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STS-122: Crew arrival

The space shuttle Atlantis astronauts arrive at the Kennedy Space Center for their countdown to launch.

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STS-122: The mission

Atlantis' trip to the station will deliver the European Space Agency's Columbus science lab module.

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STS-122: The programs

Managers from the shuttle, station and EVA programs discuss Atlantis' upcoming flight.

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STS-122: Spacewalks

Three spacewalks are planned during Atlantis' STS-122 assembly mission. Lead spacewalk officer Anna Jarvis previews the EVAs.

 Full briefing
 EVA 1 summary
 EVA 2 summary
 EVA 3 summary

The Atlantis crew

The astronauts of Atlantis' STS-122 mission meet the press in the traditional pre-flight news conference.

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Harmony's big move

The station's new Harmony module is detached from the Unity hub and moved to its permanent location on the Destiny lab.

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Delta 4-Heavy launch

The first operational Delta 4-Heavy rocket launches the final Defense Support Program missile warning satellite for the Air Force.

 Full coverage

Columbus readied

The European Space Agency's Columbus laboratory module moves to pad 39A and placed aboard shuttle Atlantis for launch.

 To pad | Installed

Station port moved

The station crew uses the robot arm to detach the main shuttle docking port and mount it to the new Harmony module Nov. 12.

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Atlantis rolls out

Space shuttle Atlantis rolls from the Vehicle Assembly Building to pad 39A for its December launch with the Columbus module.

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Atlantis goes vertical

Atlantis is hoisted upright and maneuvered into position for attachment to the external tank and boosters.

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Saturn's small moons tell the story of their origins
NASA/JPL NEWS RELEASE
Posted: December 7, 2007

Imaging scientists on NASA's Cassini mission are telling a tale of how the small moons orbiting near the outer rings of Saturn came to be. The moons began as leftover shards from larger bodies that broke apart and filled out their "figures" with the debris that made the rings.


Shown here are the highest resolution images of the moons Pan and Atlas, showing their distinctive "flying saucer" shapes, owing Credit: NASA/JPL/Space Science Institute
Download larger image version here

 
It has long been suspected that Saturn's rings formed in the disintegration of one or several large icy bodies, perhaps pre-existing moons, by giant impacts. The resulting debris quickly spread and settled into the equatorial plane to form a thin disk surrounding the planet. And the small, irregularly shaped ring-region moons were believed to be the leftover pieces from this breakup.

Now, several years' worth of cosmic images of Saturn's 14 known small moons have been used to derive the sizes and shapes of most of them, and in about half the cases, even masses and densities. This information, published in the Dec. 7 issue of the journal Science, has led to new insights into how some of these moons may have formed.

The tip-off was the very low density of the inner moons, about half that of pure water ice, and sizes and shapes that suggested they have grown by the accumulation of ring material. The trouble was, these moons are within and near the rings, where it is not possible for small particles to fuse together gravitationally. So how did they do it? They got a jump start.

"We think the only way these moons could have reached the sizes they are now, in the ring environment as we now know it to be, was to start off with a massive core to which the smaller, more porous ring particles could easily become bound," said Carolyn Porco, Cassini imaging team leader from the Space Science Institute in Boulder, Colo. Porco is the lead author of the first of two related articles published in this week's issue of Science.

Simple calculations and more complicated computer simulations have shown that ring particles will readily become bound to a larger seed having the density of water ice. By this process, a moon will grow even if it is relatively close to Saturn. The result is a ring-region moon about two to three times the size of its dense ice core, covered with a thick shell of porous, icy ring material. To make a 30-kilometer moon (19 miles) requires a seed of about 10 kilometers (6 miles).

Where did such large cores come from? And when did this all take place?

"The core may in fact be one of the remnants from the original ring-forming event," said co-author Derek Richardson, professor of astronomy at the University of Maryland, College Park, "which might have been left intact all this time and protected from additional collisional breakup by the mantle of ring particles around it."

Just exactly when the rings formed is not known. "But it is not out of the question that the moons date back to the time of ring formation," said Porco.

The researchers show that the cores of Pan and Daphnis, which orbit within gaps in the outer A ring, were large enough to open narrow gaps. Accretion, or accumulation of material, they say, probably occurred quickly. The moons grew and their gaps widened, achieving their present sizes before the gaps were completely emptied of material, and probably before the local rings reached their present thickness.

So how did Pan in the main rings, and Atlas, which orbits just beyond the outer edge of the main rings, get the prominent equatorial ridges that make them look like flying saucers? The second paper reports evidence for a secondary stage of accretion that occurred after the moons' growth was completed and after the rings flattened to their present 20-meter (66 feet) thickness.

"Our computer simulations show that the ridges must have accreted rapidly when Saturn's rings were thin, forming small accretion disks around the equators of Pan and Atlas," said Sebastien Charnoz, lead author and an associate of imaging team member Andre Brahic at the University Paris-Diderot and CEA Saclay, in France. "The ridges might be the remains of 'fossilized' accretion disks, fundamental structures seen at all scales in the universe, from planetary rings to galaxies."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the Cassini mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.