APOD Retrospective: October 20

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: October 20

Post by bystander » Thu Oct 20, 2011 3:35 am


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2014 Yesterday, a comet passed very close to Mars. In fact, Comet C/2013 A1 (Siding Spring) passed closer to the red planet than any comet has ever passed to Earth in recorded history. To take advantage of this unique opportunity to study the close interaction of a comet and a planet, humanity currently has five active spacecraft orbiting Mars: NASA's MAVEN, MRO, Mars Odyssey, as well as ESA's Mars Express, and India's Mars Orbiter. Most of these spacecraft have now sent back information that they have not been damaged by small pieces of the passing comet. These spacecraft, as well as the two active rovers on the Martian surface -- NASA's Opportunity and Curiosity -- have taken data and images that will be downloaded to Earth for weeks to come and likely studied for years to come. The featured image taken yesterday, however, was not taken from Mars but from Earth and shows Comet Siding Spring on the lower left as it passed Mars, on the upper right.

2013 Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. The scene also captures the broad tail and bright coma of Comet McNaught, the Great Comet of 2007. Currently, many sky enthusiasts are following the development of Comet ISON, a comet which might become the Great Comet of 2013.

2012 Ghostly apparitions of two fundamental planes in planet Earth's sky span this October all-sky view. The scene was captured from a lakeside campsite under dark skies in northern Maine, USA. In it, the plane of our Milky Way Galaxy arcs above faint airglow along the horizon. Zodiacal light, a band of dust scattering sunlight along the solar system's ecliptic plane, stretches almost horizontally across the wide field and intersects the Milky way near a point marked by bright planet Jupiter. Right of Jupiter, past the Pleiades star cluster, is the brightening of the Zodiacal band known as the Gegenschein, also visible to the eye on that dark night. Begirt with many a blazing star and rising above the distant mountains, Orion the hunter is reflected in the lake's calm waters.

2011 A good target for binoculars and small telescopes, Comet Garradd (C/2009 P1) now shines in planet Earth's evening skies, a steady performer but just below naked-eye visibility. Telescopic images like this composite from October 15 can find the comet with a lovely green coma, sporting multiple tails, and lingering against a background of faint stars. The field of view spans over 1 degree or about 2 full moons within the southern boundaries of the constellation Hercules. Now around 16 light minutes (2 astronomical units) away, P1 Garradd is an intrinsically large comet, but will never make a very close approach to Earth or the Sun while sweeping through the inner solar system. As a result, the comet will likely stay a sight for telescopic eyes only, moving slowly through the sky and remaining in Hercules during the coming months.

2010 Is that Venus or an airplane? A common ponderable for sky enthusiasts is deciding if that bright spot near the horizon is the planet Venus. Usually, an airplane will show itself by moving significantly in a few moments. Venus will set only slowly as the Earth turns. Still, the identification would be easier if Venus did not keep shifting its position each night. Pictured above, Venus was captured on 44 different nights during 2006 and 2007 over the Bolu mountains in Turkey, when Earth's sister planet appeared exclusively in the evening sky. The average spacing of the images was about five days, while the images were always taken with the Sun about seven degrees below the horizon. That bright spot toward the west in your evening sky this month might be neither Venus nor an airplane, but Mars.

2009 What does a solar prominence look like in three dimensions? To help find out, NASA launched the STEREO satellites to keep a steady eye on the Sun from two different vantage points. The STEREO satellites orbit the Sun nearly along Earth's orbit, but one (dubbed Ahead) currently leads the Earth, while the other (dubbed Behind) currently trails. Three weeks ago, a powerful prominence erupted and remained above the Sun for about 30 hours, allowing the STEREO satellites to get numerous views of the prominence from different angles. Pictured above is a high-resolution image of the event from the STEREO Ahead satellite. A video of the prominence erupting as seen from both spacecraft can be found here. The unusually quiet nature of the Sun over the past two years has made large prominences like this relatively rare. The combined perspective of STEREO will help astronomers better understand the mechanisms for the creation and evolution of prominences, coronal mass ejections, and flares.

2008 Why would Saturn show such strange colors? The robotic Cassini spacecraft currently orbiting Saturn has beamed back images showing that the northern hemisphere our Solar System's most spectacularly ringed planet has changed noticeably since Cassini arrived in 2004, now sporting unusual and unexpected colors. No one is sure why. Although the cause for many of Saturn's colors is unknown, the recent change in colors is thought to be related to the changing seasons. Pictured above, the unusual colors are visible just north of the dark ring shadows. The razor-thin plane of ring particles is visible nearly edge-on across the bottom of the image. The cloudy moon Titan looms large just above the rings, while close observation will reveal three other moons. Cassini arrived at Saturn in 2004, sending back data and images that have not only led to a deeper understanding of the Jovian world's atmosphere, moons, and rings, but also raised new mysteries.

2007 Inspired during a visit to Fort Davis, Texas, home of McDonald Observatory and dark night skies, photographer Larry Landolfi created this tantalizing fantasy view. The composited image suggests the Milky Way is a heavenly extension of a deserted country road. Of course, the name for our galaxy, the Milky Way (in Latin, Via Lactea), does refer to its appearance as a milky band or path in the sky. In fact, the word galaxy itself derives from the Greek for milk. Visible on moonless nights from dark sky areas, though not so colorful as in this image, the glowing celestial band is due to the collective light of myriad stars along the plane of our galaxy, too faint to be distinguished individually. The diffuse starlight is cut by dark swaths of obscuring galactic dust clouds. At the beginning of the 17th century, Galileo turned his telescope on the Milky Way and announced it to be composed of innumerable stars.

2006 South of Antares, in the tail of the nebula-rich constellation Scorpius, lies emission nebula IC 4628. Nearby hot, massive stars, millions of years young, radiate the nebula with invisible ultraviolet light, stripping electrons from atoms. The electrons eventually recombine with the atoms to produce the visible nebular glow. This narrow band image adopts a typical false-color mapping of the atomic emission, showing hydrogen emission in green hues, sulfur as red and oxygen as blue. At an estimated distance of 6,000 light-years, the region shown is about 250 light-years across. The nebula is also cataloged as Gum 56 for Australian astronomer Colin Stanley Gum, but seafood-loving astronomers might know this cosmic cloud as The Prawn Nebula.

2005 What is the Andromeda galaxy really like? To find out, astronomers looked at our largest galactic neighbor in a different light: infrared. Astronomers trained the orbiting Spitzer Space Telescope at the Messier monster (M31) for over 18 hours, creating a mosaic that incorporated 11,000 separate exposures. The result, pictured above, shows M31 in greater infrared detail than ever before. Infrared light in this 24-micron color band is particularly sensitive to dust heated up by stars. Visible above are previously undiscovered features including intricate structure in the spiral arms, a spiral arc near the center, an off center ring of star formation, and an unusual hole in the galaxy's disk. In contrast, the Andromeda galaxy appears much smoother in visible light and even ultraviolet light. Analyses and comparison of this image to other images will likely yield clues not only to the violent past of M31 but to our own Milky Way Galaxy as well.

2004 NGC 281 is a busy workshop of star formation. Prominent features include a small open cluster of stars, a diffuse red-glowing emission nebula, large lanes of obscuring gas and dust, and dense knots of dust and gas in which stars may still be forming. The open cluster of stars IC 1590 visible around the center has formed only in the last few million years. The brightest member of this cluster is actually a multiple-star system shining light that helps ionize the nebula's gas, causing the red glow visible throughout. The lanes of dust visible left of center are likely homes of future star formation. Particularly striking in the above photograph are the dark Bok globules visible against the bright nebula. The entire NGC 281 system lies about 10 thousand light years distant.

2003 How's the weather on Neptune? Tracking major weather patterns on the Solar System's outermost gas giant can help in the understanding of global weather patterns here on Earth. Each summer for the past five years, Neptune has been imaged and major weather patterns studied. The latest picture, taken on September 15, is shown above in false color. Visible in pink near Neptune's lower right is a new storm dubbed Annabelle that is several times larger than her terrestrial sister Isabel, a concurrent storm system that occurred here on Earth. Although Isabel lasted a few weeks, no one knows how long Annabelle will endure. On the upper right is Neptune's largest moon Triton, an unusual moon that sports volcanoes that spew ice.

2002 Before there was the International Space Station, the reigning orbiting spaceport was Russia's Mir. Pictured above in 1995, the United States Space Shuttle Atlantis docked with the segmented Mir. During shuttle mission STS-71, astronauts answered questions from school students over amateur radio and performed science experiments aboard Spacelab. The Spacelab experiments helped to increase understanding of the effects of long-duration space flights on the human body. Last year, after 15 years of successful service, the decaying Mir space station broke up as it entered the Earth's atmosphere.

2001 Tune your radio telescope to 408MHz (408 million cycles per second) and check out the Radio Sky! You should find that frequency on your dial somewhere between US broadcast television channels 13 and 14. In the 1970s large dish antennas at three radio observatories, Jodrell Bank, MPIfR, and Parkes Observatory, were used to do just that - the data were combined to map the entire sky. Near this frequency, cosmic radio waves are generated by high energy electrons spiraling along magnetic fields. In the resulting false color image, the galactic plane runs horizontally through the center, but no stars are visible. Instead, many of the bright sources near the plane are distant pulsars, star forming regions, and supernova remnants, while the grand looping structures are pieces of bubbles blown by local stellar activity. External galaxies like Centaurus A, located above the plane to the right of center, and the LMC (below and right) also shine in the Radio Sky.

2000 Orbiting over the north pole of planet Earth on May 5, the MODIS instrument on-board the Terra spacecraft, recorded this view of the ice cap 700 kilometers below. A radial grid centered on the pole is shown on top of the approximately true color image where each pixel covers about one square kilometer. Frozen sea ice appears whitish while open water or newly refrozen ice looks black. An impressive criss-crossing network of cracks in ice shifting above a liquid water ocean is visible, traced by the meandering dark lines. In fact, the dark network of cracks in the sea ice is reminiscent of another world in our solar system which may also harbor a liquid water ocean -- Jupiter's ice moon Europa.

1999 What causes Hubble's Variable Nebula to vary? The unusual nebula pictured above changes its appearance noticeably in just a few weeks. Discovered over 200 years ago and subsequently cataloged as NGC 2661, the remarkable nebula is named for Edwin Hubble, who studied it earlier this century. Hubble's Variable Nebula is a reflection nebula made of gas and fine dust fanning out from the star R Monocerotis. The faint nebula is about one light-year across and lies about 2500 light-years away towards the constellation of Monocerotis. A leading variability explanation for Hubble's Variable Nebula holds that dense knots of opaque dust pass close to R Mon and cast moving shadows onto the reflecting dust seen in the rest of the nebula.

1998 The Sun's third largest planet usually looks quite dull. Uranus typically appears as a featureless small spot in a small telescope or a featureless large orb in a large telescope. Last August, however, the Hubble Space Telescope was able to photograph Uranus in infrared light, where the distant planet better shows its unusual clouds, rings, and moons. Recent analysis indicates that clouds seen here in orange appear to circle Uranus at speeds in excess of 500 kilometers per hour. Comparisons to earlier photographs show a slight precession shift in the brightest of Uranus' rings. Several of Uranus' numerous small moons are visible.

1997 Can you identify this stellar nebula? How many light-years from Earth did you say? Looking like a twisting cloud of gas and dust between the stars this wispy nebulosity is actually close by - a spiral eddy formed near the North Atlantic Gulf Stream off the East coast of the US. Tens of miles across, spiral eddies are an ocean current phenomenon discovered by observations from manned spacecraft. Imaged by the Challenger space shuttle crew during the STS 41G mission this eddy is dramatically visible due to the low sun angle and strong reflection of sunlight. The reflection is caused by a very thin biologically produced oily film on the surface of the swirling water. Prior to STS 41G these eddies were thought to be rare but are now understood to be a significant dynamic feature of ocean currents. However, no good explanation of their origin or persistence exists.


1996 "Safe!" -- In September 1967 (during regular season play), while making a successful soft landing on the Moon's Mare Tranquillitatis, the Surveyor 5 lander actually slid several feet. Equipped with television cameras and some soil sampling experiments, the US Surveyor spacecraft were intended to determined if the lunar surface at chosen locations was suitable for manned landings. Surveyor 5 touched down on the inside edge of a small crater inclined at about 20 degrees. Its footpad slipped and dug the trench visible in the picture. Covered with lunar soil, the footpad is about 20 inches in diameter.

1995 Above is the best yet color image of the asteroid Gaspra based on data returned by NASA's Galileo spacecraft. Color variations have been added to high resolution images and enhanced to highlight changes in reflectivity, surface structure and composition. The illuminated portion of the asteroid is about 11 miles long. Galileo encountered Gaspra on October 29, 1991 during the cruise phase of its mission to study the Jovian system. When it arrives at Jupiter, in December 1995, the robot spacecraft's atmospheric probe will descend into Jupiter's atmosphere, becoming the first ever probe to enter the atmosphere of a gas giant planet. Updates on Galileo's progress can be found at "Online from Jupiter".

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