juliensimon/nasa-apod huggingface dataset:
In A Grand Canyon On Mars (1998-02-12)

In a grand canyon on Mars, steep slopes fall away from a smooth plateau revealing striking layered rock formations. The canyon is part of the Valles Marineris, a 2,500 mile long system of canyons cutting across the Martian equator. This view, recorded on January 1 by the orbiting Mars Global Surveyor shows a small portion of Valles Marineris in amazing detail. The image is about 6 miles wide and high resolution versions show features as small as 20 feet across. What processes caused the layering? In the Grand Canyon on planet Earth, sedimentary processes have resulted in spectacular rock layers. But similar layers of rock in canyons of the Hawaiian Islands were created by volcanoes. Regardless of the origin of layering on Mars, its extent suggests that early Mars was geologically active and complex.
Explorer I (1998-02-13)

Inaugurating the era of space exploration for the US, the First Explorer was launched into Earth orbit forty years ago (February 1, 1958) by the Army Ballistic Missle Agency. The Explorer I satellite weighed about 30 pounds, was 6 feet long, 6 inches in diameter and consisted of batteries, transmitters, and scientific instrumentation built into the fourth stage of a Jupiter-C rocket. Foreshadowing NASA and the adventurous and successful Explorer Program, Explorer I bolstered national prestige in the wake of Sputnik. The satellite also contributed to a spectacular scientific bonanza - the discovery of Earth-girdling belts of magnetically trapped charged particles now known as the Van Allen Radiation Belts.
The Rosette Nebula (1998-02-14)

Would the Rosette Nebula by any other name look as sweet? The bland New General Catalog designation of NGC 2237 doesn't appear to diminish the appearance of the this flowery emission nebula. Inside the nebula lies an open cluster of bright young stars designated NGC 2244. These stars recently formed from the nebular material and their stellar "wind" has cleared a hole in the nebula's center, insulated by a layer of dust and hot gas. Ultraviolet light from the hot cluster stars causes the surrounding nebula to glow.

Image credit: G. Greaney
Stars Without Galaxies (1998-02-15)

Galaxies are made up of stars, but are all stars found within galaxies? Using the Hubble Space Telescope, researchers exploring the Virgo Cluster of galaxies have now found about 600 red giant stars adrift in intergalactic space. Above is an artist's vision of the sky from a hypothetical planet of such a lonely sun. The night sky on a world orbiting an intergalactic star would be a stark contrast to Earth's - which features a spectacle of stars, all members of our own Milky Way galaxy. As suggested by the illustration, a setting swollen red sun would leave behind a dark sky flecked only with faint, fuzzy, apparitions of Virgo Cluster galaxies. Possibly ejected from their home galaxies during galaxy-galaxy collisions, these isolated suns may well represent part of a large, previously unseen stellar population, filling the space between Virgo Cluster galaxies.
Sagittarius Dwarf to Collide with Milky Way (1998-02-16)

Our Galaxy is being invaded. Recent observations indicate that in the next 100 million years, the Sagittarius Dwarf galaxy will move though the disk of our own Milky Way Galaxy yet again . The Sagittarius Dwarf (Sgr), shown as the extended irregular shape below the Galactic Center,  is the closest of 9 known small dwarf spheroidal galaxies that orbit our Galaxy.  Don't worry, our Galaxy is not in danger, but no such assurances are issued for the Sagittarius Dwarf: the intense gravitational tidal forces might pull it apart. Oddly, however, Sgr's orbit indicates that is has been through our Galaxy several times before, and survived!  One possibility is that Sgr contains a great deal of low-density dark matter that hold it together gravitationally during these collisions.
Shocked by Supernova 1987a (1998-02-17)

even years ago the brightest supernova of modern times was recorded. Now the expanding debris from this tremendous stellar explosion is seen to be crashing into previously expelled material.  The onset of this collision is shown by the arrow in the above picture as the yellow spot on the interior of the ring.  Although the collision is occurring at speeds near 60 million km/hour, it will appear to take years due to the vast distances involved. As the supernova blast wave moves out, it shock-heats any gas it encounters, causing it to glow.  Astronomers are thus hopeful that the blast wave will illuminate the interesting past of SN 1987a, and perhaps provide more clues about the origins of the mysterious rings.
M27: The Dumbbell Nebula (1998-02-18)

The first hint of what will become of our Sun was discovered inadvertently in 1764.  At that time, Charles Messier was compiling a list of "annoying" diffuse objects not to be confused with "interesting" comets. The 27th object on Messier's list, now known as M27 or the Dumbbell Nebula, is a planetary nebula, the type of nebula our Sun will produce when nuclear fusion stops in its core. M27 is one of the brightest planetary nebulae on the sky, and can be seen in the constellation Vulpecula with binoculars.  It takes light about 1000 years to reach us from M27.  Understanding the physics and significance of M27 was well beyond 18th century science.  Even today, many things remain mysterious about bipolar planetary nebula like M27, including the physical mechanism that expels a low-mass star's gaseous outer-envelope, leaving an X-ray hot white dwarf.
Miranda (1998-02-19)

Miranda is a bizarre world which surely had a tempestuous past. The innermost of the larger Uranian moons, Miranda is almost 300 miles in diameter and was discovered only 50 years ago (February 16, 1948) by the renown American planetary astronomer Gerard Kuiper. Examined very closely by the Voyager 2 spacecraft in 1986, this dark and distant world turned out to be quite a surprise. Miranda was found to display a unique, bewildering variety of terrain leading some to suggest that it has been fractured up to 5 times during its evolution. Along with the famous "chevron" feature, the bright V-shaped area just above center, this composite of the highest resolution images of Miranda shows wild juxtapositions of ridges and valleys, older cratered and younger smooth surfaces, and shadowy canyons perhaps 12 miles deep.  The large crater (below center) is the 15 mile wide crater Alonso.
Hale-Bopp: A Continuing Tail (1998-02-20)

Where is Hale-Bopp now? The Great Comet of 1997, one of the largest and most active comets ever, is outbound about 400 million miles from the sun. Too faint for viewing without telescopes or binoculars, Hale-Bopp is presently positioned in the very southerly constellation of Pictor. This "negative" image (black stars against white sky) of Hale-Bopp is the result of a 1 hour time exposure using the 1-metre European Southern Observatory Schmidt telescope on January 5. Clearly the comet still has a substantial tail, blown by the solar wind, that points generally away from the sunward direction. But look closely. A spiky "anti-tail" is also visible pointing toward the sun! It is likely that this anti-tail is composed of dust grains released from the comet nucleus which are too large to be easily pushed by the solar wind. As Hale-Bopp recedes from the sun its activity will subside but astronomers are still uncertain as to how long its tails will last. After a swing through the outer solar system and the Oort cloud, Hale-Bopp will pass through the inner solar system again ... around the year 5400.
Neptune: Big Blue Giant (1998-02-21)

This picture was taken by the Voyager 2 spacecraft in 1989 - the only spacecraft ever to visit Neptune. Neptune will be the farthest planet from the Sun until 1999, when the elliptical orbit of Pluto will cause it to once again resume this status. Neptune, like Uranus, is composed mostly of liquid water, methane and ammonia, is surrounded by a thick gas atmosphere of mostly hydrogen and helium, and has many moons and rings. Neptune's moon Triton is unlike any other and has active volcanoes. The nature of Triton's unusual orbit around Neptune is the focus of much discussion and speculation.
Southern Lights and Shuttle Glow (1998-02-22)

A background of distant stars, sinuous and spiky bands of Southern Lights (Aurora Australis), and the faint glow of charged plasma (ionized atomic gas) surrounding the Space Shuttle Discovery's engines give this photo from the STS-39 mission an eerie, otherworldly look. This image reflects Discovery's April 1991 mission well - its payload bay (PLB) was filled with instruments designed to study celestial objects, aurora and atmospheric phenomena, and the low Earth orbit environment around the PLB itself. The aurora seen here are at a height of about 50-80 miles. Aurora are caused by charged particles in the solar wind, channeled through the Earth's magnetic field which excite molecules in the upper atmosphere.
M104: The Sombrero Galaxy (1998-02-23)

What's going on in the center of this spiral galaxy? Named the Sombrero Galaxy for its hat-like resemblance, M104 features a prominent dust lane and a bright halo of stars and globular clusters.  Something truly energetic is going on in the Sombrero's center, as it not only appears bright in visible light, but glows prodigiously in X-ray light as well. This X-ray emission coupled with unusually high central stellar speeds cause many astronomers to speculate that a black hole lies at the Sombrero's center - a black hole possibly a billion times the mass of our Sun.
The Lyman Alpha Forest (1998-02-24)

We live in a forest.  Strewn throughout the universe are "trees" of hydrogen gas that absorb light from distant objects. These gas clouds leave numerous absorption lines in a distant quasar's spectra, together called the Lyman-alpha forest. Distant quasars appear to be absorbed by many more Lyman-alpha clouds than nearby quasars, indicating a Lyman-alpha thicket early in our universe. The above image depicts one possible computer realization of how Lyman-alpha clouds were distributed at a redshift of 3. Each side of the box measures 30 million light-years across. Much remains unknown about the Lyman-alpha forest, including the real geometry and extent of the clouds, and why there are so many fewer clouds today.

Image credit: J. Shalf, Y. Zhang 
(UIUC) et al., 
GCCC
The Solar Neighborhood (1998-02-25)

You are here. The orange dot in the above false-color drawing represents the current location of the Sun among local gas clouds in the spiral Milky Way Galaxy. These gas clouds are so thin that we usually see right through them. Nearly spherical bubbles surround regions of recent star formation. The purple filaments near the Sun are gas shells resulting from star formation 4 million years ago in the Scorpius-Centaurus Association, located to the Sun's lower left. The Sun has been between spiral arms moving through relatively low density gas for the past 5 million years. In contrast, the Sun oscillates in the Milky Way plane every 66 million years, and circles the Galactic Center every 250 million years.

Image credit: P. C. Frisch
(U. Chicago), with thanks to C. Wellman
A Southern Sky View (1998-02-26)

From horizon to horizon, the night sky above Loomberah, New South Wales, Australia was photographed by astronomer Gordon Garradd on March 22, 1996. Garradd used a home made all-sky camera with a fish-eye lens, resulting in a circular 200 degree field of view. This gorgeous sky view is dominated by the luminous band of the Milky Way cut by dramatic, dark interstellar dust clouds. Along with the bright stars of our Galaxy, the Large Magellanic Cloud is visible at the upper right (about 1 o'clock) and the long, lovely, bluish tail of comet Hyakutake can be seen toward the bottom of the image, near the bright star Arcturus. Bright city lights from nearby Tamworth glow along the Northwestern horizon.

Image credit: G. Garradd
Solar Eclipse: A Composite View (1998-02-27)

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.
Eagle EGGs in M16 (1998-02-28)

Star forming regions known as "EGGs" are uncovered at the end of this giant pillar of gas and dust in the Eagle Nebula (M16).  EGGs, short for evaporating gaseous globules, are dense regions of mostly molecular hydrogen gas that fragment and gravitationally collapse to form stars. Light from the hottest and brightest of these new stars heats the end of the pillar and causes further evaporation of gas - revealing yet more EGGs and more young stars.  This picture was taken by the Wide Field and Planetary Camera on board the Hubble Space Telescope.
A Sky Full Of Hydrogen (1998-03-01)

Interstellar space is filled with extremely tenuous clouds of gas which are mostly Hydrogen. The neutral hydrogen atom (HI in astronomer's shorthand) consists of 1 proton and 1 electron. The proton and electron spin like tops but can have only two orientations; spin axes parallel or anti-parallel. It is a rare event for Hydrogen atoms in the interstellar medium to switch from the parallel to the anti-parallel configuration, but when they do they emit radio waves with a wavelength of 21 centimeters (about 8 inches) and a corresponding frequency of exactly 1420 MHz. Tuned to this frequency radio telescopes have mapped the neutral Hydrogen in the sky. The above image represents such an all-sky HI survey with the plane of our Milky Way Galaxy running horizontally through the center. In this false color image no stars are visible, just diffuse clouds of gas tens to hundreds of light years across which cluster near the plane. The gas clouds seem to form arching, looping structures, stirred up by stellar activity in the galactic disk.
Rumors of a Strange Universe (1998-03-02)

In a meeting in California two weeks ago, unpublished results were presented indicating that most of the energy in our universe is not in stars or galaxies but is tied to space itself.  In the language of cosmologists, a large cosmological constant is directly implied by new distant supernovae observations. Suggestions of a cosmological constant (lambda) are not new -- they have existed since the advent of modern relativistic cosmology. Such claims are not usually popular with astronomers, though, because lambda is so unlike known universe components, because lambda's value appears limited by observations, and because less-strange cosmologies without lambda have historically done well in explaining the data.  Therefore most lamdba claims do not make the News Summary in a prestigious journal like Science.  What is noteworthy here is the seemingly direct and reliable method of the observations, and the good reputations of the scientists conducting the investigation. The   above picture of a supernova at redshift 0.5 in a spiral galaxy, was taken by this collaboration. However, two teams of scientists are independently studying distant supernovae, and the last official word from the other team was a result consistent with no cosmological constant, reported just two months ago.  Extraordinary claims require extraordinary evidence, and so cosmologists the world over eagerly await peer-reviewed results, further details, and more data.
560 Kilometers Above Europa (1998-03-03)

Tomorrow's picture: Aurora Over Alaska  < 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 &: Michigan Tech. U.
Aurora Over Alaska (1998-03-04)

Higher than the highest mountain, higher than the highest airplane, lies the realm of the aurora. Aurora rarely reach below 60 kilometers, and can range up to 1000 kilometers.  Aurora light results from solar electrons and protons striking molecules in the Earth's atmosphere. Frequently, when viewed from space, a complete aurora will appear as a circle around one of the Earth's magnetic poles. The above photograph was taken in January in Alaska and shows a spectacular aurora borealis above a frozen landscape which includes spruce trees and the photographer's truck.  The picture had to be taken quickly as the temperature was below -40 degrees.

Image credit: D. Hutchinson
Canaries Sky (1998-03-05)

This gorgeous view of stars, nebulae, and the Milky Way comes from the dark night sky above the lovely island of La Palma in the Canaries archipelago. The picture was made by a group of experienced astrophotographers who traveled there to take advantage of the ideal observing conditions near La Palma's Observatorio del Roque de los Muchachos. Skygazers can easily pick out several of their favorite astronomical objects in this wide angle time exposure which covers about 40 degrees on winter the sky. Faint stars along the plane of our Galaxy compose the delicate, luminous band of the Milky Way stretching across the image from the bottom left. The familiar constellation of Orion the hunter is also easy to find, with glowing nebulae highlighting the hunter's belt and sword. Orion's famous red giant star Betelgeuse, near picture center, has a yellowish cast and Rigel is the bright star in Orion at lower right. Brilliant white Sirius, near the bottom, is the brightest star in the picture (and in Earth's night sky). Sirius, is part of the constellation Canis Major (Big Dog). Across the Milky Way, above and to the left of Sirius, is slightly less brilliant Procyon, brightest star of Canis Minor. A V-shaped group of yellowish stars at the upper right, part of Taurus the bull, is dominated by the red giant Aldebaran.

Image credit: A. Vannini,
G. Li Causi,
A. Ricciardi,
A. Garatti
Water Ice At The Lunar Poles (1998-03-06)

After seven weeks in lunar orbit, instruments on board NASA's Lunar Prospector spacecraft have produced strong evidence for water ice on the Moon. While not in extensive sheets, this ice could be in the form of crystals, mixed in low concentrations with material in craters around the frigid North and South lunar poles. How much ice? So far, the estimates are from 10 to 300 million metric tons (about 3 to 90 billion gallons). The water ice is likely a result of meteoritic and cometary bombardment. Using very different instruments, the Star-Wars-technology Clementine spacecraft also detected telltale signs of ice as it orbited the Moon in 1994. Although the lunar samples returned by the Apollo missions suggested that the Moon was bone dry, the Clementine result sparked the controversy and hope that water ice might still be found - holding out the tantalizing possibility that a substantial reservoir of lunar water could be used to support human exploration. Above is a mosaic of Clementine images of the cratered terrain surrounding the Moon's North pole. Surprisingly, Lunar Prospector's data indicates that the North polar region contains about twice as much ice as the South.  As Lunar Prospector's mission continues, further data will help determine more precisely the amount and distribution of the water ice.
NGC 1818: A Young Globular Cluster (1998-03-07)

Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are perhaps 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. But things are different next door - in the neighboring LMC galaxy. Pictured above is a "young" globular cluster residing there: NGC 1818. Recent observations show it formed only about 40 million years ago - just yesterday compared to the 12 billion year ages of globular clusters in our own Milky Way
Shuttle Engine Blast (1998-03-08)

The Space Shuttle Discovery's orbital maneuvering system (OMS) engine firing produced this dramatic flare as it cruised "upside down" in low Earth orbit.  Discovery was named for a ship commanded by Captain James Cook RN, the 18th Century English astronomer and navigator. Cook's voyages of discovery established new standards in scientific exploration and brought extensive knowledge of the unknown Pacific regions, including Australia, New Zealand, and the Hawaiian Island archipelago to Europeans. The Space Shuttle Endeavor, also named after one of Cook's ships, is the most recent addition to NASA's four-orbiter shuttle fleet.
Yogi Rock on Mars (1998-03-09)

Yogi is possibly the best photographed rock on Mars. By combining many pictures taken during the Mars Pathfinder Mission last year, scientists were able to create a super-resolution, digitally enhanced image that better allows them to study Yogi's surface and more accurately determine how Yogi was formed.  The smoothness of some Martian rocks suggests previous interactions with water, implying that Mars was both warmer and wetter in the past.
Cracks and Ridges on Europa (1998-03-10)

Which way to the interstate?  What appears to be a caricature of a complex highway system on Earth is actually a system of ridges and cracks on the icy surface of Jupiter's moon Europa. The distance between parallel ridges in the above photograph is typically about 1 kilometer.  The complexity of the cracks and ridges tell a story of Europa's past that is mostly undecipherable -- planetary geologists try to understand just the general origin of the overall features.  One noteworthy feature is the overall white sheen, possibly indicating the presence of frost.  Another is the dark centers between parallel ridges, which might indicate that dirty water from an underground ocean recently welled up in the cracks and froze.
A Total Eclipse of the Sun (1998-03-11)

On February 26th, it was dark during the day. This total solar eclipse was the last visible from the Americas for this millennium. A total solar eclipse is exciting partly because it is so short. Were Earth's Moon farther away, no total eclipse would occur at all because the Moon would appear too small to block out all of the Sun.  Were the Moon much closer to the Earth, the eclipse would last much longer.  Oddly, the Sun and Moon have almost exactly the same angular size. Even for well located observers, this creates at most only a very few minutes of totality before the Moon's shadow moves away.  During a total solar eclipse in 1919, these few minutes were long enough for scientists to determine that the Sun's gravity bends light from the stars behind it, dramatically confirming the accuracy of a new theory of gravity postulated over 80 years ago.

Image credit: J. Sanford