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STS-41B: Human satellite
One of the iconic moments of the early space shuttle program was astronaut Bruce McCandless floating above the brilliantly blue Earth completely disconnected from his spacecraft. He was testing the Manned Maneuvering Unit, a jet-powered backpack that would enable spacewalkers to travel away from the space shuttle to service satellites. In this post-flight presentation, the crew of Challenger's STS-41B mission of February 1984 narrate the film highlights from their mission that also included the first shuttle landing at Kennedy Space Center.

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Shuttle launch delay
Space Shuttle Program Manager Wayne Hale announces his decision to replace suspect fuel-level sensors inside the liquid hydrogen portion of Discovery's external tank. The three-week job means Discovery will miss its May launch window, delaying the second post-Columbia test flight to the next daylight period opening July 1. Hale made the announcement during a news conference from Johnson Space Center on March 14.

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Stardust science
NASA's Stardust spacecraft returned to Earth in January with the first samples ever retrieved from a comet. This briefing with mission scientists held March 13 from the Johnson Space Center offers an update on the initial research into the comet bits.

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Exploring Enceladus
The Cassini spacecraft orbiting the planet Saturn has found evidence indicating pockets of liquid water may exist near the surface on the icy moon Enceladus, raising the question of whether the small world could support life. This movie includes stunning images of Enceladus taken by Cassini and animation of geysers seen erupting from the moon.

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MRO's orbit insertion explained
The make-or-break engine firing by the Mars Reconnaissance Orbiter to enter orbit around Mars and the subsequent aerobraking to reach the low-altitude perch for science observations are explained by project manager Jim Graf in this narrated animation package.

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Ideas on gas-giant planet formation take shape
CARNEGIE INSTITUTION NEWS RELEASE
Posted: March 22, 2006

Rocky planets such as Earth and Mars are born when small particles smash together to form larger, planet-sized clusters in a planet-forming disk, but researchers are less sure about how gas-giant planets such as Jupiter and Saturn form. Is core accretion--the process that creates their smaller, terrestrial cousins--responsible? Or could an alternate model known as disk instability--in which the planet-forming disk itself actually fragments into a number of planet-sized clumps--be at work? Could both be possible under different circumstances?

Recent work from the Carnegie Institution's Department of Terrestrial Magnetism explores both possibilities. This and other relevant work regarding planet formation is presented at the NASA Astrobiology Science Conference (AbSciCon) 2006 at the Ronald Reagan Building in Washington, D.C. March 26-30.

Carnegie Fellow Hannah Jang-Condell1 has devised a method to catch the early stages of gas-giant core accretion in the act. If actively accreting cores exist, they should leave a gravitational "dimple" in the planet-forming disk--even if the cores are only a fraction the size of Jupiter. Since disk instability would result in planet-sized fragments straight away, the existence of these young, intermediate-sized cores would be a clear indicator of core accretion.

The telltale gravitational dimples resemble craters on the Moon with sunlight shining in from the side: the inside of the edge nearest the star is shadowed, while the star-facing edge is illuminated. The bright side heats up and the shadowed side remains cool, yielding a distinct thermal pattern that an Earth-based observer should be able to see in the infrared spectrum. "If we could detect this signature in a protoplanetary disk, it would indicate the presence of a young planetary body that could go on to form a gas-giant via core accretion," Jang-Condell said.

In some situations, however, core accretion seems an unlikely model for gas-giant planet formation. For example, theoretical computer models by DTM staff member Alan Boss2 suggest that disk instability best explains planet formation around M dwarf stars, which have masses from one tenth to one half that of the Sun. Core accretion would likely take more than 10 million years around these small, gravitationally weak stars, while disk instability happens quickly enough to yield gas-giant planets in as little as 1,000 years.

"M dwarf stars dominate the stellar population in the solar neighborhood, and so are attractive targets for searching for habitable planets," Boss said. "The models show that gas-giant planets are indeed likely to form...at distances sufficiently large enough to permit the later formation of habitable, terrestrial planets."

Other talks and posters on planet formation at the conference include: A study of organic matter in the planet-forming disks of three young stars, ranging in age from less than one million to over 300 million years3; methods to detect water ice, methane ice, and silicate dust in the planet-forming disks of distant stars4; and a method to deduce the composition of far-off planets based on their mass and radius5.