juliensimon/nasa-apod huggingface dataset:
Candor and Ophir Chasmata (2003-02-18)

First imaged by the Mariner 9 spacecraft, Valles Marineris, the grand canyon of Mars, is a system of enormous depressions called chasmata that stretch some 4,000 kilometers along the Martian equator. Looking north over the canyon's central regions, Candor chasma lies in the foreground of this spectacular view with the steep walls of Ophir chasma near the top. Surface collapse and landslides are seen to be part of the complex geologic history of these dramatic features but recent high resolution images have also revealed layered deposits within the canyon system. This picture represents a mosaic of images recorded in 1978 from Martian orbit by the Viking 1 and 2 spacecraft. The full width of the picture covers about 800 kilometers.
Pauli Exclusion Principle: Why You Don't Implode (2003-02-19)

Why doesn't matter just bunch up?  The same principle that keeps neutron stars and white dwarf stars from imploding also keeps people from imploding and makes normal matter mostly empty space.  The observed reason is known as the Pauli Exclusion Principle.  The principle states that identical fermions -- one type of fundamental matter -- cannot be in the same place at the same time and with the same orientation.  The other type of matter, bosons, do not have this property, as demonstrated clearly by recently created Bose-Einstein condensates.  Recently, the Pauli Exclusion Principle was demonstrated graphically in the above picture of clouds of two isotopes of lithium -- the left cloud composed of bosons while the right cloud is composed of fermions.  As temperature drops, the bosons bunch together, while the fermions better keep their distance.  The reason why the Pauli Exclusion Principle is true and the physical limits of the principle are still unknown.

Image credit: Andrew Truscott & 
Randall Hulet 
(Rice U.)
Cold Wind from the  Boomerang Nebula (2003-02-20)

A cold wind blows from the central star of the Boomerang Nebula. Seen here in a detailed false-color image recorded in 1998 by the Hubble Space Telescope, the nebula lies about 5,000 light-years away towards the grand southern constellation of Centaurus. The symmetric cloud appears to have been created by a high-speed wind of gas and dust blowing from an aging central star at speeds of nearly 600,000 kilometers per hour. This rapid expansion has cooled molecules in the nebular gas to about one degree above absolute zero - colder than even the cosmic background radiation - making it the coldest region observed in the distant Universe. Shining with light from the central star reflected by dust, the frigid Boomerang Nebula is believed to be a star or stellar system evolving toward the planetary nebula phase.
Melting Snow and the Gullies of Mars (2003-02-21)

Tantalizing images of gullies on Mars have offered striking evidence for recent flows of liquid water. But Mars is too cold and its atmosphere too thin for liquid water to exist on the surface. Still a new and compelling explanation for gullies carved by liquid water was inspired by this recently released image from the Mars Odyssey spacecraft. Pictured is a section of what is likely a snow covered crater in the Martian southern hemisphere. North is at the top and the scene, illuminated from the left, is about 16 kilometers wide. Patches of smooth snow pack remain along the northern crater wall, while structures resembling the famous Martian gullies appear to be emerging as the snow cover gradually disappears, and are exposed along the crater's western (left) wall. Melting snow, running underneath the snow pack and down the crater walls would be protected from the extreme surface conditions, remaining liquid and eroding the gullies over time. Could life exist in a liquid water environment beneath the Martian snow?
Infrared Saturn (2003-02-22)

This delightfully detailed false-color image of Saturn was taken in January 1998 by the orbiting Hubble Space Telescope. The picture is a combination of three images from Hubble's NICMOS instrument and shows the lovely ringed planet in reflected infrared sunlight. Different colors indicated varying heights and compositions of cloud layers generally thought to consist of ammonia ice crystals. The eye-catching rings cast a shadow on Saturn's upper hemisphere. The bright stripe seen within the left portion of the shadow is infrared sunlight streaming through the large gap in the rings known as the Cassini Division. Two of Saturn's many moons have also put in an appearance, Tethys just beyond the planet's disk at the upper right, and Dione at the lower left. Presently, Saturn shines brightly in evening skies as a pale yellow "star" near the constellation Orion.
A Twisted Solar Eruptive Prominence (2003-02-23)

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.
Comet Neat Passes an Erupting Sun (2003-02-24)

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.
M42: Wisps of the Orion Nebula (2003-02-25)

The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulas.  Here, glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away.  In the above deep image, faint wisps and sheets of dust and gas are particularly evident.  The Great Nebula in Orion can be found with the unaided eye just below and to the left of the easily identifiable belt of three stars in the popular constellation Orion.  In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries.   These nurseries contain hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds.  Also known as M42, the Orion Nebula spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

Image credit: John P. Gleason 
(Celestial Images)
Anticrepuscular Rays Over Horseshoe Canyon (2003-02-26)

What's happening over the horizon?  Although the scene may appear somehow supernatural, nothing more unusual is occurring than a setting Sun and some well placed clouds.  Pictured above are anticrepuscular rays.  To understand them, start by picturing common crepuscular rays that are seen any time that sunlight pours though scattered clouds.  Now although sunlight indeed travels along  straight lines, the projections of these lines onto the spherical sky are great circles.  Therefore, the crepuscular rays from a setting (or rising) sun will appear to re-converge on the other side of the sky.  At the anti-solar point 180 degrees around from the Sun, they are referred to as anticrepuscular rays. While enjoying the sunset after dinner near Horseshoe Canyon in Utah, the photographer chanced to find that an even more spectacular sight was occurring in the other direction just over the canyon -- a particularly vivid set of anticrepuscular rays.

Image credit: Peggy Peterson
When Moons and Shadows Dance (2003-02-27)

It's no wonder Jupiter is a favorite target for even modest earthbound telescopes. The most massive planet in the solar system with four of the largest moons also boasts the famous Great Red Spot, a giant hurricane-like storm system over three hundred years old. Recorded on December 15, 2002 between 7:19 and 8:40 UT, over a thousand digital images were processed and stacked to create this spectacular 21 frame animation of the Jovian system. South is up and as the Great Red Spot tracks across the face of Jupiter, innermost Galilean moon Io enters the scene at the far right. Io occults (passes in front of) the edge of the more sedately orbiting Ganymede with Io's shadow moving quickly across the gas giant's cloud tops, just below the Red Spot. While the moon Callisto is outside the field of view, its large, dark shadow is also visible crossing the Jovian disk at the upper left. Viewed from Earth, the orbits of the Galilean moons presently lie nearly edge-on, offering many chances to observe similar dances of Jupiter's moons.

Image credit: Wes Higgins
Fox Fur, the Unicorn, and a Christmas Tree (2003-02-28)

Glowing hydrogen gas fills this gorgeously detailed sky view centered on the variable star S Mon in the faint but fanciful constellation Monoceros, the Unicorn. A star forming region (NGC 2264), the complex jumble of cosmic gas and dust is about 2,700 light-years distant and mixes reddish emission nebulae excited by energetic light from newborn stars with dark interstellar dust clouds. Where the otherwise obscuring dust clouds lie close to stars they also reflect starlight, forming blue reflection nebulae. The wide vista spans about 1.5 degrees or nearly 3 full moons, covering 70 light-years at the distance of NGC 2264. Its cast of cosmic characters includes the Cone Nebula (far left), the Fox Fur Nebula, whose convoluted pelt lies just below S Mon, and the Christmas Tree star cluster. The triangular Christmas Tree cluster appears sideways here, with its apex at the Cone Nebula and its broader base centered on S Mon.

Image credit: Russell Croman
Stereo Eros (2003-03-01)

Get out your red/blue glasses and float next to asteroid 433 Eros, 170 million kilometers away! Orbiting the Sun once every 1.8 earth-years, asteroid Eros is a diminutive 40 x 14 x 14 kilometer world of undulating horizons, craters, boulders and valleys. Its unsettling scale and bizarre shape are emphasized in this picture - a mosaic of images from the NEAR Shoemaker spacecraft processed to yield a stereo anaglyphic view.  Along with dramatic chiaroscuro, NEAR's 3-D imaging provided important measurements of the asteroid's landforms and structures, and clues to the origin of this city-sized chunk of solar system. The smallest features visible here are about 30 meters across. After spending a year in orbit around Eros, the historic Near Shoemaker spacecraft made the first ever landing on an asteroid's surface February 12, 2001.
In the Center of the Trapezium (2003-03-02)

Start with the constellation of Orion.  Near Orion's belt is a fuzzy area known as the Great Nebula of Orion or M42.  In this nebula is a bright star cluster known as the Trapezium, shown above.  New stellar systems are forming there in gigantic globs of gas and dust known as Proplyds.  Looking closely at the above picture also reveals that gas and dust surrounding some of the dimmer stars appears to form structures that point away from the brighter stars.  The above false color image was made by combining several exposures from the orbiting Hubble Space Telescope.
Will the Universe End in a Big Rip? (2003-03-03)

How will our universe end?  Recent speculation now includes a pervasive growing field of mysterious repulsive energy that rips virtually everything apart.  Although the universe started with a Big Bang, analysis of recent cosmological measurements allows a possibility that it will end with a Big Rip.  As soon as few billion years from now, the controversial scenario holds, dark energy will grow to such a magnitude that our own Galaxy will no longer be able to hold itself together.  After that, stars, planets, and then even atoms might not be able to withstand the expansive internal force.  Previously, speculation on the ultimate fate of the universe centered on either a re-collapsing Big Crunch or a Big Chill.  Although the universe's fate is still a puzzle, piecing it together will likely follow from an increased understanding of the nature of dark matter and dark energy.

Image credit: Lynette Cook
In the Center of the Lagoon Nebula (2003-03-04)

The center of the Lagoon Nebula is busy with the awesome spectacle of star formation.  Visible in the lower left, at least two long funnel-shaped clouds, each roughly half a light-year long, have been formed by extreme stellar winds and intense energetic starlight.  The tremendously bright nearby star, Hershel 36, lights the area.  Vast walls of dust hide and redden other hot young stars.  As energy from these stars pours into the cool dust and gas, large temperature differences in adjoining regions can be created generating shearing winds which may cause the funnels.  This picture, spanning about 5 light years, was taken in 1995 by the orbiting Hubble Space Telescope.  The Lagoon Nebula, also known as M8, lies about 5000 light years distant toward the constellation of Sagittarius.
Where People Live on Planet Earth (2003-03-05)

Where do people live on Planet Earth?  Although people inhabit every continent, the highest population densities occur in Asia.   Sparsely inhabited regions occur on virtually every continent, however, including the Sahara Desert in Africa, the Great White North of North America, the outback of Australia, the Amazon of South America, and the Himalayan Mountains of Asia.  The above color-coded map was produced from populations estimates made for 1994, when the world population was about 5.5 billion.  Current estimates place the world population at about 6.3 billion.
Comet NEAT in Southern Skies (2003-03-06)

After last month's dramatic swoop past the Sun, Comet NEAT (C/2002 V1) appeared as a naked-eye comet, emerging from the evening twilight in planet Earth's southern skies. On March 1st, New Zealand photographer Noel Munford captured this telephoto view of the outbound comet close to the southwestern horizon against the faint stars of the constellation Sculptor. He reports that the picture is a good representation of the comet's visual appearance on that date and estimates the impressive tail to be five or six degrees long. Discovered last November as part of the Near-Earth Asteroid Tracking program, there was some speculation that this comet would not survive its close encounter with the Sun. However, Comet NEAT is now returning to the outer solar system, diving southward and fading fast.

Image credit: Noel Munford
(Palmerston North Astronomical Society, New Zealand)
The Star Trails of Kilimanjaro (2003-03-07)

The night had no moon, but the stars were out. And camped at 16,000 feet on Mt. Kilimanjaro, photographer Dan Heller recorded this marvelous 3 1/2 hour long exposure. Here the landscape is lit mostly by the stars. Flashlights give the tents an erie internal radiance while the greenish glow from the distant city lights of Moshi, Tanzania filter through the clouds below. The view from this famous equatorial African mountain is toward the south, putting the South Celestial Pole close to the horizon on the far left, near the center of the graceful concentric star trail arcs. In the thin air and clear dark skies, even the ghostly Milky Way left a faint triangular glow as it swept across the middle of the dreamlike scene.

Image credit: Dan Heller
Solar Sail (2003-03-08)

Nearly 400 years ago astronomer Johannes Kepler observed comet tails blown by a solar breeze and suggested that vessels might likewise navigate through space using appropriately fashioned sails. It is now widely recognized that sunlight does indeed produce a force which moves comet tails and a large, reflective sail could be a practical means of propelling a spacecraft. In fact, the illustration above represents one concept explored by NASA centers to develop an interstellar probe pushed along by sunlight reflected from an ultrathin sail. Nearly half a kilometer wide, the delicate solar sail would be unfurled in space. Continuous pressure from sunlight would ultimately accelerate the craft to speeds about five times higher than possible with conventional rockets -- without requiring any fuel! If launched in 2010 such a probe could overtake Voyager 1, the most distant spacecraft bound for interstellar space, in 2018 going as far in eight years as the Voyager will have journeyed in 41 years.
Farewell Jupiter (2003-03-09)

Tomorrow's picture: Big Pinwheel  < | Archive | Index | Search | Calendar | Glossary | Education | About APOD | >  Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Technical Rep.: Jay Norris. Specific rights apply. A service of: LHEA at NASA / GSFC & NASA SEU Edu. Forum & Michigan Tech. U.
M101: The Pinwheel Galaxy (2003-03-10)

Why do many galaxies appear as spirals?  A striking example is M101, shown above, whose relatively close distance of about 22 million light years allow it to be studied in some detail.  Recent evidence indicates that a close gravitational interaction with a neighboring galaxy created waves of high mass and condensed gas which continue to orbit the galaxy center.  These waves compress existing gas and cause star formation.  One result is that M101, also called the Pinwheel Galaxy, has several extremely bright star-forming regions (called HII regions) spread across its spiral arms.  M101 is so large that its immense gravity distorts smaller nearby galaxies.

Image credit: Jean-Charles Cuillandre
Iridescent Clouds (2003-03-11)

Why would clouds appear to be different colors?  A relatively rare phenomenon known as iridescent clouds can show unusual colors vividly or a whole spectrum of colors simultaneously.  These clouds are formed of small water droplets of nearly uniform size.  When the Sun is in the right position and mostly hidden by thick clouds, these thinner clouds significantly diffract sunlight in a nearly coherent manner, with different colors being deflected by different amounts.  Therefore, different colors will come to the observer from slightly different directions.  Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too far from the Sun to exhibit striking colors. Pictured above, iridescent clouds were photographed over Norway two months ago.

Image credit: Arne Danielsen
Lunar Farside from Apollo 11 (2003-03-12)

The far side of the Moon is rough and filled with craters.  By comparison, the near side of the Moon, the side we always see, is relatively smooth.  Since the Moon is rotation locked to always point the same side toward Earth, humanity has only glimpsed the lunar farside recently -- last century.  The light highlands of the far side are older than the dark Maria of the near side.  A thinner crust on the near side that allowed for more dark lava flows is thought to be the cause of differences between the two sides.  The cause for the crust thickness differences is still being researched, however.  The large impact basin pictured above is Crater 308.  It spans about 30 kilometers and was photographed by crew of Apollo 11 as they circled the Moon in 1969.
WIRO at Jupiter (2003-03-13)

Gazing out over the mountaintops from the Wyoming InfraRed Observatory (WIRO), astronomers recently recorded this bizarre looking image of the solar system's ruling planet, gas giant Jupiter. The false-color picture is a composite of images taken to test a sophisticated digital camera operating at liquid helium temperatures and sensitive to wavelengths about three times longer than visible red light. At those infrared wavelengths (near 2.1 microns) the molecular hydrogen and methane gas in Jupiter's dense lower atmosphere strongly absorb sunlight, so the normally bright, banded planet looks very dark. But particles and haze over the equator and poles rise above the absorbing layers into Jupiter's stratosphere and reflect the infrared sunlight. Also clearly extending into the Jovian stratosphere is the famous Great Red Spot seen here in yellow just under the equatorial band at the right. North is up in this view and Jupiter's rapid 10 hour rotation will soon carry the Great Red Spot behind the planet's right limb.
DEM L71: When Small Stars Explode (2003-03-14)

Large, massive stars end their furious lives in spectacular supernova explosions -- but small, low mass stars may encounter a similar fate. In fact, instead of simply cooling off and quietly fading away, some white dwarf stars in binary star systems are thought to draw enough mass from their companions to become unstable, triggering a nuclear detonation. The resulting standard candle stellar explosion is classified as a Type Ia supernova and perhaps the best example yet of the aftermath is this expanding cloud of shocked stellar debris, DEM L71, in the nearby Large Magellanic Cloud. The sharp false-color x-ray image from the orbiting Chandra Observatory shows the predicted bright edges of the outer blast wave shock region and the x-ray glow of an inner region of reverse shock heated gas. Based on the Chandra data, estimates for the composition and total mass of expanding gas strongly suggest that this is all that remains of a white dwarf star. Light from this small star's self-destructive explosion would have first reached Earth several thousand years ago.
Apollo 12: Self-Portrait (2003-03-15)

Is it art? In November of 1969, Apollo 12 astronaut-photographer Charles "Pete" Conrad recorded this masterpiece while documenting colleague Alan Bean's lunar soil collection activities on the Oceanus Procellarum. The image is dramatic and stark. Bean is faceless. The harsh environment of the Moon's Ocean of Storms is echoed in his helmet's perfectly composed reflection of Conrad and the lunar horizon. Works of photojournalists originally intent on recording the human condition on planet Earth, such as Lewis W. Hine's images from New York City in the early 20th century, or Margaret Bourke-White's magazine photography are widely regarded as art. Similarly many documentary astronomy and space images can be appreciated for their artistic and esthetic appeal.
NGC 253: The Sculptor Galaxy (2003-03-16)

NGC 253 is not only one of the brightest spiral galaxies visible, it is also one of the dustiest.  Discovered in 1783 by Caroline Herschel in the constellation of Sculptor, NGC 253 lies only about ten million light-years distant.  NGC 253 is the largest member of the Sculptor Group of Galaxies, the nearest group to our own  Local Group of Galaxies.  The dense dark dust accompanies a high star formation rate, giving NGC 253 the designation of starburst galaxy.  Visible in the above photograph from the Hubble Space Telescope is the active central nucleus, also known to be a bright source of X-rays and gamma rays.