Astronomy Picture of the Day

Debris Disks Surround Distant Suns



In this dramatic artist's vision, debris along the outer reaches of a planet forming disk orbits in the glare of a distant sun. But inset are actual images of such disks around two nearby stars - AU Microscopii (top left; edge-on) and HD107146 (right: face-on) - as seen by the Hubble Space Telescope. Combined with infrared images from the Spitzer Space Telescope that show debris disks around known planet bearing stars, the data provide the first direct link between extrasolar disks and planets, suggesting a scenario where evolving planets scatter debris produced by collisions into giant disks. In time, the dusty disks may dwindle and become like our own Solar System's comet reservoir, the Kuiper Belt.

Doomed Star Eta Carinae



Eta Carinae may be about to explode. But no one knows when - it may be next year, it may be one million years from now. Eta Carinae's mass - about 100 times greater than our Sun - makes it an excellent candidate for a full blown supernova. Historical records do show that about 150 years ago Eta Carinae underwent an unusual outburst that made it one of the brightest stars in the southern sky. Eta Carinae, in the Keyhole Nebula, is the only star currently thought to emit natural LASER light. This image, taken in 1996, resulted from sophisticated image-processing procedures designed to bring out new details in the unusual nebula that surrounds this rogue star. Now clearly visible are two distinct lobes, a hot central region, and strange radial streaks. The lobes are filled with lanes of gas and dust which absorb the blue and ultraviolet light emitted near the center. The streaks remain unexplained. Will these clues tell us how the nebula was formed? Will they better indicate when Eta Carinae will explode?

Old Planetary Dust Disks Found by SST



Why are some older stars surrounded by dust? Observations from the Spitzer Space Telescope by a team led by George Rieke (U. Arizona) were expected to show that young stars, on the order of one million years old, have large dust disks, while relatively older stars, between 10 and 100 million years old, have none. The conventional wisdom was that the dust disks surrounding young stars were still forming planets, while in older systems these disks had dissipated after planets had already formed. Unexpectedly, they found some older stars with the infrared glow of impressive rings or disks of dust. A possible explanation is that the old disks are remnant debris from violent collisions between many forming planets of rock. Resultant dust rings from such a scenario are depicted by an artist's illustration above.

Earth Nears Asteroid Toutatis



On Wednesday, September 29, the Earth came within one million miles of the asteroid Toutatis — the closest predicted aproach of our fair planet to a sizable asteroid or comet in this century. Coming within one million miles or about 4 times the Earth-Moon distance, Earth would appear to be nearly the size of the full moon in the asteroid's sky, as suggested in this illustration. In Earth's sky, Toutatis appeared only as a faint object rapidly moving against a background of stars. Also known as Earth-crossing asteroid 4179, Toutatis is in an eccentric 4 year orbit which moves it from the asteroid belt between Mars and Jupiter to just inside Earth's orbit. When the Earth passed near it in 1992 Toutatis was imaged by radar and seen to be two irregularly shaped lumps, perhaps joined by a narrow neck. This bizarre object is up to 1.5 miles wide, 3 miles long, and is tumbling through space. Studies of Toutatis and other Earth-crossing asteroids help reveal connections between the Solar System's meteorites, main-belt asteroids and comets. These asteroids also offer tantalizing targets for robotic exploration and, over time, represent potential collision hazards for planet Earth!

An Inner Neptune for 55 Cancri



Is our Solar System unique? The discovery of a Neptune-mass planet in an sub-Mercury orbit around nearby Sun-like star 55 Cancri, announced yesterday along with the discovery of other similar systems, gives a new indication that planetary systems as complex as our own Solar System likely exist elsewhere. The planet, discovered in data from the Hobby-Eberly telescope in Texas, the Lick Observatory in California, and the orbiting Hubble Space Telescope, is one of four planets now known to orbit 55 Cancri — the others being similar in mass to Jupiter. The finding involved noting subtle changes in the speed of the star caused by its orbiting planets. The above drawing depicts what this planet might look like, assuming a mass similar to Neptune, but a composition similar to Earth. The star 55 Cancri, only 40 light-years distant, is visible with binoculars towards the constellation of Cancer.

Sedna at Noon



Standing on Sedna - the solar system's most distant known planetoid - your view of the Sun at high noon might look something like this. An artist's dramatic vision, the picture shows the Sun suspended above the nearby horizon as a bright star immersed in the dusty ecliptic plane. Within the dust-scattered sunlight are more familiar members of the solar system, including planet Earth. But at a distance of about 13 billion kilometers (8 billion miles) Earth would only be visible in binoculars or a small telescope. In Sedna's dark, daytime skies, the noonday Sun is also joined by the faint stars and obscuring dust clouds of the Milky Way, suspended on the left above stark, ruddy terrain. For Sedna-based sky gazers, all planets have interior orbits and would remain close to the Sun in Sedna's skies. Of course, for earthbound astronomers, interior planets Venus and Mercury also remain near the Sun, with Venus scheduled for a rare crossing of the solar disc on June 8.

"It has a dark side and a light side and it holds the Universe together"

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Apollo 17 Panorama: Astronaut Running



What would it be like to explore the surface of another world? In 1972 during the Apollo 17 mission, astronaut Harrison Schmitt found out first hand. In this case, the world was Earth's own Moon. In this recently compiled panorama of lunar photographs originally taken by astronaut Eugene Cernan, the magnificent desolation of the barren Moon is apparent. Visible above and by scrolling right are lunar rocks in the foreground, lunar mountains in the background, some small craters, a lunar rover, and astronaut Schmidt on his way back to the rover. A few days after this image was taken, humanity left the Moon and has yet to return.

Astronaut at Work



Did you ever have a day where everything got turned around and you just couldn't tell which way was up? Fortunately, this didn't happen to astronaut James S. Voss on 2000 May 21, who spent six hours preparing to fix and upgrade the International Space Station. Voss is shown above anchored in the clutches of Space Shuttle Atlantis' mechanical arm, maneuvering outside the shuttle's cargo bay high above planet Earth. This space walk was the 85th in US history and the fifth dedicated to the construction of the International Space Station. The STS-101 mission returned after successfully replacing the station's batteries, lifting the station into a higher orbit, and replenishing needed supplies.

Working in Space



High above planet Earth, a human helps an ailing machine. The machine, in this potentially touching story, is the Hubble Space Telescope, which is not in the picture. The human is Astronaut Steven L. Smith, and he is seen above retrieving a power tool from the handrail of the Remote Manipulator System before resuming work on HST in 1999 December. For most astronauts, space is not a place for relaxation and vacation, but rather a place for hard work. Since many space missions involve costly equipment and complicated experiments, astronauts are usually people of considerable knowledge and training. Although the hours may be long and work may be taxing, one frequently reported perk of working in space is the spectacular view.

Mir Dreams



This dream-like image of Mir was recorded by astronauts as the Space Shuttle Atlantis approached the Russian space station prior to docking during the STS-76 mission. Sporting spindly appendages and solar panels, Mir resembles a whimsical flying insect hovering about 350 kilometers above New Zealand's South Island and the city of Nelson near Cook Strait. In late March 1996, Atlantis shuttled astronaut Shannon W. Lucid to Mir for a five month visit, increasing Mir's occupancy from 2 to 3. It returned to pick Lucid up and drop off astronaut John Blaha during the STS-79 mission in August of that year. Since becoming operational in 1986, Mir has been visited by over 100 spacefarers from the nations of planet Earth including, Russia, the United States, Great Britain, Germany, France, Japan, Austria, Kazakhstan and Slovakia. After joint Shuttle-Mir training missions in support of the International Space Station, continuous occupation of Mir ended in August 1999. The Mir was deorbited in March 2001.

The Averted Side Of The Moon



This vintage 60-kopek stamp celebrates a dramatic achievement. On the 7th of October, 1959 (7/X/1959), the Soviet interplanetary station which has come to be called "Luna 3" successfully photographed the far side of the moon giving denizens of planet Earth their first ever view of this hidden hemisphere. Lacking the digital image technology familiar now, Luna 3 took the pictures on 35mm film which was automatically developed on board. The pictures were then scanned and the signal transmitted to Earth days later in what was perhaps also the first interplanetary fax. In all, seventeen pictures were received providing enough coverage and resolution to construct a far side map and identify a few major features. Depicted on the stamp are regions dubbed the Sea of Moscow, the Soviet Mountains, the Bay of Astronauts, and the Sea of Dreams.

Shadow of a Martian Robot



What if you saw your shadow on Mars and it wasn't human? Then you might be the Opportunity rover currently exploring Mars. Opportunity and sister robot Spirit have been probing the red planet since January, finding evidence of ancient water, and sending breathtaking images across the inner Solar System. Pictured above, Opportunity looks opposite the Sun into Endurance Crater and sees its own shadow. Two wheels are visible on the lower left and right, while the floor and walls of the unusual crater are visible in the background. Opportunity is cautiously edging its way into this enigmatic crater, hoping to find new clues into the wet ancient past of our Solar System's second most habitable planet.

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At the Summit of Olympus Mons



From martian orbit, the Mars Express cameras looked down on the largest volcano in the solar system. The result was this stunningly detailed overhead view of the caldera or summit crater region of Olympus Mons. Fittingly named for the lofty abode of the gods of Greek mythology, Olympus Mons rises 21 kilometers above the surrounding plain or to about 3 times the height of Mt. Everest. The area pictured is 102 kilometers across and the caldera pits are up to 3 kilometers deep. For comparison, hawaiian volcanic calderas range up to 18 kilometers in diameter. Outlined by steep cliffs, Olympus Mons itself is about 600 kilometers in diameter.

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Above the Eye of Hurricane Ivan



Ninety percent of the houses on Grenada were damaged. Such is the destructive force of Hurricane Ivan, already one of the most powerful and destructive hurricanes on record. And the storm will likely make landfall in southern USA tomorrow. Ivan is the currently the third - and largest - hurricane set to strike the US this hurricane season. The swirling eye of Hurricane Ivan was photographed above from the orbiting International Space Station (ISS) on Saturday as the storm's sustained 200 kilometer per hour winds wrecked havoc in the Caribbean. The bad news is that hurricane season in the Atlantic typically lasts until November 30, still over two months away. The more immediate bad news is that tropical storm Jeanne is next in line coming across the mid-Atlantic Ocean and could pass Puerto Rico sometime today.

Transit of Venus



The rare transit of Venus across the face of the Sun earlier this month was one of the better-photographed events in sky history. Both scientific and artistic images have been flooding in from the areas that could see the transit: Europe and much of Asia, Africa, and North America. Scientifically, solar photographers confirmed that the black drop effect is really better related to the viewing clarity of the camera or telescope than the atmosphere of Venus. Artistically, images might be divided into several categories. One type captures the transit in front of a highly detailed Sun. Another category captures a double coincidence such as both Venus and an airplane simultaneously silhouetted, or Venus and the International Space Station in low Earth orbit. A third image type involves a fortuitous arrangement of interesting looking clouds, as shown by example in the above image taken from North Carolina, USA. There the distant orb of giant Venus might have been mistaken, at first glance, for a small but unusually circular cloud.

Cone Nebula Close-Up



Cones, pillars, and majestic flowing shapes abound in stellar nurseries where natal clouds of gas and dust are buffeted by energetic winds from newborn stars. A well-known example, the Cone Nebula within the bright galactic star-forming region NGC 2264, was captured in this close-up view from the Hubble Space Telescope's Advanced Camera for Surveys. While the Cone Nebula, about 2,500 light-years away in Monoceros, is around 7 light-years long, the region pictured here surrounding the cone's blunted head is a mere 2.5 light-years across. In our neck of the galaxy that distance is just over half way from the Sun to its nearest stellar neighbor, Alpha Centauri. The massive star NGC 2264 IRS, seen by Hubble's infrared camera in 1997, is the likely source of the wind sculpting the Cone Nebula and lies off the top of the image. The Cone Nebula's reddish veil is produced by glowing hydrogen gas.

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Solstice Celebration



From the EIT instrument onboard the space-based SOHO observatory, the tantalizing picture is a false-color composite of three images all made in extreme ultraviolet light. Each individual image highlights a different temperature regime in the upper solar atmosphere and was assigned a specific color; red at 2 million, green at 1.5 million, and blue at 1 million degrees C. The combined image shows bright active regions strewn across the solar disk, which would otherwise appear as dark groups of sunspots in visible light images, along with some magnificent plasma loops and an immense prominence at the right hand solar limb.

A Solar Filament Lifts Off



Hot gas frequently erupts from the Sun. One such eruption produced the glowing filament pictured above, which was captured in 2000 July by the Earth-orbiting TRACE satellite. The filament, although small compared to the overall size of the Sun, measures over 100,000 kilometers in height, so that the entire Earth could easily fit into its outstretched arms. Gas in the filament is funneled by the complex and changing magnetic field of the Sun. After lifting off from the Sun's surface, most of the filamentary gas will eventually fall back. More powerful solar eruptions emit particles that reach the Earth and can disrupt manmade satellites. The cause and nature of solar eruptions are the topic of much research.

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Sunspot Loops in Ultraviolet



It was a quiet day on the Sun. The above image shows, however, that even during off days the Sun's surface is a busy place. Shown in ultraviolet light, the relatively cool dark regions have temperatures of thousands of degrees Celsius. Large sunspot group AR 9169 is visible as the bright area near the horizon. The bright glowing gas flowing around the sunspots has a temperature of over one million degrees Celsius. The reason for the high temperatures is unknown but thought to be related to the rapidly changing magnetic field loops that channel solar plasma. Sunspot group AR 9169 moved across the Sun during 2000 September and decayed in a few weeks.

Spicules: Jets on the Sun



Imagine a pipe as wide as a state and as long as half the Earth. Now imagine that this pipe is filled with hot gas moving 50,000 kilometers per hour. Further imagine that this pipe is not made of metal but a transparent magnetic field. You are envisioning just one of thousands of young spicules on the active Sun. Pictured above is perhaps the highest resolution image yet of these enigmatic solar flux tubes. Spicules dot the above frame of solar active region 10380 that crossed the Sun in June, but are particularly evident as a carpet of dark tubes on the right. Time-sequenced images have recently shown that spicules last about five minutes, starting out as tall tubes of rapidly rising gas but eventually fading as the gas peaks and falls back down to the Sun. These images also indicate, for the first time, that the ultimate cause of spicules is sound-like waves that flow over the Sun's surface but leak into the Sun's atmosphere.

A Prominent Solar Prominence from SOHO



One of the most spectacular solar sights is a prominence. A solar prominence is a cloud of solar gas held above the Sun's surface by the Sun's magnetic field. Last month, NASA's Sun-orbiting SOHO spacecraft imaged an impressively large prominence hovering over the surface, pictured above. The Earth would easily fit under the hovering curtain of hot gas. A quiescent prominence typically lasts about a month, and may erupt in a Coronal Mass Ejection (CME) expelling hot gas into the Solar System. Although somehow related to the Sun's changing magnetic field, the energy mechanism that creates and sustains a Solar prominence is still a topic of research.

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Solar Arcs and Halos



Have you ever seen a bright halo around the Sun? Unusual halos and arcs were so bright one recent afternoon in Trier, Germany that even casual people on the street noticed them. The fantastic sky display is pictured above and included a 22 degree halo arc, a complete parhelic circle, a circumhorizon arc and even an infralateral arc. A computer simulation has been run that mimics the above rare display. A cloud partially blocked the usually more intense direct glare of the Sun. Sunlight refracting through falling and fluttering hexagonal ice crystals creates such displays. Such atmospheric ice crystals also cause sundogs and Moon halos.

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