juliensimon/nasa-apod huggingface dataset:
A Mystery In Gamma Rays (2001-08-11)

Gamma rays are the most energetic form of light, packing a million or more times the energy of visible light photons. If you could see gamma rays, the familiar skyscape of steady stars would be replaced by some of the most bizarre objects known to modern astrophysics -- and some which are unknown. When the EGRET instrument on the orbiting Compton Gamma-ray Observatory surveyed the sky in the 1990s, it cataloged 271 celestial sources of high-energy gamma-rays. Researchers identified some with exotic black holes, neutron stars, and distant flaring galaxies. But 170 of the cataloged sources, shown in the above all-sky map, remain unidentified. Many sources in this gamma-ray mystery map likely belong to already known classes of gamma-ray emitters and are simply obscured or too faint to be otherwise positively identified. However, astronomers have called attention to the ribbon of sources winding through the plane of the galaxy, projected here along the middle of the map, which may represent a large unknown class of galactic gamma-ray emitters. In any event, the unidentified sources could remain a mystery until the planned launch of the more sensitive Gamma-ray Large Area Space Telescope in 2005.
Eagle EGGs in M16 (2001-08-12)

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 Piece of Interplanetary Dust (2001-08-13)

The dust that pervades our Solar System is not the dust that pervades our homes.  Solar System dust comes from comets and asteroids, whereas house dust is most likely lint or dead cells.  Pictured above is a piece of interplanetary dust caught by a high-flying U2-type aircraft.  It likely originates in the early days of our Solar System, being stored and later ejected by a passing comet.  The particle is composed of glass, carbon, and a conglomeration of silicate mineral grains.  It measures only 10 microns across, a tenth the width of a typical human hair.  NASA's STARDUST mission, launched in 1999, is scheduled to pass through the tail of Comet Wild 2 in 2004 and return many more interstellar dust samples to Earth in 2006.
X-Rays from the Galactic Plane (2001-08-14)

In February 2000, the orbiting Chandra X-ray Observatory spent 27 hours staring into the plane of our Milky Way galaxy. Its target was a spot in the small constellation Scutum, within the Milky Way's zone of avoidance where galactic gas and dust clouds block visible light, making a poor window for optical telescopes. However the penetrating x-ray observations looked through the obscurations revealing the Milky Way and the Universe beyond. The x-ray view is reconstructed above in false color. Distant active galaxies emitting high energy x-rays appear as blue dots, while reddish dots are sources of lower energy x-rays, likely stars within the Milky Way itself. Intriguing is the diffuse blue glow of high energy x-rays, distinct from the individual sources in the picture. Astronomers have long debated whether our galactic plane's apparently extended x-ray emission was due to discrete sources or diffuse hot gas. As these results suggest diffuse interstellar gas with a temperature of tens of millions of degrees Celsius is indeed the answer, other questions arise. What heats the gas to these incredible temperatures? Why does this energetic gas linger in the galactic plane?
Mars: 3-D Dunes (2001-08-15)

Get out your red/blue glasses and treat yourself to this dramatic 3-D view of sand dunes on Mars! The field of undulating dunes is found in Nili Patera, a volcanic depression in central Syrtis Major, the most prominent dark feature on the Red Planet. Two different images from the orbiting Mars Global Surveyor spacecraft were combined to make this stereo picture, one taken in March 1999 and the other recorded in April 2001. Sculpted by winds like the sand dunes of Earth, these particular Martian dunes show no change in shape over the time separating the two images, a period equivalent to about one Martian year. This cropped version of the 3-D picture spans an area around 2 kilometers across. Walking, you might cover that distance in about 20 minutes.
Centaurus A: X-Rays from an Active Galaxy (2001-08-16)

Its core hidden from optical view by a thick lane of dust, the giant elliptical galaxy Centaurus A was among the first objects observed by the orbiting Chandra X-ray Observatory. Astronomers were not disappointed, as Centaurus A's appearance in x-rays makes its classification as an active galaxy easy to appreciate. Perhaps the most striking feature of this Chandra false-color x-ray view is the jet, 30,000 light-years long. Blasting toward the upper left corner of the picture, the jet seems to arise from the galaxy's bright central x-ray source -- suspected of harboring a black hole with a million or so times the mass of the Sun. Centaurus A is also seen to be teeming with other individual x-ray sources and a pervasive, diffuse x-ray glow. Most of these individual sources are likely to be neutron stars or solar mass black holes accreting material from their less exotic binary companion stars. The diffuse high-energy glow represents gas throughout the galaxy heated to temperatures of millions of degrees C. At 11 million light-years distant in the constellation Centaurus, Centaurus A (NGC 5128) is the closest active galaxy.
The 47 Ursae Majoris System (2001-08-17)

Watching and waiting, astronomers have uncovered the presence of more than 70 planets orbiting stars other than the Sun. So far almost all these extrasolar planets have crazy elongated orbits, lie uncomfortably close to their parent stars, or are found in bizarre, inhospitable systems. Yet a reported new planet discovery indicates for the first time that a nearby sun-like star, 47 Ursae Majoris (47 UMa), has at least two planets in nearly circular orbits more reminiscent of Jupiter and Saturn in our own familiar Solar System. The planets are too distant and faint to be photographed directly. Still, 13 years of spectroscopic observations of 47 UMa have revealed the wobbling signature of a second planet intertwined with one previously known. In this artist's illustration, the worlds of 47 UMa hang over the rugged volcanic landscape of a hypothetical moon. The moon orbits the newly discovered planet, imagined here with Saturn-like rings, while the previously known planet is visible as a tiny crescent, close to the yellowish star. Closer still to 47 UMa is another tiny dot, a hypothetical Earth-like water world. About 51 light-years distant, 47 UMa can be found in planet Earth's sky near the Big Dipper.
Phobos: Doomed Moon of Mars (2001-08-18)

Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this 1978 Viking 1 orbiter image, the largest moon, Phobos, is indeed seen to be a heavily cratered asteroid-like object. About 17 miles across, Phobos really zips through the Martian sky. Actually rising above Mars' western horizon and setting in the east, it completes an orbit in less than 8 hours. But Phobos is doomed. Phobos orbits so close to Mars, (about 3,600 miles above the surface compared to 250,000 miles for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it will likely crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.
Mercury: A Cratered Inferno (2001-08-19)

Mercury's surface looks similar to our Moon's.  Each is heavily cratered and made of rock.  Mercury's diameter is about 4800 km, while the Moon's is slightly less at about 3500 km (compared with about 12,700 km for the Earth).  But Mercury is unique in many ways.  Mercury is the closest planet to the Sun, orbiting at about 1/3 the radius of the Earth's orbit.  As Mercury slowly rotates, its surface temperature varies from an unbearably cold -180 degrees Celsius to an unbearably hot 400 degrees Celsius.  The place nearest the Sun in Mercury's orbit changes slightly each orbit - a fact used by Albert Einstein to help verify the correctness of his then newly discovered theory of gravity: General Relativity.   The above picture was taken by the only spacecraft ever to pass Mercury: Mariner 10 in 1974.
The Lagoon Nebula in Three Colors (2001-08-20)

The bright Lagoon Nebula is home to a diverse array of astronomical objects.  Particularly interesting sources include a bright open cluster of stars and several energetic star-forming regions.  When viewed by eye, cluster light is dominated by an overall red glow that is caused by luminous hydrogen gas, while the dark filaments are caused by absorption by dense lanes of dust. The above picture, from the Curtis-Schmidt Telescope, however, shows the nebula's emission in three exact colors specifically emitted by hydrogen, oxygen, and sulfur. The Lagoon Nebula, also known as M8 and NGC 6523, lies about 5000 light-years away.  The Lagoon Nebula can be located with binoculars in the constellation of Sagittarius spanning a region over three times the diameter of a full Moon.

Image credit: R. Barba, 
N. Morrell et al. 
(UNLP),
CTIO, 
NOAO, 
NSF
Dark Spots on Neptune (2001-08-21)

Neptune has spots.  The Solar System's outermost gas giant shows a nearly uniform blue hue created by small amounts of methane drifting in a thick atmosphere of nearly colorless hydrogen and helium.  Dark spots do appear, however, that are anti-cyclones: large high-pressure systems that swirl in Neptune's cold cloud tops.  Two dark spots are visible in the above picture taken by the robot Voyager 2 spacecraft in 1989: an Earth-sized Great Dark Spot located on the far left, and Dark Spot 2 located near bottom.  A bright cloud dubbed Scooter accompanies the Great Dark Spot.  Recent computer simulations indicate that scooters are methane clouds that might commonly be found near dark spots.  Subsequent images of Neptune by the Hubble Space Telescope in 1994 indicated that both of these dark spots had dissipated, but another had been created.
The Bubbling Cauldron of NGC 3079 (2001-08-22)

dge-on spiral galaxy NGC 3079 is a mere 50 million light-years away toward the constellation Ursa Major. Shown in this stunning false-color Hubble Space Telescope image, the galaxy's disk - composed of spectacular star clusters in winding spiral arms and dramatic dark lanes of dust - spans some 70,000 light-years. Still, NGC 3079's most eye-catching features are the pillars of gas which tower above a swirling cosmic cauldron of activity at the galaxy's center. Seen in the close-up inset at lower right, the pillars rise to a height of about 2,000 light-years and seem to lie on the surface of an immense bubble rising from the galactic core. Measurements indicate that the gaseous pillars are streaming away from the core at 6 million kilometers per hour. What makes this galaxy's cauldron bubble? Astronomers are exploring the possibility that the superbubble is formed by winds from massive stars. If so, these massive stars were likely born all at once as the galactic center underwent a sudden burst of star formation.
Distortion from a Distant Cluster (2001-08-23)

This stunning color deep sky view toward the constellation Pisces was made with data from a fast, sensitive, digital detector known as the Big Throughput Camera operating at Cerro Tololo Inter-American Observatory in Chile. Hardly noticeable in the original picture is the small cluster of about 15 galaxies nearly 3 billion light-years distant, circled at the lower right. In fact, this distant cluster was not discovered by noticing its appearance in the image at all, but instead by mapping the subtle distortions created by its gravity. As predicted by Einstein's General Relativity theory, the cluster's gravitational mass acts like a lens, bending light and distorting the shape of background galaxies. The effect is known as gravitational lensing. Computer mapping of weak distortions of background galaxy shapes across the Big Throughput image revealed that the large scale distribution of mass in that part of the sky was concentrated in a small region. That region turned out to correspond to the galaxy cluster -- the first time such an object has been discovered on the basis of its mass properties rather than its light.
NEAR at Eros: Before Touchdown (2001-08-24)

On 12 February, 2001, the NEAR-Shoemaker spacecraft gently touched-down on the the surface of Eros -- the first ever landing on an asteroid. During the descent, the spacecraft's camera recorded successive images of the diminutive world's surface, revealing fractured boulders, dust filled craters, and a mysterious collapsed channel. The last frame, seen in the above montage at the far left, was taken at a range of 128 meters. Expanded in the inset, it shows surface features a few centimeters across. Stereo experimenter Patrick Vantuyne, constructed this montage from the final images in the landing sequence, carefully identifying the overlapping areas in successive frames. Frames which overlap were taken by the spacecraft from slightly different viewpoints, allowing Vantuyne to construct close-up stereo images of the surface of asteroid 433 Eros.
Pioneer 10: The First 7 Billion Miles (2001-08-25)

Q: What was made by humans and is 7.3 billion miles away? A: Pioneer 10 -- and 1997 was the 25th anniversary of its launch. Almost 11 light-hours distant, Pioneer 10 is presently about twice as far from the Sun as Pluto, and bound for interstellar space at 28,000 miles per hour. The distinction of being the first human artifact to venture beyond the known planets of the Solar System is just one in a long list of firsts for this spacefaring ambassador, including; the first spacecraft to travel through the asteroid belt and explore the outer Solar System, the first spacecraft to visit Jupiter, and the first to use a planet's gravity to change its course and to reach solar-system-escape velocity. Pioneer 10's mission is nearing an end. Now exploring the distant reaches of the heliosphere it will soon run out of sufficient electrical power to operate science instruments. However, the 570 lb. spacecraft will continue to coast and in 300,000 years or so it will pass within about 3 light years of nearby star Ross 248. Ross 248 is a faint red dwarf just over 10 light years distant in the constellation Taurus. (Note: In 1998 Voyager 1, launched 5 years later but traveling faster than Pioneer 10, became humanity's most distant spacecraft.)
Uranus: The Tilted Planet (2001-08-26)

Uranus is the third largest planet in our Solar System after Jupiter and Saturn.  Uranus is composed mostly of rock and ices, but with a thick hydrogen and helium atmosphere. The blue hue of Uranus' atmosphere arises from the small amount of methane which preferentially absorbs red light.  This picture was snapped by the Voyager 2 spacecraft in 1986 - the only spacecraft ever to visit Uranus.  Uranus has many moons and a ring system.  Uranus, like Venus, has a rotation axis that is greatly tilted and sometimes points near the Sun.  It remains an astronomical mystery why Uranus' axis is so tilted.  Uranus and Neptune are quite similar: Uranus is slightly larger but less massive.
Artificial Night Sky Brightness (2001-08-27)

Where have all the dim stars gone?  From many places on the Earth including major cities, the night sky has been reduced from a fascinating display of hundreds of stars to a diffuse glow through which only a handful of stars are visible.  The above map indicates the relative amount of light pollution that occurs across the Earth.  The cause of the pollution is artificial light reflecting off molecules and aerosols in the atmosphere.  Parts of the Eastern United States and Western Europe colored red have an artificial night sky glow over nine times that of the natural sky.  In any area marked orange or red, the central band of our Milky Way Galaxy is no longer visible. The International Dark Sky Association suggests common types of fixtures that provide relatively little amounts of light pollution.

Image credit: P. Cinzano
Jagged Hills on Jupiter's Callisto (2001-08-28)

Why does Jupiter's moon Callisto have unusual jagged hills?  This mystery came to light after the robot spacecraft Galileo, in orbit around Jupiter since 1995, swooped past the dark moon in May.  The resulting pictures were the highest resolution yet obtained for a Jovian moon: objects as small as 3 meters across are discernable.  The strange landscapes pictured above show areas rich in bright sharp mounds about 100 meters tall.  A likely formation hypothesis holds that these hills are the result of ejecta thrown billions of years ago during a violent impact.  The lower inset region apparently has undergone an epoch of relatively high ice-erosion, where dark rock has filled in some of the inter-hill regions.  NASA has recently cleared Galileo to continue swooping Jupiter's moons until 2003, when it will end its journey with a spectacular dive into Jupiter's atmosphere.
AFGL 2591: A Massive Star Acts Up (2001-08-29)

Young star AFGL 2591 is putting on a show.  The massive star is expelling outer layers of dust-laced gas as gravity pulls inner material toward the surface.  AFGL 2591 is estimated to be about one million years old -- much younger than our own Sun's 5 billion-year age -- and has created a nebula over 500 times the diameter of our Solar System in just the past 10,000 years.  The above image in infrared light is one of the first from the new NIRI instrument mounted on one of the largest ground-based optical telescopes in the world: Gemini North.  Sharp details are discernable that are blocked by opaque dust in visible-light images.  Close inspection of the image reveals at least four expanding rings, indicating an episodic origin to the mysterious activity.  AFGL 2591 lies about 3000 light years away toward the constellation of Cygnus.
How Big Is 2001 KX76? (2001-08-30)

Newly discovered minor planet 2001 KX76 is circled in the top panel above, a recent composite image from the European Southern Observatory's 2.2 meter telescope at La Silla, Chile. Though 2001 KX76 appears here as single point of light in an unremarkable star field, its orbit has been accurately measured by Astrovirtel, a newly operational "virtual telescope" capable of mining many years of archival data for previously unrecognized images of 2001 KX76. The results show this minor planet to be very distant, now orbiting just beyond Pluto and Charon in the realm of the Kuiper Belt. At its distance, apparent brightness, and assuming a reasonable surface reflectivity, 2001 KX76 would be 1,200 kilometers or more across -- larger than the largest main-belt asteroid, Ceres. In fact, the illustration in the bottom panel graphically compares this size estimate to Pluto, Charon, and the largest previously known Kuiper Belt objects, indicating the newfound minor planet is second only to Pluto in diameter. Along with other evidence, the comparison suggests that Pluto and Charon are closely related to Kuiper Belt worlds like 2001 KX76.
The Flight Of Helios (2001-08-31)

Solar-powered, remotely piloted, and flying at about 25 miles per hour, NASA's Helios aircraft, is pictured above at 10,000 feet in skies northwest of Kauai, Hawaii on August 13. This ultralight propeller driven aircraft, essentially a flying wing with 14 electric engines, was built by AeroVironment Inc. Covered with solar cells, Helios' impressive 247 foot wide wing exceeds the wing span and even overall length of a Boeing 747 jet airliner. Climbing during daylight hours, the prototype aircraft ultimately reached an altitude just short of 100,000 feet, breaking records for non-rocket powered flight. Helios is intended as a technology demonstrator, but regular, long-duration flights at that altitude could be used for environmental monitoring missions and, communications relays. In the extremely thin air 100,000 feet above Earth's surface, the flight of Helios also simulates conditions for winged flight in the atmosphere of Mars.
Magnetars In The Sky (2001-09-01)

Indicated on this infrared image of the galactic center region are positions of candidate magnetars -- believed to be the strongest magnets in the galaxy. Classified by observers as Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs), these cosmic powerhouses are likely city-sized, spinning, highly-magnetized neutron stars. How strong is a magnetar's magnetic field? The Earth's magnetic field which deflects compass needles is measured to be about 1 Gauss, while the strongest fields sustainable in earthbound laboratories are about 100,000 Gauss. A magnetar's monster magnetic field is estimated to be as high as 1,000,000,000,000,000 Gauss. A magnet this strong, located at about half the distance to the Moon would easily erase your credit cards and suck pens out of your pocket. In 1998, from a distance of about 20,000 light-years, one magnetar, SGR 1900+14 generated a powerful flash of gamma-rays detected by many spacecraft. That blast of high-energy radiation is now known to have had a measurable effect on Earth's ionosphere. At the surface of the magnetar, its powerful magnetic field is thought to buckle and shift the neutron star crust generating the intense high-energy flares.
Deimos: A Small Martian Moon (2001-09-02)

Mars has two tiny moons, Phobos and Deimos.  Pictured above is Deimos, the smaller moon of Mars.  In fact, Deimos is one of the smallest known moons in the Solar System measuring only nine miles across.  The diminutive Martian moons were discovered in 1877 by Asaph Hall, an American astronomer working at the US Naval Observatory in Washington D.C.  The existence of two Martian moons was predicted around 1610 by Johannes Kepler, the astronomer who derived the laws of planetary motion.  In this case, Kepler's prediction was not based on scientific principles, but his writings and ideas were so influential that the two Martian moons are discussed in works of fiction such as Jonathan Swift's Gulliver's Travels, written in 1726, over 150 years before their actual discovery.
The Making of the Rotten Egg Nebula (2001-09-03)

Fast expanding gas clouds mark the end for a central star in the Rotten Egg Nebula.  The once-normal star has run out of nuclear fuel, causing the central regions to contract into a white dwarf.  Some of the liberated energy causes the outer envelope of the star to expand.  In this case, the result is a photogenic proto- planetary nebula.  As the million-kilometer per hour gas rams into the surrounding interstellar gas, a supersonic shock front forms where ionized hydrogen and nitrogen glow blue.  The complex shock front had been hypothesized previously but never so clearly imaged.  Thick gas and dust hide the dying central star.  The Rotten Egg Nebula, also known as the Calabash Nebula and OH231.8+4.2, will likely develop into a full bipolar planetary nebula over the next 1000 years.  The nebula, pictured above, is about 1.4 light-years in extent and located about 5000 light-years away toward the constellation of Puppis.