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This date in history
On April 4, 1983, space shuttle Challenger launched on its maiden voyage. The ship carried a crew of four astronauts and NASA's first Tracking and Data Relay Satellite. (2min 08sec file)
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Thursday's Mars rover update
New pictures and science results from the Spirit and Opportunity rovers on Mars are presented at this briefing from Thursday, April 1. (52min 57sec file)
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X-43A launched
NASA's experimental X-43A hypersonic research aircraft is successfully launched by a Pegasus rocket off the coast of California on March 27. (2min 40sec file)
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Mars water discovery
Scientists present evidence from the Mars rover Opportunity during this Tuesday news conference that shows the landing site was once the bottom of a salty sea. (76min 48sec file)
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Armstrong speech
Neil Armstrong accepts the Rotary National Space Trophy for career contributions in aerospace. He says President Bush's plan to return to the Moon is economically feasible and has "substantial merit and promise." (12min 10sec)
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Saturn moon casts 'once-in-a-lifetime' shadow
CHANDRA X-RAY CENTER NEWS RELEASE
Posted: April 5, 2004

A rare celestial event was captured by NASA's Chandra X-ray Observatory as Titan -- Saturn's largest moon and the only moon in the Solar System with a thick atmosphere -- crossed in front of the X-ray bright Crab Nebula. The X-ray shadow cast by Titan allowed astronomers to make the first X-ray measurement of the extent of its atmosphere.


A rare celestial event was captured by Chandra as Titan crossed in front of the X-ray bright Crab Nebula. The X-ray shadow cast by Titan allowed astronomers to make the first X-ray measurement of the extent of its atmosphere. Credit: NASA/CXC/Penn State/K.Mori et al.
 
On January 5, 2003, Titan transited the Crab Nebula, the remnant of a supernova explosion that was observed to occur in the year 1054. Although Saturn and Titan pass within a few degrees of the Crab Nebula every 30 years, they rarely pass directly in front of it.

"This may have been the first transit of the Crab Nebula by Titan since the birth of the Crab Nebula," said Koji Mori of Pennsylvania State University in University Park, and lead author on an Astrophysical Journal paper describing these results. "The next similar conjunction will take place in the year 2267, so this was truly a once in a lifetime event."

Chandra's observation revealed that the diameter of the X-ray shadow cast by Titan was larger than the diameter of its solid surface. The difference in diameters gives a measurement of about 550 miles (880 kilometers) for the height of the X-ray absorbing region of Titan's atmosphere. The extent of the upper atmosphere is consistent with, or slightly (10-15%) larger, than that implied by Voyager I observations made at radio, infrared, and ultraviolet wavelengths in 1980.

"Saturn was about 5% closer to the Sun in 2003, so increased solar heating of Titan may account for some of this atmospheric expansion," said Hiroshi Tsunemi of Osaka University in Japan, one of the coauthors on the paper.


This graphic shows how closely to the Crab Nebula's center Titan passed during the transit. Although Titan passes within a few degrees of the Crab every 30 years, it rarely passes directly in front of it. Credit: Titan's Path: NASA/CXC/Penn State/K.Mori et al.; Crab Nebula: NASA/CXC/ASU/J.Hester et al.
 
The X-ray brightness and extent of the Crab Nebula made it possible to study the tiny X-ray shadow cast by Titan during its transit. By using Chandra to precisely track Titan's position, astronomers were able to measure a shadow one arcsecond in diameter, which corresponds to the size of a dime as viewed from about two and a half miles.

Unlike almost all of Chandra's images which are made by focusing X-ray emission from cosmic sources, Titan's X-ray shadow image was produced in a manner similar to a medical X-ray. That is, an X-ray source (the Crab Nebula) is used to make a shadow image (Titan and its atmosphere) that is recorded on film (Chandra's ACIS detector).

Titan's atmosphere, which is about 95% nitrogen and 5% methane, has a pressure near the surface that is one and a half times the Earth's sea level pressure. Voyager I spacecraft measured the structure of Titan's atmosphere at heights below about 300 miles (500 kilometers), and above 600 miles (1000 kilometers). Until the Chandra observations, however, no measurements existed at heights in the range between 300 and 600 miles.

Understanding the extent of Titan's atmosphere is important for the planners of the Cassini-Huygens mission. The Cassini-Huygens spacecraft will reach Saturn in July of this year to begin a four-year tour of Saturn, its rings and its moons. The tour will include close flybys of Titan that will take Cassini as close as 600 miles, and the launching of the Huygens probe that will land on Titan's surface.

"If Titan's atmosphere has really expanded, the trajectory may have to be changed," said Tsunemi.

The paper on these results has been accepted and is expected to appear in a June 2004 issue of The Astrophysical Journal. Other members of the research team were Haroyoski Katayama (Osaka University), David Burrows and Gordon Garmine (Penn State University), and Albert Metzger (JPL). Chandra observed Titan from 9:04 to 18:46 UT on January 5, 2003, using its Advanced CCD Imaging Spectrometer instrument.

NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the Office of Space Science, NASA Headquarters, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Final Shuttle Mission Patch

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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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This beautiful one piece set features the Apollo program emblem surrounded by the individual mission logos.
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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
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Apollo 11 special patch
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Inside Apollo mission control
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The ultimate Apollo 11 DVD
This exceptional chronicle of the historic Apollo 11 lunar landing mission features new digital transfers of film and television coverage unmatched by any other.
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Next ISS crew
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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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An insider's view of how Apollo flight controllers operated and just what they faced when events were crucial.
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