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Cassini preview
The Cassini spacecraft's arrival at Saturn is previewed in this detailed news conference from NASA Headquarters on June 3. (50min 01sec file)
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Saturn arrival explained
Cassini's make-or-break engine firing to enter orbit around Saturn is explained with graphics and animation. Expert narration is provided by Cassini program manager Robert Mitchell. (3min 33sec file)
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Cassini mission science
The scientific objectives of the Cassini mission to study the planet Saturn, its rings and moons are explained by Charles Elachi, director of the Jet Propulsion Laboratory. (4min 54sec file)
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Huygens mission science
After entering orbit around Saturn, the Cassini spacecraft will launch the European Huygens probe to make a parachute landing on the surface of the moon Titan. The scientific objectives of Huygens are explained by probe project manager Jean-Pierre Lebreton. (3min 14sec file)
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Saturn's moon Titan
Learn more about Saturn's moon Titan, which is believed to harbor a vast ocean, in this narrated movie. (4min 01sec file)
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Relive Cassini's launch
An Air Force Titan 4B rocket launches NASA's Cassini spacecraft at 4:43 a.m. October 15, 1997 from Cape Canaveral, Florida. (5min 15sec file)
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Exploring the hills
"A brand new mission" is beginning for the Mars Exploration Rover Spirit as it nears the Columbia Hills as described in this presentation by science team member James Rice. (5min 57sec file)
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Exploring Endurance
New pictures from the Mars rover Opportunity as it drives around the rim of Endurance Crater are presented with narration by science team member Wendy Calvin. (5min 25sec file)
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Mars rover update
Mission officials and scientists discuss the condition and progress of Mars rovers Spirit and Opportunity plus the latest science news in this briefing from June 2. (40min 55sec file)
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Options to save Hubble
NASA Administrator Sean O'Keefe announces plans to examine a robotic servicing mission to the Hubble Space Telescope. (33min 51sec file)
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Galaxy formation theory no longer conflicts
UNIVERSITY OF CHICAGO NEWS RELEASE
Posted: June 11, 2004

Astrophysicists led by the University of Chicago's Andrey Kravtsov have resolved an embarrassing contradiction between a favored theory of how galaxies form and what astronomers see in their telescopes.

Astrophysicists base their understanding of how galaxies form on an extension of the big bang theory called the cold dark matter theory. In this latter theory, small galaxies collide and merge, inducing bursts of star formation that create the different types of massive and bright galaxies that astronomers see in the sky today. (Dark matter takes its name from the idea that 85 percent of the total mass of the universe is made of unknown matter that is invisible to telescopes, but whose gravitational effects can be measured on luminous galaxies.)

This theory fits some key data that astrophysicists have collected in recent years. Unfortunately, when astrophysicists ran supercomputer simulations several years ago, they ended up with 10 times more dark matter satellites-clumps of dark matter orbiting a large galaxy-than they expected.

"The problem has been that the simulations don't match the observations of galaxy properties," said David Spergel, professor of astrophysics at Princeton University. "What Andrey's work represents is a very plausible solution to this problem."

Kravtsov and his collaborators found the potential solution in new supercomputer simulations they will describe in a paper that will appear in the July 10 issue of the Astrophysical Journal. "The solution to the problem is likely to be in the way the dwarf galaxies evolve," Kravtsov said, referring to the small galaxies that inhabit the fringes of large galaxies.

In general, astrophysicists believe that formation of very small dwarf galaxies should be suppressed. This is because gas required for continued formation of stars can be heated and expelled by the first generation of exploding supernovae stars. In addition, ultraviolet radiation from galaxies and quasars that began to fill the universe approximately 12 billion years ago heats the intergalactic gas, shutting down the supply of fresh gas to dwarf galaxies.

In the simulations, Kravtsov, along with Oleg Gnedin of the Space Telescope Science Institute and Anatoly Klypin of New Mexico State University, found that some of the dwarf galaxies that are small today have been more massive in the past and could gravitationally collect the gas they need to form stars and become a galaxy.

"The systems that appear rather feeble and anemic today could, in their glory days, form stars for a relatively brief period," Kravtsov said. "After a period of rapid mass growth, they lost the bulk of their mass when they experienced strong tidal forces from their host galaxy and other galaxies surrounding them."

This galactic "cannibalism" persists even today, with many of the "cannibalized" dwarf galaxies becoming satellites orbiting in the gravitational pull of larger galaxies.

"Just like the planets in the solar system surrounding the sun, our Milky Way galaxy and its nearest neighbor, the Andromeda galaxy, are surrounded by about a dozen faint 'dwarf' galaxies," Kravtsov said. "These objects were pulled in by the gravitational attraction of the Milky Way and Andromeda some time ago during their evolution."

The simulations had succeeded where others had failed because Kravtsov's team analyzed simulations that were closely spaced in time at high resolution. This allowed the team to track the evolution of individual objects in the simulations. "This is rather difficult and is not often done in analyses of cosmological simulations. But in this case it was the key to recognize what was going on and get the result," Kravtsov said.

The result puts the cold dark matter scenario on more solid ground. Scientists had attempted to modify the main tenets of the scenario and the properties of dark matter particles to eliminate the glaring discrepancy between theory and observation of dwarf galaxies. "It turns out that the proposed modifications introduced more problems than they solved," Kravtsov said.

The simulations were performed at the National Center for Supercomputer Applications, University of Illinois at Urbana-Champaign, with grants provided by the National Science Foundation and the National Aeronautics and Space Administration.

Mars rover collectible patch

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This commemorative patch celebrates NASA's Curiosity rover mission of the Mars Science Laboratory in search of clues whether the Red Planet was once hospitable to life.
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Soviet Space
For the first time ever available in the West. Rocket & Space Corporation Energia: a complete pictorial history of the Soviet/Russian Space Program from 1946 to the present day all in full color. Available from our store.
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Viking patch
This embroidered mission patch celebrates NASA's Viking Project which reached the Red Planet in 1976.
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Apollo 7 DVD
For 11 days the crew of Apollo 7 fought colds while they put the Apollo spacecraft through a workout, establishing confidence in the machine what would lead directly to the bold decision to send Apollo 8 to the moon just 2 months later.
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Gemini 12
Gemini 12: The NASA Mission Reports covers the voyage of James Lovell and Buzz Aldrin that capped the Gemini program's efforts to prove the technologies and techniques that would be needed for the Apollo Moon landings. Includes CD-ROM.
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Columbia Report
The official accident investigation report into the loss of the space shuttle Columbia and its crew of seven. Includes CD-ROM.
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Final Shuttle Mission Patch

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The crew emblem for the final space shuttle mission is now available in our store. Get this piece of history!
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STS-134 Patch

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The final planned flight of space shuttle Endeavour is symbolized in the official embroidered crew patch for STS-134. Available in our store!
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Ares 1-X Patch
The official embroidered patch for the Ares 1-X rocket test flight, is available for purchase.
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Apollo Collage
This beautiful one piece set features the Apollo program emblem surrounded by the individual mission logos.
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Project Orion
The Orion crew exploration vehicle is NASA's first new human spacecraft developed since the space shuttle a quarter-century earlier. The capsule is one of the key elements of returning astronauts to the Moon.
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Fallen Heroes Patch Collection
The official patches from Apollo 1, the shuttle Challenger and Columbia crews are available in the store.
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