juliensimon/nasa-apod huggingface dataset:
Fireball, Smoke Trail, Meteor Storm (2001-11-22)

Returning from orbit, space shuttles enter the atmosphere at about 8 kilometers per second as friction heats their protective ceramic tiles to over 1,400 degrees Celsius. By contrast, the bits of comet dust which became the Leonid meteors seen on November 18, were moving at 70 kilometers per second, completely vaporizing at altitudes of around 100 kilometers. In this single 5 minute time exposure, three Leonid meteors are shooting through skies above Spruce Knob, West Virginia, USA.  Background stars are near the constellation Orion. The brightest meteor, a fireball, dramatically changes colors along its path and leaves a smokey persistant trail drifting in high-altitude winds. From that extremely dark site, at an elevation of 1,200 meters, astrophotographer Jerry Lodriguss reports, "We observed a [zenithal hourly rate] of about 3,600 at 10:30 UT and very high rates from 9:30 UT until well into the start of astronomical twilight at 10:50 UT. It was quite a spectacular storm, with bolides going off like flashbulbs, green and red fireballs and other fainter Leonids in all parts of the sky."

Image credit: Jerry Lodriguss
Counting Falling Stardust (2001-11-23)

In the clear, dark and moonless predawn hours of November 18, Greenbelt, Maryland's local baseball field was packed. The crowd stared skyward and occasionally conversed in hushed and reverent tones. "How many did you count?" a man asked. Some had long since lost track ... but others were keeping score, counting hundreds of Leonid meteors in a short hour's worth of skygazing. Farther to the west, near Florence, Arizona, recreational astronomers also gathered to enjoy the celestial show. Taken from that location, this single, 10 minute, wide-angle exposure captured a dozen or so Leonid meteors. The shooting stars are clearly seen to be streaming from the shower's radiant point in the constellation Leo.

Image credit: Rick Scott
Mariner's Mercury (2001-11-24)

Mercury, the closest planet to the Sun, remains the most mysterious of the Solar System's inner planets. Hiding in the Sun's glare it is a difficult target for Earth bound observers. The only spacecraft to explore Mercury close-up was Mariner 10 which executed three flybys of Mercury in 1974 and 1975, surveying approximately 45 percent of its surface. Mariner 10 deftly manuevered to photograph part of the sunlit hemisphere during each approach, passed behind the planet, and continued to image the sun-facing side as the spacecraft receded. Its highest resolution photographs recorded features approximately a mile across. A reprocessing of the Mariner 10 data has resulted in this dramatic mosaic.  Like the Earth's Moon, Mercury's surface shows the scars of impact cratering - the smooth vertical band and patches visible above represent regions where no image information is available.
M16: Stars from Eagle's EGGs (2001-11-25)

Newborn stars are forming in the Eagle Nebula.  This image, taken with the Hubble Space Telescope in 1995, shows evaporating gaseous globules (EGGs) emerging from pillars of molecular hydrogen gas and dust.  The giant pillars are light years in length and are so dense that interior gas contracts gravitationally to form stars.  At each pillars' end, the intense radiation of bright young stars causes low density material to boil away, leaving stellar nurseries of dense EGGs exposed.  The Eagle Nebula, associated with the open star cluster M16, lies about 7000 light years away.
Leonids From the Road (2001-11-26)

Sometimes you just have to stop and watch the meteors.  In the early morning hours of November 18, a band of eleven people searched for a flat and cloudless site to see the 2001 Leonids Meteor Shower.  Starting in central Iowa, weather satellite images indicated that southern Minnesota might be their best chance, and so off they drove.  Although they couldn't shake off all of the clouds, they found a dark gravel road, pulled off, and settled in for the rare celestial light show.  "By about 4 am, we were visually counting 1000 per hour. After that they started to increase dramatically."  The photographer was just a little too late to catch a really bright fireball, but did catch several other bright Leonid meteors above one of the cars in the convoy.  The exposure lasted a few minutes.    News: APOD is now also available in Spanish.

Image credit: Tom Bailey
Ancient Layered Rocks on Mars (2001-11-27)

Is this a picture of Mars or Earth?  Oddly enough, it is a picture of Mars.  What may appear to some as a terrestrial coastline is in fact a formation of ancient layered rocks and wind-blown sand on Mars.  The above-pictured region spans about three kilometers in Schiaparelli Crater.  What created the layers of sediment is still a topic of research.  Viable hypotheses include ancient epochs of deposit either from running water or wind-blown sand.  Winds and sandstorms have smoothed and eroded the structures more recently.  The "water" that appears near the bottom is actually dark colored sand.  The image was taken with the Mars Global Surveyor spacecraft that has now returned over 100,000 images.   News: APOD is now also available in Spanish.
Extra-Solar Planetary Atmosphere Detected (2001-11-28)

By directly detecting the atmosphere of a planet outside our Solar System, humanity has taken another small step toward finding extraterrestrial life. The unexpected detection by David Charbonneau (Caltech) and associates came from Hubble Space Telescope observations of Sun-like star HD 209458.  As an orbiting planet crossed between that star and the Earth, sodium in the planet's atmosphere absorbed starlight at very specific colors.  The planet, originally discovered two years ago, has about 70 percent the mass of Jupiter but orbits very close in.  A long-term goal of this type of research is the detection of planetary biomarkers that would indicate life, such as oxygen, water, or methane.
Coronal Inflow (2001-11-29)

The active Sun has thrown a lot our way lately, including storms of particles streaming outward in the solar wind and clouds of plasma which triggered awesome auroral displays. Still, a growing body of intriguing observations from the LASCO instrument on board the space-based SOlar and Heliospheric Observatory (SOHO) indicates material also flows back toward the Sun, starting from over 2 million kilometers above its visible surface. Relatively hard to detect against the outflowing solar corona, a dark inflowing cloud's relative motion is tracked above in two highly processed images recorded an hour apart. The solar surface, graphically shown by the yellow quarter circle at the lower right, is blocked from view by a smooth occulting disk. Fighting against a solar wind outflow of about 120 kilometers per second the cloud seems to be moving inward at 50-100 kilometers per second. Occasionally appearing as often as once per hour, the clouds, seem to be dragged in by collapsing magnetic field loops rather than gravity alone. Researchers are now working to relate this surprising inflow to the solar wind and magnetic environment of the Sun.
Meteor Storm Sights and Sounds (2001-11-30)

This dramatic four-frame animation shows a fireball meteor and its developing persistent "smoke" train, recorded two weeks ago in skies near Salvador, Brazil. Indeed similar sights are astonishingly familiar world-wide to witnesses of this November's fireball-rich Leonid meteor storm. A few skygazers even discovered that some bright Leonid fireballs made faint, gentle, hissing sounds(!), a surprising effect only recently appreciated and understood. Accounts of fireball meteors making noise have long been viewed with skepticism, particularly because sounds were reportedly heard just as the meteor was seen overhead. But light travels much faster than sound so, like delayed thunder from a distant lightning stroke, a meteor produced sound should only be heard long after the meteor streak was seen. A sound explanation supported by laboratory tests is that turbulent plasma created by the meteor's passage generates very low frequency radio waves. Traveling at light speed the radio waves reach the ground simultaneously with visible light where they are strong enough to induce oscillating currents and audible vibrations in common objects like grass, leaves, wire-frame glasses, and perhaps even dry, frizzy hair.

Image credit: Paulo Raymundo
(Reaiche
Observatory)

 animation courtesy Tony Phillips
(SpaceWeather.com)
Neptune's Great Dark Spot: Gone But Not Forgotten (2001-12-01)

When NASA's Voyager 2 spacecraft flew by distant Neptune in August of 1989, astronomers were shocked. Since Neptune receives only 3 percent the sunlight Jupiter does, they expected to find a dormant, dark, frigid planet. Instead, the Voyager images revealed evidence of a dynamic and turbulent world. One of the most spectacular discoveries was of the Great Dark Spot, shown here in close-up. Surprisingly, it was comparable in size and at the same relative southern latitude as Jupiter's Great Red Spot, appearing to be a similar rotating storm system. Winds near the spot were measured up to 1500 miles per hour, the strongest recorded on any planet. The Voyager data also revealed that the Great Dark Spot varied significantly in size during the brief flyby. When the Hubble Space Telescope viewed the planet in 1994, the spot had vanished -- only to be replaced by another dark spot in the planet's northern hemisphere!
Rumors of a Strange Universe (2001-12-02)

Three years ago results were first 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 other observations, and because less-strange cosmologies without lambda have previously done well in explaining the data.  What is noteworthy here is the seemingly direct and reliable method of the observations and the good reputations of the scientists conducting the investigations.  Over the past three years, two independent teams of astronomers have continued to accumulate data that appears to confirm the unsettling result.  The above picture of a supernova that occurred in 1994 on the outskirts of a spiral galaxy was taken by one of these collaborations.  Still, extraordinary claims require extraordinary evidence, and so cosmologists the world over continue to await more data and confirmation by independent methods.
Dueling Auroras (2001-12-03)

Tomorrow's picture: Star Flame  < | 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: Phil Hoffman
AE Aurigae: The Flaming Star (2001-12-04)

Is star AE Aurigae on fire?  Although surrounded by what may look like smoke, the object known as the "flaming star" creates energy primarily by nuclear fusion, like other stars.  Fire, typically defined as the rapid molecular acquisition of oxygen, happens only when sufficient oxygen is present and is not important in such high-energy, low-oxygen environments such as stars.  The material that appears as smoke is mostly interstellar hydrogen, but does contain smoke-like dark filaments of carbon-rich dust grains.  The AE Aurigae region was imaged by the KPNO 0.9-meter telescope and is shown above in false but representative colors.  The star AE Aurigae itself is very bright, young, blue, and known as a runaway star since it appears to have been ejected from the Orion Nebula region about 2.7 million years ago.
A Sky Filled with Leonids (2001-12-05)

In the early morning hours of November 19, amateur Chen Huang-Ming caught a sky filled with astronomical wonders.  With his fisheye camera set up on Ho-Huan Mountain in Taiwan for a half-hour exposure, he started the above image a local time of 2:33 am.  First, the many famous stars and nebulas captured are too numerous to count.  Planets Jupiter and Saturn are visible, while the plane of our Milky Way Galaxy sweeps diagonally across the image.  What makes this image most spectacular, however, are the over 100 bright meteors visible from the 2001 Leonids Meteor Shower.  The meteor shower is caused by the Earth plowing through a stream of sand-sized ice particles shed years ago by Comet Tempel-Tuttle.  Note that the meteors can all be tracked back to a radiant in the constellation Leo, the direction from which the particles orbit the Sun.

Image credit: Chen Huang-Ming
Comet Linear (WM1) Brightens (2001-12-06)

A comet bright enough to be seen with binoculars is swooping into southern skies.  Comet C/2000 WM1 (LINEAR) continues to brighten and develop tails as it nears its closest approach of the Sun in late January 2002.  Comet LINEAR WM1 was discovered over a year ago when it was out past Jupiter and still very faint.  In the above picture from the Curtis Schmidt 0.6-meter Telescope in Chile, a 30-second exposure in red on December 4 captured detail in Comet LINEAR WM1's emerging dust tail.  Optimistic sky watchers hope that Comet LINEAR WM1 will undergo an even greater (and unexpected) brightening to the point where its coma and tails are easily discernable to the unaided eye.  Comet LINEAR WM1 is being watched with particular interest by astronomers because its ion tail might yield clues to understanding the solar wind expelled from near the Sun's poles.
Mediterranean Leonid 2001 (2001-12-07)

A road trip from Ankara to the Mediterranean coast southeast of Antalya, Turkey found clear skies and splendid scenery for astrophotographer Tunc Tezel's viewing of the 2001 Leonid meteor storm. There he captured this dream-like image of a fireball meteor near the horizon's twilight glow, reflected in calm ocean waters. Lights from coastal dwellings and nearby islands are seen in the foreground with brilliant Sirius shining as the brightest star in the heavens, visible in the constellation Canis Major at the upper right. Many enthusiasts who made special trips to view this November's Leonids were rewarded with similar spectacles of the fireball-rich storm. Airborne astronomers too had much to be thankful for as Leonid observations from a specially equiped aircraft flying at 40,000 feet produced bountiful data on the chemical composition of these dust grains from a comet's tail.

Image credit: Tunc Tezel
Moon Struck (2001-12-08)

Craters produced by ancient impacts on the airless Moon have long been a familiar sight. But only since 1999 have observers seen elusive optical flashes on the lunar surface - likely explosions resulting from impacting meteoroids. These startling observations were made with modest telescopes and video equipment during the 1999 and 2001 Leonid meteor showers. Six confirmed flashes, some initially as bright as a third magnitude star, were all seen within hours of the peak of the 1999 shower. At least two more lunar flashes, the brightest one at about fourth magnitude, have been confirmed during this November's Leonid storm. The 1999 locations are indicated by the red Xs on the dark lunar night side in this projection of the Moon from November 18 of that year. Similar flashes would have been difficult to see if viewed against the Moon's brightly lit portion. It is estimated that the flashes were made by meteoroids with masses in the range of 1 to 10 kilograms, producing craters a few meters across.
The Belt of Venus (2001-12-09)

Although you've surely seen it, you might not have noticed it.  During a cloudless twilight, just before sunrise or after sunset, part of the atmosphere above the horizon appears slightly off-color, slightly pink.  Visible in the above photograph, this off-color band between the dark eclipsed sky and the blue sky can best be seen in the direction opposite the Sun and is called the Belt of Venus.  Straight above, blue sky is normal sunlight reflecting off the atmosphere.  In the Belt of Venus, however, the atmosphere reflects light from the setting (or rising) Sun which appears more red.  The Belt of Venus can be seen from any location with a clear horizon.  It is frequently caught by accident in other photographs.
Globular Cluster M15 (2001-12-10)

Stars, like bees, swarm around the center of bright globular cluster M15.  This ball of over 100,000 stars is a relic from the early years of our Galaxy, and continues to orbit the Milky Way's center.  M15, one of about 150 globular clusters remaining, is noted for being easily visible with only binoculars, having at its center one of the densest concentrations of stars known, and containing a high abundance of unusual variable stars and pulsars.  The above image, taken in ultraviolet light with the WIYN Telescope, spans about 120 light years and shows the gradual increase in stars toward the cluster's center.  M15 lies about 35,000 light years away toward the constellation of Pegasus.  Recent evidence indicates that a massive black hole might reside as the center of M15.
Venusian Half-Shell (2001-12-11)

Venus, second planet from the Sun, appears above imaged for the first time ever in x-rays (left) by the orbiting Chandra Observatory. Chandra's smoothed, false-color, x-ray view is compared to an optical image (right) from a small earthbound telescope. Both show Venus illuminated by the Sun from the right, with only half the sunward hemisphere visible, but at least one striking difference is apparent. While the optical image in reflected sunlight is filled and bright at the center, Venus in x-rays is bright around the edge. Venus' x-rays are produced by fluorescence rather than reflection. About 120 kilometers or so above the surface, incoming solar x-rays excite atoms in the Venusian atmosphere to unstable energy levels. As the atoms rapidly decay back to their stable ground states they emit a "fluorescence" x-ray, creating a glowing x-ray half-shell above the sunlit hemisphere. More x-ray emitting material can be seen looking at the edge of the shell, so the edge appears brighter in the x-ray image.
Leonids Over Korean Observatory (2001-12-12)

There were two peaks to this year's Leonid Meteor Shower.  The first peak was best seen during the early morning hours of November 18 in North America, while the second peak, almost twice the intensity of the first, occurred eight hours later and was best seen from Asia.  Pictured above is an image of SoBaekSan National Observatory in Korea during the second Leonid peak, starting at 3:50 am local time.  Visible in the background are numerous Leonid meteors bright enough to be seen over background light even during the 40-minute exposure.  Local observers reported an average of over one meteor per second during this outburst. Next year's Leonid Meteor Shower might be even more intense but will have to compete will the glare of a nearly full moon.

Image credit: O Chul Kwon
(SoBaekSan National Observatory)
The South Pole of Mars (2001-12-13)

The south pole of Mars is the bright area near the center of the detailed, subtly shaded color image above. Recorded in September of this year by the Mars Global Surveyor (MGS) spacecraft, the picture shows a region surrounding the 400 kilometer wide martian polar cap in the midst of southern hemisphere spring. During this season the ice cap, predominantly layers of frozen carbon dioxide (dry ice) plus some water ice, begins to shrink as the ices change directly from solid to gas (sublimate). Hazy clouds of ice crystals and fog, extend across the bottom of the picture and a darker, more defrosted area is visible at the upper right, near the Red Planet's night side. A wealth of MGS data has allowed changes in the extent and density of the ice cap to be tracked over time. Now, researchers are also reporting indications that, in addition to seasonal changes, overall the martian southern ice cap has been dwindling in recent years -- dramatic evidence of a changing martian climate. At the measured rate, the increasing amount of carbon dioxide released could gradually raise Mars' atmospheric pressure, doubling it over hundreds to thousands of martian years.
NGC 7023: The Iris Nebula (2001-12-14)

Like delicate cosmic petals, these clouds of interstellar dust and gas have blossomed 1,300 light-years away in the fertile star fields of the constellation Cepheus. Sometimes called the Iris Nebula and dutifully cataloged as NGC 7023, this is not the only nebula in the sky to evoke the imagery of flowers. Still, the beautiful digital image shows off the Iris Nebula's range of colors and symmetries in impressive detail. Within the Iris, dusty nebular material surrounds a massive, hot, young star in its formative years. Central filaments of cosmic dust glow with a reddish photoluminesence as some dust grains effectively convert the star's invisible ultraviolet radiation to visible red light. Yet the dominant color of the nebula is blue, characteristic of dust grains reflecting starlight. Dark, obscuring clouds of dust and cold molecular gas are also present and can lead the eye to see other convoluted and fantastic shapes. Infrared observations indicate that this nebula may contain complex carbon molecules known as PAHs. As shown here, the Iris Nebula is about 6 light-years across.

Image credit: Brian Lula
Ganymede: Torn Comet - Crater Chain (2001-12-15)

This striking line of 13 closely spaced craters on Jupiter's moon Ganymede was photographed by the Galileo spacecraft in 1997. The picture covers an area about 120 miles wide and the chain of craters cuts across a sharp boundary between dark and light terrain. What caused this crater chain? Remarkably, the exploration of the Solar System, has shown that crater chains like this one are not unique, though they were considered mysterious until a dramatic object lesson was offered by comet Shoemaker-Levy 9. In 1994 many denizens of planet Earth watched as huge pieces of this torn comet slammed into Jupiter itself in a spectacular series of sequential impacts. It is very likely that similar torn comets from the early history of the Solar System are responsible for this and other crater chains.
The Horsehead Nebula (2001-12-16)

One of the most identifiable nebulae in the sky, the Horsehead Nebula in Orion, is part of a large, dark, molecular cloud.  Also known as Barnard 33, the unusual shape was first discovered on a photographic plate in the late 1800s.  The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis.  The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the Horsehead's neck casts a shadow to the left.  Streams of gas leaving the nebula are funneled by a strong magnetic field.  Bright spots in the Horsehead Nebula's base are young stars just in the process of forming.  Light takes about 1500 years to reach us from the Horsehead Nebula.   The above image was taken with the 0.9-meter telescope at Kitt Peak National Observatory.
Leaving the International Space Station (2001-12-17)

It was time to go home.  During their eight days aboard the Earth-orbiting International Space Station (ISS), ESA Flight Engineer Claudie Haigner, Russian Commander Victor Afanasyev, and Russian Flight Engineer Konstantin Kozeev had completed several experiments and successfully delivered a new lifeboat.  The lifeboat was actually the new Soyuz capsule they arrived in -- they returned home in an older Soyuz capsule that had been left six months ago.  Haigner, an expert in rheumatology and neuroscience, studied the development of frog embryos under microgravity conditions.  Pictured above on October 31, their Soyuz spacecraft undocks from the ISS while dark space and a blue Earth hover in the background.