juliensimon/nasa-apod huggingface dataset:
Sandy Gas Jets Hypothesized on Mars (2006-08-23)

What's causing seasonal dark spots on Mars?  Every spring, strange dark spots appear near the Martian poles, and then vanish a few months later.  These spots typically span 50 meters across and appear fan shaped.  Recent observations made with THEMIS instrument onboard NASA's Mars Odyssey, currently orbiting Mars, found the spots to be as cold as the carbon dioxide (CO2) ice beneath them.  Based on this evidence, a new hypothesis has been suggested where the spots are caused by explosive jets of sand-laden CO2.  As a pole warms up in the spring, frozen CO2 on the surface thins, perforates, and begins to vent gaseous CO2 held underneath.   Within this hypothesis, interspersed dark sand would explain the color of the spots, while the underlying frozen CO2 would explain the coolness of the spots.  Pictured above, an artist depicts what it might be like to stand on Mars and witness the venting of these tremendous gas and dust jets.

Image credit: Ron Miller (ASU)
The Matter of the Bullet Cluster (2006-08-24)

The matter in galaxy cluster 1E 0657-56, fondly known as the "bullet cluster", is shown in this composite image. A mere 3.4 billion light-years away, the bullet cluster's individual galaxies are seen in the optical image data, but their total mass adds up to far less than the mass of the cluster's two clouds of hot x-ray emitting gas shown in red. Representing even more mass than the optical galaxies and x-ray gas combined, the blue hues show the distribution of dark matter in the cluster. Otherwise invisible to telescopic views, the dark matter was mapped by observations of gravitational lensing of background galaxies. In a text book example of a shock front, the bullet-shaped cloud of gas at the right was distorted during the titanic collision between two galaxy clusters that created the larger bullet cluster itself. But the dark matter present has not interacted with the cluster gas except by gravity. The clear separation of dark matter and gas clouds is considered direct evidence that dark matter exists.
Blue Lagoon (2006-08-25)

Stars come and go as you slide your cursor over this engaging image of M8, aka the Lagoon Nebula. Of course, the nebula is itself a star-forming region, but the stars that appear and disappear here include background and foreground stars that by chance lie along the same line of sight. In this "for fun" comparison of two nearly identical digital images, the stellar point sources were removed from one image by computer processing to leave only the diffuse emission from the glowing gas clouds. In both pictures, red emission (H-alpha emission) from atomic hydrogen dominates the cosmic lagoon's visible light, but narrow band filters were used to record the image data and map the hydrogen emission to green hues, with emission from sulfur atoms in red and oxygen in blue. The lovely Lagoon Nebula spans about 30 light-years at an estimated distance of 5,000 light-years toward the constellation Sagittarius.

Image credit: Russell Croman
Apollo 17: VIP Site Anaglyph (2006-08-26)

Get out your red/blue glasses and check out this stereo scene from Taurus-Littrow valley on the Moon! The color anaglyph features a detailed 3D view of Apollo 17's Lunar Rover in the foreground -- behind it lies the Lunar Module and distant lunar hills. Because the world was  going to be able to watch the Lunar Module's ascent stage liftoff via the rover's TV camera, this parking place was also known as the VIP Site. In December of 1972, Apollo 17 astronauts Eugene Cernan and Harrison Schmitt spent about 75 hours on the Moon, while colleague Ronald Evans orbited overhead. The crew returned with 110 kilograms of rock and soil samples, more than from any of the other lunar landing sites. Cernan and Schmitt are still the last to walk (or drive) on the Moon.
Barred Spiral Galaxy NGC 1300 (2006-08-27)

Big, beautiful, barred spiral galaxy NGC 1300 lies some 70 million light-years away on the banks of the constellation Eridanus. This Hubble Space Telescope composite view of the gorgeous island universe is one of the largest Hubble images ever made of a complete galaxy. NGC 1300 spans over 100,000 light-years and the Hubble image reveals striking details of the galaxy's dominant central bar and majestic spiral arms. In fact, on close inspection the nucleus of this classic barred spiral itself shows a remarkable region of spiral structure about 3,000 light-years across. Unlike other spiral galaxies, including our own Milky Way, NGC 1300 is not presently known to have a massive central black hole.
Eight Planets and New Solar System Designations (2006-08-28)

How many planets are in the Solar System?  This popular question now has a new formal answer according the International Astronomical Union (IAU): eight.  Last week, the IAU voted on a new definition for planet and Pluto did not make the cut.  Rather, Pluto was re-classified as a dwarf planet and is considered as a prototype for a new category of trans-Neptunian objects.  The eight planets now recognized by the IAU are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.  Solar System objects now classified as dwarf planets are: Ceres, Pluto, and the currently unnamed 2003 UB313.  Planets, by the new IAU definition, must be in orbit around the sun, be nearly spherical, and must have cleared the neighborhood around their orbits.  The demotion of Pluto to dwarf planet status is a source of continuing dissent and controversy in the astronomical community.
Supernova Remnant E0102 from Hubble (2006-08-29)

It's the blue wisp near the bottom that's the remnant of a tremendous recent supernova explosion.  The large pink structure looming to the upper right is part of N76, a large star forming region in our neighboring Small Magellanic Cloud (SMC) galaxy.   The supernova remnant wisp, with full coordinate name 1E0102.2-7219 and frequently abbreviated as E0102, also lies in the SMC, about 50 light years away from N76.   The above image is a composite of several images taken by the Hubble Space Telescope.  E0102 is of research interest because we see it as it appeared only 2,000 years after its explosion.  Examination of E0102 therefore gives clues about how an enigmatic supernova works and what materials it dispersed into the surrounding interstellar medium.
A Backward Sunspot and the New Solar Cycle (2006-08-30)

Why is sunspot 905 backwards?  Perhaps it is a key marker for the beginning of a new magnetic cycle on our Sun.  Every 11 years, our Sun goes through a magnetic cycle, at the end of which its overall magnetic orientation is reversed.   An 11-year solar cycle has been observed for hundreds of years by noting peaks and valleys in the average number of sunspots. Just now, the Sun is near Solar Minimum, and likely to start a long progression toward the most active time, called Solar Maximum, in about 5.5 years.  An indicator that the sun's magnetic field is reversing is the appearance of sunspots with the reverse magnetic polarity than normal.  A few weeks ago, one small candidate reverse sunspot was sighted but faded quickly.  Now, however, a larger sunspot with negative polarity is being tracked.  This sunspot, numbered 905, appears as the unusual white spot in the above magnetic image of the Sun taken with the SOHO spacecraft a few days ago.   In the past few days, Sunspot 905 has actually begun to break apart and might also become the source of coronal mass ejections and explosive solar flares. Solar astronomers predict that the coming Solar Maximum will be unusually active.
Extra Galaxies (2006-08-31)

Careful inspection of the full field of view for this sharp composite image reveals a surprising number of galaxies both near and far toward the constellation Ursa Major. The most striking is clearly NGC 3718, a warped spiral galaxy found near picture center. NGC 3718's faint spiral arms look twisted and extended, its bright central region crossed by obscuring dust lanes. A mere 150 thousand light-years to the right is another large spiral galaxy, NGC 3729. The two are likely interacting gravitationally, accounting for the peculiar appearance of NGC 3718. While this galaxy pair lies about 52 million light-years away, the remarkable Hickson Group 56 can also be seen clustered just below NGC 3718. Hickson Group 56 consists of five interacting galaxies and lies over 400 million light-years away.

Image credit: 2006 Astr. Campers,
Adam Block
(Caelum Obs.),
CSS,
U. Arizona
 Alum. Assoc.
Gemini South Star Trails (2006-09-01)

Stars seem to arc through southern skies in this surrealistic time exposure -- recorded before moonrise from the Gemini South Observatory, Cerro Pachon, Chile, Planet Earth. During the one hour 40 minute exposure camera and tripod were fixed, so the concentric star trails are a reflection of Earth's daily rotation about its axis. The view looks to the south and includes the Gemini telescope enclosure in the foreground. At the apparent center of the curving trails, the South Celestial Pole lies just off the upper left edge. Two faint, wide streaks track the Magellanic Clouds, satellites of the Milky Way Galaxy, while a meteor flashes throught the scene just left of the observatory.

Image credit: Elke Schulz
Dusty Spiral M66 (2006-09-02)

When morning twilight came to the Paranal Observatory in Chile, astronomers Mark Neeser and Peter Barthel interrupted their search for faint quasars, billions of light-years away. And just for a moment, they used Very Large Telescopes at the European Southern Observatory to appreciate the beauty of the nearby Universe. One result was this stunning view of beautiful spiral galaxy M66, a mere 35 million light-years away. About 100 thousand light-years across with striking dust lanes and bright star clusters along sweeping spiral arms, M66 is well known to astronomers as a member of the Leo Triplet of galaxies. Gravitational interactions with its neighborhood galaxies have likely influenced the shape of dusty spiral M66.
Pluto in True Color (2006-09-03)

Pluto is mostly brown.  The above picture captures the true colors of Pluto as well as the highest surface resolution so far recovered.  Although no spacecraft has yet visited this distant world, the New Horizons spacecraft launched early this year is expected to reach Pluto in 2015. Pluto recent reclassification, by the International Astronomical Union, from planet to dwarf planet remains a topic of much debate. The above map was created by tracking brightness changes from Earth of Pluto during times when it was being partially eclipsed by its moon Charon.  The map therefore shows the hemisphere of Pluto that faces Charon.  Pluto's brown color is thought dominated by frozen methane deposits metamorphosed by faint but energetic sunlight. The dark band below Pluto's equator is seen to have rather complex coloring, however, indicating that some unknown mechanisms may have affected Pluto's surface.
The Large Magellanic Cloud in Infrared (2006-09-04)

Where does dust collect in galaxies? To help find out, a team of researchers took the most detailed image ever of gas clouds and dust in the neighboring Large Magellanic Cloud (LMC) galaxy.  The composite image, shown above, was taken by the Spitzer Space Telescope in infrared light, which highlights the natural glow of the warm materials returned to the interstellar medium by stars. The above mosaic combines 300,000 individual pointings to create a composite 1,000-times sharper than any previous LMC image. Visible are vast clouds of gas and dust, showing in graphic detail that dust prefers regions near young stars (red-tinted bright clouds), scattered unevenly between the stars (green-tinted clouds), and in shells around old stars (small red dots).  Also visible are huge caverns cleared away by the energetic outflows of massive former stars. The faint blue (false-color) glow across the bottom is the combined light from the old stars in the central bar of the LMC.   The LMC is a satellite galaxy to our own Milky Way Galaxy, spans about 70,000 light years, and lies about 160,000 light years away toward the southern constellation of the Swordfish (Dorado).
Bright Cliffs Across Saturn's Moon Dione (2006-09-05)

What causes the bright streaks on Dione?  Recent images of this unusual moon by the robot Cassini spacecraft now orbiting Saturn are helping to crack the mystery.  Close inspection of Dione's trailing hemisphere, pictured above, indicates that the white wisps are composed of deep ice cliffs dropping hundreds of meters. The cliffs may indicate that Dione has undergone some sort of tectonic surface displacements in its past. The bright ice-cliffs run across some of Dione's many craters, indicating that the process that created them occurred later than the impacts that created those craters.  Dione is made of mostly water ice but its relatively high density indicates that it contains much rock inside.  Giovanni Cassini discovered Dione in 1684.  The above image was taken at the end of July from a distance of about 263,000 kilometers. Other high resolution images of Dione were taken by the passing Voyager spacecraft in 1980.
Green Aurora Over Lake Superior (2006-09-06)

What if your horizon was green?  If you've got a camera, take a picture!  That was the experience of Jeff Hapeman last week when visiting the Pictured Rocks National Lakeshore in Michigan.  On a quiet night toward the northern horizon over Lake Superior was a long lasting diffuse green aurora.  The above image was taken in an effort to capture the sense of wonder one gets when watching an auroral display. Auroras are sparked by energetic particles from the Sun impacting the magnetic environment around the Earth.  Resultant energetic particles such as electrons and protons rain down near the Earth's poles and impact the air.  The impacted air molecules temporarily lose electrons, and when oxygen molecules among them reacquire these electrons, they emit green light.  Auroras are known to have many shapes and colors.

Image credit: Jeff Hapeman
Colorful Moon Mosaic (2006-09-07)

No single exposure can easily capture faint stars along with the subtle colors of the Moon. But this dramatic composite view highlights both. The mosaic digitally stitches together fifteen carefully exposed high resolution images of a bright, gibbous Moon and a representative background star field. The fascinating color differences along the lunar surface are real, though highly exaggerated, corresponding to regions with different chemical compositions. And while these color differences are not visible to the eye even with a telescope, moon watchers can still see a dramatic lunar presentation tonight. A partial eclipse of the Moon will be visible from Europe, Africa, Asia, and Australia.

Image credit: Noel Carboni
Messier 110 (2006-09-08)

This very sharp telescopic vista features the last object in the modern version of Charles Messier's catalog of bright clusters and nebulae - Messier 110. A dwarf elliptical galaxy, M110 (aka NGC 205) is actually a bright satellite of the large spiral galaxy Andromeda, making M110 a fellow member of the local group of galaxies. Seen through a foreground of nearby stars, M110 is about 15,000 light-years across. That makes it comparable in size to satellite galaxies of our own Milky Way, the Large and Small Magellanic Clouds. Though elliptical galaxies are normally thought to be lacking in gas and dust to form new stars, M110 is known to contain young stars, and faint dust clouds can easily be seen in this detailed image at about the 7 and 11 o'clock positions relative to the galaxy center.

Image credit: Johannes Schedler
(Panther Observatory)
Shadow Play (2006-09-09)

During September 7th's lunar eclipse, the Moon slid through the Earth's shadow. Extending into space, Earth's cone-shaped shadow has two distinct parts, the lighter, outer part or penumbra, and the darker, inner shadow called the umbra. For this eclipse, the lunar disk just grazed the shadow's dark inner umbra. As a result, only a small part of the Moon was noticeably eclipsed, but the performance still attracted the attention of Moon watchers along the Earth's night side. In this creative scene, eclipse enthusiasts have matched the curve of a hoop to the eclipsed portion of the Moon - demonstrating the apparent size and position of the lunar disk relative to the umbra and penumbra. Of course, it's only shadow play.

Image credit: Laurent Laveder
(PhotoAstronomique.net)
Star Clusters Young and Old (2006-09-10)

Many stars form in clusters. Galactic or open star clusters are relatively young swarms of bright stars born together near the plane of our Milky Way Galaxy. Separated by about a degree on the sky, two nice examples are M46 (upper left) 5,400 light-years in the distance and M47 (lower right) only 1,600 light-years away toward the nautical constellation Puppis. Around 300 million years young M46 contains a few hundred stars in a region about 30 light-years across. Aged 80 million years, M47 is a smaller but looser cluster of about 50 stars spanning 10 light-years. But this portrait of stellar youth also contains an ancient interloper. The small, colorful patch of glowing gas in M46 is actually the planetary nebula NGC 2438 - the final phase in the life of a sun-like star billions of years old. NGC 2438 is estimated to be only 3,000 light-years distant and likely represents a foreground object, only by chance appearing along our line of sight to youthful M46.

Image credit: Chris Hetlage
Eclipsed Moon Rising Over England (2006-09-11)

Last Thursday, part of our Moon turned dark.  The cause, this time, was not a partial lunar phase -- the Moon was full -- but rather that part of the Moon went into Earth's shadow.  The resulting partial lunar eclipse was visible from the eastern Atlantic Ocean through Europe, Africa, and Asia and into the western Pacific Ocean.  The darkest part of the lunar eclipse, when part of the Moon was completely shielded from sunlight, lasted about 90 minutes. Pictured above, a partially eclipsed Moon is seen rising over an estate in Huddersfield, England.  The above image was taken far away from the house in the foreground, as only this would allow it to appear as angularly small as the half-degree Moon far in the background.   A setting twilight Sun lit the foreground. The next eclipse of the Moon will occur in March 2007.

Image credit: Gain Lee
Saturn at Night (2006-09-12)

This is what Saturn looks like at night.  In contrast to the human-made lights that cause the nighttime side of Earth to glow faintly, Saturn's faint nighttime glow is primarily caused by sunlight reflecting off of its own majestic rings.  The above image of Saturn at night was captured in July by the Cassini spacecraft now orbiting Saturn.  The above image was taken when the Sun was far in front of the spacecraft.  From this vantage point, the northern hemisphere of nighttime Saturn, visible on the left, appears eerily dark.  Sunlit rings are visible ahead, but are abruptly cut off by Saturn's shadow. In Saturn's southern hemisphere, visible on the right, the dim reflected glow from the sunlit rings is most apparent. Imprinted on this diffuse glow, though, are thin black stripes not discernable to any Earth telescope -- the silhouetted C ring of Saturn. Cassini has been orbiting Saturn since 2004 and its mission is scheduled to continue until 2008.
Atlantis to Orbit (2006-09-13)

Birds don't fly this high.  Airplanes don't go this fast.  The Statue of Liberty weighs less.  No species other than human can even comprehend what is going on, nor could any human just a millennium ago.  The launch of a rocket bound for space is an event that inspires awe and challenges description.  Pictured above, the Space Shuttle Atlantis lifted off to visit the International Space Station during the morning of 2006 September 9.  From a standing start, the two million kilogram rocket ship left to circle the Earth where the outside air is too thin to breathe and where there is little noticeable onboard gravity.  Rockets bound for space are now launched from somewhere on Earth about once a week.
M33: Spiral Galaxy in Triangulum (2006-09-14)

The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this detailed, wide field image nicely shows off M33's blue star clusters and pinkish star forming regions which trace the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 1 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Image credit: Thomas V. Davis
11 Hour Star Trails (2006-09-15)

Fix your camera to a tripod, lock the shutter open, and you can make an image of star trails - graceful concentric arcs traced by the stars as planet Earth rotates on its axis. Of course, the length of the star trails will depend on the exposure time. While exposures lasting just five minutes produce a significant arc, in about 12 hours a given star would trace out half a circle. But in any long exposure, the background glow from light-polluted skies can build up to wash out the trails. Still, astronomer Josch Hambsch produced this stunning composite of star trails around the South Celestial Pole with an effective "all night" exposure time of almost 11 hours. To do it, he combined 128 consecutive five minute long digital exposures recorded in very dark night skies above Namibia. In his final image, the background glow on the right is due in part to the faint, arcing Milky Way.

Image credit: Josch Hambsch
Discovery Orbiter Anaglyph (2006-09-16)

Approaching the International Space Station on STS-121 in July, the Shuttle Orbiter Discovery posed for a series of photographs. The process was part of an inspection to check for damage to the orbiter, but against the backdrop of planet Earth 300 kilometers below, the pictures themselves are stunning. Stereo artist Patrick Vantuyne has combined two of them (ISS013e48787 and ISS013e48788) to produce this dramatic 3D image. The stereo anaglyph is intended to be viewed with red/blue glasses. Details visible along the forward fuselage include high temperature (black) and low temperature (white) insulation tiles, thrusters used for steering and attitude control, and crew compartment windows.
Anticrepuscular Rays Over Florida (2006-09-17)

What's happening over the horizon?  Although the scene may appear somehow supernatural, nothing more unusual is occurring than a setting Sun and some well placed clouds.  Strangely, the actual sunset was occurring in the opposite direction from where the camera was pointing.  Pictured above are anticrepuscular rays.  To understand them, start by picturing common crepuscular rays that are seen any time that sunlight pours though scattered clouds.  Now although sunlight indeed travels along straight lines, the projections of these lines onto the spherical sky are great circles.  Therefore, the crepuscular rays from a setting (or rising) sun will appear to re-converge on the other side of the sky.  At the anti-solar point 180 degrees around from the Sun, they are referred to as anticrepuscular rays. While enjoying the sunset after visiting NASA's Kennedy Space Center in Florida,  the photographer chanced to find that an even more spectacular sight was occurring in the other direction just over the Atlantic Ocean -- a particularly vivid set of anticrepuscular rays.

Image credit: Daniel Herron