NASA Astronomy Picture of the Day 2011-8

Shuttle Reentry Streak from Orbit

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.

Asteroid Vesta Full Frame

Why is the northern half of asteroid Vesta more heavily cratered than the south? No one is yet sure. This unexpected mystery has come to light only in the past few weeks since the robotic Dawn mission became the first spacecraft to orbit the second largest object in the asteroid belt between Mars and Jupiter. The northern half of Vesta, seen on the upper left of the above image, appears to show some of the densest cratering in the Solar System, while the southern half is unexpectedly smooth. Also unknown is the origin of grooves that circle the asteroid nears its equator, particularly visible on this Vesta rotation movie, and the nature of dark streaks that delineate some of Vesta's craters, for example the crater just above the the image center. As Dawn spirals in toward Vesta over the coming months, some answers may emerge, as well as higher resolution and color images. Studying 500-km diameter Vesta is yielding clues about its history and the early years of our Solar System.

The Leo Triplet Galaxies from VST

This popular group is famous as the Leo Triplet - a gathering of three magnificent galaxies in one field of view. Crowd pleasers when imaged with even modest telescopes, these galaxies can be introduced individually as NGC 3628 (left), M66 (bottom right), and M65 (top right). All three are large spiral galaxies. They tend to look dissimilar because their galactic disks are tilted at different angles to our line of sight. NGC 3628 is seen edge-on, with obscuring dust lanes cutting across the plane of the galaxy, while the disks of M66 and M65 are both inclined enough to show off their spiral structure. Gravitational interactions between galaxies in the group have also left telltale signs, including the warped and inflated disk of NGC 3628 and the drawn out spiral arms of M66. This gorgeous deep view of the region was taken by the new VLT Survey Telescope (VST) and spans about one degree (two full moons) on the sky. The field covers over 500 thousand light-years at the trio's estimated distance of 30 million light-years.

A Dusty Iris Nebula

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. Surrounding it, obscuring clouds of dust and cold molecular gas are also present and can suggest other convoluted and fantastic shapes. Within the Iris, the 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 cosmic dust 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. At the estimated distance of the Iris Nebula this remarkable wide field view is over 30 light-years across.

A Summer Night's Dream

Fix your digital camera to a tripod, start a long series of exposures, and you too can record star trails. The concentric arcs traced by the stars as planet Earth rotates on its axis often produce dreamlike scenes in otherwise familiar situations. Fall asleep, though, and the results might surprise you. Setting up on a summer night, photographer Mike Rosinski began his exposures, initially planning to capture about 45-55 minutes worth of star trails from his yard in Hartland, Michigan, USA. But he dozed, only to awaken some 3 hours later to find his camera had continued to run until the battery died. Composing the resulting images, the graceful concentric star trails were expected, along with light from a late rising Moon glinting on windows. Still, as he slept on the warm night a blizzard of yellow streaks flooded the scene, not left by fairies but fireflies.

Comet Garradd and Messier 15

Recorded on August 2, this telescopic composite image catches Comet Garradd (C/2009 P1) in the same field of view as globular star cluster M15. The celestial scene would have been a rewarding one for influential 18th century comet hunter Charles Messier. While Messier scanned French skies for comets, he carefully cataloged positions of things which might be fuzzy and comet-like in appearance but did not move against the background stars and so were definitely not comets. M15 (lower right), the 15th entry in his famous not-a-comet catalog, is now understood to be a cluster of over 100,000 stars some 35,000 light-years distant. The comet, discovered in August 2009 by astronomer G. J. Garradd (Siding Spring Observatory, Australia) is currently sweeping across the constellation Pegasus, some 13 light-minutes from Earth. Shining faintly around 9th magnitude, comet Garradd will brighten in the coming months, predicted to be just below naked eye visibility near its peak in February 2012.

MyCn18: An Hourglass Planetary Nebula

The sands of time are running out for the central star of this hourglass-shaped planetary nebula. With its nuclear fuel exhausted, this brief, spectacular, closing phase of a Sun-like star's life occurs as its outer layers are ejected - its core becoming a cooling, fading white dwarf. In 1995, astronomers used the Hubble Space Telescope (HST) to make a series of images of planetary nebulae, including the one above. Here, delicate rings of colorful glowing gas (nitrogen-red, hydrogen-green, and oxygen-blue) outline the tenuous walls of the hourglass. The unprecedented sharpness of the HST images has revealed surprising details of the nebula ejection process that are helping to resolve the outstanding mysteries of the complex shapes and symmetries of planetary nebulas.

Seasonal Dark Streaks on Mars

What is causing these dark streaks on Mars? A leading hypothesis is flowing -- but quickly evaporating -- water. The streaks, visible in dark brown near the image center, appear in the Martian spring and summer but fade in the winter months, only to reappear again the next summer. These are not the first markings on Mars that have been interpreted as showing the effects of running water, but they are the first to add the clue of a seasonal dependence. The above picture, taken in May, digitally combines several images from the the HiRISE instrument on the Mars Reconnaissance Orbiter (MRO). The image is color-enhanced and depicts a slope inside Newton crater in a mid-southern region of Mars. The streaks bolster evidence that water exists just below the Martian surface in several locations, and therefore fuels speculation that Mars might harbor some sort of water-dependent life. Future observations with robotic spacecraft orbiting Mars, such as MRO, Mars Express, and Mars Odyssey will continue to monitor the situation and possibly confirm -- or refute -- the exciting flowing water hypothesis.

Juno Rockets Toward Jupiter

Next stop: Jupiter. Last week included one of the few times in history that humanity launched something completely off the Earth, moving away so fast that it will never return. Well, almost -- Juno's planned trajectory actually brings it homeward bound in about two years, zipping by, this time using the Earth's gravity to pull it to an even higher speed, high enough to reach Jupiter. The above video depicts the launch of Juno aboard a Atlas V rocket. When the robotic Juno spacecraft reaches Jupiter in 2016, it will spend just over a year circling the Solar System's largest planet, using its unique cadre of instruments to probe the planet, sending back clues of its structure and origin. Then Juno will be instructed to dive into the thick atmosphere of the Jovian giant, taking as much data as it can before it melts.

The Summer Triangle Over Catalonia

Can you find the Summer Triangle? It's not hard to find this famous triangle of stars this time of year from northern locations. Just look straight up after sunset and find three of the brightest stars in the sky that nearly form a triangle. Then compare these stars to sky images like the one shown above, or hold up a smart phone running a good sky labelling application. The three stars that form the vertexes of the Summer Triangle are Vega, Deneb, and Altair. Pictured above is a 360 degree full sky projection framing not only the Summer Triangle but the great arch of our Milky Way Galaxy. The image was taken last week in front of a small river that encircles the historic town of Sant Llorenç de la Muga in Catalonia, northeastern Spain.

The Snows of Paranal

Recorded last week, this dawn portrait of snowy mountain and starry sky captures a very rare scenario. The view does feature a pristine sky above the 2,600 meter high mountain Cerro Paranal, but clear skies over Paranal are not at all unusual. That's one reason the mountain is home to the European Southern Observatory's Very Large Telescope. Considering the number of satellites now in orbit, the near sunrise streak of a satellite glinting at the upper left isn't rare either. And the long, bright trail of a meteor can often be spotted this time of year too. The one at the far right is associated with the annual Perseid meteor shower whose peak is expected tomorrow (Friday, August 12). In fact, the rarest aspect of the picture is just the snow. Cerro Paranal rises above South America's Atacama desert, known as the driest place on planet Earth.

NGC 7331 and Beyond

Big, beautiful spiral galaxy NGC 7331 is often touted as an analog to our own Milky Way. About 50 million light-years distant in the northern constellation Pegasus, NGC 7331 was recognized early on as a spiral nebula and is actually one of the brighter galaxies not included in Charles Messier's famous 18th century catalog. Since the galaxy's disk is inclined to our line-of-sight, long telescopic exposures often result in an image that evokes a strong sense of depth. The effect is further enhanced in this sharp image by galaxies that lie beyond the gorgeous island universe. The background galaxies are about one tenth the apparent size of NGC 7331 and so lie roughly ten times farther away. Their close alignment on the sky with NGC 7331 occurs just by chance. Seen here through faint foreground dust clouds lingering above the plane of Milky Way, this visual grouping of galaxies is also known as the Deer Lick Group.

Castle and Meteor by Moonlight

ch August, as planet Earth swings through dust trailing along the orbit of periodic comet Swift-Tuttle, skygazers enjoy the Perseid Meteor Shower. As Earth moves through the denser part of the comet's wide dust trail this year's shower peaks around 6:00 UT August 13 (this morning), when light from a nearly full Moon masks all but the brighter meteor streaks. Still, Perseid meteors can be spotted in the days surrounding the peak. Moonlight and a Perseid meteor created this gorgeous skyscape, recorded in a simple, single, 10 second long exposure on the morning of August 12. Below the moonlit clouds in the foreground are the ruins of a medieval castle near Veszprem, Hungary, seen against the Bakony mountain range. In the night sky above the clouds, the Perseid meteor's trail is joined by bright planet Jupiter near the center of the frame along with the lovely Pleiades star cluster at the left.

Tornado and Rainbow Over Kansas

The scene might have been considered serene if it weren't for the tornado. During 2004 in Kansas, storm chaser Eric Nguyen photographed this budding twister in a different light -- the light of a rainbow. Pictured above, a white tornado cloud descends from a dark storm cloud. The Sun, peeking through a clear patch of sky to the left, illuminates some buildings in the foreground. Sunlight reflects off raindrops to form a rainbow. By coincidence, the tornado appears to end right over the rainbow. Streaks in the image are hail being swept about by the high swirling winds. Over 1,000 tornadoes, the most violent type of storm known, occur on Earth every year, many in tornado alley. If you see a tornado while driving, do not try to outrun it -- park your car safely, go to a storm cellar, or crouch under steps in a basement.

Rover Arrives at Endeavour Crater on Mars

What can the present-day terrain in and around large Endeavour crater tell us about ancient Mars? Starting three years ago, NASA sent a coffee-table sized robot named Opportunity on a mission rolling across the red planet's Meridiani Planum to find out. Last week, it finally arrived. Expansive Endeavour crater stretches 22 kilometers from rim to rim, making it the largest crater ever visited by a Mars Exploration Rover (MER). It is hypothesized that the impact that created the crater exposed ancient rock that possibly formed under wet conditions, and if so, this rock may yield unique clues to the watery past of Mars. Pictured above, the west rim of Endeavour looms just ahead of the Opportunity rover. Opportunity may well spend the rest of its operational life exploring Endeavour, taking pictures, spinning its wheels, and boring into intriguing rocks.

Shapley 1: An Annular Planetary Nebula

What happens when a star runs out of nuclear fuel? For stars about the mass of our Sun, 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, pictured above and designated Shapley 1 after the famous astronomer Harlow Shapley, has a very apparent annular ring like structure. Although some of these nebulas appear like planets on the sky (hence their name), they actually surround stars far outside our Solar System.

Perseid Below

Denizens of planet Earth watched this year's Perseid meteor shower by looking up into the moonlit night sky. But this remarkable view captured by astronaut Ron Garan looks down on a Perseid meteor. From Garan's perspective onboard the International Space Station orbiting at an altitude of about 380 kilometers, the Perseid meteors streak below, swept up dust left from comet Swift-Tuttle heated to incandescence. The glowing comet dust grains are traveling at about 60 kilometers per second through the denser atmosphere around 100 kilometers above Earth's surface. In this case, the foreshortened meteor flash is right of frame center, below the curving limb of the Earth and a layer of greenish airglow. Out of the frame, the Sun is on the horizon beyond one of the station's solar panel arrays at the upper right. Seen above the meteor near the horizon is bright star Arcturus and a star field that includes the constellations Bootes and Corona Borealis. The image was recorded on August 13 while the space station orbited above an area of China approximately 400 kilometers to the northwest of Beijing. Poll: Is APOD a blog?

A Sun Pillar Over Ontario

What is that on the horizon? No, it's not an alien starship battling distant Earthlings, but rather a sun pillar. When driving across Ontario, Canada in early June, the photographer was surprised to encounter such an "eerie and beautiful" vista, and immediately took pictures. When the atmosphere is cold, ice sometimes forms flat six-sided crystals as it falls from high-level clouds. Air resistance then causes these crystals to lie nearly flat much of the time as they flutter to the ground. If viewed toward a rising or setting Sun, these flat crystals will reflect sunlight and create an unusual column of light -- a sun pillar as seen above. Such columns of light are not uncommon to see, and a retrospective of past APODs that have featured picturesque sun pillars can be found here. Challenge: Find blogs running longer than APOD.

Herschel's Cocoon

In this remarkable infrared skyscape of interstellar clouds adrift in the high flying constellation Cygnus, the eye is drawn to the Cocoon Nebula. Also known as IC5146, the dusty star forming region is shown in blue hues in the Herschel Space Observatory false color image, at wavelengths more than 100 times longer than visible red light. And while visible light images show the Cocoon nebula at the end of long dark nebula Barnard 168, Hershel's infrared view finds the cosmic Cocoon punctuating a trail of filamentary clouds of glowing dust. The filaments have widths that suggest they are formed as shockwaves from exploding stars travel through the medium, sweeping up and compressing the interstellar dust and gas. Herschel data also indicate stars are forming along the dusty filaments. The Cocoon Nebula itself is about 15 light-years wide and 4,000 light-years away.

Stereo Vesta

Get out your red/blue glasses and float next to 4 Vesta. A 500 kilometer diameter world, Vesta lies in the main asteroid belt between the orbits of Mars and Jupiter. This stereo anaglyph was constructed from two separate images recorded on July 24 by the just arrived Dawn spacecraft's framing camera with a resolution of about 500 meters per pixel. The 3D view features Vesta's newly discovered terrain, including long equatorial ridges and troughs and the prominent string of three craters at the upper right dubbed Snowman. Highlighted in 3D, steep sides of many of the craters show streaks of both bright and dark material. Of course, the ion-driven Dawn spacecraft is not marooned off Vesta. After a year exploring the asteroid from orbit, Dawn is scheduled to depart, beginning its journey to Ceres.

The Fairy of Eagle Nebula

The dust sculptures of the Eagle Nebula are evaporating. As powerful starlight whittles away these cool cosmic mountains, the statuesque pillars that remain might be imagined as mythical beasts. Pictured above is one of several striking dust pillars of the Eagle Nebula that might be described as a gigantic alien fairy. This fairy, however, is ten light years tall and spews radiation much hotter than common fire. The greater Eagle Nebula, M16, is actually a giant evaporating shell of gas and dust inside of which is a growing cavity filled with a spectacular stellar nursery currently forming an open cluster of stars. The above image in scientifically re-assigned colors was released in 2005 as part of the fifteenth anniversary celebration of the launch of the Hubble Space Telescope.

TrES-2b: Dark Planet

Why is this planet so dark? Planet TrES-2b reflects back less than one percent of the light it receives, making it darker than any known planet or moon, darker even than coal. Jupiter-sized TrES-2b orbits extremely close to a sun-like star 750 light years away, and was discovered producing slight eclipses in 2006 using the modest 10-cm telescopes of the Trans-Atlantic Exoplanet Survey (TrES). The alien world's strange darkness, however, was only uncovered recently by observations indicating its slight reflective glow by the Sun-orbiting Kepler satellite. An artist's drawing of planet is shown above, complete with unsubstantiated speculation on possible moons. Reasons for TrES-2b's darkness remain unknown and are an active topic of research.

Aurora Over Greenland

This aurora arched from horizon to horizon. During the current Shelios expedition to observe and learn about the northern lights, the sky last weekend did not disappoint. After sunset and some careful photographic planning, the above image was taken from the expedition's Qaleraliq campsite in southern Greenland. Visible straight through the center of the aurora, found with a careful eye, is the Big Dipper and the surrounding constellation of the Big Bear (Ursa Major). The brightest orb on the far right is the Moon, while Jupiter can be seen even further to the right. The Shelios expedition is scheduled to last until the end of August and include live broadcasts of ongoing auroras.

A Pileus Iridescent Cloud Over Ethiopia

Yes, but how many dark clouds have a multicolored lining? Pictured, behind this darker cloud, is a pileus iridescent cloud, a group of water droplets that have a uniformly similar size and so together diffract different colors of sunlight by different amounts. The above image was taken just after the picturesque sight was noticed by chance by a photographer in Ethiopia. A more detailed picture of the same cloud shows not only many colors, but unusual dark and wavy bands whose origins are thought related to wave disturbances in the cloud.

Portrait of NGC 281

Look through the cosmic cloud cataloged as NGC 281 and it's almost easy to miss stars of open cluster IC 1590. But, formed within the nebula, that cluster's young, massive stars ultimately power the pervasive nebular glow. The eye-catching shapes looming in this portrait of NGC 281 are sculpted columns and dense dust globules seen in silhouette, eroded by intense, energetic winds and radiation from the hot cluster stars. If they survive long enough, the dusty structures could also be sites of future star formation. Playfully called the Pacman Nebula because of its overall shape, NGC 281 is about 10,000 light-years away in the constellation Cassiopeia. This composite image was made through narrow-band filters, but combines emission from the nebula's hydrogen, sulfur, and oxygen atoms in a visible spectrum palette. It spans over 80 light-years at the estimated distance of NGC 281.

A Young Supernova in the Nearby Pinwheel Galaxy

A nearby star has exploded and telescopes all over the world are turning to monitor it. The supernova, dubbed PTF 11kly, was discovered by computer only two days ago as part of the Palomar Transient Factory (PTF) sky survey utilizing the wide angle 1.2-meter Samuel Oschin Telescope in California. Its rapid recovery makes it one of the supernovas caught most soon after ignition. PTF 11kly occurred in the photogenic Pinwheel galaxy (M101), which, being only about 21 million light years away, makes it one of the closest supernovas seen in decades. Rapid follow up observations have already given a clear indication that PTF 11kly is a Type Ia supernova, a type of white dwarf detonation that usually progresses in such a standard manner than it has helped to calibrate the expansion history of the entire universe. Studying such a close and young Type Ia event, however, may yield new and unique clues. If early indications are correct, PTF 11kly should brighten to about visual magnitude 10 in the coming weeks, making it possible to monitor with even moderately sized telescopes. APOD Retrospective: The best of the spiral galaxy M101

Hurricane Irene Forms

How does a hurricane form? Although a complete picture is still being researched, insight into this process might be gleaned by watching the above time lapse movie of the formation of Hurricane Irene, a large storm system currently threatening the eastern seaboard of the USA. Starting as a slight pressure difference visible as nondescript clouds on the lower right, Hurricane Irene is shown growing into large spiraling storm system of low pressure off the coast of South Carolina. A hurricane is powered by evaporating ocean water, and so typically gains strength over warm water and loses strength over land. Besides Earth, other planets that have hurricane-like storm systems include Venus, Saturn, Jupiter, Uranus, and Neptune. Much remains unknown about hurricanes and cyclones, including the exact path they will take.

A Jet from Galaxy M87

What's causing a huge jet to emanate from the center of galaxy M87? Although the unusual jet was first noticed early in the twentieth century, the exact cause is still debated. The above picture taken by the Hubble Space Telescope in 1998 shows clear details, however. The most popular hypothesis holds that the jet is created by energetic gas swirling around a massive black hole at the galaxy's center. The result is a 5000 light-year long blowtorch where electrons are ejected outward at near light-speed, emitting eerily blue light during a magnetic spiral. M87 is a giant elliptical galaxy residing only 50 million light-years away in the Virgo Cluster of Galaxies. The faint dots of light surrounding M87's center are large ancient globular clusters of stars.

Hickson 44 in Leo

Scanning the skies for galaxies, Canadian astronomer Paul Hickson and colleagues identified some 100 compact groups of galaxies, now appropriately called Hickson Compact Groups. The four prominent galaxies seen in this intriguing telescopic skyscape are one such group, Hickson 44, about 100 million light-years distant toward the constellation Leo. The two spiral galaxies in the center of the image are edge-on NGC 3190 with its distinctive, warped dust lanes, and S-shaped NGC 3187. Along with the bright elliptical, NGC 3193 at the right, they are also known as Arp 316. The spiral in the upper left corner is NGC 3185, the 4th member of the Hickson group. Like other galaxies in Hickson groups, these show signs of distortion and enhanced star formation, evidence of a gravitational tug of war that will eventually result in galaxy mergers on a cosmic timescale. The merger process is now understood to be a normal part of the evolution of galaxies, including our own Milky Way. For scale, NGC 3190 is about 75,000 light-years across at the estimated distance of Hickson 44. APOD Retrospective: Small Groups of Galaxies

The Coldest Brown Dwarf

This cosmic snapshot composed with image data from NASA's Wide-field Infrared Survey Explorer (WISE) satellite captures a multitude of faint stars and distant galaxies toward the constellation Lyra at wavelengths longer than visible light. But the object circled at the center is not quite a star. Cataloged as WISE 1828+2650, it lies within 40 light-years of the Sun and is currently the coldest brown dwarf known. A brown dwarf begins like a star, with the gravitational collapse of a dense cloud of gas and dust, but is not massive enough to achieve the core temperatures and densities that trigger hydrogen fusion, the stable source of a star's energy. Instead the failed star ultimately cools and emits most of its light at infrared wavelengths. Remarkably, brown dwarfs are roughly the size of the planet Jupiter. How cold is WISE 1828+2650? While brown dwarfs have measured surface temperatures of up to 1,400 degrees C (2,600 degress F), this brown dwarf , assigned to spectral class Y, has the estimated temperature of a warm room, less than about 27 degrees C (80 degrees F).

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