APOD Retrospective: September 19

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: September 19

Post by bystander » Mon Sep 19, 2011 6:08 am

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2015 This eerie landscape of incandescent plasma suspended in looping and twisted magnetic fields stretched toward the Sun's eastern horizon on September 16. Captured through a backyard telescope and narrowband filter in light from ionized hydrogen, the scene reveals a gigantic prominence lofted above the solar limb. Some 600,000 kilometers across, the magnetized plasma wall would dwarf worlds of the Solar System. Ruling gas giant Jupiter can only boast a diameter of 143,000 kilometers or so, while planet Earth's diameter is less than 13,000 kilometers. Known as a hedgerow prominence for its appearance, the enormous structure is far from stable though, and such large solar prominences often erupt.
2014 For astrobiologists, these may be the four most tantalizing moons in our Solar System. Shown at the same scale, their exploration by interplanetary spacecraft has launched the idea that moons, not just planets, could have environments supporting life. The Galileo mission to Jupiter discovered Europa's global subsurface ocean of liquid water and indications of Ganymede's interior seas. At Saturn, the Cassini probe detected erupting fountains of water ice from Enceladus indicating warmer subsurface water on even that small moon, while finding surface lakes of frigid but still liquid hydrocarbons beneath the dense atmosphere of large moon Titan. Now looking beyond the Solar System, new research suggests that sizable exomoons, could actually outnumber exoplanets in stellar habitable zones. That would make moons the most common type of habitable world in the Universe.

2013 In this engaging scene from planet Earth, the Moon shines through cloudy skies following sunset on the evening of September 8. Despite the fading light, the camera's long exposure still recorded a colorful, detailed view of a shoreline and western horizon looking toward the island San Gabriel from Colonia del Sacramento, Uruguay. Lights from Buenos Aires, Argentina are along the horizon on the left, across the broad Rio de la Plata estuary. The long exposure strongly overexposed the Moon and sky around it, though. So the photographer quickly snapped a shorter one to merge with the first image in the area around the bright lunar disk. As the the second image was made with a telephoto setting, the digital merger captures both Earth and sky, exaggerating the young Moon's slender crescent shape in relation to the two nearby bright stars. The more distant is bluish Spica, alpha star of the constellation Virgo. Closest to the Moon is Earth's evening star, planet Venus, emerging from a lunar occultation.

2012 Next stop: Ceres. Last week the robotic Dawn spacecraft ended its year-long mission to asteroid Vesta, becoming the first spacecraft ever to visit this far off world located between Mars and Jupiter, in the Solar System's main asteroid belt. Many of the best images taken by Dawn at Vesta have been compiled into the above encompassing view. Vesta shows evidence of being a leftover from the early years of our Solar System, a building block for rocky planets like Earth. Vesta's ancient surface shows heavy cratering and long troughs likely created by huge impacts. The minor planet's low gravity allows for surface features like huge cliffs and a large mountain that reaches twice the height of Earth's Mount Everest, visible at the image bottom. Vesta, however, spanning about 500 kilometers across, is only the second most massive object in the asteroid belt. And so, two weeks ago, Dawn fired its gentle ion rockets and has begun chasing the most massive: Ceres. If everything goes as planned, Dawn will reach Ceres in 2015. Ceres looks quite different to the distant telescope -- but what will Dawn find?

2011 What created the circular structure around the south pole of asteroid Vesta? Pictured above, the bottom of the second largest object in the asteroid belt was recently imaged for the first time by the robotic Dawn satellite that arrived last month. A close inspection of the 260-meter resolution image shows not only hills and craters and cliffs and more craters, but ragged circular features that cover most of the lower right of the 500-kilometer sized object. Early speculation posits that the structure might have been created by a collision and coalescence with a smaller asteroid. Alternatively, the features might have originated in an internal process soon after the asteroid formed. New clues might come in the next few months as Dawn spirals down toward the rocky world and obtains images of increasingly high resolution.

2010 What dark forms lurk in the mists of the Carina Nebula? These ominous figures are actually molecular clouds, knots of molecular gas and dust so thick they have become opaque. In comparison, however, these clouds are typically much less dense than Earth's atmosphere. Pictured above is part of the most detailed image of the Carina Nebula ever taken, a part where dark molecular clouds are particularly prominent. The image has recently been retaken and then re-colored based on light emitted by oxygen. The entire Carina Nebula spans over 300 light years and lies about 7,500 light-years away in the constellation of Carina. NGC 3372, known as the Great Nebula in Carina, is home to massive stars and changing nebula. Eta Carinae, the most energetic star in the nebula, was one of the brightest stars in the sky in the 1830s, but then faded dramatically. Wide-field annotated and zoomable versions of the larger image composite are also available.

2009 Far beyond the local group of galaxies lies NGC 3621, some 22 million light-years away. Found in the multi-headed southern constellation Hydra, the winding spiral arms of this gorgeous island universe are loaded with luminous young star clusters and dark dust lanes. Still, for earthbound astronomers NGC 3621 is not just another pretty face-on spiral galaxy. Some of its brighter stars have been used as standard candles to establish important estimates of extragalactic distances and the scale of the Universe. This beautiful image of NGC 3621 traces the loose spiral arms far from the galaxy's brighter central regions that span some 100,000 light-years. Spiky foreground stars in our own Milky Way Galaxy and even more distant background galaxies are scattered across the colorful skyscape.

2008 Located just 500 light-years away toward the constellation Scorpius, this star is only slightly less massive and a little cooler than the Sun. But it is much younger, a few million years old compared to the middle-aged Sun's 5 billion years. This sharp infrared image shows the young star has a likely companion positioned above and left - a hot planet with about 8 times the mass of Jupiter, orbiting a whopping 330 times the Earth-Sun distance from its parent star. The young planetary companion is still hot and relatively bright in infrared light due to the heat generated during its formation by gravitational contraction. In fact, such newborn planets are easier to detect before they age and cool, becoming much fainter. Though over 300 extrasolar planets have been found using other techniques, this picture likely represents the first direct image of a planet belonging to a star similar to the Sun.

2007 What does the surface of Saturn's mysterious moon Iapetus look like? To help find out, the robotic Cassini spacecraft now orbiting Saturn was sent soaring last week just 2,000 kilometers from the unique equatorial ridge of the unusual walnut-shaped two-toned moon. The above image from Cassini is from about 4,000 kilometers out and allows objects under 100-meters across to be resolved. Cassini found an ancient and battered landscape of craters, sloping hills, and mountains as high as 10 kilometers and so rival the 8.8-kilometer height of Mt. Everest on Earth. Just above the center of this image is a small bright patch where an impacting rock might have uncovered deep clean water ice. Space scientists will be studying flyby images like this for clues to the origin of Iapetus' unusual shape and coloring with particular emphasis because no more close flybys of the enigmatic world are planned.

2006 What have we found on the way to large Victoria Crater? Smaller Beagle Crater. The robotic Opportunity rover rolling across Mars stopped at Beagle Crater early last month and took an impressively detailed 360-degree panorama of the alien Martian landscape. Beagle crater appears in the center as a dip exposing relatively dark sand. Surrounding 35-meter Beagle Crater are many of the rocks ejected during its creation impact. Opportunity's detailed images show significant erosion on the rocks and walls of Beagle Crater, indicating that the crater is not fresh. Beagle Crater's unofficial name derives from the ship HMS Beagle where Charles Darwin observations led him to postulate his theory of natural selection. That ship was named after the dog breed of beagle. Opportunity is scheduled to roll up to expansive Victoria Crater this week.

2005 What are asteroids made of? To help find out, Japan's JAXA space agency launched the Hayabusa mission to rendezvous with asteroid Itokawa. Last week, the small robotic Hayabusa spacecraft arrived at asteroid Itokawa and stationed itself only 20 kilometers away. Although a long term goal is to find out how much ice, rock and trace elements reside on the asteroid's surface, a shorter term goal is to determine the mass of the asteroid by measuring the attraction of the drifting Hayabusa spacecraft. During the next few months, Hayabusa will also image and map asteroid Itokawa as it orbits the Sun. The above time-lapse image sequence was taken by Hayabusa upon final approach, showing the general oblong shape of the asteroid. In November, a small coffee-can sized robot dubbed MINERVA is scheduled for release and is expected to hop around the asteroid taking pictures. Also in November, Hayabusa will fire pellets into asteroid Itokawa and collect some of the debris in a return capsule. In December, Hayabusa will fire its rockets toward Earth and drop the return capsule to Earth in 2007 June.

2004 A magnetic compass does not point toward the true North Pole of the Earth. Rather, it more closely points toward the North Magnetic Pole of the Earth. The North Magnetic Pole is currently located in northern Canada. It wanders in an elliptical path each day, and moves, on the average, more than forty meters northward each day. Evidence indicates that the North Magnetic Pole has wandered over much of the Earth's surface in the 4.5 billion years since the Earth formed. The Earth's magnetic field is created by Earth's partially ionized outer core, which rotates more rapidly than the Earth's surface. Indicated in the above picture is Ellef Ringnes Island, the location of Earth's North Magnetic Pole in 1994.

2003 Launched in 1989 and looping through the jovian system since late 1995, the voyage of NASA's Galileo spacecraft will soon come to an end. The spacecraft has been targeted to plunge directly into Jupiter this Sunday, September 21st, at about 30 miles per second. Its components will be vaporized in the gas giant's outer atmosphere. While Galileo's long voyage of exploration has resulted in a spectacular scientific legacy, the spacecraft's ultimate fate is related to perhaps its most tantalizing discovery -- strong evidence for a liquid ocean beneath the frozen surface of Jupiter's moon Europa. Galileo is now almost completely out of fuel for maneuvers, so this intentional collision with Jupiter will prevent any unintentional future collision with Europa and the possibility of contaminating the jovian moon with microbes from Earth hardy enough to survive in interplanetary space. Color image data from the Galileo mission recorded between 1995 and 1998 was used to create this depiction of Europa's cracked and icy surface. The inset shows dark reddish, disrupted regions dubbed Thera and Thrace.

2002 A day is just under 11 minutes long on 1998 KY26, a 30 meter wide, fast-spinning, water-rich asteroid. This computer simulated view of its lumpy surface has a resolution of about 3 meters and is based on radar and optical observations (click on the image for a series of surface views). The observations were made shortly after the discovery of the diminutive world which passed within about 800,000 kilometers of Earth, or about 2 times the Earth-Moon distance, in June of 1998. Around 10 million asteroids of similar size may exist in orbits that also come near Earth's, but little is known about them. However, spinning so fast, tiny 1998 KY26 can not be a loose conglomerate held together by gravity alone. Instead it is likely a monolithic chunk fragmented from a larger asteroid. As the radar and optical data suggest 1998 KY26 has a high water content, this relatively accessible asteroid could be a literal oasis for future space explorers.

2001 NASA is preparing to launch its next Great Observatory in 2002, but it does not yet have a proper name. Can you help? Currently referred to only as the Space Infrared Telescope Facility (SIRTF), NASA seeks to add something more significant. Previously, NASA named its Great Observatories for scientists of the recent past, including the Hubble Space Telescope, the Compton Gamma Ray Observatory and the Chandra X-ray Observatory. SIRTF will be the most powerful infrared telescope ever launched, imaging everything from nearby planetary disks to distant galaxies. To enter the contest, one must conform to all rules including the submission of an essay of 250 words or less. The contest ends on December 20.

2000 In the depths of the dark clouds of dust and molecular gas known as M17, stars continue to form. Visible in the above recently released representative-color photograph of M17 by the New Technology Telescope are clouds so dark that they appear almost empty of near infrared light. The darkness of these molecular clouds results from background starlight being absorbed by thick carbon-based smoke-sized dust. As bright massive stars form, they produce intense and energetic light that slowly boils away the dark shroud. M17's unusual appearance has garnered it such nicknames as the Omega Nebula, the Horseshoe Nebula, and the Swan Nebula. M17, visible with binoculars towards the constellation of Sagittarius, lies 5000 light-years away and spans 20 light-years across.

1999 What is going on in NGC 891? This galaxy appeared previously to be very similar to our own Milky Way Galaxy: a spiral galaxy seen nearly edge-on. However, recent high-resolution images of NGC 891's dust show unusual filamentary patterns extending well away from its Galactic disk. This interstellar dust was probably thrown out of the galactic disk toward the halo by stellar supernovae explosions. Because dust is so fragile, its appearance after surviving disk expulsion can be very telling. Newly discovered phenomena, however, sometimes appear so complex that more questions are raised than are answered.

1998 Diminutive Deimos is the smallest of the two tiny Moons of Mars. Potato-shaped and barely 6 miles wide this asteroid-like body was visited by the Viking 2 orbiter in 1977. This image was made when the spacecraft approached to within 18 miles of Deimos' surface. One of the most detailed pictures of a celestial body ever taken by an orbiting spacecraft, the field of view is less than a square mile and features just under 10 feet across are visible. Craters and large chunks of rock are seen scattered on the surface while some of the craters appear to be covered by a layer of powdery soil or "regolith".

1997 Stars come in bunches. Of the over 200 globular star clusters that orbit the center of our Milky Way Galaxy, 47 Tucanae is the second brightest globular cluster (behind Omega Centauri). Known to some affectionately as 47 Tuc or NGC 104, it is only visible from the Southern Hemisphere. Light takes about 20,000 years to reach us from 47 Tuc which can be seen near the SMC in the constellation of Tucana. Red Giant stars are particularly easy to see in the above photograph. The dynamics of stars near the center of 47 Tuc are not well understood, particularly why there are so few binary systems there.

1996 A clear blue summer sky finally grows dark and the new telescope, hastily set up in the backyard, generates excitment and anticipation. "I bought it for the kids ...", Dad assures himself as he over-anxiously supervises the two young boys' efforts to center a bright, first quarter Moon, in the finder. The evening's first target acquired, James adjusts the focus knob and falls silent. Suddenly, "Wow, looks just like on Apollo 13!". His younger brother Christopher takes his turn. "Do you see the Moon?", James asks, eager to provide guidance based on his own observing experience. Christopher echoes his brother's enthusiasm, "Yes, and I see all the crashes too!". The view they shared was not too different from the above image of a six day old moon, recorded in July 1995 by Rhode Island amateur astronomer Jim Hendrickson. Along the terminator, the line between lunar night and day, the shadows outline to advantage the spectacular craters -- caused by all the crashes.

1995 Almost unknown to casual observers in the northern hemisphere, the southern sky contains two diffuse wonders known as the Magellanic Clouds. The Magellanic Clouds are small irregular galaxies orbiting our own larger Milky Way spiral galaxy. The Small Magellanic Cloud (SMC), pictured here, is about 250,000 light years away and contains a preponderance of young, hot, blue stars indicating it has undergone a recent period of star formation. There is evidence that the SMC is actually two galaxies superposed to appear as one. The bright blob near the right hand edge of the frame is a globular cluster near the outskirts of the Milky Way.

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