juliensimon/nasa-apod huggingface dataset:
A Tether in Space (2005-03-28)

One of the greatest unrequited legends of outer space is the tether.  Tethers, long strands of material, hold the promise of stabilizing satellites, generating electricity, and allowing easy transportation.  Possibly the most extreme vision of the space tether is the space elevator popularized by Arthur C. Clarke, where a tether is constructed that connects the ground to geosynchronous orbit.  One problem is strength - it is difficult to make a long useful tether that does not snap.  To help realize more of the tremendous potential of space tethers, NASA and the SpaceWard Foundation have issued the Centennial Tether Challenge, promising a monetary reward for the best space tether that exceeds a minimum standard.  Pictured above is the deployment of the Tethered Satellite System 1 (TSS-1) by the space shuttle Altantis in 1992.  Like other tested tethers, TSS-1 failed to live up to its promise, although many valuable lessons were learned.
Crescents of Titan and Dione (2005-03-29)

What would it be like to see a sky with many moons?  Such is the sky above Saturn.  When appearing close to each other, moons will show a similar phase. A view with two of the more famous moons of Saturn in crescent phase was captured last month by the robot spacecraft Cassini now orbiting Saturn.  Titan, on the lower left, is among the largest moons in the Solar System and is perpetually shrouded in clouds.  Recently, the Huygens probe landed on Titan and gave humanity its first view of its unusual surface.  Dione, on the upper right, has less than a quarter of Titan's diameter and has no significant atmosphere.  Dione, although appearing smaller, was only half the distance to Titan when the above image was taken.
ULXs in M74 (2005-03-30)

In visual appearance, M74 is a nearly perfect face-on spiral galaxy, about 30 million light-years away toward the constellation Pisces.  The red blotches seen in this composite view are ultraluminous x-ray sources (ULXs) mapped by the Chandra X-ray Observatory. The ULXs are so called because they actually do radiate 10 to 1,000 times more x-ray power than "ordinary" x-ray binary stars, which harbor a neutron star or stellar mass black hole. In fact, watching these ULXs change their x-ray brightness over periods of 2 hours or so, astronomers conclude that ULXs could well be intermediate mass black holes -- black holes with masses 10,000 times or so greater than the Sun, but still much less than the million solar mass black holes which lurk in the centers of large spiral galaxies. How did these intermediate mass black holes get there? One intriguing suggestion is that they are left over from the cores of much smaller galaxies that are merging with spiral galaxy M74.
Gamma-Ray Earth (2005-03-31)

The pixelated planet above is actually our own planet Earth seen in gamma rays - the most energetic form of light. In fact, the gamma rays used to construct this view pack over 35 million electron volts (MeV) compared to a mere two electron volts (eV) for a typical visible light photon. The Earth's gamma-ray glow is indeed very faint, and this image was constructed by combining data from seven years of exposure during the life of the Compton Gamma Ray Observatory, operating in Earth orbit from 1991 to 2000. Brightest near the edge and faint near the center, the picture indicates that the gamma rays are coming from high in Earth's atmosphere. The gamma rays are produced as the atmosphere interacts with high energy cosmic rays from space, blocking the harmful radiation from reaching the surface.  Astronomers need to understand Earth's gamma-ray glow well as it can interfere with observations of cosmic gamma-ray sources like pulsars, supernova remnants, and distant active galaxies powered by supermassive black holes.   APOD Update: APOD mirror now on-line from the Beijing Planetarium in Chinese.
Water On Mars (2005-04-01)

Can you help discover water on Mars?  Finding water on different regions on Mars has implications for understanding its complex geologic history, the possible existence of past life and the sustenance of potential future astronauts.  Many space missions have taken photographs of the surface of the red planet, and some of them might show a subtle clue pointing to water on Mars that has been missed.  By close inspection of images, following curiosity, applying scientific principles, applying knowledge about features on the Martian surface, and applying principles of planetary geology, such clues might be brought to light.  In the meantime, happy April Fool's Day from the folks at APOD!

Image credit: Ellen Roper (GCC)
Cyg X-1: Can Black Holes Form in the Dark? (2005-04-02)

The formation of a black hole from the collapsing core of a massive star is thought to be heralded by a spectacular supernova explosion. Such an extremely energetic collapse is also a leading explanation for the mysterious cosmic gamma-ray bursts. But researchers now suggest that the Milky Way's most famous black hole, Cygnus X-1, was born when a massive star collapsed -- without any supernova explosion at all. Their dynamical evidence is summarized in this color image of a gorgeous region in Cygnus, showing Cyg X-1 and a cluster of massive stars (yellow circles) known as Cygnus OB3. Arrows compare the measured direction and speed of Cyg X-1 and the average direction and speed of the massive stars of Cyg OB3. The similar motions indicate that Cyg X-1's progenitor star was itself a cluster member and that its path was not altered at all when it became a black hole. In contrast, if Cyg X-1 were born in a violent supernova it would have likely received a fierce kick, changing its course. If not a supernova, could the formation of the Cyg X-1 black hole have produced a dark gamma-ray burst in the Milky Way?
The Galactic Center Radio Arc (2005-04-03)

What causes this unusual structure near the center of our Galaxy? The long parallel rays slanting across the top of the above radio image are known collectively as the Galactic Center Radio Arc and jut straight out from the Galactic plane.  The Radio Arc is connected to the Galactic center by strange curving filaments known as the Arches.  The bright radio structure at the bottom right likely surrounds a black hole at the Galactic center and is known as Sagittarius A*.  One origin hypothesis holds that the Radio Arc and the Arches have their geometry because they contain hot plasma flowing along lines of constant magnetic field.  Recent images from the Chandra X-ray Observatory appear to show this plasma colliding with a nearby cloud of cold gas.
NGC 1316: After Galaxies Collide (2005-04-04)

How did this strange-looking galaxy form?  Astronomers turn detectives when trying to figure out the cause of unusual jumbles of stars, gas, and dust like NGC 1316.   A preliminary inspection indicates that NGC 1316 is an enormous elliptical galaxy that includes dark dust lanes usually found in a spiral.  The above image taken by the Hubble Space Telescope shows details, however, that help in reconstructing the history of this gigantic jumble.  Close inspection finds fewer low mass globular clusters of stars toward NGC 1316's center. Such an effect is expected in galaxies that have undergone collisions or merging with other galaxies in the past few billion years.  After such collisions, many star clusters would be destroyed in the dense galactic center.  The dark knots and lanes of dust indicate that one or more of the devoured galaxies were spiral galaxies.  NGC 1316 spans about 60,000 light years and lies about 75 million light years away toward the constellation of the Furnace.
Light From A Distant Planet (2005-04-05)

Light emitted by a planet far beyond our Solar System has been identified for the first time.  The planet, illustrated in the above drawing, had its light detected by comparing the brightness of only the parent star, when the planet was behind the star, to the light emitted when both the planet and its parent star were visible.   The Earth-trailing Spitzer Space Telescope made the observation in infrared light, where the intrinsic glow of the planet outshines the light it reflects from its central star.  The direct observation of light allowed a measurement of both the temperature and size of the planet: HD 209458b.  Planet HD 209458b was confirmed to be larger than expected for its mass and on an orbit around its parent star that was unexpectedly close to a circle.
The M7 Open Star Cluster in Scorpius (2005-04-06)

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 recently at the Kitt Peak National Observatory in Arizona, USA as part of the Advanced Observers Program. 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: Adam Block
Solar Eclipse in View (2005-04-07)

Friday's solar eclipse will be a rare hybrid - briefly appearing as either an annular eclipse or a total eclipse when viewed from along the narrow track of the Moon's shadow. Unfortunately that track, never more than about 30 kilometers wide, lies mostly across the Pacific Ocean, beginning south of New Zealand and just ending in Venezuela. Skywatchers along the beginning and end of the shadow track will see an annular eclipse of the Sun, with the Moon's silhouette briefly surrounded by a bright ring of fire, while observers along the middle of the track will witness a total eclipse phase. But the good news is that over a much broader region of the globe, including New Zealand and much of South and North America, a partial eclipse can be seen as the Moon appears to take a bite out of the Sun. If you want to view the eclipse, take care to do it safely, and check the times for your specific location. So, what location is this solar eclipse view from? The picture above was recorded in November of 2003 from within the track of the Moon's shadow across Antarctica, of course.

Image credit: Fred Bruenjes 
(moonglow.net)
Sideways Galaxy NGC 3628 (2005-04-08)

Dark dust lanes cutting across the middle of this gorgeous island universe strongly hint that NGC 3628 is a spiral galaxy seen sideways. About 35 million light-years away in the northern springtime constellation Leo, NGC 3628 also bears the distinction of being the only member of the well known Leo triplet of galaxies not in Charles Messier's famous catalog. Otherwise similar in size to our Milky Way galaxy, the disk of NGC 3628 is clearly seen to fan out near the galaxy's edge. A faint arm of material also extends up and to the left in this deep view of the region. The distorted shape and tidal tail suggest that NGC 3628 is interacting gravitationally with the other spiral galaxies in the Leo triplet, M66 and M65.  News: Solar Eclipse Today

Image credit: Russell Croman
Inside the Elephant's Trunk (2005-04-09)

In December of 2003, the world saw spectacular first images from the Spitzer Space Telescope, including this penetrating interior view of an otherwise opaque dark globule known as the Elephant's Trunk Nebula. Seen in a composite of infrared image data recorded by Spitzer's instruments, the intriguing region is embedded within the glowing emission nebula IC 1396 at a distance of 2,450 light-years toward the constellation Cepheus. Previously undiscovered protostars hidden by dust at optical wavelengths appear as bright reddish objects within the globule. Shown in false-color, winding filaments of infrared emission span about 12 light-years and are due to dust, molecular hydrogen gas, and complex molecules called polycyclic aromatic hydrocarbons or PAHs. The Spitzer Space Telescope was formerly known as the Space Infrared Telescope Facility (SIRTF) and is presently exploring the Universe at infrared wavelengths. Spitzer follows the Hubble Space Telescope, the Compton Gamma-ray Observatory, and the Chandra X-ray Observatory as the final element in NASA's space-borne Great Observatories Program.
Venus' Once Molten Surface (2005-04-10)

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.
Clouds, Plane, Sun, Eclipse (2005-04-11)

How can part of the Sun just disappear?  When that part is really hiding behind the Moon.  Last Friday, the first partial solar eclipse of 2005 and the last total eclipse of the Sun until March 2006 was visible.  During a solar eclipse, the Sun, Moon and Earth are aligned.  The total solar eclipse was primarily visible from the Southern Pacific Ocean, while a partial solar eclipse was discoverable across South America and lower North America.  The above image composite was taken with a handheld digital camera last Friday.  After a day of rain in Mt. Holly, North Carolina, USA, a partially eclipsed Sun momentarily peeked through a cloudy sky.  After taking a sequence of images, the best eclipse shot was digitally combined with a less good eclipse shot that featured a passing airplane.

Image credit: Dorothy Verdin
Earth or Mars? (2005-04-12)

Which image is Earth, and which is Mars?  One of the above images was taken by the robot Spirit rover currently climbing Husband Hill on Mars.  The other image was taken by a human across the desert south of Morocco on Earth.  Both images show vast plains covered with rocks and sand.  Neither shows water or obvious signs of life.  Each planet has a surface so complex that any one image does not do that planet justice.  Understanding either one, it turns out, helps understand the other.  Does the one on the left look like home?  Possibly not, but it is Earth.

Image credit: Filipe Alves; 
 Mars Image Credit: 
Mars
Exploration Rover Mission,
JPL,
NASA
A Window to the Once Secret Sky (2005-04-13)

If there were a window nearby to the distant universe -- would you look through it?  Quite possibly, there is, in the form of a small telescope.  A local skykeeper could be a relative or a stranger and is frequently proud to show off the sky free of charge.  Through a window called an eyepiece, on a dark cloudless night, you can see clusters of stars, rings around Saturn, glowing nebulas of gas, craters on the Moon, and galaxies across the universe. The technology to create this window -- and the secret sky it reveals -- was unknown only 400 years ago.  Modern sky opportunities may occur this Saturday, Astronomy Day, at local amateur astronomy clubs, universities, science centers, or planetariums.  Pictured above is a small telescope being deployed at picturesque Hohe Wand, about 50 kilometers south of Vienna, Austria.  The spin of the Earth is visible in the above photo as the long star trails.

Image credit: Peter Wienerroither  
(U. Wien)
April's Moon and the Pleiades (2005-04-14)

After parting with the Sun late last week, April's moon graced the early evening sky. Its slender, three-day-old crescent shares this lovely telescopic skyview with the nearby Pleiades star cluster. Here, the Moon's sunlit crescent is overexposed while the lunar terminator, or boundary between lunar night and day, is jagged with craters and mountains. Lunar surface features can also be seen in the dim lunar night illuminated by earthshine - light from sunlit planet Earth. The sister stars of the Pleiades are grouped at the right, but their alluring blue reflection nebulae, usually highlighted in telescopic images of the cluster, are washed-out in the much brighter moonlight.

Image credit: Jerry Lodriguss
(Catching the Light)
RCW 79: Stars in a Bubble (2005-04-15)

A cosmic bubble of gas and dust, RCW 79 has grown to about 70 light-years in diameter, blown by the winds and radiation from hot young stars. Infrared light from the dust embedded in the nebula is tinted red in this gorgeous false-color view from the Spitzer Space Telescope. A good 17 thousand light-years away in the grand southern constellation Centaurus, the expanding nebula itself has triggered star formation as it plows into the gas and dust surrounding it. In fact, this penetrating infrared picture reveals groups of new stars as yellowish points scattered along the bubble's edge. One remarkable group still lies within its own natal bubble at about 7 o'clock (lower left), while another can be seen near the upper gap at about 3 o'clock (right) from the bubble's center.
Celebrating Hubble With NGC 6751 (2005-04-16)

Planetary nebulae can 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. It was selected in April of 2000, 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 high-flying constellation Aquila.
Asteroids in the Distance (2005-04-17)

Rocks from space hit Earth every day.  The larger the rock, though, the less often Earth is struck.  Many kilograms of space dust pitter to Earth daily. Larger bits appear initially as a bright meteor.  Baseball-sized rocks and ice-balls streak through our atmosphere daily, most evaporating quickly to nothing.  Significant threats do exist for rocks near 100 meters in diameter, which  strike the Earth roughly every 1000 years.  An object this size could cause significant tsunamis were it to strike an ocean, potentially devastating even distant shores.  A collision with a Massive asteroid, over 1 km across, is more rare, occurring typically millions of years apart, but could have truly global consequences.  Many asteroids remain undiscovered.  In fact, one was discovered in 1998 as the long blue streak in the above archival image taken by the Hubble Space Telescope. In 2002 June, the small 100-meter asteroid 2002 MN was discovered only after it whizzed by the Earth, passing well within the orbit of the Moon.  2002 MN passed closer than any asteroid since 1994 XM1, but not as close as 2004 MN4 will pass in 2029.  A collision with a large asteroid  would not affect Earth's orbit so much as raise dust that would affect Earth's climate.  One likely result is a global extinction of many species of life, possibly dwarfing the ongoing extinction occurring now.
Saturnian Moon and Rings (2005-04-18)

When can a robot produce art?  When it glides past the rings of Saturn. As the robot spacecraft Cassini orbiting Saturn crossed outside the famous photogenic ring plane of the expansive planet, the rings were imaged from the outside, nearly edge on, and in the shadow of Saturn.  From the upper left, ring features include the A ring, the Cassini gap, the B ring, and the darker C ring that includes the Titan gap and a gap yet unnamed.  Last month when the above image was taken, the gliding spacecraft was about one million kilometers from foreground Enceladus, a small Saturnian moon only about 500 kilometers across.  Cassini is scheduled to continue its 70 orbit tour of Saturn over the next three years, sending back images of the gas giant, its rings, and its moons that will be studied for decades to come.
Orion in Infrared (2005-04-19)

Do you recognize the constellation Orion?  This striking but unfamiliar looking picture of the familiar Orion region of the sky was produced using survey data from the now-defunct InfraRed Astronomical Satellite (IRAS).  The above image combines information recorded at three different invisible infrared wavelengths and covers about 30x24 degrees on the sky.  Most of Orion's visually impressive stars don't stand out, but bright Betelgeuse does appear as a small bright purplish dot on the lower left. The bright region on the right contains the Great Nebula in Orion, while the bright region just above the image bottom is the Rosette Nebula.  Surrounding these regions are a jumble of chaotic glowing gas and dark dust jettisoned by stars forming and exploding over millions of years.
Barnard's Loop Around Orion (2005-04-20)

Why is the belt of Orion surrounded by a bubble?  Although glowing like an emission nebula, the origin of the bubble, known as Barnard's Loop, is currently unknown.  Progenitor hypotheses include the winds from bright Orion stars and the supernovas of stars long gone.  Barnard's Loop is too faint to be identified with the unaided eye.  The nebula was discovered only in 1895 by E. E. Barnard on long duration film exposures.  Orion's belt is seen as the three bright stars across the center of the image, the upper two noticeably blue.  Just to the right of the lowest star in Orion's belt is a slight indentation in an emission nebula that, when seen at higher magnification, resolves into the Horsehead Nebula.  To the right of the belt stars is the bright, famous, and photogenic Orion Nebula.

Image credit: W. H. Wang
G21.5-0.9: A Supernova's Cosmic Shell (2005-04-21)

The picture is lovely, but this pretty cosmic shell was produced by almost unbelievable violence - created when a star with nearly 20 times the mass of the sun blasted away its outer layers in a spectacular supernova explosion. As the expanding debris cloud swept through surrounding interstellar material, shock waves heated the gas causing the supernova remnant to glow in x-rays. In fact, it is possible that all supernova explosions create similar shells, some brighter than others. Cataloged as G21.5-0.9, this shell supernova remnant is relatively faint, requiring about 150 hours of x-ray data from the orbiting Chandra Observatory to create this false-color image. G21.5-0.9 is about 20,000 light-years distant in the constellation Scutum and measures about 30 light-years across. Based on the remnant's size, astronomers estimate that light from the original stellar explosion first reached Earth several thousand years ago.
Albert Einstein's Miraculous Year (2005-04-22)

In 1905 Albert Einstein had a miraculous year. One hundred years ago, he wrote four papers which revolutionized our understanding of the Universe. The papers outlined; the idea that light could behave as a quantized particle (a photon), an explanation of the thermal motion of atoms and molecules (at a time when atoms themselves were just theories), a theory reconciling motion and the constant speed of light (Special Relativity), and the idea of mass-energy equivalence (E=mc²). Virtually every facet of our modern exploration of the Universe is touched by his now century old insights, along with his later theory of gravity and space-time - General Relativity. In centennial celebration, consider this thoughtful view of a small telescope beside the Einstein Memorial on the grounds of the National Academy of Sciences in Washington DC, USA. The marble platform at the bronze statue's feet is embedded with a map showing the positions of the planets, sun, moon and stars on the memorial's dedication date, 100 years after Einstein's birth in 1879. Albert Einstein died 50 years ago, on April 18, 1955.

Image credit: Greg Piepol