"The initial phase of aerobraking has gone exceedingly well," said David A. Spencer, Odyssey mission manager. "By skimming through the upper reaches of the Mars atmosphere during each orbit, we have reduced our orbital period by more than three hours in the past two weeks."

The orbital period is the time required to make one trip around Mars. The first orbit took more than 18 hours. It is now a 15-hour trip. By using the Martian atmosphere to slow itself down, Odyssey uses less fuel, a factor that kept the cost of the mission down.

Odyssey arrived at Mars Oct. 23. Since then, it has been making long, looping orbits around the Red Planet that carried it up to 17,000 miles away.

The spacecraft's closest approach to the planet, known as periapsis, has been lowered to 110 kilometers (68 miles) above the surface.

"This closest approach occurs over the north polar region on Mars, in a relatively low density region surrounded by strong winds like the jet stream on Earth," said JPL's Richard Zurek. "Like Earth, Mars has distinct seasons, and this low density area, called the polar vortex, develops each fall and will persist until spring, long after Odyssey has finished aerobraking."

On Earth, the jet stream is a high-altitude river of air that circles the globe. It typically stays well north, but sometimes dips deep into the United States and Europe. Storms tend to track along the jet stream.

NASA's Mars Global Surveyor spacecraft, was already orbiting Mars when Odyssey arrived, is monitoring the lower atmosphere of Mars, observing the entire planet each day to watch for dust storms and weather events that could affect the upper atmosphere.

Though Surveyor is healthy, mission managers are concerned enough about possible dust storms that they've planned for how Odyssey could be used as a backup, if needed, to watch for storms itself.

Flight controllers have completed the calibration and testing of Odyssey's thermal emission imaging system, in case it is needed for this task, according to a NASA press release.

The aerobraking phase is planned for completion in late January 2002. By then, Odyssey should be in its desired circular orbit, roughly 250 miles above the surface.

The science portion of the mission will begin sometime in early February, mission managers said. Odyssey will use its cameras and other instruments to search for subsurface water and ice, to understand the chemistry of Martian soil, and to measure the deadly radiation environment of the Red Planet.

SPECIAL REPORT: Full Coverage of the Odyssey Mission to Mars