SAN FRANCISCO (AP) _ As NASA's 2001 Mars Odyssey spacecraft neared completion Wednesday of its 100th orbit of the Red Planet, scientists had to contend with a surprisingly fickle Martian atmosphere in guiding and slowing the robotic probe.
Odyssey entered orbit around Mars on Oct. 23. Since then, scientists have guided the spacecraft on a series of controlled skims through the atmosphere, using the drag provided by the carbon dioxide-rich shroud to slow the spacecraft and shape its orbit.
With each pass, the drag produced by the tenuous atmosphere shaves about five minutes from a full orbit. As of Wednesday, it was taking Odyssey 6.32 hours to whip around Mars, down from the 18.6 hours it took in October.
Odyssey skims through the atmosphere of Mars for anywhere from three to four minutes during each orbit. But where and at what altitude it does so has made a big difference, scientists said Wednesday at the fall meeting of the American Geophysical Union in San Francisco.
The goal is to hit atmosphere dense enough to slow the spacecraft but not so thick that it overheats the craft's solar panel through increased atmospheric friction.
``It's that balancing act we deal with each day,'' said Richard Zurek, the National Aeronautics and Space Administration's lead scientist for the aerobraking phase of the mission.
From orbit to orbit, however, scientists have discovered wider than expected swings in the density of the Martian atmosphere as the probe passes over the planet at varying latitudes, longitudes and altitudes. The changes in density seen so far have been up to 100 percent and have been most dramatic over the north pole.
``We're seeing differing atmospheric effects,'' said Steve Saunders, project scientist for the mission, based at NASA's Jet Propulsion Laboratory in Pasadena.
On each orbit, Odyssey passes near the Martian north pole, currently at an altitude of about 60 miles at its closest. At that height, the Martian atmosphere is about 1/100,000th as dense as the Earth's atmosphere at sea level, Zurek said.
To achieve the desired drag, engineers are constantly tinkering with the altitude at which Odyssey comes in over Mars. Typically, such changes are small _ about one to two miles, but enough to translate into swings of 15 percent to 30 percent in the thickness of the atmosphere.
``We can fly to a certain density in the Martian atmosphere no matter where it occurs,'' Zurek said.
Odyssey has another 200-plus orbits left before it will settle into its final science mapping orbit. That should occur in mid-January, when the probe will swing around Mars once every two hours.
Beginning in February, the $297 million Odyssey mission will remotely prospect for chemicals and elements on the surface, seek out frozen reservoirs of water, and later serve as a communications relay satellite for future Mars spacecraft.
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