Astronomy Picture of the Day

[Steve Gardner]

Source: NASA

Space Station Over the Ionian Sea
Credit: STS-118 Shuttle Crew, NASA

Explanation: Last August, the Space Shuttle Endeavour crew captured this shot of the International Space Station (ISS) against the backdrop of Planet Earth. During that trip to the ISS, the space shuttle crew re-supplied the station, repaired the station, and even built more of the station. Its primary mission complete, the crew took the premier spaceship on a tour around the premier space station. Pictured during this inspection tour, the ISS is visible in front the Ionian Sea. The boot of Italy is visible on the left, while the western coastlines of Greece and Albania stretch across the top. The dorsal fin of the upside-down shuttle orbiter pokes into the very top of the image. The Space Shuttle Discovery subsequently visited the ISS in October while the next shuttle mission to the ISS is scheduled for next week.

[Steve Gardner]

Source: NASA

Comet Holmes from the Hubble Space Telescope
Credit: A. Dyer (Ground based image); NASA, ESA, and H. Weaver (JHU APL) (Hubble image)

Explanation: Why did Comet Holmes brighten? The unexpectedly bright Comet 17P/Holmes continues to grace northern skies as a naked-eye addition to the constellation Perseus. Any northern sky enthusiast with a dark sky, a bright curiosity, and a recent sky map should still be able to locate the comet in a few minutes. What is seen, however, is primarily the sun-light reflecting dust coma. It surrounds an iceberg nucleus too small and too faint to discern. Clues to the nearly million-fold brightness increase are therefore being sought in dramatic images of the enigmatic comet's central regions taken earlier this month by the Hubble Space Telescope. One such Hubble image, shown above, indicates a still unresolved dense central dust cloud near the nucleus, surrounded by a more complex, anisotropic coma. The Hubble images do not show any obvious fragmentation of the nucleus, however, as was seen last year in Comet Schwassman-Wachmann 3. Observers around and above the world will continue to study this unusual addition to the night sky.

[Steve Gardner]

Source: NASA

Aristarchus Plateau
Credit & Copyright: Alan Friedman

Explanation: Anchored in the vast lava flows of the Moon's Oceanus Procellarum lies the Aristarchus Plateau. The bright impact crater at the corner of the plateau is Aristarchus, a young crater 42 kilometers wide and 3 kilometers deep. Only slightly smaller, lava flooded Herodotus crater is above and to the left. A valley or rille feature likely carved by rapidly flowing lava or a collapsed lava tunnel, Vallis Schroteri begins just to the right of Herodotus and winds across the plateau for about 160 kilometers, eventually turning toward the top of the picture. Aristarchus Plateau itself is like a rectangular island about 200 kilometers across, raised up to 2 kilometers or so above the smooth surface of the lunar Ocean of Storms. Recorded from a backyard observatory in Buffalo, New York, the contrast of light-colored ejecta around Aristarchus with surrounding dark, smooth, lava flooded surfaces suggests more familiar snowy scenes of planet Earth.

[Steve Gardner]

Source: NASA

M74: The Perfect Spiral
Credit: NASA, ESA, and the Hubble Heritage (STScI / AURA)- ESA / Hubble Collaboration
Acknowledgment: R. Chandar (Univ. Toledo) and J. Miller (Univ. Michigan)

Explanation: If not perfect, then this spiral galaxy is at least one of the most photogenic. An island universe of about 100 billion stars, 32 million light-years away toward the constellation Pisces, M74 presents a gorgeous face-on view. Classified as an Sc galaxy, the grand design of M74's graceful spiral arms are traced by bright blue star clusters and dark cosmic dust lanes. Constructed from image data recorded in 2003 and 2005, this sharp composite is from the Hubble Space Telescope's Advanced Camera for Surveys. Spanning about 30,000 light-years across the face of M74, it includes exposures recording emission from hydrogen atoms, highlighting the reddish glow of the galaxy's large star-forming regions.

[Steve Gardner]

Source: NASA

Gibbous Europa
Credit: Galileo Project, JPL, NASA; reprocessed by Ted Stryk

Explanation: Although the phase of this moon might appear familiar, the moon itself might not. In fact, this gibbous phase shows part of Jupiter's moon Europa. The robot spacecraft Galileo captured this image mosaic during its mission orbiting Jupiter from 1995 - 2003. Visible are plains of bright ice, cracks that run to the horizon, and dark patches that likely contain both ice and dirt. Raised terrain is particularly apparent near the terminator, where it casts shadows. Europa is nearly the same size as Earth's Moon, but much smoother, showing few highlands or large impact craters. Evidence and images from the Galileo spacecraft, indicated that liquid oceans might exist below the icy surface. To test speculation that these seas hold life, ESA has started preliminary development of the Jovian Europa Orbiter, a spacecraft proposed to orbit Europa. If the surface ice is thin enough, a future mission might drop hydrobots to burrow into the oceans and search for life.

[Steve Gardner]

Source: NASA

A Complete Solar Cycle from SOHO
Credit: SOHO - EIT Consortium, ESA, NASA

Explanation: Every eleven years, our Sun goes through a solar cycle. A complete solar cycle has now been imaged by the sun-orbiting SOHO spacecraft, celebrating the 12th anniversary of its launch yesterday. A solar cycle is caused by the changing magnetic field of the Sun, and varies from solar maximum, when sunspot, coronal mass ejection, and flare phenomena are most frequent, to solar minimum, when such activity is relatively infrequent. Solar minimums occurred in 1996 and 2007, while the last solar maximum occurred in 2001 Pictured above is a SOHO image of the Sun in extreme ultraviolet light for each year of the last solar cycle, with images picked to illustrate the relative activity of the Sun.

[Steve Gardner]

Source: NASA

Movie: Analemma Over New Jersey
Credit & Copyright: Tom Matheson (Guidescope.net)

Explanation: An analemma is that figure-8 curve that you get when you mark the position of the Sun at the same time each day throughout planet Earth's year. Above, 26 separate exposures were recorded to illustrate the regular solar motion -- a difficult project performed mostly during the calendar year 2006. The images were taken at 8 am in the morning in northern New Jersey, USA, and digitally combined with a single foreground image later. The individual images have since been combined into a movie. Solstices correspond to the top and bottom of the figure-8, indicating the northern and southernmost excursions of the Sun in the sky. The tilt of planet Earth's axis and the variation in speed as it moves around its orbit combine to produce the graceful analemma curve.

[Steve Gardner]

Source: NASA

Comet Holmes Over Hungary
Credit & Copyright: Tamas Ladanyi

Explanation: Comet Holmes refuses to fade. The unusual comet that surprisingly brightened nearly a million-fold in late October continues to remain visible to the unaided eye from dark locations. Night to night, Comet 17P/Holmes is slowly gliding through the constellation Perseus, remaining visible to northern observers during much of the night right from sunset. Pictured above, Comet Holmes was captured from Hungary last week. The remarkable snowball continues to retain a huge coma, but now shows very little of a tail. To the far right is the open cluster of stars NGC 1245. How much longer Comet Holmes will remain visible to the unaided eye is unknown.

[Steve Gardner]

Source: NASA

Mars in View
Credit & Copyright: Jean-Luc Dauvergne, Francois Colas

Explanation: Very good telescopic views of Mars can be expected in the coming weeks as the Red Planet nears opposition on December 24th. Of course, opposition means opposite the Sun in planet Earth's sky - an arrangement that occurs every 26 months for Mars. Because of Mars' more elliptical orbit, the actual date of closest approach to Earth will be December 18, when Mars will be within about 88 million kilometers of our fair planet. Situated in the constellation Gemini and rising after twilight, the bright, ruddy disk of Mars will reach nearly 16 arcseconds in diameter. In this already exceptional image taken on November 18, north is down and surface markings around the sprawling, dark, albedo feature Syrtis Major are remarkably clear. The image was recorded with a video camera and filters on a 1 meter telescope at Pic Du Midi, a mountain top observatory in the French Pyrenees. Taking advantage of the shorter travel distance near opposition, NASA launched the Phoenix lander to Mars in August, scheduled to arrive in May 2008.

[Steve Gardner]

Source: NASA

Double Cluster in Perseus
Credit & Copyright: Volker Wendel, Josef Popsel, Stefan Binnewies

Explanation: Skygazers recently following Comet Holmes have probably also chanced across this lovely starfield, not far from the comet on the sky in the constellation Perseus. Some 7,000 light-years away, this pair of open or galactic star clusters is an easy binocular target and is visible to the unaided eye from dark sky areas. In fact, it was cataloged in 130 BC by Greek astronomer Hipparchus. Now known as h and chi Persei, or NGC 869(left) and NGC 884, the clusters themselves are separated by only a few hundred light-years and contain stars much younger and hotter than the Sun. In addition to being physically close together, the clusters' ages based on their individual stars are similar - evidence that both clusters were likely a product of the same star-forming region.

[Steve Gardner]

Source: NASA

Star Trails at Dawn
Credit & Copyright: Koen van Gorp

Explanation: Just fix your camera to a tripod and you too can make an image of graceful trails traced by the stars as planet Earth rotates on its axis. Made on September 14 from Montlaux, France, this wide-angle view nicely shows the stars near the celestial equator tracing nearly straight lines in projection, while stars north and south of the equator, respectively, appear to circle the north and south celestial poles. Featured are the stars of Orion (right of center), brilliant Venus rising (left) as bright star Sirius rises in the south (bottom center), and a polar orbiting Iridium satellite (upper left). Beautiful dawn sky colors seem painted along the horizon. This remarkable picture was constructed from 477 consecutive 30 second digital exposures recorded over 4.3 hours and later combined.

[Steve Gardner]

Source: NASA

The Fairy of Eagle Nebula
Credit: The Hubble Heritage Team, (STScI/AURA), ESA, NASA

Explanation: The dust sculptures of the Eagle Nebula are evaporating. As powerful starlight whittles away these cool cosmic mountains, the statuesque pillars that remain might be imagined as mythical beasts. Pictured above is one of several striking dust pillars of the Eagle Nebula that might be described as a gigantic alien fairy. This fairy, however, is ten light years tall and spews radiation much hotter than common fire. The greater Eagle Nebula, M16, is actually a giant evaporating shell of gas and dust inside of which is a growing cavity filled with a spectacular stellar nursery currently forming an open cluster of stars. The above image in scientifically re-assigned colors was released as part of the fifteenth anniversary celebration of the launch of the Hubble Space Telescope.

[Steve Gardner]

Source: NASA

A Jet from the Sun
Credit & Copyright: Hinode, JAXA, NASA

Explanation: What powers the solar wind? Our Sun is known to emit a powerful wind of particles with gusts that can even affect astronauts and satellites orbiting Earth. The cause of the solar wind has been debated for decades but is thought to be rooted in Alfvén waves generated by the ever changing magnetic field of the Sun. Newly released images from the Japanese Hinode satellite appear to bolster this hypothesis, imaging an average of 240 daily plasma jets that are excellent candidates to fuel the outwardly moving Alfvén waves. The jets and waves are themselves ultimately created by magnetic reconnection events, rapid events where lines of constant magnetic field suddenly move extremely rapidly, dragging electrons and protons along with them. On the image left, one such jet is visible in X-ray light. Bright spots show relatively energetic regions elsewhere on the Sun.

[Steve Gardner]

Source: NASA

The Universe Nearby
Credit & Copyright: 2MASS, T. H. Jarrett, J. Carpenter, & R. Hurt

Explanation: What does the universe nearby look like? This plot shows over one and a half million of the brightest stars and galaxies in the nearby universe detected by the Two Micron All Sky Survey (2MASS) in infrared light. The resulting image is an incredible tapestry of stars and galaxies that provides limits on how the universe formed and evolved. Across the center are stars that lie in the plane of our own Milky Way Galaxy. Away from the Galactic plane, vast majority of the dots are galaxies, color coded to indicate distance, with blue dots representing the nearest galaxies in the 2Mass survey, and red dots indicating the most distant survey galaxies that lie at a redshift near 0.1. Named structures are annotated. Many galaxies are gravitationally bound together to form clusters, which themselves are loosely bound into superclusters, which in turn are sometimes seen to align over even larger scale structures.

[Steve Gardner]

Source: NASA

Mars Rover Races to Survive
Credit: NASA, JPL-Caltech, USGS, UNM, HiRise

Explanation: The Martian rover Spirit is now in the race of its life. The rolling robot is trying to reach an outpost to spend the winter, but it keeps getting bogged down in soft sand on Mars. Earth scientists hope that Spirit can reach a slope on the northern edge of the unusual feature dubbed Home Plate, before the end of this month when northern winter will be phasing in on Mars. Reaching this slope will likely allow the rover to tilt enough toward the Sun to create a needed increase in the efficiency of its energy-absorbing solar panels. This map shows the path of Spirit from July 2004 until just last month.