The Orbiter will fire its main engine at about 1:50 a.m. Pacific Daylight Time (spacecraft time) on Thursday, Sept. 23, to slow itself down so that it can be captured in orbit around the planet.

"The curtain goes up on this year's Mars missions with the orbit insertion of Mars Climate Orbiter," said Dr. Sam Thurman, flight operations manager for the Orbiter at JPL.

"Hopefully, the happily-ever-after part of the play will be the successful mission of the Mars Polar Lander that begins in December, followed by the mapping mission of the Orbiter that is set to begin next March," said Thurman.

Once captured in orbit around Mars, the Orbiter will begin a period of aerobraking. During each of its long, elliptical loops around Mars, the Orbiter will pass through the upper layers of the atmosphere each time it makes its closest approach to the planet.

Friction from the atmosphere on the spacecraft and its wing-like solar array will cause the spacecraft to lose some of its momentum during each close approach. As the spacecraft slows during each close approach, the maximum altitude of the orbit will decrease and the orbit will become more circular.

"The period of time from Mars orbit insertion for the Orbiter through the end of surface operations for the Mars Polar Lander is the big crunch time for us," Thurman said.

"If you look at the number of individual engineering jobs we have to do in this six-month period and how the Orbiter and the Lander interact to accomplish their respective missions-all with a team of 80 people to do it-to me that's where we're breaking new ground. "It's going further and faster with fewer people and with a smaller budget. If we're successful, I think we'll raise the bar on the whole faster, better, cheaper mantra to a new level-to a level that's not been attained by anyone else," said Thurman.

Mars Climate Orbiter's first assignment after it completes aerobraking will be to serve as the communications relay for its sister spacecraft, the Mars Polar Lander.

After the Lander's surface mission ends in February 2000, the Orbiter's science mission begins with routine monitoring of the atmosphere, surface and polar caps for a complete Martian year (687 Earth days) or the equivalent of almost two Earth years.

"We're interested in what happens during all the seasons of a Mars year. Weather is what happens from day-to-day and the year-long effect of all of that is climate," said Dr. Richard Zurek, project scientist for the Orbiter at JPL.

"Mars Climate Orbiter will do what weather satellites do-it will take pictures of clouds, it will look for storms, and it will try to understand the atmospheric winds by measuring temperature and pressure and by watching how the atmospheric distributions of dust and water vapor change with time," added Zurek.

Today the Martian atmosphere is so thin and cold that it does not rain; liquid water placed on the surface would quickly freeze into ice or evaporate into the atmosphere. The temporary polar frosts that advance and retreat with the seasons are made mostly of condensed carbon dioxide, the major constituent of the Martian atmosphere.

But the planet also hosts both water-ice clouds and dust storms, the latter ranging in scale from local to global. If typical amounts of atmospheric dust and water were concentrated today in the polar regions, they might deposit a fine layer every year, so that the top meter (or yard) of the polar layered terrains could be a well-preserved record showing tens of thousands of years of Martian geology and climatology.

The Orbiter carries two science instruments: the Pressure Modulator Infrared Radiometer (PMIRR), a copy of the atmospheric sounder on the Mars Observer spacecraft lost in 1993, and the Mars Color Imager (MARCI), a new, light-weight imager combining wide- and medium-angle cameras.

The radiometer will measure temperatures, dust, water vapor and clouds by using a mirror to scan the atmosphere from the Martian surface up to 80 kilometers (50 miles) above the planet's limb. The PMIRR instrument was provided by JPL, supported by Oxford University and the Space Research Institute (Russia); its principal investigator is Dr. Daniel McCleese (JPL).

Meanwhile, the imager will gather horizon-to-horizon images at up to kilometer-scale (half-mile-scale) resolutions, which will then be combined to produce daily global weather images. The camera will also image surface features and produce a map with 40-meter (130-foot) resolution in several colors, to provide unprecedented views of Mars' surface. The MARCI instrument was provided by Malin Space Science Systems (MSSS); its principal investigator is Dr. Michael Malin, who also leads the Mars Orbiter Camera investigation on Mars Global Surveyor.