Mars Climate Orbiter, the first of two NASA spacecraft to reach Mars this year, is set to go into orbit around the red planet to become the first interplanetary weather satellite and a communications relay for the next lander mission to explore Mars.

The orbiter will fire its main engine at 2:01 a.m. Pacific Daylight Time on Thursday, September 23, 1999, 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 NASA's Jet Propulsion Laboratory, Pasadena, CA. "If all goes well, 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."

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 with each pass, the maximum altitude of the orbit will decrease and the orbit will become more circular.

Mars Climate Orbiter's first assignment after it completes aerobraking will be to serve as the communications relay for its sibling spacecraft, Mars Polar Lander, set to land near the south pole on December 3. 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), 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 longterm 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."

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 which 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 on the ground year after year. Consequently, the top meter (or yard) of the polar layered terrains could be a well- preserved tree-ring-like record showing tens of thousands of years of Martian geology and climatology.

The orbiter carries two science instruments: the Pressure Modulator Infrared Radiometer, a copy of the atmospheric sounder on the Mars Observer spacecraft lost in 1993; and the Mars Color Imager, 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 radiometer was provided by JPL, supported by Oxford University and Russia's Space Research Institute; its principal investigator is Dr. Daniel McCleese of JPL.

Meanwhile, the imager will gather horizon-to-horizon images at up to kilometer-scale (half-mile) resolutions, which will then be combined to produce daily global weather images. The camera will also image surface features and produce a map showing objects the size of a football field with 40-meter (130-foot) resolution in several colors, providing global views that will help create a season-to-season portrait of the planet. The camera was provided by Malin Space Science Systems, San Diego; CA; its principal investigator is Dr. Michael Malin, who also provided the camera for the currently orbiting Mars Global Surveyor.

The complete press kit is at:

Mars Climate Orbiter is managed by the Jet Propulsion Laboratory for NASA's Office of Space Science, Washington, DC. Lockheed Martin Astronautics, Denver, CO, is the agency's industrial partner for development and operation of the Orbiter. JPL is a division of the California Institute of Technology.

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