juliensimon/nasa-apod huggingface dataset:
Weak Lensing Distorts the Universe (2000-03-15)

Is the distant universe really what it appears to be?  Astronomers hope not.  Intervening dark matter, which is normally invisible, might show its presence by distorting images originating in the distant universe, much the way an old window distorts images originating on the other side.  By noting the degree to which background galaxies appear unusually flat and unusually similar to neighbors, the dark matter distribution producing these weak gravitational lensing distortions can be estimated.  Recently released analysis of the shapes of 200,000 distant galaxies imaged with the Canada-France-Hawaii Telescope (CFHT) does indicate the presence of a massive network of distributed dark matter.  Future results may even be able to discern details of the distribution.  The above computer generated simulation image shows how dark matter, shown in red, distorts the light path from and apparent shape of distant galaxies, depicted in blue.
NEAR Shoemaker Views Eros (2000-03-16)

Orbiting asteroid 433 Eros, 145 million miles from Earth, NASA's NEAR spacecraft has been returning stunning views as its year long mission of exploration gets underway. A mosaic of recent NEAR images recorded at a range of about 127 miles, this picture illustrates some of the amazing contrasts which apparently exist within the domain of this diminutive world. Features as small as 65 feet are visible here, while long shadows emphasize the differences in the cratered regions at the left and smooth groved terrain at right. Up close, the undulating surface seems flecked with bright deposits and peppered with enormous boulders. As NEAR is poised to investigate mysteries of the formation of asteroids and the origins of the solar system from this unprecedented vantage point, NASA has renamed the spacecraft NEAR Shoemaker in honor of the late Dr. Eugene M. Shoemaker, legendary geologist, comet hunter, and inspirational pioneer in the field of interplanetary science.
Martian Dust Devil Trails (2000-03-17)

Who's been marking up Mars? This portion of a recent high-resolution picture from the orbiting Mars Global Surveyor spacecraft shows twisting dark trails criss-crossing a relatively flat rippled region about 3 kilometers wide on the martian surface. Newly formed trails like these presented researchers with a tantalizing martian mystery but have now been identified as likely the work of miniature wind vortices known to occur on the red planet - martian dust devils. Another example of wind processes on an active Mars, dust devils had been detected passing near the Viking and Mars Pathfinder landers. Such spinning columns of rising air heated by the warm surface are common in dry and desert areas on planet Earth. Typically lasting only a few minutes, they becoming visible as they pick up loose dust. On Mars, dust devils can be up to 8 kilometers high and leave dark trails as they disturb the bright, reflective surface dust.
A Wind From The Sun (2000-03-18)

A wind from the Sun blows through our Solar System. The behaviour of comet tails as they flapped and waved in this interplanetary breeze gave astronomers the first hint of its existence. Streaming outward at 250-400 miles/second, electrons and ions boiling off the Sun's incredibly hot but tenuous corona account for the Solar Wind - now known to affect the Earth and other planets along with voyaging spacecraft. Rooted in the Solar Magnetic Field, the structure of the corona is visible extending a million miles above the Sun's surface in this composite image from the EIT and UVCS instruments onboard the SOHO spacecraft. The dark areas, known as coronal holes, represent the regions where the highest speed Solar Wind originates.
Apollo 16: Exploring Plum Crater (2000-03-19)

Apollo 16 spent three days on Earth's Moon in April 1972.  The fifth lunar landing mission out of six, Apollo 16 was famous for deploying and using an ultraviolet telescope as the first lunar observatory, and for collecting rocks and data on the mysterious lunar highlands.   In the above picture, astronaut John W. Young photographs Charles M. Duke, Jr. collecting rock samples at the Descartes landing site.  Duke stands by Plum Crater while the Lunar Roving Vehicle waits parked in the background. The Lunar Roving Vehicle allowed the astronauts to travel great distances to investigate surface features and collect rocks. High above, Thomas K. Mattingly orbits in the Command Module.
Mercury on the Horizon (2000-03-20)

Have you ever seen the planet Mercury?  Because Mercury orbits so close to the Sun, it is never seen far from the Sun, and so is only visible near sunrise or sunset.  If trailing the Sun, Mercury will be visible for several minutes before it follows the Sun behind the Earth.  If leading the Sun, Mercury will be visible for only several minutes before the Sun rises and hides it with increasing glare.  An informed skygazer can usually pick Mercury out of a dark horizon glow with little more than determination.  Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during the middle of last month.  Each picture was taken from the same location in Spain when the Sun was 10 degrees below the horizon and superposed on the single most photogenic sunset.
HH111's 12 Light-Year Star Jet (2000-03-21)

Tomorrow's picture: Spherule from Space  < | 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.
A Spherule from Outer Space (2000-03-22)

When a meteorite strikes the Moon, the energy of the impact melts some of the splattering rock, a fraction of which might cool into tiny glass beads.  Many of these glass beads were present in lunar soil samples returned to Earth by the Apollo missions.  Pictured above is one such glass spherule that measures only a quarter of a millimeter across.  This spherule is particularly interesting because it has been victim to an even smaller impact.  A miniature crater is visible on the upper left, surrounded by a fragmented area caused by the shockwaves of the small impact.  By dating many of these impacts, some astronomers estimate that cratering on our Moon increased roughly 500 million years ago and continues even today.
Inside Mars (2000-03-23)

What's inside Mars? From orbit, the Mars Global Surveyor (MGS) spacecraft has recorded detailed images of the red planet since July 1997. Still, its cameras can not look beneath the surface. But minute changes in the spacecraft's orbital velocity are produced by variations in the planet's gravitational field, and these changes are related to interior density fluctuations. When the subtle orbital changes were measured using MGS radio science experiments and combined with the accurate Mars Orbiter Laser Altimeter topographical data, researchers were able to produce a map of the thickness of the martian crust. In this color cut-away diagram of the results, red colors correspond to thin and blue to thick areas of the crust which rides above the martian mantle. From the global map, the crust is seen to range from about 20 to 50 miles thick and shows a dramatic difference between the generally thinner northern hemisphere to thicker southern hemisphere crust. For the newly formed planet, the thin crust would have promoted rapid cooling and may have given rise to a large northern ocean on early Mars.
A Mystery In Gamma Rays (2000-03-24)

Gamma rays are the most energetic form of light, packing a million or more times the energy of visible light photons. What if you could see gamma rays? If you could, 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. These sources are very different from the powerful gamma-ray bursters that flash and fade rapidly from view, and 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 the already known classes of gamma-ray emitters and are simply obscured or too faint to be otherwise positively identified. However, astronomers recently 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.
The Earth Also Rises (2000-03-25)

The Lunar Orbiter 1 spacecraft was launched in 1966 to map the lunar surface in preparation for the Apollo moon landings. NASA's plucky robotic explorer performed its job well and pioneered this classic view of the Earth poised above the lunar horizon. The first humans to directly witness a similar scene were the Apollo 8 astronauts. As they orbited the Moon in December of 1968 they also recorded Earth rise in a photograph that was to become one of the most famous images in history - a moving portrait of our world from deep space.
Venus' Once Molten Surface (2000-03-26)

If you could look at Venus with radar eyes - this is what you might see. This computer reconstruction of the surface of Venus was created from data from the Magellan spacecraft.  Magellan orbited Venus and used radar to map our neighboring planet's surface between 1990 and 1994. Magellan found many interesting surface features, including the large circular domes, typically 25-kilometers across, that are depicted above.  Volcanism is thought to have created the domes, although the precise mechanism remains unknown. Venus' surface is so hot and hostile that no surface probe has lasted more than a few minutes.
Flying Over Asteroid Eros (2000-03-27)

What would it look like to fly over an asteroid?  Spacecraft NEAR Shoemaker in orbit around asteroid 433 Eros found out earlier this month when it recorded its first fly-over sequence.  The saddle region of the Sun-orbiting space-mountain appears to zip past the camera in this condensed hour-long time-lapse sequence.  The spacecraft was orbiting about 200 kilometers above the asteroid.  Movies such as this are scientifically useful for discerning between regions that are naturally dark and regions that have their brightness dominated by shadows.  The week before, a bright X-ray burst from the Sun allowed NEAR's X-ray spectrometer to detect the presence of several elements on Eros' surface by their X-ray fluorescent signatures.
M20: The Trifid Nebula (2000-03-28)

Unspeakable beauty and unimaginable bedlam can be found together in the Trifid Nebula. Also known as M20, this photogenic nebula is visible with good binoculars towards the constellation of Sagittarius. The energetic processes of star formation create not only the colors but the chaos.  The red-glowing gas results from high-energy starlight striking interstellar hydrogen gas.  The dark dust filaments that lace M20 were created in the atmospheres of cool giant stars and in the debris from supernovae explosions.  Which bright young stars light up the blue reflection nebula is still being investigated.  The light from M20 we see today left perhaps 3000 years ago, although the exact distance remains unknown.  Light takes about 50 years to cross M20.

Image credit: Todd Boroson
Fullerenes as Miniature Cosmic Time Capsules (2000-03-29)

Scientists have found, unexpectedly, tiny time capsules from billions of years in the past.  The discovery involves small molecules that can apparently become trapped during the formation of large enclosed molecules known as fullerenes, or buckyballs.  Luann Becker  (U. Hawaii) and collaborators recently found fullerenes in an ancient meteorite that fell to Earth about 30 years ago.  Extra-terrestrial fullerenes inside the meteorite survived, and upon inspection, were not empty inside.  The small molecules trapped inside are giving a glimpse of what the Solar System was like during its formation.  Pictured above is a computer simulation showing a relatively small fullerene (60-atoms of carbon) situated above a hydrogenated silicon surface.  How these fullerenes formed, how they survived, where else they can be found, and what else might be found inside these tiny time capsules is developing into an exciting  area of research.

Image credit: Keith Beardmore 
(LANL)
Saturn-Sized Worlds Discovered (2000-03-30)

The last decade saw the profound discovery of many worlds beyond our solar system, but none analogs of our home planet Earth. Exploiting precise observational techniques, astronomers inferred the presence of well over two dozen extrasolar planets, most nearly as massive as gas giant Jupiter or more, in close orbits around sun-like stars. Less massive planets must certainly exist, and yesterday preeminent planet-finders announced the further detection of two more new worlds -- each a potentially smaller, saturn-sized planet.  The parent suns are 79 Ceti (constellation Cetus), at a distance of 117 light-years, and HD46375 (constellation Monoceros), 109 light-years away. With at least 70 percent the mass of Saturn, 79 Ceti's planet orbits on average 32.5 million miles from the star compared to 93 million miles for the Earth-Sun distance. This arresting artist's vision depicts the newly discovered world with rings and moons, known characteristics of giant planets in our solar system. HD46375's planet is at least 80 percent Saturn's mass, orbiting only 3.8 million miles from its parent star. While Saturn's mass is only one third of Jupiter's, it is still about 100 times that of Earth, and dramatic discoveries in the search for smaller planets are still to come.
Free-Floating Planets In Orion (2000-03-31)

This false-colour image of the young Trapezium star cluster in the Orion Nebula was made with an infrared camera at wavelengths about twice as long as visible light. The infrared data are part of a sensitive survey of this nearby star forming region in which astronomers have identified over 100 extremely low mass objects -- candidates for elusive brown dwarf stars. Brown dwarfs are failed stars with masses so low (about 8% of the Sun's) that they can not sustain nuclear hydrogen burning, a sun-like star's main energy source. While brown dwarfs are thought to be still massive enough to burn deuterium for energy, thirteen of the low mass objects show evidence of lying below even the deuterium burning limit (about 1.3% of the Sun's mass) falling in a range more commensurate with giant planets. These drifting, "free-floating planets" are perhaps as little as 8 times as massive as Jupiter and likely formed along with the cluster stars a million or so years ago. They are detectable in the infrared because they are still hot from formation, but will eventually cool and fade. If the Trapezium is typical of young star clusters, then the survey results suggest that brown dwarfs and free-floating planets may be fairly common, but there are not enough to solve the mystery of dark matter in the Universe.
Planet Earth from TIROS 1: First TV Image (2000-04-01)

The Television InfraRed Observational Satellite (TIROS) 1 was the first weather satellite. Launched into a polar orbit 40 years ago (April 1, 1960) equipped with two TV cameras, TIROS 1 was operational for only 78 days but demonstrated the feasibility of monitoring planet Earth's cloud cover and weather patterns from space. TIROS satellites eventually began continuous coverage in 1962 and enabled accurate worldwide weather forecasts and alerts. Above is the first TIROS TV image, taken from an altitude of about 700 kilometers. Crude by contemporary standards, it represents the beginning of what is still one of the most important continuing applications of space technology.
Eagle EGGs in M16 (2000-04-02)

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 Twisted Solar Eruptive Prominence (2000-04-03)

A huge eruptive prominence is seen moving out from our Sun in this condensed half-hour time-lapse sequence.  Ten Earths could easily fit in the "claw" of this seemingly solar monster.  This large prominence, though, is significant not only for its size, but its shape.  The twisted figure eight shape indicates that a complex magnetic field threads through the emerging solar particles.  Recent evidence of differential rotation inside the Sun might help account for the surface explosion.  The sequence was taken early this year by the Sun-orbiting SOHO satellite.  Although large prominences and energetic Coronal Mass Ejections (CMEs) are relatively rare, they are occurring more frequently now that we are near the Solar Maximum, a time of peak sunspot and solar activity in the eleven-year solar cycle.
A Superwind from the Cigar Galaxy (2000-04-04)

What's lighting up the Cigar Galaxy?  M82, as this irregular galaxy is also known, was stirred up by a recent pass near large spiral galaxy M81.  This doesn't fully explain the source of the red-glowing outwardly expanding gas, however.  Recent evidence indicates that this gas is being driven out by the combined emerging particle winds of many stars, together creating a galactic "superwind."  The above recently released photograph from the new Subaru Telescope highlights the specific color of red light strongly emitted by ionized hydrogen gas, showing detailed filaments of this gas.  The filaments extend for over 10,000 light years.  The 12-million light-year distant Cigar Galaxy is the brightest galaxy in the sky in infrared light, and can be seen in visible light with a small telescope towards the constellation of Ursa Major.
The M7 Open Star Cluster in Scorpius (2000-04-05)

M7 is one of the most prominent open clusters of stars on the sky.  The cluster, dominated by bright blue stars, can be seen with the naked eye in a dark sky in the tail of the constellation of Scorpius.  M7 contains about 100 stars in total, is about 200 million years old, spans 25 light-years across, and lies about 1000 light-years away.   This color picture was taken in 1995 at the Burrell-Schmidt Telescope at Kitt Peak National Observatory in Arizona.  The M7 star cluster has been known since ancient times, being noted by Ptolemy in the year 130 AD.  Also visible is a dark dust cloud near the bottom of the frame, and literally millions of unrelated stars towards the Galactic center.

Image credit: N. A. Sharp
Venus, Moon, and Neighbors (2000-04-06)

Rising before the Sun on February 2nd, astrophotographer Joe Orman anticipated this apparition of the bright morning star Venus near a lovely crescent Moon above a neighbor's house in suburban Phoenix, Arizona, USA. Fortunately, the alignment of bright planets and the Moon is one of the most inspiring sights in the night sky and one that is often easy to enjoy and share without any special equipment. Take tonight, for example. Those blessed with clear skies can simply step outside near sunset and view a young crescent Moon very near three bright planets in the west Jupiter, Mars, and Saturn. Jupiter will be the unmistakable brightest star near the Moon with a reddish Mars just to Jupiter's north and pale yellow Saturn directly above. Of course, these sky shows create an evocative picture but the planets and Moon just appear to be near each other -- they are actually only approximately lined up and lie in widely separated orbits. Unfortunately, next month's highly publicized alignment of planets on May 5th will be lost from view in the Sun's glare but such planetary alignments occur repeatedly and pose no danger to planet Earth.

Image credit: Joe Orman
Celebrating Hubble With NGC 6751 (2000-04-07)

Planetary nebulae do look simple, round, and planet-like in small telescopes. But images from the orbiting Hubble Space Telescope have become well known for showing these fluorescent gas shrouds of dying Sun-like stars to possess a staggering variety of detailed symmetries and shapes. This composite color Hubble image of NGC 6751 is a beautiful example of a classic planetary nebula with complex features and was selected to commemorate the tenth anniversary of Hubble in orbit. The colors were chosen to represent the relative temperature of the gas - blue, orange, and red indicating the hottest to coolest gas. Winds and radiation from the intensely hot central star (140,000 degrees Celsius) have apparently created the nebula's streamer-like features. The nebula's actual diameter is approximately 0.8 light-years or about 600 times the size of our solar system. NGC 6751 is 6,500 light-years distant in the constellation Aquila.
Compton Observatory In Orbit (2000-04-08)

Nine years ago the massive Compton Gamma Ray Observatory was deployed in low earth orbit. Sparkling interior reflections and the bright limb of the Earth are visible in this 1991 window view of Compton's release by the crew of the Space Shuttle Atlantis. Lofted above the protective atmosphere, Compton's instruments could explore the extreme high-energy Universe in gamma rays, photons with 100,000 times or more the energy of visible light. The premier gamma-ray observatory far exceeded expectations for a two- to five-year mission, but a recent gyroscope failure has prompted NASA to decide to steer the satellite safely back into the atmosphere. Compton's lasting legacy of discovery will include the detection of more than 400 celestial gamma-ray sources, 10 times more than were previously known; and more than 2,500 gamma-ray bursts.
Mysterious Pluto and Charon (2000-04-09)

Pluto is the only planet in our Solar System remaining unphotographed by a passing spacecraft. Distant Pluto and its moon Charon therefore remain somewhat mysterious.  In addition to direct imaging by the Hubble Space Telescope, careful tracking of brightness changes that occur as each object eclipses the other have allowed astronomers to build up the above black & white surface maps.  These maps depict the face of Pluto (left) that always faces Charon, and the face of Charon that always faces away from Pluto. The rectangular pixels are an artifact of the mapping software. The Pluto-Kuiper Express mission is tentatively planned for launch in 2004 and might encounter Pluto as early as 2012.
Aurora in Red and Yellow (2000-04-10)

The past week brought some spectacular aurora to northern skies.  These aurorae were caused by a large interplanetary shock wave that exploded from the Sun on April 4.  When the shock wave reached the Earth on April 6, the resulting aurora could be seen in clear skies as far south as North Carolina.  As the aurorae occurred high in the Earth's atmosphere, they were accompanied by an unusual alignment of planets far in the background.  Pictured above that night, an unusual multicolored auroral display graced the skies above the domes of the Brno Observatory in the Czech Republic.

Image credit: Jan Safar
(Brno Observatory)