposures to offer different perspectives.

APOD: 2001 March 1 - Maximum Sun
Explanation: Astronomers recently witnessed an astounding, large scale solar event as the Sun's north and south magnetic poles changed places! But, this complete solar magnetic field flip was actually anticipated. It occurs every 11 years during the maximum of the solar activity cycle. Plagues of sun spots, flaring active regions, and huge prominences are also hard-to-miss signs that the solar maximum is here. On February 12, the sungazing SOHO spacecraft captured this dramatic image of a magnificent prominence above the Sun's limb. Seen at the lower right, streams of relatively cool dense plasma were lofted along looping magnetic field lines extending outward about 30 times the diameter of planet Earth. Far above the limb at the upper right, a disconnected ghostly arc surrounds a dark cavity with bright central emission. These features are telltale signs of a coronal mass ejection -- yet another violent expulsion of material from the active Sun. Enormous, intensely bright active regions also mottle the solar surface in this image, recorded in the light of energetic Helium atoms by SOHO's Extreme ultraviolet Imaging Telescope.

APOD: 2001 January 20 - Helios Helium
Explanation: This image of the active Sun was made using ultraviolet light emitted by ionized Helium atoms in the Solar chromosphere. Helium was first discovered in the Sun in 1868, its name fittingly derived from from the Greek word Helios, meaning Sun. Credit for the discovery goes to astronomer Joseph Norman Lockyer (born May 17, 1836). Lockyer relied on a then recently developed technique of spectroscopy, dissecting sunlight into a spectrum, and the idea that each element produces a characteristic spectral pattern of bright lines. He noticed a yellow line in a solar spectrum made during an eclipse which could not be accounted for by elements known on Earth. Almost 27 years later terrestrial Helium was finally discovered when the spectrum of a Helium bearing mineral of Uranium provided an exact match to the previously detected element of the Sun. Helium is now known to be the second most abundant element (after Hydrogen) in the Universe.

APOD: 2000 September 1 - SOHO Sungrazer
Explanation: SOHO, the space-based SOlar and Heliospheric Observatory, has become by far the reigning champion facility for discovering comets, its total having recently reached 200. As might be expected of a solar observatory, most of the SOHO discovered comets are sungrazers, destined to dive within a mere 50 thousand kilometers or so of the solar photosphere. At that range the intense heat and gravitational forces make it unlikely these primitive chunks of ice and dust will survive. Based on their similar orbits, as first worked out by 19th century German astronomer Heinrich Kreutz, all sungrazers are believed to originate from a single large parent comet which broke up during a perihelion passage perhaps 2,000 years ago. Over time, pieces have continued to split off producing a family of smaller comets which seem to travel in the same orbit. These frames from SOHO's coronograph were taken two hours apart on April 29 of this year. They show a sungrazer (SOHO comet discovery number 111) with a long, bright tail headed toward its fiery encounter. The sun itself is hidden behind the coronograph's occulting disk at each frame's upper right.

APOD: 2000 June 21 - Solstice Celebration
Explanation: Season's greetings! At 01:48 Universal Time on June 21 the Sun reaches its northernmost point in planet Earth's sky marking a season change and the first solstice of the year 2000. In celebration, consider this delightfully detailed, brightly colored image of the active Sun. From the EIT instrument onboard the space-based SOHO observatory, the tantalizing picture is a false-color composite of three images all made in extreme ultraviolet light. Each individual image highlights a different temperature regime in the upper solar atmosphere and was assigned a specific color; red at 2 million, green at 1.5 million, and blue at 1 million degrees C. The combined image shows bright active regions strewn across the solar disk, which would otherwise appear as dark groups of sunspots in visible light images, along with some magnificent plasma loops and an immense prominence at the righthand solar limb.

APOD: 2000 May 24 - Pleiades, Planets, And Hot Plasma
Explanation: Bright stars of the Pleiades, four planets, and erupting solar plasma are all captured in this spectacular image from the space-based SOlar and Heliospheric Observatory (SOHO). In the foreground of the 15 degree wide field of view, a bubble of hot plasma, called a Coronal Mass Ejection (CME), is blasting away from the active Sun whose position and relative size is indicated by the central white circle. Beyond appear four of the five naked-eye planets -- courtesy of the planetary alignment which did not destroy the world! In the background are distant stars and the famous Pleiades star cluster, also easily visible to the unaided eye when it shines in the night sky. Distances for these familiar celestial objects are; the Sun, 150 million kilometers away; Mercury, Venus, Jupiter, and Saturn, about 58, 110, 780, and 1,400 million kilometers beyond the Sun respectively; and the Pleiades star cluster at a mere 3,800 trillion kilometers (400 light-years). SOHO itself orbits 1.5 million kilometers sunward of planet Earth. The image was recorded by the Large Angle and Spectrometric COronagraph (LASCO) instrument on board SOHO on Monday, May 15 at 10:42 UT.

APOD: 2000 May 20 - Sungrazer
Explanation: Arcing toward a fiery fate, this Sungrazer comet was recorded by the SOHO spacecraft's Large Angle Spectrometric COronagraph (LASCO) on Dec. 23rd, 1996. LASCO uses an occulting disk, partially visible at the lower right, to block out the otherwise overwhelming solar disk allowing it to image the inner 5 million miles of the relatively faint corona. The comet is seen as its coma enters the bright equatorial solar wind region (oriented vertically). Spots and blemishes on the image are background stars and camera streaks caused by charged particles. Positioned in space to continuously observe the Sun, SOHO has detected 7 sungrazing comets. Based on their orbits, they are believed to belong to a family of comets created by successive break ups from a single large parent comet which passed very near the sun in the twelfth century. The bright comet of 1965, Ikeya-Seki, was also a member of the Sungrazer family, coming within about 400,000 miles of the Sun's surface. Passing so close to the Sun, Sungrazers are subjected to destructive tidal forces along with intense solar heat. This comet, known as SOHO 6, did not survive.

APOD: 2000 May 5 - Planets In The Sun
Explanation: Today, all five naked-eye planets (Mercury, Venus, Mars, Jupiter, Saturn) plus the Moon and the Sun will at least approximately line-up. As viewed from planet Earth, they will be clustered within about 26 degrees, the closest alignment for all these celestial bodies since February 1962, when there was a solar eclipse! Such planetary alignments are not dangerous, except of course that the Sun might hurt your eyes when you look at it. So it might be easier to appreciate today's solar system spectacle if you use a space-based coronagraph ... like the LASCO instrument onboard the SOHO observatory. In this recent LASCO image, an occulting disk supported by a structure seen projecting from the lower left blocks out the overwhelming sunlight. It shows three of the planets along with the Sun's location and bright solar wind regions against a background of stars, but Mars and Venus are unfortunately outside LASCO's roughly 15 degree field of view. The horizontal bars through the planets are digital image artifacts. And what about the Moon? The SOHO spacecraft is positioned well beyond lunar orbit where its view of the Sun is never interrupted by the Moon.

APOD: 2000 April 3 - A Twisted Solar Eruptive Prominence
Explanation: A huge eruptive prominence is seen moving out from our Sun in this condensed half-hour time-lapse sequence. Ten Earths could easily fit in the "claw" of this seemingly solar monster. This large prominence, though, is significant not only for its size, but its shape. The twisted figure eight shape indicates that a complex magnetic field threads through the emerging solar particles. Recent evidence of differential rotation inside the Sun might help account for the surface explosion. The sequence was taken early this year by the Sun-orbiting SOHO satellite. Although large prominences and energetic Coronal Mass Ejections (CMEs) are relatively rare, they are occurring more frequently now that we are near the Solar Maximum, a time of peak sunspot and solar activity in the eleven-year solar cycle.

APOD: 2000 March 18 - A Wind From The Sun
Explanation: A wind from the Sun blows through our Solar System. The behaviour of comet tails as they flapped and waved in this interplanetary breeze gave astronomers the first hint of its existence. Streaming outward at 250-400 miles/second, electrons and ions boiling off the Sun's incredibly hot but tenuous corona account for the Solar Wind - now known to affect the Earth and other planets along with voyaging spacecraft. Rooted in the Solar Magnetic Field, the structure of the corona is visible extending a million miles above the Sun's surface in this composite image from the EIT and UVCS instruments onboard the SOHO spacecraft. The dark areas, known as coronal holes, represent the regions where the highest speed Solar Wind originates.

APOD: 2000 March 9 - Sun Storm: A Coronal Mass Ejection
Explanation: Late last month another erupting filament lifted off the active solar surface and blasted this enormous bubble of magnetic plasma into space. Direct light from the sun is blocked in this picture of the event with the sun's relative position and size indicated by a white half circle at bottom center. The field of view extends 2 million kilometers or more from the solar surface. While hints of these explosive events, called coronal mass ejections or CMEs, were discovered by spacecraft in the early 70s this dramatic image is part of a detailed record of this CME's development from the presently operating SOlar and Heliospheric Observatory (SOHO) spacecraft. Near the minimum of the solar activity cycle CMEs occur about once a week, but as we approach solar maximum rates of two or more per day are anticipated. Though this CME was clearly not headed for Earth, strong CMEs are seen to profoundly influence space weather, and those directed toward our planet and can have serious effects.

APOD: 2000 February 25 - The Comets Of SOHO
Explanation: After four years of successful sun-gazing, the space-based SOlar and Heliospheric Observatory (SOHO) has also become the most successful comet-hunter in history, racking up 102 new comets. Above are examples of SOHO's comet discoveries imaged by LASCO, an on-board coronagraph. LASCO was designed to monitor the solar corona out to a distance of about 12 million miles while blotting out the bright solar disk. Most of the comets discovered with SOHO belong to a special class known as sungrazers - thought to be returning fragments of a large comet which broke up during its historic passage near the sun. Subject to intense solar heat and tidal forces, sungrazers (examples in the left column) are unlikely to survive their close passage. However, ten of SOHO's new comets, like those in the right column, are more "typical" comets and pass the sun at safe distances. In fact, on its outbound journey, the comet at the top right was bright enough to be seen with the unaided eye. Discovered this year on February 4th, the comet at the bottom right holds the distinction of being SOHO comet number 100.

APOD: October 24, 1999 - The Magnetic Carpet Of The Sun
Explanation: The Sun has a magnetic carpet. Its visible surface appears to be carpeted with tens of thousands of magnetic north and south poles joined by looping field lines which extend outward into the Solar Corona. Recently, researchers have revealed maps of large numbers of these small magnetic concentrations produced using data and images from the space-based SOHO observatory. The above computer generated sunscape highlights these effects, with white and black field lines drawn in joining regions of strong magnetism. A close-up of the Solar surface is illustrated in the inset. These small magnetic regions emerge, fragment, drift, and disappear over periods of only 40 hours or so. Their origin is mystifying and their dynamic behavior is difficult to reconcile with present theories of rotationally driven large-scale solar magnetism. Is some unknown process at work? Possibly, but the source of this mystery may well be the solution to another -- the long standing mystery of why the outer Solar Corona is over 100 times hotter than the sun's visible surface! The SOHO data reveal that energy released as these loops break apart and interact seems to be heating the coronal plasma.

APOD: October 21, 1999 - Follow The Spots
Explanation: The Sun rotates on its axis about once every 27 days. How can you tell? Just follow the sunspots. This composite picture was constructed from solar images recorded daily by the MDI instrument on board the space-based SOlar and Heliospheric Observatory (SOHO). It shows the Sun's visible surface for most days of August 1999 so that the same sunspots appear many times as the solar rotation carries them across the face of the Sun. Sunspot temperatures are around 5,000 degrees C. but the spots appear dark as they are actually cooler than the surrounding regions of the solar surface. The sequential images of the sunspot groups show how these regions with high magnetic fields change from day to day.

APOD: September 23, 1999 - Equinox and Eruptive Prominence
Explanation: Today, the Sun crosses the celestial equator and seasons change from Summer to Fall in the north and Winter to Spring in the southern hemisphere. Defined by the Sun's position in sky the event is known as an equinox - the length of daylight is equal to the length of night. Just last week the active Sun produced the dramatic eruptive prominence seen in this extreme ultraviolet picture from the space-based SOHO observatory. The hot plasma is lofted above the solar surface by twisting magnetic fields. How big is the prominence? Click on the image to view the larger full-sun picture. At the same scale, planet Earth would likely still appear smaller than your cursor.

APOD: September 8, 1999 - A Superior Conjunction Of Mercury
Explanation: In astronomical parlance, an interior planet is at superior conjunction when it is located on the opposite side of the Sun from Earth. Mercury, the solar system's innermost planet, zips past this point in its orbit today. In fact, this recent picture from a solar coronagraph on board the the space-based SOHO observatory shows Mercury positioned very close to the Sun as seen from a near Earth vantage point. The coronagraph uses an internal occulting disk to block the intense solar glare which otherwise hides this sight from ground-based observers. The shadow of the occulting disk is at the center with the Sun's size and position indicated by the white circle. Mercury is the bright dot with a horizontal line (a digital artifact), while faint dots scattered throughout the field are stars. Bright regions of the sun's outer atmosphere are also visible. As Mercury continues in its orbit, on November 15 it will actually appear to cross the disk of the Sun as viewed from Earth.

APOD: August 12, 1999 - Deploying Spartan
Explanation: Last October the Space Shuttle Discovery deployed Spartan 201, a spacecraft that monitored the corona of the Sun. Instruments on Spartan 201 were used to estimate the density of electrons emitted into the solar corona, calibrate data from the Solar and Heliospheric Observatory (SOHO) satellite, and study how the Sun is changing as it reaches maximum activity over the next few years. Pictured above, the space shuttle's robot arm (top left) releases Spartan (center) into space. The tail fin of the space shuttle is visible on the right, while the Earth hovers in the background. Spartan floated near the shuttle for two days before it was picked up again and returned to Earth.

APOD: July 16, 1999 - Solar Surfin
Explanation: The sun's corona is a tenuous outer atmosphere composed of streams of energetic charged particles, but it is only easily seen from Earth during a total solar eclipse. For example, this 1991 image of totality from atop Mauna Kea, Hawaii forms a fleeting snapshot of the mysterious corona's beautiful, intricate structures and streams. However in space, instruments can use occulting disks to simulate eclipses and more readily monitor the corona beyond the sun's edge. Combined observations from the space-based SOHO UCVS and shuttle-borne Spartan 201 experiments have recently contributed to a major advance in understanding the high-speed component of the wind of particles in the corona. They reveal evidence for magnetic waves within the corona itself that push solar wind particles along, like an ocean wave gives a surfer a ride. Surprisingly, heavier charged particles can surf the magnetic waves faster - oxygen ions were found to achieve speeds of up to 500 miles per second, faster than the lighter hydrogen ions which make up most of the solar wind.

APOD: July 8, 1999 - Eruptive Prominence
Explanation: Activity on our parent star continues to increase as the sun approaches a maximum in its 11-year solar cycle, expected in the year 2000. On June 14 - only a week before the solstice - the space-based SOHO observatory recorded this stunning view of an immense prominence erupting from the sun's southern latitudes (south is up). The false-color image was made in the extreme Ultraviolet light produced by ionized Helium atoms in the solar plasma. Earth dwellers fortunate enough to be well located in Europe, the Middle East, Asia may be able to view for themselves activity above the solar limb during the upcoming August solar eclipse - the last total eclipse of the second millennium.

APOD: July 2, 1999 - Shadow Of A Comet
Explanation: Hale-Bopp, the Great Comet of 1997, may have been the most viewed comet in history - visible even from bright metropolitan skies. Astronomers are now reporting that this magnificent comet also cast a shadow against the glare of the solar system's ultraviolet haze. This false-color image represents a slice of the sky viewed by the SWAN (Solar Wind ANisotropy) instrument aboard the space-based SOHO observatory. Recorded on March 8, 1997 it shows a general haze of solar ultraviolet light scattered by interstellar hydrogen. The sun itself is positioned below the bottom center of the cropped image and the large bright spot is ultraviolet sunlight scattered by the cloud of hydrogen gas surrounding Hale-Bopp's nucleus. Just above and to the left is a broad, diffuse, dark streak - the 150 million kilometer long shadow produced by the denser regions of this hydrogen envelope. Why are comets surrounded by hydrogen? The hydrogen comes from the breakup of water (H20) vapor released as the comet nucleus approaches the sun. These observations indicate that Hale-Bopp's nucleus was producing about 300 tons of water per second.

APOD: April 30, 1999 - Solar Shock Wave
Explanation: On September 24, 1997 a shock wave blasted across the surface of the sun at speeds of 250 to 600 kilometers per second. On planet Earth, observer Barry Reynolds photographed the expanding shock front (left) in the light emitted by hydrogen atoms at the solar surface. His discovery image was nicely confirmed by a space-based extreme ultraviolet image (right) of the shock ramming through the sun's upper atmosphere as recorded by the SOHO satellite observatory. In both pictures a bright solar flare is seen near the center of a circular arc-like feature representing a shock front. The shock front is dark in the ground based photo and bright in the ultraviolet image. These shock fronts are believed to be tracers of a 3-dimensional disturbance caused by the flare but researchers are uncertain as to the exact physical mechanisms which produced it. Along with other violent events called coronal mass ejections, solar flares are known to generate streams of energetic particles which can affect the Earth's magnetosphere and Earth orbiting satellites.

APOD: February 8, 1999 - The Solar Wind Emerges
Explanation: Winds of fast particles blow out from the Sun, but why? Astronomers came a step closer to answering this question recently by making detailed observations of the high-speed wind source with the space-borne Solar and Heliospheric Observatory (SOHO). Images like those shown above isolate regions of inflowing gas, shown in red, and outflowing gas, shown in blue. Particles such as electrons and protons flow out at speeds near 3 million kilometers per hour. This wind will typically enter one of the Sun's coronal holes before flowing out into the Solar System. Analysis indicates that the high-speed wind escapes at the edges of large convection cells, drawn in black. SOHO has recently been revived to run without the use of any orienting gyroscopes.

APOD: December 12, 1998 - Driving To The Sun
Explanation: How long would it take to drive to the Sun? Brittany, age 7, and D.J., age 12, ponder this question over dinner one evening. James, also age 7, suggests taking a really fast racing car while Christopher, age 4, eagerly agrees. Jerry, a really old guy who is used to estimating driving time on family trips based on distance divided by speed, offers to do the numbers. "Let's see ... the Sun is 93 million miles away. So, if we drove 93 miles per hour the trip would only take us 1 million hours." How long is 1 million hours? One year is 365 days times 24 hours per day, or 8,760 hours. One hundred years would be 876,000 hours, still a little short of the 1 million hour drive time -- so the Sun is really quite far away. Christopher is not impressed, but as he grows older he will be. You've got to be impressed by something that's 93 million miles away and still hurts your eyes when you look at it!

APOD: October 15, 1998 - A Great Day For SOHO
Explanation: The last 10 days have been great days for SOHO, the space-based SOlar and Heliospheric Observatory. Contact was completely lost with this international research spacecraft over 3 months ago but recovery teams have reacquired control of SOHO and, beginning October 5th, have been successfully switching on its scientific instruments. This October 13th view of the Sun in the light of ionized Helium atoms was recorded by the restored EIT instrument. It shows bright active regions and lofty prominences above the solar limb. North is toward the left rather than the top as the spacecraft's orientation has not yet been fully adjusted. (For a full Sun / full resolution view, click on the picture!) With the solar cycle approaching a maximum in the coming years, excitement continues to build as it becomes very likely that SOHO will be able to resume its unprecedented exploration of solar phenomena.

APOD: August 20, 1998 - SOHO Composite: Coronal Mass Ejection
Explanation: This complex composite image of an ominous and spectacular event - an expanding storm of energetic particles from the Sun - was constructed using data recorded by the SOHO spacecraft on November 6, 1997. Four images from two SOHO (Solar Orbiting Heliospheric Observatory) instruments have been nested to show the ultraviolet Sun at center and a large eruption of material from the right-hand solar limb. Known as a Coronal Mass Ejection or CME, the expanding cloud has become relatively cool and dark in the middle with bright edges still connected to the solar surface. High energy protons have peppered the SOHO detectors causing the crazed streaks and blemishes. The picture covers a region extending about 13.5 million miles from the Sun (32 Solar Radii).

On June 25, after successfully completing its planned mission, contact with SOHO was lost -- but has recently been re-established! Hopefully SOHO will soon be able to continue operating in an extended mission phase.

APOD: June 29, 1998 - Solar Magnetic Bananas
Explanation: Is that our Sun? The unusual banana-shaped loops shown above are actually part of a computer-generated snap-shot of our Sun's magnetic field. This animated frame was constructed using data from the ground-based U.S. Solar Vector Magnetograph and the space-based Japanese X-Ray Telescope Yohkoh. Surfaces of constant magnetic field strength loop through the Sun's corona, break through the Sun's surface, and connect regions of magnetic activity such as sunspots. Recently, contact has been interrupted with the Sun-watching SOHO satellite. Although SOHO had completed its two year mission, attempts are still being made to re-establish communication.

APOD: June 11, 1998 - SOHOs Twin Sungrazers
Explanation: This four frame animation (courtesy D. Biesecker) shows two comets arcing toward a fatal fiery encounter with the Sun. These discovery images were recorded by the LASCO instrument on board the space-based SOHO solar observatory on June 1-2. A portion of LASCO's circular occulting disk - which blocks the blinding direct sunlight - is seen at the upper left along with a bright solar wind region extending to the right. For scale, the size and position of the Sun's edge are outlined by the white quarter circle on the occulting disk. The Sungrazer comets approach from below and have visible tails. The lower comet's coma is bright enough to cause a horizontal blemish in the digital image, while the tail of the upper comet grows dramatically as it closes with the Sun. The pair are "twins" or at least "siblings" in the sense that they are both likely members of a family of comets thought to result from the breakup of a single large parent comet. Members of the Sungrazer family can pass within 400,000 miles or less of the solar surface and many, like this pair, do not survive.

APOD: June 4, 1998 - Comet SOHO and Nebulae in Orion
Explanation: Astrophotographer Michael Horn captured this gorgeous view of comet SOHO in the dark night sky above Wandibindle, Queensland, Australia on May 23rd. On this date, comet SOHO was moving against the background of the nebula-rich constellation of Orion. South is up in the picture which shows SOHO's bright head or coma and long tail extending past the glowing gas clouds and dark dust lanes of the Flame and Horsehead nebulae. Alnitak, the bright star above and to the right of the cometary coma, is also known as Zeta Orionis, the eastern-most of the three stars in Orion's belt. Southern Hemisphere observers report that comet SOHO has recently undergone a dramatic increase in brightness.

APOD: June 1, 1998 - Solar Flares Cause Sun Quakes
Explanation: An 11th magnitude quake has been recorded on the Sun, immediately following a moderate solar flare. The quake was the first ever recorded on the Sun, but only because astronomers have only recently figured out when and how to find them using the orbiting SOHO spacecraft. Dark waves from the quake can be seen in the above picture spreading out from an explosive bright flare. The solar ripples are similar in appearance to waves caused by a rock thrown into a pond. The magnitude and evolution of these quakes gives information about the physical nature of solar flares, the surface of the Sun, and even the Sun's interior.

APOD: May 21, 1998 - Bright Comet SOHO
Explanation: Discovered this month with an orbiting solar observatory, bright Comet SOHO has now emerged from the Sun's glare. This telephoto picture of the new naked-eye comet was taken by astrophotographer Michael Horn after sunset in the western twilight above Lake Samsonvale, Brisbane, Australia on May 18. The comet is seen in the constellation Orion. Its long lovely tail stretches nearly 5 degrees to the bright star Bellatrix, near the top of the image. For Southern Hemisphere comet watchers, views of Comet SOHO (1998J1) will improve as this month draws to a close and the comet climbs to the south and east on its journey outward bound. In February 1999, NASA plans to launch the Stardust mission to fly close to a comet and return samples of dust from a comet's tail.

APOD: May 20, 1998 - Discovery Image: Comet SOHO (1998 J1)
Explanation: Staring at the Sun from a vantage point in space (Kids, don't try this at home!), the Solar and Heliospheric Observatory (SOHO) has enabled the discovery of much about our closest star. It has also been used to discover about 50 comets. While not competing with Hale-Bopp, one of SOHO's recently discovered comets has proved to be bright enough to see with the unaided eye. The May 4th discovery image is shown above with an enlarged inset of the comet. This colorized image is from SOHO's solar coronagraph (LASCO) which views the region around the Sun by blocking out the overwhelming sunlight with an occulting disk. The disk is visible near the bottom left, with the Sun's size and position indicated by the white circle. Bright solar wind regions can also be seen along with the the planet Mars and a background of stars. The comet itself is just entering the field of view at the upper right. Observers report that Comet SOHO (1998 J1) has now been seen low in the western sky following sunset and is moving south and east becoming more visible as this month progresses, particularly from the Southern Hemisphere.

APOD: May 16, 1998 - Helios Helium
Explanation: This image of the relatively quiet Sun was made using ultraviolet light emitted by ionized Helium atoms in the Solar chromosphere. Helium was first discovered in the Sun in 1868, its name fittingly derived from from the Greek word Helios, meaning Sun. Credit for the discovery goes to astronomer Joseph Norman Lockyer (born May 17, 1836). Lockyer relied on a then recently developed technique of spectroscopy, dissecting sunlight into a spectrum, and the idea that each element produces a characteristic spectral pattern of bright lines. He noticed a yellow line in a solar spectrum made during an eclipse which could not be accounted for by elements known on Earth. Almost 27 years later terrestrial Helium was finally discovered when the spectrum of a Helium bearing mineral of Uranium provided an exact match to the previously detected element of the Sun. Helium is now known to be the second most abundant element (after Hydrogen) in the Universe.

APOD: April 29, 1998 - Tornadoes on the Sun
Explanation: Giant spinning clouds of gas, similar to Earth's tornadoes, have been found on the Sun. Solar tornadoes, however, can be larger than the entire Earth, and sustain wind gusts over 1000 times stronger than their Earth counterparts. The SOHO spacecraft has found that solar tornadoes start low in the Sun's atmosphere and spiral outwards, gathering speed as they enter the Solar System. Earthlings have more to fear from Earth's own weather phenomena, though, because the high speed particles that result from solar tornadoes are easily stopped by the Earth's thick atmosphere. Earthlings may have much to learn from solar tornadoes, including details of how the solar wind and corona are powered, and how to better predict future solar particle storms that could damage sensitive satellites.

APOD: February 27, 1998 - Solar Eclipse: A Composite View
Explanation: Yesterday, the Moon's shadow reached out and touched the Earth, treating a large portion of the Western Hemisphere to an Eclipse of the Sun. This composite image combines pictures of the Sun made from both Earth and space. The central direct image of the solar surface was recorded yesterday by the Extreme Ultraviolet Imaging Telescope on board the space based SOHO observatory. It is surrounded by a telescopic picture of the Sun as seen from the island of Aruba during the total eclipse. The surrounding view of the eclipsed Sun reveals the gleaming solar corona, visible to ground based observers during totality. Such combined images can help connect explosive events and features on the Sun's surface with the corona and solar wind.

APOD: December 21, 1997 - A Winter Solstice
Explanation: Today is the Winter Solstice, the shortest day of the year in the Northern Hemisphere. The yearly cycle of Seasons on planet Earth once again finds the Sun at its lowest point in the Northern Sky. The Sun's own 11 year cycle of activity is progressing toward a maximum which will occur in 2000-2001. This image of the Sun in the light of ionized Helium was recorded by the space-based SOHO observatory only three days ago and shows many prominences and active regions.

APOD: November 20, 1997 - Escape From The Sun
Explanation: Twisted magnetic fields arching from the solar surface can trap ionized gas, suspending it in huge looping structures. These majestic plasma arches are seen as prominences above the solar limb. On September 14, this dramatic and detailed image was recorded by the EIT experiment on board the space-based SOHO observatory in the light emitted by ionized Helium. It shows hot plasma escaping into space as a fiery prominence breaks free from magnetic confinement a hundred thousand miles above the Sun. These awesome events bear watching as they can affect communications and power systems ninety three million miles away on Planet Earth.

APOD: November 6, 1997 - The Magnetic Carpet Of The Sun
Explanation: The Sun has a magnetic carpet. Its visible surface appears to be carpeted with tens of thousands of magnetic north and south poles joined by looping field lines which extend outward into the Solar Corona. Recently, researchers have revealed maps of large numbers of these small magnetic concentrations produced using data and images from the space-based SOHO observatory. The above computer generated sunscape highlights these effects, with white and black field lines drawn in joining regions of strong magnetism. A close-up of the Solar surface is illustrated in the inset. These small magnetic regions emerge, fragment, drift, and disappear over periods of only 40 hours or so. Their origin is mystifying and their dynamic behavior is difficult to reconcile with present theories of rotationally driven large-scale Solar Magnetism. Is some unknown process at work? Possibly, but the source of this mystery may well be the solution to another -- the long standing mystery of why the outer Solar Corona is over 100 times hotter than the sun's visible surface! The SOHO data reveal that energy released as these loops break apart and interact seems to be heating the coronal plasma.

APOD: September 18, 1997 - Erupting Sun
Explanation: On August 27th twisting magnetic fields propelled this huge eruptive prominence a hundred thousand miles above the Sun's surface. The seething plasma of ionized gases is at a temperature of about 150,000 degrees Farenheit and spans over 200,000 miles (about 27 Earths). The Extreme ultraviolet Imaging Telescope (EIT) onboard the space-based SOHO observatory recorded this exquisitely detailed image in the light of ionized Helium atoms from its vantage point in a Halo orbit. This is the largest solar prominence observed by SOHO instruments since they began exploring solar phenomena in early 1996.

APOD: September 4, 1997 - Rivers in the Sun
Explanation: The surface of the Sun is shifting. By watching sunspots, it has long been known that our Sun rotates. It was also known that the center of the Sun rotates faster than the poles. Now, recent measurements by the Solar Oscillations Investigations group of the SOHO Observatory have found that the surface of the sun moves in other ways, too. Hot, electrically charged gas flows along and beneath the Sun's surface as depicted in the above computer generated diagram. The speed of these solar rivers is false-color coded with red hydrogen moving faster than blue. Over the course of a year plasma moves from the equator to the poles, while internal eddies circulate gas from deep inside the Sun. One surprise is the similarity to the motion of air in the Earth's atmosphere - indicating that scientists might learn more about Earth's global weather by studying the Sun.

APOD: April 18, 1997 - Solar Storm Causes X-Ray Aurora
Explanation: On April 7, the SOHO spacecraft spotted a Solar Storm ejecting a cloud of energetic particles toward planet Earth. The plasma cloud's center missed Earth, but high energy particles swept up by Earth's magnetosphere still created a geomagnetic storm! Residents of northerly lattitudes were treated to the spectacle of brilliant aurora as curtains of green and white light danced across the sky. In this image from April 11, the Polar Ionospheric X-ray Imaging Experiment (PIXIE) onboard NASA's orbiting POLAR spacecraft records the strongest X-ray aurora seen in more than a year of operation. The false color image overlaying a map of North America reveals X-rays generated in the upper atmosphere by showers of high energy electrons.

APOD: April 11, 1997 - The Sun Puffs
Explanation: The Earth has once again endured a burst of particles from the Sun. The latest storm, which began Monday, was one of the best documented solar storms to date. At 10 am (EDT) ground monitors of the SOHO spacecraft, which continually monitors the Sun, noticed a weak spot in the solar corona was buckling again, this time letting loose a large, explosive Coronal Mass Ejection (CME). Almost simultaneously, NASA's WIND spacecraft began detecting bursts of radio waves from electrons involved in this magnetic storm. Supersonic waves rippled though the solar corona as a puff of high energy gas shot out into the Solar System. The above image shows two photographs of the Sun taken about 15 minutes apart and subtracted, highlighting the explosion. The CME gas will have little lasting effect on the Earth, but might make this a good weekend to see an aurora.

APOD: March 4, 1997 - Solar Wind And Milky Way
Explanation: The Sun is bright, so bright that it overwhelms the light from other stars even for most satellite-borne telescopes. But LASCO, a coronograph onboard the space-based SOHO Observatory, uses occulting disks to block the intense solar light and examine the tenuous, hot gases millions of miles above the Sun's surface. In this LASCO image from December 24, 1996, an occulting disk (center) and mechanical support (extending from the lower left) are visible along with the billowing Solar Wind. Appearing in the background are faint stars and obscuring dust clouds toward the center of our Milky Way Galaxy! The field of view covers about 16 degrees, corresponding to 28 million miles at the distance of the Sun - just under half the diameter of Mercury's orbit. A prominent dark interstellar dust cloud cuts through the Milky Way starfield running approximately south (lower right) to north. Blemishes on the image are camera streaks caused by charged particles.

APOD: February 26, 1997 - Sungrazer
Explanation: Arcing toward a fiery fate, this Sungrazer comet was recorded by the SOHO spacecraft's Large Angle Spectrometric COronagraph (LASCO) on Dec. 23rd, 1996. LASCO uses an occulting disk, partially visible at the lower right, to block out the otherwise overwhelming solar disk allowing it to image the inner 5 million miles of the relatively faint corona. The comet is seen as its coma enters the bright equatorial solar wind region (oriented vertically). Spots and blemishes on the image are background stars and camera streaks caused by charged particles. Positioned in space to continuously observe the Sun, SOHO has detected 7 sungrazing comets. Based on their orbits, they are believed to belong to a family of comets created by successive break ups from a single large parent comet which passed very near the sun in the twelfth century. The bright comet of 1965, Ikeya-Seki, was also a member of the Sungrazer family, coming within about 400,000 miles of the Sun's surface. Passing so close to the Sun, Sungrazers are subjected to destructive tidal forces along with intense solar heat. This comet, known as SOHO 6, did not survive.

APOD: February 17, 1997 - A Wind From The Sun
Explanation: A wind from the Sun blows through our Solar System. The behaviour of comet tails as they flapped and waved in this interplanetary breeze gave astronomers the first hint of its existence. Streaming outward at 250-400 miles/second, electrons and ions boiling off the Sun's incredibly hot but tenuous corona account for the Solar Wind - now known to affect the Earth and other planets along with voyaging spacecraft. Rooted in the Solar Magnetic Field, the structure of the corona is visible in this composite image from the EIT and UVCS instruments onboard the SOHO spacecraft, extending a million miles above the Sun's surface. The dark areas, known as coronal holes, represent the regions where the highest speed Solar Wind originates.

APOD: January 7, 1997 - Red Sun Streaming
Explanation: The Sun is leaking. In fact, it is gushing: particles stream away from the Sun at hundreds of kilometers per second. Some of these particles strike the Earth and cause aurora. Most particles, however, either surround the Sun as a huge solar corona or glide into interstellar space as the solar wind. Don't worry about the Sun totally evaporating - it loses too little mass to have any lasting effect. The above false-color picture was taken with the Large Angle and Spectrometric Coronograph on board the SOlar and Heliospheric Observatory (SOHO). This instruments blocks out the central solar disk so it can image the regions surrounding the Sun. The large streamers visible are typical. Where these charged ions get their enormous streaming energy is still a mystery!

APOD: January 6, 1997 - Blue Sun Glaring
Explanation: The Sun is a bubbling ball of extremely hot gas. In this false-color picture, light blue regions are extremely hot - over 1 million degrees, while dark blue regions are slightly cooler. The camera filter used was highly sensitive to the emission of highly charged iron ions, which trace the magnetic field of the Sun. The rich structure of the image shows the great complexity of the Sun's inner corona. A small active region can be seen just to the right and above center. This picture was taken in ultraviolet (extremely blue) light by the Extreme-ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) spacecraft, which is orbiting the Sun just ahead of the Earth, at the L1 point. SOHO was launched in 1995 and will continually monitor the Sun for several years.

APOD: December 21, 1996 - Sun and Winter Solstice 1996
Explanation: Today is the Winter Solstice for 1996. After steadily sinking in Northern Hemisphere skies, the Sun is now at its lowest declination - marking the first day of Northern Winter (but Southern Summer!). The Earth is actually closer to the Sun during this season, a fact not usually appreciated by those who dwell on the planet's Northern half. Two days ago, the EIT camera onboard the SOHO spacecraft recorded this image of the Sun in the light of highly ionized Iron atoms. This extreme ultraviolet picture emphasizes magnetic field lines and active regions in the hot plasma above the solar surface. For today's Solar images at many wavelengths, check out the Solar Data Analysis Center's web site.

APOD: July 27, 1996 - Driving to the Sun
Explanation: How long would it take to drive to the Sun? Brittany, age 7, and D.J., age 12, ponder this question over dinner one evening. James, also age 7, suggests taking a really fast racing car while Christopher, age 4, eagerly agrees. Jerry, a really old guy who is used to estimating driving time on family trips based on distance divided by speed, offers to do the numbers. "Let's see ... the Sun is 93 million miles away. So, if we drove 93 miles per hour the trip would only take us 1 million hours." How long is 1 million hours? One year is 365 days times 24 hours per day, or 8,760 hours. One hundred years would be 876,000 hours, still a little short of the 1 million hour drive time -- so the Sun is really quite far away. Christopher is not impressed, but as he grows older he will be. You've got to be impressed by something that's 93 million miles away and still hurts your eyes when you look at it!

APOD: May 21, 1996 - The Iron Sun
Explanation: The ultraviolet light emitted by eleven times ionized iron at temperatures over 2 million degrees Farenheit was used to record the above picture of the Sun on May 16. The image was made by the EIT camera onboard the SOHO spacecraft, a space observatory which can continuously observe the Sun. Eleven times ionized iron is atomic iron with eleven of its electrons stripped away. Here the electrons are stripped by the frantic collisions with other atoms and electrons which occur at the extreme temperatures in the Solar Corona. Since electrons are negatively charged, the resulting ionized iron atom is highly positively charged. Astronomer's "shorthand" for eleven times ionized iron is written "Fe XII", the chemical symbol for iron followed by a Roman numeral 12 (Fe I is neutral iron).

APOD: May 20, 1996 - Helios Helium
Explanation: Above is an image of the relatively quiet Sun made on May 18 in light emitted by ionized Helium atoms in the Solar chromosphere. Helium was first discovered in the Sun in 1868, its name fittingly derived from from the Greek word Helios, meaning Sun. Credit for the discovery goes to astronomer Joseph Lockyer. Lockyer relied on a recently developed technique of spectroscopy, dissecting sunlight into a spectrum, and the idea that each element produces a characteristic spectral pattern of bright lines. He noticed a yellow line in a solar spectrum made during an eclipse which could not be accounted for by elements known on Earth. Almost 27 years later Helium was finally discovered on Earth when the spectrum of a Helium bearing mineral of Uranium provided an exact match to the previously detected element of the Sun. Helium is now known to be the second most abundant element (after Hydrogen) in the Universe.

APOD: May 17, 1996 - Comet Hyakutake and a Solar Flare
Explanation: A rare coincidence was recently captured by the orbiting SOHO spacecraft. During the closest approach to the Sun of Comet Hyakutake on May 1, SOHO photographed the comet. By accident -- during the time this photograph was being taken -- a solar flare was being ejected from the Sun. Therefore, at the top of this false-color picture, Comet Hyakutake is visible, while emission to the left of the Sun is a solar flare. The Sun, at the center of the picture, was blocked by an artificial occulter in the LASCO telescope, allowing objects much dimmer than the Sun to be observed. SOHO was launched in December of 1995 and contains many instruments which study the Sun.

APOD: May 16, 1996 - Comet Hyakutake Passes the Sun
Explanation: On May 1, Comet Hyakutake made its closest approach to the Sun. During this time it was not possible to view the comet with most astronomical instruments because of the brightness of the nearby Sun. But the orbiting Solar and Heliospheric Observatory (SOHO) spacecraft contains telescopes meant to look directly at the Sun and so were able to track the comet during this crucial time. This picture, taken May 2, shows the tails of Comet Hyakutake pointing away from the Sun, as expected. During its closest approach, Comet Hyakutake passed inside the orbit of Mercury. Comet Hyakutake will not return to the inner Solar System for another 14,000 years.

APOD: December 14, 1995 - An Atlas Centaur Rocket Launches
Explanation: 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.