APOD Retrospective: November 13

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: November 13

Post by bystander » Sun Nov 13, 2011 4:50 am


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2014 Yesterday, the first soft landing on a comet took place some 500 million kilometers from planet Earth as the Rosetta mission lander Philae settled on the nucleus of C67/P Churyumov-Gerasimenko. The landing site, dubbed Agilkia, is located near the center of this remarkable image snapped by Philae's ROLIS (ROsetta Lander Imaging System) camera. Taken from a distance of about 3 kilometers the image has a resolution of about 3 meters per pixel at the surface. After Philae's release from the orbiter, its seven-hour long descent was made without propulsion or guidance. Following its descent the lander is in place, though its anchoring harpoon system did not fire. For 2.5 days the lander is intended to conduct its main science mission returning extensive images and data. An extended surface mission may be possible if sunlight and dust conditions allow solar panels to recharge Philae's battery.

2013
[imghover6=http://apod.nasa.gov/apod/image/1311/sa ... ed_960.jpg]http://apod.nasa.gov/apod/image/1311/sa ... ni_960.jpg[/imghover6]Image Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

In the shadow of Saturn, unexpected wonders appear. The robotic Cassini spacecraft now orbiting Saturn drifted in giant planet's shadow earlier this year and looked back toward the eclipsed Sun. Cassini saw a unique and celebrated view. First, the night side of Saturn is seen to be partly lit by light reflected from its own majestic ring system. Next, Saturn's expansive ring system appears as majestic as always even from this odd angle. Ring particles, many glowing only as irregular crescents, slightly scatter sunlight toward Cassini in this natural color image. Several moons and ring features are also discernible. Appearing quite prominently is Saturn's E ring, the ring created by the unusual ice-fountains of the moon Enceladus and the outermost ring visible above. To the upper left, far in the distance, are the planets Mars and Venus. To the lower right, however, is perhaps the most wondrous spectacle of all: the almost invisible, nearly ignorable, pale blue dot of Earth.

2012 Some people are so inspired by solar eclipses that they quilt. Pictured above is a resulting textile from one such inspiration. The 38x38 inch quilt offers impressions of a total annular eclipse, when the Moon is too far from the Earth to cover the entire Sun, witnessed in Spain in October of 2005. Today, however, a full total solar eclipse will occur, although it will only be visible to eclipse chasers and those who live in a thin swath of Australia. For a few minutes, those near the center of the eclipse path will see the entire Sun blocked by the Moon, causing the day to become unusually dark. Just before -- and just after -- totality occurs, sunlight may stream between mountains on the Moon's edge creating a diamond ring effect. The next total eclipse of the Sun will occur in November 2013.

2011 Few butterflies have a wingspan this big. The bright clusters and nebulae of planet Earth's night sky are often named for flowers or insects, and NGC 6302 is no exception. With an estimated surface temperature of about 250,000 degrees C, the central star of this particular planetary nebula is exceptionally hot though -- shining brightly in ultraviolet light but hidden from direct view by a dense torus of dust. This dramatically detailed close-up of the dying star's nebula was recorded by the Hubble Space Telescope soon after it was upgraded in 2009. Cutting across a bright cavity of ionized gas, the dust torus surrounding the central star is near the center of this view, almost edge-on to the line-of-sight. Molecular hydrogen has been detected in the hot star's dusty cosmic shroud. NGC 6302 lies about 4,000 light-years away in the arachnologically correct constellation of the Scorpion (Scorpius).

2010 Like delicate cosmic petals, these clouds of interstellar dust and gas have blossomed 1,300 light-years away in the fertile star fields of the constellation Cepheus. Sometimes called the Iris Nebula and dutifully cataloged as NGC 7023, this is not the only nebula in the sky to evoke the imagery of flowers. Still, this beautiful digital image shows off the Iris Nebula's range of colors and symmetries in impressive detail. Within the Iris, dusty nebular material surrounds a hot, young star. The dominant color of the brighter reflection nebula is blue, characteristic of dust grains reflecting starlight. Central filaments of the dusty clouds glow with a faint reddish photoluminesence as some dust grains effectively convert the star's invisible ultraviolet radiation to visible red light. Infrared observations indicate that this nebula may contain complex carbon molecules known as PAHs. As shown here, the bright blue portion of the Iris Nebula is about six light-years across.

2009 This beautiful telescopic skyscape features spiral galaxy NGC 918. The island universe is about 50,000 light-years across and lies some 60 million light-years away toward the constellation Aries. An artistic presentation, the image shows spiky foreground stars in our own Milky Way Galaxy and convoluted dust clouds that hang hundreds of light-years above our galactic plane, dimly reflecting starlight. It also captures NGC 918 in a cosmic moment important to astrophysicists on planet Earth. Light from supernova SN2009js, absent in previous images, is indicated by the two lines just below and left of the galaxy's center. The supernova itself is the death explosion of a massive star within the plane of galaxy NGC 918. It was just discovered in October by supernova search teams in Japan and the US.

2008 The northern Martian summer is waning. As predicted, a decline in daylight hours, deteriorating weather, and dust storms are preventing solar arrays on the Phoenix Mars Lander from providing power. Phoenix's last signal was received on November 2, its successful mission ending after more than five months in the arctic region of the Red Planet, a run that exceeded its planned operational lifetime. Attempting to discover if Mars' surface has ever been able to support microbial life, Phoenix performed an extensive analysis of the soil and returned a wealth of image data. Of course, one of the lander's most exciting results was the detection of water-ice near the Martian surface. Recorded in October, this picture from the lander's Robotic Arm Camera shows the region under the Phoenix with flat, exposed icy patches. That area caused researchers to exclaim "Holy Cow!" when it was first imaged a few days after the May 25 touchdown of the Phoenix Mars Lander.

2007 Where do cosmic rays come from? A major step toward answering this century old question may have just come in from the Auger Observatory project, the world's premier cosmic ray observatory. That high energy fundamental particles are barreling through the universe has been known for about a century. Because ultra high energy cosmic rays are so rare and because their extrapolated directions are so imprecise, no progenitor objects have ever been unambiguously implied. New results from Auger, however, indicate that 12 of 15 ultra high energy cosmic rays have sky directions statistically consistent with the positions of nearby active galactic nuclei. These galactic centers are already known to emit great amounts of light and are likely powered by large black holes. The Auger results also indicate that the highest energy cosmic rays are protons, since the electric charge of higher energy nuclei would force the Milky Way Galaxy's magnetic field to deflect and effectively erase progenitor source direction. Pictured above, an artist illustrates a cosmic ray striking the Earth's atmosphere and creating a shower of secondary particles detectable on the surface. The image of Centaurus A digitally superposed near the top signifies one such active galaxy from which cosmic rays might originate.

2006 Three thousand light-years away, a dying star throws off shells of glowing gas. This image from the Hubble Space Telescope reveals the Cat's Eye Nebula to be one of the most complex planetary nebulae known. In fact, the features seen in the Cat's Eye are so complex that astronomers suspect the bright central object may actually be a binary star system. The term planetary nebula, used to describe this general class of objects, is misleading. Although these objects may appear round and planet-like in small telescopes, high resolution images reveal them to be stars surrounded by cocoons of gas blown off in the late stages of stellar evolution.

2005 This panorama of the cratered lunar surface was constructed from images returned by the US Surveyor 6 lander. Surveyor 6 was not the first spacecraft to accomplish a soft landing on the Moon ... but it was the first to land and then lift off again! After the spacecraft touched down near the center of the Moon's nearside in November of 1967, NASA controllers commanded it to hop. Briefly firing its rocket engine and lifting itself some 4 meters above the surface, the Surveyor moved about 2.5 meters to one side before setting down again. The hopping success of Surveyor 6 essentially marked the completion of the Surveyor series main mission - to determine if the lunar terrain was safe for the planned Apollo landings.

2004 The critics rave - "Amazing!", "Unbelievable!", "The best I've ever seen!" They aren't talking about a movie, though. Instead, even casual sky critics are remarking on November's stunning auroral displays, visible with surprising intensities well beyond the confines of high latitudes where auroral activity is normally observed. In fact, in this example of an unforgettable performance a green ribbon of auroral light stretches from horizon to horizon - recorded on November 7th with a fisheye lens near Warrensburg, Missouri, USA. Want to see an aurora? Relatively wide-spread displays may continue, triggered by activity from an energetic sunspot region.

2003 Does Mars have canals? A hot debate topic of the late 1800s, several prominent astronomers including Percival Lowell not only claimed to see an extensive system of long straight canals on Mars, but used them to indicate that intelligent life exists there. The relatively close opposition of 1894 was used to make drawings like the one digitally re-scaled on the above left. The above map was originally prepared by Eugene Antoniadi and redrawn by Lowell Hess for the book Exploring Mars, by Roy A. Gallant. In more modern times, the latest Mars opposition has allowed the Hubble Space Telescope to capture a picture of similar orientation. Comparison of the two images shows that large features were impressively recorded, but that an extensive system of long and straight canals just does not exist. Satellites orbiting Mars have now shown conclusively that the red planet does indeed have surface features similar to canals, but that these are usually smaller, curved, and less extensive than that previously claimed. Real canyon systems like Noctis Labyrinthus are most likely cracks caused by surface stress.

2002 Dominating the top third of Africa is the largest band of dry land on Earth: the Sahara Desert. Stretching across the Sahara are vast plains of sand and gravel, seas of sand dunes, and barren rocky mountains. Only 10,000 years ago, however, grasses covered the region, then rich in mammals such as lions and elephants. Now only two percent of the Sahara are oases, patches of land where crops will grow and where nearly two million people live. Oases are usually centered on natural water springs. Pictured above is colorful and rocky land spanning about 50 kilometers near the Terkezi Oasis in Chad.

2001 What is that unusual object? Astronomers can identify most objects that are imaged on the sky, but not all. Pictured above is one that currently defies classification. Attributes of the object include that it has unusual colors, appears to be fading as months go by, and appears to be associated with a distant galaxy. Its discoverers hold hope that they have uncovered the first known orphan afterglow, an explosion that would have been classified as a gamma-ray burst if the gamma-rays were beamed in our direction. Orphan afterglows, if they exist, could have unparalleled brightness, and hence be visible so far away that they yield key information about the early years of our universe. A bit of caution might be merited, however, as the last well-publicized mystery object turned out not to be a new member of the astronomical zoo, but rather an unusual type of quasar. Follow-up observations and analysis over the next year may find more objects like this and/or solve this mystery.

2000 We live in a forest. Strewn throughout the universe are "trees" of hydrogen gas that absorb light from distant objects. These gas clouds leave numerous absorption lines in a distant quasar's spectra, together called the Lyman-alpha forest. Distant quasars appear to be absorbed by many more Lyman-alpha clouds than nearby quasars, indicating a Lyman-alpha thicket early in our universe. The above image depicts one possible computer realization of how Lyman-alpha clouds were distributed at a redshift of 3. Each side of the box measures 30 million light-years across. Much remains unknown about the Lyman-alpha forest, including the real geometry and extent of the clouds, and why there are so many fewer clouds today.

1999 Will this be the year? Last year's Leonid meteor shower did not produce the meteor storm many had hoped for. Still, it put on a dazzling show with many bright fireball meteors. For example, this Leonid fireball, photographed through light clouds, eerily flashed across the skies of Monteromano, Italy on November 17, 1998. This year, the chances for a storm with thousands of meteors per hour are considered good ... but experts are quick to acknowledge that such predictions are tricky. Want to see for yourself? The predicted peak should occur on early Thursday, November 18 (UTC) but meteor activity will certainly be observable days before and after. If the night is clear, just grab a lawn chair and a warm jacket, go outside and look up!

1998 This simulated image models the intensities of gamma rays with over 40 million times the energy of visible light, and represents how the sky might appear to the proposed Gamma-ray Large Area Space Telescope (GLAST) after its first year in orbit. Familiar steady stars are absent from the dramatic 80x80 degree field which looks directly away from the center of the Galaxy. Instead, the Geminga and Crab pulsars - bizarre, spinning stellar corpses known to be neutron stars - are the two brightest gamma-ray sources. These and other bright objects in the field, dense pulsars, monstrous active galaxies, and still unknown sources, have been detected by the Energetic Gamma-Ray Experiment Telescope (EGRET) on the orbiting Compton Gamma-Ray Observatory. However, most of the simulated point sources are new - extrapolating current ideas and anticipating discoveries resulting from GLAST's improved gamma-ray vision. The central broad band of faint gamma-ray emission is due to high-energy cosmic rays colliding with interstellar gas in the outer spiral arms of the Milky Way, while below is a diffuse energetic glow from prominent molecular clouds in Monoceros, Orion, Auriga, and Taurus. Intended to explore the most extreme energy sources in the distant cosmos and planned for launch in 2005, the GLAST mission is under development by NASA and a collaboration of U. S. and international partners.

1997 El Niño is a temporary global climate change resulting from unusually warm water in the central Pacific Ocean. El Niño can cause unusual or severe weather for some locations over the next few months. Warm water is shown in white in the above false-color picture taken by the orbiting TOPEX/Poseidon satellite in late October. The Pacific Ocean is color coded by sea surface height relative to normal ocean levels. The large white area represents a mass of warm water 30 times greater than all the Great Lakes, flowing toward the Americas. Although El Niños occur every decade or so, this year's is the first ever predicted. The cause and full effects of El Niños are still under study.

1996 Comet Hale-Bopp continues its slow trek across the night sky, and can now be seen superposed near the bright globular cluster M14. Will Comet Hale-Bopp become as bright in early 1997 as Comet Hyakutake did in early 1996? It is still too early to tell. Currently Hale-Bopp is curiously holding at about 5th magnitude - just barely bright enough to see without binoculars from a dark location. Because of the size of coma, some speculate that the nucleus of Hale-Bopp is unusually large. The actual nucleus is obscured, however, and recent speculation includes that the nucleus is comparable in size to Comet Halley - about 10-15 km across.

1995 Recently two new types of lightning have been verified: red sprites and blue jets. These atmospheric discharges occur very high in the Earth's atmosphere - much higher than the familiar form of lightning. Blue jets appear blue in color and go from the tops of clouds to a height of about 50 kilometers. Because blue jets typically last almost a full second, one can easily see the jet rise from the clouds with a fast video camera. Blue jets appear as narrow cones and travel about 100 kilometers per second. The existence of blue jets has been suggested previously, but only in 1994 were aircraft flown above massive thunderstorms with the high speed video equipment necessary to verify these spectacular events. The above black & white picture shows several blue jets rising from a thunderstorm simultaneously. Part of the aircraft is visible in silhouette on the right. Scientists are unsure of the cause and nature of blue jets.

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