juliensimon/nasa-apod huggingface dataset:
Hoag's Object: A Strange Ring Galaxy (2002-09-09)

Is this one galaxy or two?  This question came to light in 1950 when astronomer Art Hoag chanced upon this unusual extragalactic object.  On the outside is a ring dominated by bright blue stars, while near the center lies a ball of much redder stars that are likely much older.  Between the two is a gap that appears almost completely dark.  How Hoag's Object formed remains unknown, although similar objects have now been identified and collectively labeled as a form of ring galaxy.  Genesis hypotheses include a galaxy collision billions of years ago and perturbative gravitational interactions involving an unusually shaped core.  The above photo taken by the Hubble Space Telescope in July 2001 reveals unprecedented details of Hoag's Object and may yield a better understanding.   Hoag's Object spans about 100,000 light years and lies about 600 million light years away toward the constellation of Serpens.  Coincidentally, visible in the gap (at about one o'clock) is yet another ring galaxy that likely lies far in the distance.
Venus Beyond the Storm (2002-09-10)

A thunderstorm, lightning, a bright star and a bright planet all graced an evening sky for a short while near Bismarck, North Dakota, USA two weeks ago.  Thick thunderclouds from a passing storm are the origin of a strong cloud to ground lightning strike.  Small areas of rain darken portions of the orange sunset, visible at the horizon above the vast prairie.  The planet Venus peeks below the clouds on the lower left of the image.  Blue sky shines high above the distant storm, streaked with high white cirrus clouds.  The bright star Arcturus glitters near the image top, just left of center.  Just a few minutes later, only a memory and this picture remained.

Image credit: Lyndon Anderson 
(Prairiejournal.com)
Pluto & Charon Eclipse a Triple Star (2002-09-11)

Occasionally, a planet in our Solar System will pass in front of a bright star.  Since stars and planets take up so little space on the sky, such events are quite rare.  Two months ago, however, Pluto and its large moon Charon passed in front of a comparatively bright triple star system known as P126.  By noting how P126 A dimmed, the event was useful for studying Pluto's relatively unknown atmosphere.  A Very Large Telescope in Chile using a deformable mirror to counter the blurring effect of Earth's atmosphere captured the above image.
X-Rays From Tycho's Supernova Remnant (2002-09-12)

In 1572, Danish astronomer Tycho Brahe recorded the sudden appearance of a bright new star in the constellation Cassiopeia. The new star faded from view over a period of months and is believed to have been a supernova, one of the last stellar explosions seen in our Milky Way galaxy. Now known as Tycho's Supernova Remnant, the expanding debris cloud is shown in this detailed false-color x-ray image from the orbiting Chandra Observatory. Represented in blue, the highest energy x-rays come from shocked regions along the outer edges of the supernova remnant, corresponding to gas at temperatures of 20 million degrees Celsius. X-rays from cooler gas (only 10 million degrees or so!) dominate the remnant's interior. Unlike some other supernova remnants, no hot central point source can be found, supporting the theory that the origin of this stellar explosion was a runaway nuclear detonation that ultimately destroyed a white dwarf star. At a distance of about 7,500 light-years, Tycho's Supernova Remnant appears to be nearly 20 light-years across. This x-ray picture's field of view slightly cuts off the bottom of the generally spherical cloud.
Aristarchus Plateau (2002-09-13)

Anchored in the vast lava flows of the Moon's Oceanus Procellarum lies the Aristarchus Plateau. Recorded from a backyard observatory on planet Earth, this sharp, amazingly colorful view nicely captures the geologically diverse area, including the brownish plateau, Aristarchus and Herodotus craters, and the meandering Vallis Schroteri. The bright impact crater at the corner of the plateau is Aristarchus, a young crater 42 kilometers wide and 3 kilometers deep, surrounded by a radial system of light-colored rays. Only slightly smaller, lava flooded Herodotus crater is above and to the left. A valley or rille feature likely carved by rapidly flowing lava or a collapsed lava tunnel, Vallis Schroteri begins just to the right of Herodotus and winds across the plateau for about 160 kilometers, eventually turning toward the top of the picture and the shadow of the lunar terminator. Aristarchus Plateau itself is like a rectangular island about 200 kilometers across, raised up to 2 kilometers or so above the smooth surface of the lunar Ocean of Storms.

Image credit: Russell Croman
X-Ray Moon (2002-09-14)

This x-ray image of the Moon was made by the orbiting ROSAT (Röntgensatellit) Observatory in 1990. In this digital picture, pixel brightness corresponds to x-ray intensity. Consider the image in three parts: the bright hemisphere of the x-ray moon, the darker half of the moon, and the x-ray sky background. The bright lunar hemisphere shines in x-rays because it scatters x-rays emitted by the sun. The background sky has an x-ray glow in part due to the myriad of distant, powerful active galaxies, unresolved in the ROSAT picture but recently detected in Chandra Observatory x-ray images. But why isn't the dark half of the moon completely dark? It's true that the dark lunar face is in shadow and so is shielded from direct solar x-rays. Still, the few x-ray photons which seem to come from the moon's dark half are currently thought to be caused by energetic particles in the solar wind bombarding the lunar surface.
Zodiacal Light and the False Dawn (2002-09-15)

An unusual triangle of light will be particularly bright near the eastern horizon before sunrise during the next two months for observers in Earth's northern hemisphere.  Once considered a false dawn, this triangle of light is actually Zodiacal Light, light reflected from interplanetary dust particles.  The triangle is clearly visible on the left of the above frame taken from Mauna Kea in Hawaii on September 8 by one of the developing global network of fisheye nighttime web cameras, called CONCAMs, of the Night Sky Live Project.  Zodiacal dust orbits the Sun predominantly in the same plane as the planets: the ecliptic.  Zodiacal light is so bright this time of year because the dust band is oriented nearly vertical at sunrise, so that the thick air near the horizon does not block  out relatively bright reflecting dust.  Zodiacal light is also bright for people in Earth's northern hemisphere in March and April just after sunset.
An Atlas V Rocket Prepares to Launch (2002-09-16)

Tomorrow's picture: Hold the Mayo  < | 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.

Image credit: Ecliptic 
Enterprises Corporation
A Force from Empty Space: The Casimir Effect (2002-09-17)

This tiny ball provides evidence that the universe will expand forever.  Measuring slightly over one tenth of a millimeter, the ball moves toward a smooth plate in response to energy fluctuations in the vacuum of empty space.  The attraction is known as the Casimir Effect, named for its discoverer, who, 50 years ago, was trying to understand why fluids like mayonnaise move so slowly.  Today, evidence is accumulating that most of the energy density in the universe is in an unknown form dubbed dark energy.  The form and genesis of dark energy is almost completely unknown, but postulated as related to vacuum fluctuations similar to the Casimir Effect but generated somehow by space itself.  This vast and mysterious dark energy appears to gravitationally repel all matter and hence will likely cause the universe to expand forever.  Understanding vacuum fluctuations is on the forefront of research not only to better understand our universe but also for stopping micro-mechanical machine parts from sticking together.

Image credit: Umar Mohideen 
(U. California at Riverside)
A Sagittarius Starscape (2002-09-18)

Many vast star fields in the plane of our Milky Way Galaxy are rich in clouds of stars, dust, and gas.  First and foremost, visible in the above picture are millions of stars, many of which are similar to our Sun.  Next huge filaments of dark interstellar dust run across the image and block the light from millions of more stars yet further across our Galaxy.  The bright red region on the left is the Omega Nebula, an emission nebula of mostly hot hydrogen gas also known as M17.  A small bright grouping of stars near the image center is the open cluster M18, while the long bright streak of stars just right of center is M24.  On the far right of the image is the picturesque red emission nebula IC 1283 flanked by two blue reflection nebulas NGC 6589 and NGC 6590.  These objects are visible with a small telescope toward the constellation of Sagittarius.

Image credit: AAO
Asteroid 1998 KY26 (2002-09-19)

A day is just under 11 minutes long on 1998 KY26, a 30 meter wide, fast-spinning, water-rich asteroid. This computer simulated view of its lumpy surface has a resolution of about 3 meters and is based on radar and optical observations (click on the image for a series of surface views). The observations were made shortly after the discovery of the diminutive world which passed within about 800,000 kilometers of Earth, or about 2 times the Earth-Moon distance, in June of 1998. Around 10 million asteroids of similar size may exist in orbits that also come near Earth's, but little is known about them. However, spinning so fast, tiny 1998 KY26 can not be a loose conglomerate held together by gravity alone. Instead it is likely a monolithic chunk fragmented from a larger asteroid. As the radar and optical data suggest 1998 KY26 has a high water content, this relatively accessible asteroid could be a literal oasis for future space explorers.
The Crab Nebula Pulsar Shrugs (2002-09-20)

How does a city-sized neutron star power the vast Crab Nebula?  The expulsion of wisps of hot gas at high speeds appears to be at least part of the answer.  Yesterday time-lapse movies taken from both the Chandra X-ray Observatory and the Hubble Space Telescope were released showing a wisp of gas moving out at about half the speed of light.  Wisps like this likely result from tremendous electric voltages created by the central pulsar, a rapidly rotating, magnetized, central neutron star. The hot plasma strikes existing gas, causing it glow in colors across the electromagnetic spectrum.  Pictured above is a composite image of the center of the Crab Nebula where red represents radio emission, green represents visible emission, and blue represents X-ray emission.  The dot at the very center is the hot pulsar spinning 30 times per second.
Moonset, Planet Earth (2002-09-21)

During the Astro-1 astronomy mission of December, 1990, Space Shuttle astronauts photographed this stunning view of the setting full moon poised above the Earth's limb. In the foreground, towering clouds of condensing water vapor mark the extent of the troposphere, the lowest layer of the planet's life-sustaining atmosphere. Strongly scattering blue sunlight, the upper atmospheric layer, the stratosphere, fades dramatically to the black background of space. Moon and clouds are strong visual elements of many well known portraits of planet Earth, including Ansel Adams' famous "Moonrise, Hernandez, New Mexico", photographed in 1941.
Two Hours Before Neptune (2002-09-22)

Two hours before closest approach to Neptune in 1989, the Voyager 2 robot spacecraft snapped this picture.  Clearly visible for the first time were long light-colored cirrus-type clouds floating high in Neptune's atmosphere. Shadows of these clouds can even be seen on lower cloud decks.  Most of Neptune's atmosphere is made of hydrogen and helium, which is invisible.  Neptune's blue color therefore comes from smaller amounts of atmospheric methane, which preferentially absorbs red light.  Neptune has the fastest winds in the Solar System, with gusts reaching 2000 kilometers per hour.   Speculation holds that diamonds may be created in the dense hot conditions that exist under the clouds-tops of Uranus and Neptune.
The Milky Way Over the French Alps (2002-09-23)

Have you ever seen the band of our Milky Way Galaxy?  Chances are you have never seen it like this -- nor could you.  In a clear sky from a dark location at the right time, a faint band of light is visible across the sky.  This band is the disk of our spiral galaxy.  Since we are inside this disk, the band appears to encircle the Earth.  The above spectacular picture is a bit of a digital trick, though.  A first shot was taken in July 2000 with the camera counter-rotating from the Earth so that the stars appear fixed.  This allowed a long exposure from which a great amount of detail could emerge from the background star field.  Later, after moonrise, a much shorter image was taken from the same location catching details of the French Alps near Mount Blanc, the highest mountain in Western Europe.  Reflections in the water were later enhanced digitally.

Image credit: Marc Sylvestre
(Universia)
To Fly Free in Space (2002-09-24)

What would it be like to fly free over the seas and clouds of Earth?  The first to experience such an "untethered space walk" were NASA astronauts Bruce McCandless and Robert Stewart during Space Shuttle mission 41-B in 1984.  McCandless, pictured above, used a Manned Maneuvering Unit (MMU) to move in and around the cargo bay of the space shuttle.  The MMU works by shooting jets of nitrogen and has since been used to help deploy and retrieve satellites.  With a mass over 140 kilograms, an MMU is heavy on Earth, but, like everything, is weightless in space. The MMU was replaced in 2001 with the SAFER backpack propulsion unit.
Jupiter, Moons and Bees (2002-09-25)

Rising before the Sun on September 4, Jupiter and an old cresent Moon gathered in the dim constellation of Cancer. Watching from a hillside near Austin, Texas, planet Earth, astrophotographer Russell Croman recorded this view of their passing as clouds gracefully dimmed the brilliant moonlight. Earthshine illuminates the lunar night side and on close inspection, bright Jupiter at the lower right appears tightly flanked by its own four large Galilean moons. Next to Jupiter lies a loose swarm of stars just below the clouds. The stars are the brighter members of the nearby star cluster M44, popularly known as the Beehive cluster. Croman's remarkable digital image has been processed only slightly to improve the visibility of the earthshine region and Jupiter's moons.

Image credit: Russell Croman
Rocket Trail at Sunset (2002-09-26)

Bright light from a setting Sun and pale glow from a rising Moon both contribute to this stunning picture of a rocket exhaust trail twisting and drifting in the evening sky. Looking west, the digital telephoto view was recorded from Table Mountain Observatory near Wrightwood California, USA on September 19, four days before the autumnal equinox. The rocket, a Minuteman III solid fuel missile, was far down range when the image was taken. Launched from Vandenberg Air Force Base it carried its test payload thousands of miles out over the Pacific Ocean. The red/orange color from the setting Sun dramatically intensifies near the top of the rocket trail, but below the sunset line, the very bottom of the trail is faintly illuminated from the east by a nearly full Moon.  Still in full sunlight, the bright diffuse cloud at the top of the trail, the result of a rocket stage separation, is tinged with rainbows likely produced by high altitude ice crystals forming in the exhaust plume.  Astronomer James Young comments that the cloud takes on the appearance of a white dove flying from right to left across the sky.
Accretion Disk Simulation (2002-09-27)

Don't be fooled by the familiar symmetry. The graceful spiral structure seen in this computer visualization does not portray winding spiral arms in a distant galaxy of stars. Instead, the graphic shows spiral shock waves in a three dimensional simulation of an accretion disk -- material swirling onto a compact central object that could represent a white dwarf star, neutron star, or black hole. Such accretion disks power bright x-ray sources within our own galaxy. They form in binary star systems which consist of a donor star (not shown above), supplying the accreting material, and a compact object whose strong gravity ultimately draws the material towards its surface. For known x-ray binary systems the size of the accretion disk itself might fall somewhere between the diameter of the Sun (about 1,400,000 kilometers) and the diameter of the Moon's orbit (800,000 kilometers). One interesting result of the virtual reality astrophysics illustrated here is that the simulated disk develops instabilities which tend to smear out the pronounced spiral shocks.
X-Ray Rainbows (2002-09-28)

A drop of water or prism of glass can spread out visible sunlight into a rainbow of colors. In order of increasing energy, the well known spectrum of colors in a rainbow runs red, orange, yellow, green, blue, indigo, violet. X-ray light too can be spread out into a spectrum ordered by energy ... but not by drops of water or glass. Instead, the orbiting Chandra X-ray Observatory uses a set of 540 finely ruled, gold gratings to spread out the x-rays, recording the results with digital detectors. The resulting x-ray spectrum reveals much about the compositions, temperatures, and motions within cosmic x-ray sources. This false color Chandra image shows the x-ray spectrum of a star system in Ursa Major cataloged as XTE J1118+480 and thought to consist of a sun-like star orbiting a black hole. Unlike the familiar appearance of a prism's visible light rainbow, the energies here are ordered along radial lines with the highest energy x-rays near the center and lowest energies near the upper left and lower right edges of the image. The central spiky region itself is created by x-rays from the source which are not spread out by the array of gratings.
Venus: Just Passing By (2002-09-29)

Venus, the second closest planet to the Sun, is a popular way-point for spacecraft headed for the gas giant planets in the outer reaches of the solar system. Why visit Venus first? Using a "gravity assist " maneuver, spacecraft can swing by planets and gain energy during their brief encounter saving fuel for use at the end of their long interplanetary voyage. This colorized image of Venus was recorded by the Jupiter-bound Galileo spacecraft shortly after its gravity assist flyby of Venus in February of 1990. Galileo's glimpse of the veiled planet shows structure in swirling sulfuric acid clouds. The bright area is sunlight glinting off the upper cloud deck. A recent intriguing but controversial hypothesis holds that living microbes might exist in the upper clouds of Venus.
D. rad Bacteria: Candidate Astronauts (2002-09-30)

These bacteria could survive on another planet.  In an Earth lab, Deinococcus radiodurans (D. rad) survive extreme levels of radiation, extreme temperatures, dehydration, and exposure to genotoxic chemicals.  Amazingly, they even have the ability to repair their own DNA, usually with 48 hours.  Known as an extremophile, bacteria such as D. rad are of interest to NASA partly because they might be adaptable to help human astronauts survive on other worlds.  A recent map of D. rad's DNA might allow biologists to augment their survival skills with the ability to produce medicine, clean water, and oxygen.  Already they have been genetically engineered to help clean up spills of toxic mercury.  Likely one of the oldest surviving life forms, D. rad was discovered by accident in the 1950s when scientists investigating food preservation techniques could not easily kill it.  Pictured above, Deinococcus radiodurans grow quietly in a dish.
Rectangular Ridges on Mars (2002-10-01)

What could cause rectangular ridges on Mars?  As data flows in from the two spacecraft currently orbiting Mars, surface structures are seen that are not immediately understood.  These structures pose puzzles that planetary geologists are eager to solve, as they might provide clues to past processes that have shaped Mars over billions of years.  On the right of the above image is an unusual array of ridges first spotted in Mariner 9 data in 1972.  A ridge wall runs for about 5 kilometers.  Two competing progenitor theories include hardened sand dunes and once-molten rock that seeped through surface cracks and cooled.  Dubbed "Inca City" for their resemblance to stone walls of an ancient Earth civilization, the new Mars Global Surveyor images now show them to be part of a larger circular pattern, indicating an origin possibly related to the impact crater.  (Non-natural origin hypotheses are not invoked by conservative scientists unless clear indications exist that natural processes could not work.)
Star Clouds Toward the Southern Crown (2002-10-02)

The flowing trails of dust toward Corona Australis, the constellation of the Southern Crown, are visible here not because they glow, but because they absorb and reflect.  The dust appears bluer when seen near bright stars because it preferentially reflects blue light.  The densest knots of dust are so thick that no background stars or interior stars can be seen through them.  These knots indicate molecular clouds filled with cool gas where future stars will likely form.  The above image spans about 5 degrees while the reflection nebula lies about 500 light-years distant.

Image credit: David Malin
V838 Mon: Mystery Star (2002-10-03)

A leading candidate for the most mysterious star found in recent times is variable star V838 Monocerotis. At a distance of about 8,000 light-years, V838 Mon was discovered to be in outburst in January of this year. Initially thought to be a familiar type of classical nova, astronomers quickly realized that instead, V838 Mon may be a totally new addition to the astronomical zoo. Observations indicate that the erupting star transformed itself over a period of months from a small under-luminous star a little hotter than the Sun, to a highly-luminous, cool supergiant star undergoing rapid and complex brightness changes. The transformation defies the conventional understanding of stellar life cycles. A most notable feature of V838 Mon is the "expanding" nebula which now appears to surround it. Seen above in two separate images from the South African Astronomical Observatory's 1 meter telescope, the nebula is probably a light echo from shells of formerly unseen material lost by the star during its previous evolution. Light-years in diameter, the shells progressively reflect the light from V838 Mon's outbursts, providing an opportunity to look back at the history of this remarkable star's behaviour.
Facing NGC 6946 (2002-10-04)

From our vantage point in the Milky Way Galaxy, we see NGC 6946 face on. The big beautiful spiral galaxy is located just 10 million light-years away, behind a veil of foreground stars in the high and far-off constellation of Cepheus. Looking from the bright core outward along the loose, fragmented spiral arms, the galaxy's colors show a striking change from the yellowish light of old stars in the galaxy's center to young blue star clusters and reddish star forming regions. NGC 6946 is also bright in infrared light and rich in gas and dust, exhibiting a high star birth and death rate. In fact, during the 20th century, at least six supernovae, the death explosions of massive stars, were discovered in NGC 6946. In this sharp composite color digital image, a small barred structure is just visible at the gorgeous galaxy's core.

Image credit: Robert Gendler