Astronomy Picture of the Day |
APOD: 2022 September 26 - All the Water on Planet Earth
Explanation:
How much of planet Earth is made of water?
Very little, actually.
Although
oceans of water cover about 70 percent of Earth's surface, these oceans are
shallow compared to the Earth's radius.
The featured illustration
shows what would happen if all of
the water on or near the surface of the Earth were bunched up into a
ball.
The radius of this ball would be only about 700 kilometers,
less than half the radius of the
Earth's Moon, but slightly larger than Saturn's moon
Rhea which, like many moons in our outer
Solar System, is mostly water ice.
The next smallest ball depicts all of
Earth's liquid fresh water, while the tiniest ball
shows the volume of all of Earth's
fresh-water lakes and
rivers.
How any of this
water came to be on
the Earth and whether any significant amount is
trapped far
beneath Earth's surface remain topics of research.
APOD: 2022 January 4 - Moons Beyond Rings at Saturn
Explanation:
What's happened to that moon of Saturn?
Nothing -- Saturn's moon
Rhea is just partly hidden behind Saturn's rings.
In 2010, the robotic Cassini spacecraft then orbiting Saturn took
this narrow-angle view looking across the
Solar System's most
famous rings.
Rings visible in the foreground include the thin
F ring on the outside and the much wider
A and B rings just interior to it.
Although it seems to be hovering
over the rings, Saturn's moon
Janus is actually far behind them.
Janus is one of
Saturn's smaller
moons
and measures only about 180 kilometers across.
Farther out from the camera is the heavily cratered
Rhea, a much larger moon
measuring 1,500 kilometers across.
The top of Rhea is visible only through
gaps in the rings.
After more than a decade of exploration and discovery, the
Cassini spacecraft ran
low on fuel in 2017 and was directed to
enter Saturn's atmosphere,
where it surely
melted.
APOD: 2019 October 17 - Moons of Saturn
Explanation:
On July 29, 2011 the Cassini spacecraft's
narrow-angle camera
took
this snapshot and captured 5
of
Saturn's moons, from just above the ringplane.
Left to right are small moons Janus and Pandora respectively
179 and 81 kilometers across,
shiny 504 kilometer diameter Enceladus,
and Mimas, 396 kilometers across, seen just next to Rhea.
Cut off by the right edge of the frame, Rhea is Saturn's
second largest moon at 1,528 kilometers across.
So
how many moons
does Saturn have?
Twenty new found outer satellites bring its total to 82 known moons,
and since Jupiter's moon total stands at 79, Saturn is the Solar System's new
moon king.
The newly announced
Saturnian satellites are all very small, 5
kilometers or so in diameter, and most are in retrograde orbits inclined
to Saturn's ringplane.
You can help
name
Saturn's new moons, but you should
understand the rules.
Hint: A knowledge of Norse, Inuit, and Gallic mythology will help.
APOD: 2016 September 11 - All the Water on Planet Earth
Explanation:
How much of planet Earth is made of water?
Very little, actually.
Although
oceans of water cover about 70 percent of Earth's surface, these oceans are
shallow compared to the Earth's radius.
The featured illustration
shows what would happen if all of
the water on or near the surface of the Earth were bunched up into a ball.
The radius of this ball would be only about 700 kilometers, less than half the radius of the
Earth's Moon, but slightly larger than Saturn's moon
Rhea which, like many moons in our outer Solar System, is mostly water ice.
How even this much
water came to be on
the Earth and whether any significant amount is
trapped far
beneath Earth's surface remain topics of research.
APOD: 2015 January 4 - Crescent Rhea Occults Crescent Saturn
Explanation:
Soft hues, partially lit orbs, a thin trace of the ring, and slight shadows highlight this understated view of the majestic surroundings of the giant planet Saturn.
Looking nearly back toward the Sun, the
robot Cassini
spacecraft now orbiting Saturn captured
crescent phases of
Saturn
and its moon Rhea in color a few years ago.
As striking as the
above image is, it is but a single frame from a
60-frame silent movie where Rhea can be seen gliding in front of its parent world.
Since Cassini was nearly in the plane of
Saturn's rings, the normally impressive rings are visible here only as a
thin line across the image center.
APOD: 2013 March 29 - Ringside with Rhea
Explanation:
Orbiting in the plane of Saturn's rings,
Saturnian moons have a perpetual
ringside view of the gas giant planet.
Of course, while passing near the ring plane
the Cassini spacecraft also shares
their stunning perspective.
The thin rings themselves slice across the middle of
this Cassini snapshot from April 2011.
The scene looks toward the dark
night side of Saturn, in the frame at the
left, and the still sunlit side of the rings from just above the
ringplane.
Centered, over 1,500 kilometers across, Rhea is
Saturn's second largest moon and
is closest to the spacecraft, around 2.2 million kilometers away.
To Rhea's right, shiny, 500 kilometer diameter
Enceladus is about 3 million kilometers distant.
Dione, 1,100 kilometers wide,
is 3.1 million kilometers from Cassini's camera
on the left, partly blocked by Saturn's night side.
APOD: 2012 May 15 - All the Water on Planet Earth
Explanation:
How much of planet Earth is made of water?
Very little, actually.
Although
oceans of water cover about 70 percent of Earth's surface, these oceans are
shallow compared to the Earth's radius.
The above illustration shows what would happen if all of
the water on or near the surface of the Earth were bunched up into a ball.
The radius of this ball would be only about 700 kilometers, less than half the radius of the
Earth's Moon, but slightly larger than Saturn's moon
Rhea which, like many moons in our outer Solar System, is mostly water ice.
How even this much
water came to be on
the Earth and whether any significant amount is
trapped far
beneath Earth's surface remain topics of research.
APOD: 2012 April 14 - Six Moons of Saturn
Explanation:
How many moons does Saturn have?
So far 62 have
been discovered, the smallest only a fraction
of a kilometer across.
Six of its largest satellites can be seen here, though, in a sharp
Saturnian family portrait
taken on March 9.
Larger than Earth's Moon and even slightly larger than Mercury,
Titan
has a diameter of 5,150 kilometers and starts the line-up
at the lower left.
Continuing to the right across the frame are
Mimas,
Tethys, [Saturn],
Enceladus,
Dione, and
Rhea at far right.
Saturn's first known natural satellite, Titan was
discovered in 1655 by
Dutch astronomer Christiaan Huygens, while most recently the
satellite provisionally designated
S/2009 S1 was found
by the Cassini Imaging Science Team in 2009.
Tonight,
Saturn reaches opposition
in planet Earth's sky, offering
the best telescopic views of the ringed planet and moons.
APOD: 2010 July 12 - Moons Beyond the Rings of Saturn
Explanation:
What's happened to that moon of Saturn?
Nothing -- Saturn's moon Rhea is just partly hidden behind Saturn's rings.
In April, the robotic Cassini spacecraft now orbiting Saturn took this
narrow-angle view looking across the
Solar System's most
famous rings.
Rings visible in the foreground include the thin
F ring on the outside and the much wider
A and B rings just interior to it.
Although it seems to be hovering
over the rings, Saturn's moon
Janus is actually far behind them.
Janus is one of
Saturn's smaller
moons
and measures only about 180 kilometers across.
Farther out from the camera is the heavily cratered
Rhea, a much larger moon
measuring 1,500 kilometers across.
The top of Rhea is visible only through
gaps in the rings.
The Cassini mission around Saturn has
been extended to
2017
to better study the complex planetary system as its season changes from
equinox to
solstice.
APOD: 2010 May 31 - Moons and Rings Before Saturn
Explanation:
While cruising around Saturn,
be on the lookout for picturesque juxtapositions of moons and rings.
Another striking alignment occurred last March in the view of humanity's
Saturn-orbiting Cassini spacecraft.
Rhea, one of Saturn's larger moons,
was caught passing
Epimetheus, one of Saturn's smaller moons.
Epimetheus, as
pictured above, is actually well behind the heavily cratered Rhea.
Further back, several of the
complex rings of Saturn can be seen crossing the image horizontally.
Behind both the
moons and rings is giant Saturn itself,
showing expansive but featureless clouds in the
green light where the above image was taken.
The Cassini mission around Saturn has now
been extended to
2017
to better study the complex planetary system as its season changes from
equinox to
solstice.
APOD: 2010 February 1 - Shepherd Moon Prometheus from Cassini
Explanation:
Another moon of Saturn has been imaged in detail by the Cassini spacecraft.
Orbiting Saturn since 2004, the
robotic Cassini
got its closest look yet at Saturn's small moon
Prometheus last week.
Visible above in an unprocessed image from 36,000 kilometers away,
Prometheus' 100-km long surface was revealed to have an
interesting system of bulges, ridges, and craters.
These features, together with the moon's oblong shape and high reflectivity,
are now being studied to help better understand the history of
Prometheus and Saturn's rings.
Prometheus is one of the few
shepherd satellites known, as its gravity, along with its companion moon
Pandora, confines many smaller ice chucks into
Saturn's F Ring.
Cassini's next major
targeted flyby is of the moon
Rhea on March 2.
APOD: 2008 July 20 - Crescent Rhea Occults Crescent Saturn
Explanation:
Soft hues, partially lit orbs, a thin trace of the ring, and slight shadows highlight this understated view of the majestic surroundings of the giant planet Saturn.
Looking nearly back toward the Sun, the
robot Cassini
spacecraft now orbiting Saturn captured
crescent phases of
Saturn and its
moon Rhea in color a few years ago.
As striking as the
above image is, it is but a single frame from a recently released
60-frame silent movie where Rhea can be seen gliding in front of its parent world.
Since Cassini was nearly in the plane of
Saturn's rings, the normally impressive rings are visible here only as a
thin line across the image center.
Although Cassini has now concluded its
primary mission,
its past successes and opportunistic location have prompted
NASA to start a two-year
Equinox Mission, further exploring not only Saturn's enigmatic moons
Titan and
Enceladus, but Saturn herself as her grand
rings tilt right at the Sun in August 2009.
APOD: 2008 May 13 - Ancient Craters of Southern Rhea
Explanation:
Saturn's ragged moon Rhea has one of the oldest surfaces known.
Estimated as changing little in the past billion years,
Rhea shows
craters
so old they no longer appear round – their
edges have become compromised by more recent cratering.
Like Earth's Moon,
Rhea's rotation is locked on Saturn, and the
above image shows part of
Rhea's surface that always faces Saturn.
Rhea's leading surface is more highly cratered than its trailing surface.
Rhea is composed mostly of water-ice but is thought to include about 25 percent rock and metal.
The above image
was taken by the
robot Cassini spacecraft now orbiting Saturn.
Cassini swooped past
Rhea
last month
and captured the
above image
from about 350,000 kilometers away.
Rhea
spans 1,500 kilometers making it Saturn's second largest moon after
Titan.
Several
surface features on Rhea
remain unexplained including large light patches like those seen near the image top.
APOD: 2006 July 11 - Crescent Rhea Occults Crescent Saturn
Explanation:
Soft hues, partially lit orbs, a thin trace of the ring, and slight shadows highlight this understated view of the majestic surroundings of the giant planet Saturn.
Looking nearly back toward the Sun, the
robot Cassini
spacecraft now orbiting Saturn captured
crescent phases of
Saturn and its
moon Rhea in color a few months ago.
As striking as the
above image is, it is but a single frame from a recently released
60-frame silent movie where Rhea can be seen gliding in front of its parent world.
Since Cassini was nearly in the plane of
Saturn's rings, the normally impressive rings are visible here only as a
thin line across the image center.
Cassini has now passed the official half-way mark of its mission around Saturn, but is well situated to complete
another two years investigating this complex and surprising system.
APOD: 2006 June 27 - The Moving Moons of Saturn
Explanation:
The moons of Saturn never stop.
A space traveler orbiting the
ringed giant planet
would witness a continuing
silent dance where Saturn's
multiple moons pass near each other in numerous combinations.
Like a miniature
Solar System,
the innermost moons
orbit Saturn the fastest.
The above movie was centered on Saturn's moon
Rhea, so that the moons
Mimas and
Enceladus appear to glide by.
At 1,500 kilometers across,
Rhea
is over three times larger than the comparably sized
Mimas and
Enceladus.
The Sun illuminates the scene from the lower right,
giving all of the moons the same
crescent phase.
The above time lapse movie was created by the Saturn-orbiting robotic
Cassini spacecraft
over a period of about 40 minutes.
APOD: 2006 May 30 - Ancient Craters on Saturn's Rhea
Explanation:
Saturn's ragged moon Rhea has one of the oldest surfaces known.
Estimated as changing little in the past billion years,
Rhea shows
craters
so old they no longer appear round – their
edges have become compromised by more recent cratering.
Like Earth's Moon,
Rhea's rotation is locked on Saturn, and the
above image shows part of
Rhea's surface that always faces Saturn.
Rhea's leading surface is more highly cratered than its trailing surface.
Rhea is composed mostly of water-ice but is thought to have a
small rocky core.
The above image was taken by the
robot Cassini spacecraft now orbiting Saturn.
Cassini swooped past Rhea two months ago and captured the
above image from about 100,000 kilometers away.
Rhea
spans 1,500 kilometers making it Saturn's second largest moon after
Titan.
Several
surface features on Rhea
remain unexplained including large light patches.
APOD: 2006 April 5 - Slightly Beneath Saturn's Ring Plane
Explanation:
When orbiting Saturn, be sure to watch for
breathtaking superpositions of
moons,
rings, and
shadows.
One such
picturesque vista was visible recently to the
robot Cassini spacecraft now orbiting Saturn.
In late February, Cassini captured
Rhea,
the second largest moon of Saturn,
while looking up from slightly beneath Saturn's expansive
ring plane.
Signature dark gaps are visible in the nearly edge-on rings.
A shadow of
Saturn's F ring
cuts across the cratered ice-moon.
Cassini is scheduled to continue sending back images from the orbit of Saturn until at least 2008.
APOD: 2005 December 13 - 620 Kilometers Above Rhea
Explanation:
What does the surface of Saturn's moon Rhea look like?
To help find out, the robot
Cassini spacecraft currently orbiting
Saturn
was directed to fly right past the second largest moon of the
gas giant planet late last month.
Pictured above
is an image taken only 620 kilometers above
Rhea's icy surface,
spanning about 90 kilometers.
The rim of an old crater crosses the middle of the image, with many smaller and younger
craters scattered throughout.
A linear depression
-- possibly a tectonic fault -- is visible toward the right, crossing the likely loose
material that composes Rhea's surface
regolith.
The origins of many features on
Rhea
are currently unexplained and being
researched.
APOD: 2005 August 18 - Sylvia, Romulus and Remus
Explanation:
Discovered in 1866,
main belt
asteroid 87 Sylvia lies
3.5 AU from the Sun, between the orbits of Mars and Jupiter.
Also shown in recent years to be one in a growing list of
double asteroids, new observations
during August and
October 2004 made at the
Paranal Observatory convincingly
demonstrate that 87 Sylvia in fact has two moonlets - the
first known triple asteroid system.
At the center of this composite of the image data, potato-shaped
87 Sylvia
itself is about 380 kilometers wide.
The
data show inner moon, Remus,
orbiting Sylvia at a distance of about 710 kilometers
once every 33 hours, while
outer moon Romulus orbits at 1360 kilometers in 87.6 hours.
Tiny Remus and Romulus are 7 and 18 kilometers across respectively.
Because 87 Sylvia was named after
Rhea
Silvia, the mythical mother
of the founders of Rome,
the discoverers proposed Romulus and
Remus as fitting names for the two moonlets.
The triple system is thought to be the not uncommon result of
collisions producing low density,
rubble
pile asteroids that are loose aggregations of debris.
APOD: 2005 May 30 - A Great White Spot on Rhea
Explanation:
What caused this great white spot on the surface of Saturn's moon Rhea?
The spot was first noticed last year by the
robot Cassini spacecraft now orbiting
Saturn.
Cassini's flyby of Rhea in April imaged in the spot in great detail.
Astronomers hypothesize that the light-colored spot is the
result of a relatively recent
impact on the surface of the icy moon.
The impact that likely created the crater also splashed
light-colored material from the interior onto the darker surface.
Rhea spans 1,500 kilometers across and is the second
largest moon of Saturn after
Titan.
Rhea sports several other
light colored surface features
that are, as yet, not well understood.
APOD: 2005 February 15 - Saturns Moon Rhea from Cassini
Explanation:
Each moon of Saturn seems to come with its own
mystery.
Rhea, Saturn's second largest moon behind
Titan, shows unusual wisps, visible above as light colored streaks.
Higher resolution images of similar wisps on Dione indicate that they
might be made of long braided fractures.
Rhea is composed mostly of
water ice,
but likely has a
small rocky core.
Rhea's rotation and orbit are locked together, just like
Earth's Moon, so that one
side always faces Saturn.
A consequence of this is that one side always leads the other.
Rhea's leading surface is much more heavily
cratered than the trailing surface, pictured above.
The above image in natural color was taken last month by the
Cassini robot spacecraft in orbit around Saturn.
APOD: 2003 June 8 - Rhea: Saturn's Second Largest Moon
Explanation:
Rhea
is the second largest moon of
Saturn,
behind Titan,
and the largest without an atmosphere.
It is composed mostly of water ice, but has a small rocky core.
Rhea's
rotation and orbit are locked together (just like
Earth's Moon)
so that one side always faces Saturn.
A consequence of this is that one side always leads the other.
Rhea's leading surface is much more heavily cratered than its trailing
surface.
The above photograph was taken with the
Voyager 1 spacecraft in 1980.
NASA's Cassini spacecraft is currently on route to Saturn and
will arrive in 2004.
APOD: December 5, 1999 - Rhea: Saturns Second Largest Moon
Explanation:
Rhea is the second largest moon of
Saturn, behind Titan, and the largest without an atmosphere.
It is composed mostly of water
ice, but has a small rocky core.
Rhea's rotation and orbit are locked together
(just like
Earth's Moon)
so that one side always faces Saturn.
A consequence of this is that one side always leads the other.
Rhea's leading surface is much more heavily
cratered than its trailing surface.
The
above photograph was taken with the
Voyager 1 spacecraft in 1980.
APOD: January 18, 1998 - Saturn, Rings, and Two Moons
Explanation:
NASA's robot spacecraft
Voyager 2 made this image of Saturn as it
began to explore the Saturn system in 1981.
Saturn's famous rings are visible along with two of
its moons,
Rhea and
Dione
which appear as faint dots on the right
and lower right part of the picture.
Astronomers believe that Saturn's moons play a
fundamental role in sculpting its elaborate ring system.
A robot spacecraft named
Cassini was
launched last October and is expected to
rendezvous with the giant gas planet in 2004.
APOD: May 12, 1996 - Tracking Saturn's Moons
Explanation:
These five pairs of
Hubble Space Telescope images track some of
Saturn's moons as they orbit the ringed planet.
A pair of images was taken every 97 minutes on November 21, 1995 with
the Wide Field Planetary Camera-2, the normally bright ring system
appearing nearly edge-on. In the top pair, the large
bright moon Dione hangs above center while the smaller moons
Pandora, Prometheus,
and Mimas (top right image)
appear near the planet's disk close to the outer ring. By the second and third
pair of images, moons Rhea
and Epimetheus have joined the dance.
During the Saturn ring plane crossings, the reduction in light from
the edge-on rings provided an opportunity for astronomer's to
explore Saturn's complex moon system and search for elusive
undiscovered satellites.
APOD: October 15, 1995 - Iapetus: Saturn's Disappearing Moon
Explanation:
Iapetus has an unusual surface, one half of which is very dark, the other
half very light. This caused it's discoverer
Cassini to remark that
Iapetus could only be seen when on one side of Saturn but not the other.
The reason for the difference between hemispheres is presently unknown.
Iapetus is the third largest of
Saturn's moon behind
Titan and
Rhea. Iapetus
is composed predominantly of water ice.
APOD: October 14, 1995 - Rhea: Saturn's Second Largest Moon
Explanation:
Rhea is the second largest moon of
Saturn, behind
Titan,
and the largest without an atmosphere. It is composed mostly of water
ice, but has a small rocky core.
Rhea's
rotation and orbit are locked together (just like Earth's Moon)
so that one side always faces Saturn. A consequence of this is that one
side always leads the other. Rhea's leading surface is much more heavily
cratered than its trailing surface. The above photograph was taken with the
Voyager 1 spacecraft in 1980.
APOD: July 6, 1995 - Saturn, Rings, and Two Moons
Explanation:
This image of Saturn was made by NASA's robot spacecraft
Voyager
2 as it began to explore the Saturn system in 1981.
Saturn's famous rings are visible along with two of its moons, Rhea and
Dione which appear as faint dots in the right and lower right
part of the picture.
Astronomers believe that Saturn's moons play a fundamental
role in sculpting its elaborate ring system.