APOD Retrospective: August 1

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: August 1

Postby bystander » Sat Jul 30, 2011 11:02 pm

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Spiral galaxy ESO 137-001 hurtles through massive galaxy cluster Abell 3627 some 220 million light years away. The distant galaxy is seen in this colorful Hubble/Chandra composite image through a foreground of the Milky Way's stars toward the southern constellation Triangulum Australe. As the spiral speeds along at nearly 7 million kilometers per hour, its gas and dust are stripped away when ram pressure with the cluster's own hot, tenuous intracluster medium overcomes the galaxy's gravity. Evident in Hubble's near visible light data, bright star clusters have formed in the stripped material along the short, trailing blue streaks. Chandra's X-ray data shows off the enormous extent of the heated, stripped gas as diffuse, darker blue trails stretching over 400,000 light-years toward the bottom right. The significant loss of dust and gas will make new star formation difficult for this galaxy. A yellowish elliptical galaxy, lacking in star forming dust and gas, is just to the right of ESO 137-001 in the frame.

An alluring night skyscape, this scene looks west across the Grand Teton National Park, Wyoming, USA, Planet Earth. The Snake River glides through the foreground, while above the Tetons' rugged mountain peaks the starry sky is laced with exceptionally strong red and green airglow. That night, the luminous atmospheric glow was just faintly visible to the eye, its color and wavey structure captured only by a sensitive digital camera. In fact, this contemporary digital photograph matches the location and perspective of a well-known photograph from 1942 - The Tetons and The Snake River , by Ansel Adams, renowned photographer of the American West. Adams' image is one of 115 images stored on the Voyager Golden Record. Humanity's message in a bottle, golden records were onboard both Voyager 1 and Voyager 2 spacecraft, launched in 1977 and now headed toward interstellar space.

The Great Spiral Galaxy in Andromeda (aka M31), a mere 2.5 million light-years distant, is the closest large spiral to our own Milky Way. Andromeda is visible to the unaided eye as a small, faint, fuzzy patch, but because its surface brightness is so low, casual skygazers can't appreciate the galaxy's impressive extent in planet Earth's sky. This entertaining composite image compares the angular size of the nearby galaxy to a brighter, more familiar celestial sight. In it, a deep exposure of Andromeda, tracing beautiful blue star clusters in spiral arms far beyond the bright yellow core, is combined with a typical view of a nearly full Moon. Shown at the same angular scale, the Moon covers about 1/2 degree on the sky, while the galaxy is clearly several times that size. The deep Andromeda exposure also includes two bright satellite galaxies, M32 and M110 (bottom).

You don't have to be at Monument Valley to see the Milky Way arch across the sky like this -- but it helps. Only at Monument Valley USA would you see a picturesque foreground that includes these iconic rock peaks called buttes. Buttes are composed of hard rock left behind after water has eroded away the surrounding soft rock. In the above image taken about two months ago, the closest butte on the left and the butte to its right are known as the Mittens, while Merrick Butte can be seen just further to the right. High overhead stretches a band of diffuse light that is the central disk of our spiral Milky Way Galaxy. The band of the Milky Way can be spotted by almost anyone on almost any clear night when far enough from a city and surrounding bright lights.

What's that strange bright streak? It is the last image ever of a space shuttle from orbit. A week and a half ago, after decoupling from the International Space Station, the Space Shuttle Atlantis fired its rockets for the last time, lost its orbital speed, and plummeted back to Earth. Within the next hour, however, the sophisticated space machine dropped its landing gear and did what used to be unprecedented -- landed like an airplane on a runway. Although the future of human space flight from the USA will enter a temporary lull, many robotic spacecraft continue to explore our Solar System and peer into our universe, including Cassini, Chandra, Chang'e 2, Dawn, Fermi, Hubble, Kepler, LRO, Mars Express, Messenger, MRO, New Horizons, Opportunity, Planck, Rosetta, SDO, SOHO, Spitzer, STEREO, Swift, Venus-Express, and WISE.

If you could look across Venus with radar eyes, what might you 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.

A new star, likely the brightest supernova in recorded human history, lit up planet Earth's sky in the year 1006 AD. The expanding debris cloud from the stellar explosion, found in the southerly constellation of Lupus, still puts on a cosmic light show across the electromagnetic spectrum. In fact, this composite view includes X-ray data in blue from the Chandra Observatory, optical data in yellowish hues, and radio image data in red. Now known as the SN 1006 supernova remnant, the debris cloud appears to be about 60 light-years across and is understood to represent the remains of a white dwarf star. Part of a binary star system, the compact white dwarf gradually captured material from its companion star. The buildup in mass finally triggered a thermonuclear explosion that destroyed the dwarf star. Because the distance to the supernova remnant is about 7,000 light-years, that explosion actually happened 7,000 years before the light reached Earth in 1006. Shockwaves in the remnant accelerate particles to extreme energies and are thought to be a source of the mysterious cosmic rays.

The Moon's measured diameter is around 3,476 kilometers (2,160 miles). But apparent angular size, or the angle covered by an object, can also be important to Moon enthusiasts. Angular size depends on distance, the farther away an object is, the smaller an angle it covers. Since the Moon is 400,000 kilometers away, its angular size is only about 1/2 degree, a span easily covered by the tip of your finger held at arms length, or a measuring tape held in the distance by a friend. Of course the Sun is much larger than the Moon, 400 times larger in fact, but today the New Moon will just cover the Sun. The total solar eclipse can be seen along a track across northern Canada, the Arctic, Siberia, and northern China. (A partial eclipse is visible from a broader region). Solar eclipses illustrate the happy coincidence that while the Sun is 400 times the diameter of the Moon, it is also 400 times farther away giving the Sun and Moon exactly the same angular size.

Why does one half of Dione have more craters than the other? Start with the fact that Saturn's moon Dione always has one side that faces Saturn, and always has one side that faces away. This is similar to Earth's Moon. This tidal locking means that one side of Dione always leads as the moon progresses in its orbit, while the other side always trails. Dione should therefore have undergone a significant amount of impacts on its leading half. But the current leading half of Dione is less cratered than the trailing half! A possible explanation is that some impacts were so large they spun Dione, sometimes changing the part that suffered the highest impact rate before the moon's spin again became locked. Pictured above, it is the top part of Dione that appears significantly more cratered than the bottom half.

If sometimes it appears that the entire Milky Way Galaxy is raining down on your head, do not despair. It happens twice a day. As the Sun rises in the East, wonders of the night sky become less bright than the sunlight scattered by our own Earth's atmosphere, and so fade from view. They will only rotate back into view when the Earth again eclipses our bright Sun at dusk. This battle between heaven and Earth was captured dramatically over a rock formation at Capitol Reef National Park Utah, USA in 2003 May. Dark dust, millions of stars, and bright glowing red gas highlight the plane of our Milky Way Galaxy, which lies on average thousands of light years behind Earth's mountains.

Has a tenth planet been discovered? A newly discovered object, designated 2003 UB313 and located more than twice the distance of Pluto, is expected to be at least as large as Pluto and probably larger, given current measurements. 2003 UB313's dimness and highly tilted orbit (44 degrees) prevented it from being discovered sooner. Many astronomers speculate that numerous other icy objects larger than Pluto likely exist in the Kuiper Belt of the far distant Solar System. If so, and if some are found closer in than 2003 UB313, it may be premature to call 2003 UB313 the tenth planet. Illustrated above is an artist's drawing showing how 2003 UB313 might look. The unusually bright star on the right is the Sun. Much of the world eagerly await the decision by the International Astronomical Union on whether 2003 UB313 will be designated a planet or given a name without subscripts.

This tiny ball provides evidence that the universe will expand forever. Measuring slightly over one tenth of a millimeter, the ball moves toward a smooth plate in response to energy fluctuations in the vacuum of empty space. The attraction is known as the Casimir Effect, named for its discoverer, who, 50 years ago, was trying to understand why fluids like mayonnaise move so slowly. Today, evidence is accumulating that most of the energy density in the universe is in an unknown form dubbed dark energy. The form and genesis of dark energy is almost completely unknown, but postulated as related to vacuum fluctuations similar to the Casimir Effect but generated somehow by space itself. This vast and mysterious dark energy appears to gravitationally repel all matter and hence will likely cause the universe to expand forever. Understanding vacuum fluctuations is on the forefront of research not only to better understand our universe but also for stopping micro-mechanical machine parts from sticking together.

In this serene view, the moons of Earth along with the bright planet Mars shine above the city of Turku near the southwestern tip of Finland. Of course Earth's large natural satellite, the Moon, at a distance of 400,000 kilometers, is by far the brightest object in this sky. But growing brighter and closer by the hour, Mars appears as the impressively bright "star" at the right, about 64 million kilometers from Turku. Streaking across the twilight sky between the two celestial beacons, Earth's largest artificial moon, the International Space Station, orbits about 400 kilometers above the planet's surface. To capture the moment, amateur astronomer Petteri Kankaro used a digital camera and combined exposures beginning at 23:34 Universal Time on July 17th.

July was a good month for sunspots ... really big sunspots. In fact, the full disk and inset pictures above show three large groups of spots, photographed only a few days ago on July 28. Together the sunspots span a region about thirty times the diameter of planet Earth. Now rotating behind the Sun's visible edge, these groups followed close on the heels of another enormous sunspot group which appeared in mid July. All of July's monster sunspot groups could be viewed without magnification, using safe solar observing methods of course. While individual sunspots are not numbered or cataloged, groups of sunspots, designated solar active regions, are given consecutive numbers as they appear on the visible solar disk. That numbering began on January 5, 1972 and on June 14, 2002, reached active region number 10,000. At the 10k mark no door prizes were given and no disasters plagued our fair solar system. But since June 14, active region reports often drop at least the leading digit, making these three active regions AR 0050, AR 0039, and AR 0044 (top left to bottom right).

What caused the pits, ridges, and gullies on otherwise smooth Martian terrain? One hypothesis is water. The lack of craters at this mid-latitude location indicates that the terrain is quite young by geological standards, perhaps only 100,000 years old. Were the terrain since saturated by water ice, that ice would soon evaporate into the thin Martian air. Left over, however, might be fragile cake-like sand that can be broken up by wind into pits and ridges. Consequences of this hypothesis include that even the Martian equator undergoes epochs of relative wet and dry, and that future spacefarers might be able to find water (ice) in a relatively mild climate near the Martian equator. Pictured above is young-ridged terrain that also shows evidence of a downhill flow.

Why do comets emit X-rays? First discovered during the passing of Comet Hyakutake in 1996, the reason a cold comet would produce hot X-rays has since remained a mystery. On July 14, however, the orbiting Chandra X-ray Observatory was able to provide an image of passing Comet LINEAR, shown above, in enough detail to unravel the mystery. The key to the solution turns out to be the unusual wind of fast ions emitted by our Sun. These ions apparently collide with gas recently emitted by the comet and cause some ions to acquire a new electron. An electron that starts in a high-energy state will emit an X-ray as it falls in closer to the ion nucleus. As other comets move into the inner Solar System, this discovery should allow future study of the continually evolving gas cloud that surrounds comets as well as the composition of the solar wind.

Pictured above is the first american astronaut to walk in space: Edward White. White is seen floating outside the Gemini 4 capsule in 1965. The term spacewalk is deceiving since astronauts do not actually walk - they float - usually without their feet touching anything solid. White was connected to the spaceship only by a thick tether. He carried a Hand-Held Self-Maneuvering Unit which expelled gas allowing him to move around. A maneuvering device is necessary in the free-fall of space since there is nothing (besides the spacecraft) to push off of to guide movements.

Comet Shoemaker-Levy 9, named after its co-discoverers, was often referred to as the "string of pearls" comet. It is famous for its suggestive appearance as well as its collision with the planet Jupiter! The comet's original single nucleus was torn to pieces by Jupiter's strong gravity during a close encounter with the solar system's largest planet in 1992. The pieces are seen in this composite of Hubble Space Telescope images to be "pearls" strung out along the comet's orbital path. In July of 1994 these pieces collided with Jupiter in a rare and spectacular series of events.

You've had a hard day rolling past Martian rocks, so now just relax your APXS and enjoy the Martian sunset. The above pictures taken by Mars Pathfinder highlight how clouds vary during the end of a Martian day. The atmosphere on Mars is much thinner than on Earth and dominated by carbon dioxide rather than nitrogen. Clouds on Mars can be water or carbon dioxide based, depending on conditions, whereas Earth's clouds are all water based. At night the temperature at Sagan Memorial Station will dip from about -15 degrees Celsius (+4 Fahrenheit) to -77 degrees Celsius (-107 Fahrenheit). Temperatures only this cold won't bother Sojourner.

You are flying through space and come to ... the Hydra Cluster of Galaxies. Listed as Abell 1060, the Hydra Cluster contains well over 100 bright galaxies. Clusters of galaxies are the largest gravitationally-bound objects in the universe. All of the bright extended images in the above picture are galaxies in the Hydra Cluster with the exception of unrelated diffraction crosses centered on bright stars. Several proximate clusters and galaxy groups might together create an even larger entity - a supercluster - but these clumps of matter are not (yet) falling toward each other. In fact, the Hydra cluster is thought to be part of the Hydra-Centaurus Supercluster of galaxies. Similarly, our own Milky Way Galaxy is part of the Local Group of Galaxies which is part of the Virgo Supercluster of Galaxies.

"I do not know what to say in a case so surprising, so unlooked for and so novel." announced Galileo when Saturn's rings appeared to vanish in 1612. In fact, every 15 years Saturn's rings seem to almost disappear as viewed from the Earth. This happens just as the orbiting Earth crosses the plane of Saturn's rings. The edge on perspective temporarily robs astronomers of a spectacular sight, however, the ring plane crossing affords them the opportunity to measure the rings' thickness and search for undiscovered moons. In this image of Saturn, produced on May 22, 1995 by NASA's Hubble Space Telescope, two of Saturn's known moons are visible as star like objects to the left of the planet. For more information about the ring plane crossing see the Saturn Ring Plane Crossing web page.

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