APOD Retrospective: February 15

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: February 15

Post by bystander » Wed Feb 15, 2012 4:43 am


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2015 Two hours before closest approach to Neptune in 1989, the Voyager 2 robot spacecraft snapped this picture. Clearly visible for the first time were long light-colored cirrus-type clouds floating high in Neptune's atmosphere. Shadows of these clouds can even be seen on lower cloud decks. Most of Neptune's atmosphere is made of hydrogen and helium, which is invisible. Neptune's blue color therefore comes from smaller amounts of atmospheric methane, which preferentially absorbs red light. Neptune has the fastest winds in the Solar System, with gusts reaching 2000 kilometers per hour. Speculation holds that diamonds may be created in the dense hot conditions that exist under the cloud tops of Uranus and Neptune. Twenty-six years later, NASA's New Horizons is poised to be the first spacecraft to zoom past Pluto this July.

2014 This helmet-shaped cosmic cloud with wing-like appendages is popularly called Thor's Helmet. Heroically sized even for a Norse god, Thor's Helmet is about 30 light-years across. In fact, the helmet is more like an interstellar bubble, blown as a fast wind from the bright, massive star near the bubble's center sweeps through a surrounding molecular cloud. Known as a Wolf-Rayet star, the central star is an extremely hot giant thought to be in a brief, pre-supernova stage of evolution. Cataloged as NGC 2359, the nebula is located about 15,000 light-years away in the constellation Canis Major. The sharp image, made using broadband and narrowband filters, captures striking details of the nebula's filamentary structures. It shows off a blue-green color from strong emission due to oxygen atoms in the glowing gas.

2013 Two dark shadows loom across the banded and mottled cloud tops of Jupiter in this sharp telescopic view. In fact, captured on January 3rd, about a month after the ruling gas giant appeared at opposition in planet Earth's sky, the scene includes the shadow casters. Visible in remarkable detail at the left are the large Galilean moons Ganymede (top) and Io. With the two moon shadows still in transit, Jupiter's rapid rotation has almost carried its famous Great Red Spot (GRS) around the planet's limb from the right. The pale GRS was preceded by the smaller but similar hued Oval BA, dubbed Red Spot Jr., near top center. North is down in the inverted image.

2012 Reflection nebulas reflect light from a nearby star. Many small carbon grains in the nebula reflect the light. The blue color typical of reflection nebula is caused by blue light being more efficiently scattered by the carbon dust than red light. The brightness of the nebula is determined by the size and density of the reflecting grains, and by the color and brightness of the neighboring star(s). NGC 1435, pictured above, surrounds Merope (23 Tau), one of the brightest stars in the Pleiades (M45). The Pleiades nebulosity is caused by a chance encounter between an open cluster of stars and a dusty molecular cloud.

2011
[imghover6=http://apod.nasa.gov/apod/image/1102/no ... ir_900.jpg]http://apod.nasa.gov/apod/image/1102/no ... _v_900.jpg[/imghover6]Credit: NASA, JPL-Caltech, L. Rebull (SSC, Caltech); Optical Rollover: DSS, D. De Martin

The North America Nebula can do what most North Americans cannot -- form stars. Precisely where in the nebula these stars are forming has been mostly obscured by some of the nebula's thick dust that is opaque to visible light. However, a new view of the North America Nebula in infrared light by the orbiting Spitzer Space Telescope has peered through much of the dust and uncovered thousands of newly formed stars. Rolling your cursor over the above scientifically-colored infrared image will bring up a corresponding optical image of the same region for comparison. The new infrared image neatly captures young stars in many stages of formation, from being imbedded in dense knots of gas and dust, to being surrounded by disks and emitted jets, to being clear of their birth cocoons. The North America Nebula (NGC 7000) spans about 50 light years and lies about 1,500 light years away toward the constellation of the Swan (Cygnus). Still, of all the stars known in the North America Nebula, which massive stars emit the energetic light that gives the ionized red glow is still debated.

2010 If this is Saturn, where are the rings? When Saturn's "appendages" disappeared in 1612, Galileo did not understand why. Later that century, it became understood that Saturn's unusual protrusions were rings and that when the Earth crosses the ring plane, the edge-on rings will appear to disappear. This is because Saturn's rings are confined to a plane many times thinner, in proportion, than a razor blade. In modern times, the robot Cassini spacecraft orbiting Saturn now also crosses Saturn's ring plane. A series of plane crossing images from 2005 February was dug out of the vast online Cassini raw image archive by interested Spanish amateur Fernando Garcia Navarro. Pictured above, digitally cropped and set in representative colors, is the striking result. Saturn's thin ring plane appears in blue, bands and clouds in Saturn's upper atmosphere appear in gold. Since Saturn just passed its equinox, today the ring plane is pointed close to the Sun and the rings could not cast the high dark shadows seen across the top of this image, taken back in 2005. Moons appear as bumps in the rings.

2009 It's all gone but the mountains. Most of the sprawling landscape of ice that lies between the mountains visible above has now disintegrated. The above picture was taken in Antarctica from the top of Grey Nunatak, one of three Seal Nunatak mountains that border the Larsen B Ice-Shelf. The other two nunataks are visible in the picture taken in 1994. Over the past several years large chunks of the 200-meter thick Larsen B Ice-Shelf have been breaking off and disintegrating. The cause is the local high temperatures of recent years, part of a planet wide climate change called global warming. Over the past few years, the area that has disintegrated is roughly the size of Luxembourg. As ice-shelves break up, they unblock other ice sheets that fall onto the ocean, raising sea levels everywhere. Scientists are watching the much-larger Ross Ice Shelf, which, if it fully collapses, could cause global sea levels to rise five meters over the next few hundred years.

2008 Cosmic dust clouds and embedded newborn stars glow at infrared wavelengths in this tantalizing false-color view from the Spitzer Space Telescope. Pictured is of one of the closest star forming regions, part of the Rho Ophiuchi cloud complex some 400 light-years distant near the southern edge of the pronounceable constellation Ophiuchus. The view spans about 5 light-years at that estimated distance. After forming along a large cloud of cold molecular hydrogen gas, newborn stars heat the surrounding dust to produce the infrared glow. An exploration of the region in penetrating infrared light has detected some 300 emerging and newly formed stars whose average age is estimated to be a mere 300,000 years -- extremely young compared to the Sun's age of 5 billion years.

2007 Planetary nebula NGC 2440 has an intriguing bow-tie shape in this stunning view from space. The nebula is composed of material cast off by a dying sun-like star as it enters its white dwarf phase of evolution. Details of remarkably complex structures are revealed within NGC 2440, including dense ridges of material swept back from the nebula's central star. Near the center of the view, the star itself is one of the hottest known, with a surface temperature of about 200,000 kelvins. About 4,000 light-years from planet Earth toward the nautical constellation Puppis, the nebula spans over a light-year and is energized by ultraviolet light from the central star. The false-color image was recorded earlier this month using the Hubble's Wide-Field Planetary Camera 2(WFPC2), demonstrating still impressive imaging capabilities following the failure of the Advanced Camera for Surveys.

2006 Titan is one of the strangest places in our Solar System. The only moon known with thick clouds, this unusual satellite of Saturn shows evidence of evaporating lakes created by methane rain. The clouds that make Titan featureless in visible light have now been pierced several times in infrared light by the robot Cassini spacecraft currently orbiting Saturn. These images have been compiled into the above time-lapse movie. Like Earth's Moon, Titan always shows the same face toward its central planet. It therefore takes Titan about 16 days to complete one rotation. Titan has numerous areas of light terrain with some large areas of dark terrain visible near the equator. Small areas of brightest terrain might arise from ice-volcanoes and have a high amount of reflective frozen water-ice. Titan's surface was imaged for the first time early last year by the Huygens probe, which survived for three hours on a cold and sandy dark region.

2005 Each moon of Saturn seems to come with its own mystery. Rhea, Saturn's second largest moon behind Titan, shows unusual wisps, visible above as light colored streaks. Higher resolution images of similar wisps on Dione indicate that they might be made of long braided fractures. Rhea is composed mostly of water ice, but likely has a small rocky core. Rhea's rotation and orbit are locked together, just like Earth's Moon, so that one side always faces Saturn. A consequence of this is that one side always leads the other. Rhea's leading surface is much more heavily cratered than the trailing surface, pictured above. The above image in natural color was taken last month by the Cassini robot spacecraft in orbit around Saturn.

2004 How did this spherule come to be on the Moon? When a meteorite strikes the Moon, the energy of the impact melts some of the splattering rock, a fraction of which might cool into tiny glass beads. Many of these glass beads were present in lunar soil samples returned to Earth by the Apollo missions. Pictured above is one such glass spherule that measures only a quarter of a millimeter across. This spherule is particularly interesting because it has been victim to an even smaller impact. A miniature crater is visible on the upper left, surrounded by a fragmented area caused by the shockwaves of the small impact. By dating many of these impacts, some astronomers estimate that cratering on our Moon increased roughly 500 million years ago and continues even today.

2003 One hundred seventy-five years ago (on February 8th), Jules Verne was born in Nantes, France. Inspired by a lifelong fascination with machines, Verne wrote visionary works about "Extraordinary Voyages" including such terrestrial travels as Around the World in 80 Days, Journey to the Centre of the Earth, and Twenty Thousand Leagues Under the Sea. In 1865 he published the story of three adventurers who undertook a journey From the Earth to the Moon. Verne's characters rode a "projectile-vehicle" fired from a huge cannon constructed in Florida, USA. Does that sound vaguely familiar? A century later, the Saturn V rocket and NASA's Apollo program finally turned this work of fiction into fact, propelling adventuresome trios on what was perhaps Verne's most extraordinary voyage. This dramatic view shows the moonbound Apollo 11 space-vehicle riding top a Saturn V rocket as it blasts skyward. Launched from a spaceport in Florida, the Apollo 11 crew traveled to the moon and back again in 1969, making humanity's first landing on the lunar surface.

2002 Born on today's date in 1564, Galileo used a telescope to explore the Solar System. In 1610, he became the first to be amazed by Saturn's rings. After nearly 400 years, Saturn's magnificent rings still offer one of the most stunning astronomical sights. Uniquely bright compared to the rings of the other gas giants, Saturn's ring system is around 250,000 kilometers wide but in places only a few tens of meters thick. Modern astronomers believe the rings are perhaps only a hundred million years young. But accumulating dust and dynamically interacting with Saturn's moons, the rings may eventually darken and sag toward the gas giant, losing their lustre over the next few hundred million years. Since Galileo, astronomers have subjected the entrancing rings to intense scrutiny to unlock their secrets. Still mesmerized, some will take advantage of next week's (February 20th) favorable lunar occultation of Saturn to search for evidence of ring material outside the well known boundaries of the ring system. The presence of such a "lost" ring of Saturn was first hinted at in reports dating back to the early 20th century.

2001 Slice Jupiter from pole to pole, peel back its outer layers of clouds, stretch them onto a flat surface ... and for all your trouble you'd end up with something that looks a lot like this. Scrolling right will reveal the full picture, a color mosaic of Jupiter from the Cassini spacecraft. The mosaic is actually a single frame from a fourteen frame movie constructed from image data recorded by Cassini during its leisurely flyby of the solar system's largest planet late last year. The engaging movie approximates Jupiter's cloud motions over 24 jovian rotations. To make it, a series of observations covering Jupiter's complete circumference 60 degrees north and south of the equator were combined in an animated cylindrical projection map of the planet. As in the familiar rectangular-shaped wall maps of the Earth's surface, the relative sizes and shapes of features are correct near the equator but become progressively more distorted approaching the polar regions. In the Cassini movie, which also features guest appearances by moons Io and Europa, the smallest cloud structures visible at the equator are about 600 kilometers across. (Note: Downloading a large gif or quicktime version of the movie may take 15 minutes or longer.)

2000 What's happening at the center of spiral galaxy M106? A swirling disk of stars and gas, M106's appearance is dominated by two bright spiral arms and dark dust lanes near the nucleus. Bright newly formed stars near their outer tips distinguish the spiral arms in the above photograph. The core of M106 glows brightly in radio waves and X-rays where twin jets have been found running the length of the galaxy. An unusual central glow makes M106 one of the closest examples of the Seyfert class of galaxies, where vast amounts of glowing gas are thought to be falling into a central massive black hole. M106, also designated NGC 4258, is a relatively close 25 million light years away, spans 30 thousand light years across, and can be seen with a small telescope towards the constellation of Canes Venatici.

1999 La Niña is a temporary climate change caused by unusually cold water in the central Pacific Ocean. Cold water topping an unusually low sea level is shown as purple in the above false-color picture taken by the orbiting TOPEX/Poseidon satellite in mid-January. Such cold water tends to deflect winds around it, changing the course of weather systems locally and the nature of weather patterns globally. This year's La Niña appears to have weakened over the past few months, indicating a slow return to more normal seasonal weather. The full effects of the preceding El Niño and the present La Niña are still under study.

1998 Galaxies are made up of stars, but are all stars found within galaxies? Using the Hubble Space Telescope, researchers exploring the Virgo Cluster of galaxies have now found about 600 red giant stars adrift in intergalactic space. Above is an artist's vision of the sky from a hypothetical planet of such a lonely sun. The night sky on a world orbiting an intergalactic star would be a stark contrast to Earth's - which features a spectacle of stars, all members of our own Milky Way galaxy. As suggested by the illustration, a setting swollen red sun would leave behind a dark sky flecked only with faint, fuzzy, apparitions of Virgo Cluster galaxies. Possibly ejected from their home galaxies during galaxy-galaxy collisions, these isolated suns may well represent part of a large, previously unseen stellar population, filling the space between Virgo Cluster galaxies.

1997 What happens when a star runs out of nuclear fuel? The center condenses into a white dwarf while the outer atmospheric layers are expelled into space and appear as a planetary nebula. This particular planetary nebula, designated Shapley 1 after the famous astronomer Harlow Shapley, has a very apparent annular ring like structure. Although some of these nebula appear like planets on the sky (hence their name), they actually surround stars far outside our Solar System.

1996 Excitement mounts as NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft nears launch - currently scheduled for 3:53 ET on February 16. NEAR's mission is to become the first spacecraft to rendezvous with and orbit an asteroid, the asteroid designated 433 Eros. After achieving Eros orbit in 1999, project plans are to explore the asteroid for 1 year from this premiere vantage point, perhaps approaching to within 15 miles of the surface. For comparison, above is an image of the limb of asteroid Ida made by the Galileo spacecraft from a distance of about 1,500 miles, the highest resolution image of an asteroid surface - so far. It is hoped that NEAR will go far towards answering questions about the nature and origin of near Earth asteroids. These objects are thought to contain clues to the formation of the inner planets and influence the evolution of the atmosphere and life on Earth. Are asteroids and meteorites related? Do asteroids ever strike the Earth?

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