Five minutes after the engine begins its burn, communication with the orbiter will drop as spacecraft passes behind Mars.

Those will be anxious moments, said Thurman, the flight operations manager for the climate orbiter and its companion spacecraft, the Mars Polar Lander, which is also on its way to Mars. If all goes well, the orbiter will emerge from the far side of the planet after 20 minutes to begin a two-month procedure called aerobraking. It will reduce the orbiter's velocity by almost as much as the engine burn.

By early November, the orbiter should have slowed and circularized its orbit enough that controllers can raise its closest-approach altitude to a level where it will not encounter any atmospheric interference as it orbits Mars.

The orbiter will then be ready for its initial phase of duty, which is to serve as a communications and data-transfer station in support of the Mars Polar Lander. That spacecraft is scheduled to touch down near the martian southern pole Dec. 3.

Using the orbiter as a relay station for communications between Earth and the polar lander means the lander will not have to waste precious power for the long-distance broadcasts to Earth, said Richard Zurek, project scientist for the two spacecraft.

By mid-January, with the lander's mission complete, the climate orbiter will start the main course of its mission: to observe the martian climate and weather patterns for a full martian year, 687 Earth days.

Weather appears to be the most dynamic process shaping the planet's surface, but scientists know very little about it. Within the past year, cameras have detected an enormous cyclone in the planet's northern hemisphere and swirling dust devils that can reach 5 miles (8 kilometers) above the landscape carrying several tens of tons of fine red dust. Scientists know little about how these systems circulate or how they fit into the overall picture of martian climate.

The polar ice caps play crucial roles in Mars' global climate. Composed chiefly of carbon dioxide -- the same component that makes up most of the atmosphere -- the ice caps are formed each winter primarily by gas condensing out of the atmosphere. So much frost forms this way that the mass of the atmosphere changes significantly during the course of a year, Zurek said, with global atmospheric pressure potentially dropping by as much as 30 percent during the peak of winter at one of the poles.

If such a condition were to occur on Earth, Zurek said, "Instead of 1000 millibars for surface pressure, you'd get, say, 700 millibars. On Earth when we get a depression of even a few millibars, that's a pretty good weather system."

On Mars the effect would be tremendous.

The climate orbiter is carrying two instruments that will help scientists understand these atmospheric phenomena. One is a camera that will both track weather systems in the atmosphere, and map the surface. The other is an instrument that uses infrared radar to build a vertical profile of temperatures in the atmosphere, and the amount of dust and water vapor held in the air.

Dust in the atmosphere significantly affects weather and climate, Zurek explained. When it is suspended, the atmosphere warms up, which can affect wind patterns and lead to "immense wind storms that cover most of the planet," he said.

"We're all trying to understand how water and dust are moved about the planet as a way of understanding the present climate of Mars, and also how the climate may have changed in the past," Zurek said.