The latest twin missions are Mars Climate Orbitor slated for launch Decmeber 10, and Mars Polar Lander set for launch January 3. Both will be launched atop Boeing's Delta 2 vehicle from Cape Canaveral.

Mars Orbitor will circle the red planet, and conduct a detailed study of the Martian climate and atmosphere, while Mars Lander will send land an automated science station near the edge of Mars' south polar cap. Piggybacking on the lander will be two small probes that will smash into the Martian surface to test new technologies.

Both Mars Climate Orbiter and Mars Polar Lander will seek clues to the history of climate change on Mars and map the planet's surface, profile the structure of the atmosphere, detect surface ice reservoirs and dig for traces of water beneath Mars' rust colored surface.

The lander also carries a pair of basketball-sized microprobes that will be released as the lander approaches Mars and dive toward the planet's surface, penetrating up to about 1 meter (3 feet) underground to test 10 new technologies, including a science instrument to search for traces of water ice. The microprobe project, called Deep Space 2, is part of NASA's New Millennium Program.

The missions are the second installment in NASA's long-term program of robotic exploration of Mars, which was initiated with the 1996 launches of the currently orbiting Mars Global Surveyor and the Mars Pathfinder lander and rover.

The 1998 missions will advance our understanding of Mars' climate history and the planet's current water resources by digging into the enigmatic layered terrain near one of its poles for the first time. Instruments onboard the orbiter and lander will analyze surface materials, frost, weather patterns and interactions between the surface and atmosphere to better understand how the climate of Mars has changed over time.

Key scientific objectives are to determine how water and dust move about the planet and where water, in particular, resides on Mars today. Water once flowed on Mars, but where did it go? Clues may be found in the geologic record provided by the polar layered terrain, whose alternating bands of color seem to contain different mixtures of dust and ice. Like growth rings of trees, these layered geological bands may help reveal the secret past of climate change on Mars and help determine whether it was driven by a catastrophic change, episodic variations or merely a gradual evolution in the planet's environment.

Today the Martian atmosphere is so thin and cold that it does not rain; liquid water does not last on the surface, but quickly freezes into ice or evaporates and resides in 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 every year, so that the top meter (or yard) of the polar layered terrains could be a well-preserved record showing 100,000 years of Martian geology and climatology.

Nine and a half months after launch, in September 1999, Mars Climate Orbiter will fire its main engine to put itself into an elliptical orbit around Mars. The spacecraft will then skim through Mars' upper atmosphere for several weeks in a technique called aerobraking to reduce velocity and circularize its orbit. Friction against the spacecraft's single, 5.5-meter-long (18-foot) solar array will slow the spacecraft as it dips into the atmosphere each orbit, reducing its orbit period from more than 14 hours to 2 hours.

Finally, the spacecraft will use its thrusters to settle into a polar, nearly circular orbit averaging 421 kilometers (262 miles) above the surface. From there, the orbiter will await the arrival of Mars Polar Lander and serve as a radio relay satellite during the lander's surface mission. After the lander's mission is over, the orbiter will begin routine monitoring of the atmosphere, surface and polar caps for a complete Martian year (687 Earth days), the equivalent of almost two Earth years.

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.

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.

Mars Polar Lander, launched a month after the orbiter is on its way, will arrive in December 1999, two to three weeks after the orbiter has finished aerobraking. The lander is aimed toward a target sector within the edge of the layered terrain near Mars' south pole. The exact landing site coordinates will be adjusted as late as August 1999, based on images and altimeter data from the currently orbiting Mars Global Surveyor.

Like Mars Pathfinder, Mars Polar Lander will dive directly into the Martian atmosphere, using an aeroshell and parachute scaled down from Pathfinder's design to slow its initial descent. The smaller Mars Polar Lander will not use airbags, but instead will rely on onboard guidance and retro-rockets to land softly on the layered terrain near the south polar cap a few weeks after the seasonal carbon dioxide frosts have disappeared. After the heat shield is jettisoned, a camera will take a series of pictures of the landing site as the spacecraft descends.

As it approaches Mars about 10 minutes before touchdown, the lander will release the two Deep Space 2 microprobes. Once released, the projectiles will collect atmospheric data before they crash at about 200 meters per second (400 miles per hour) and bury themselves beneath the Martian surface. The microprobes will test the ability of very small spacecraft to deploy future instruments for soil sampling, meteorology and seismic monitoring. A key instrument will draw a tiny soil sample into a chamber, heat it and use a miniature laser to look for signs of vaporized water ice.

About 100 kilometers (60 miles) away from the microprobe impact sites, Mars Polar Lander will dig into the top of the terrain using a 2-meter-long (6-1/2-foot) robotic arm. A camera mounted on the robotic arm will image the walls of the trench, viewing the texture of the surface material and looking for fine-scale layering. The robotic arm will also deliver soil samples to a thermal and evolved gas analyzer, an instrument that will heat the samples to detect water and carbon dioxide. An onboard weather station will take daily readings of wind temperature and pressure, and seek traces of water vapor. A stereo imager perched atop a 1.5-meter (5-foot) mast will photograph the landscape surrounding the spacecraft. All of these instruments are part of an integrated science payload called the Mars Volatiles and Climate Surveyor.

Also onboard the lander is a light detection and ranging (lidar) experiment provided by Russia's Space Research Institute. The instrument will detect and determine the altitude of atmospheric dust hazes and ice clouds above the lander. Inside the instrument is a small microphone, furnished by the Planetary Society, Pasadena, CA, which will record the sounds of wind gusts, blowing dust and mechanical operations onboard the spacecraft itself.

The lander is expected to operate on the surface for 60 to 90 Martian days through the planet's southern summer (a Martian day is 24 hours, 37 minutes). The mission will continue until the spacecraft can no longer protect itself from the cold and dark of lengthening nights and the return of the Martian seasonal polar frosts.

The Mars Climate Orbiter, Mars Polar Lander and Deep Space 2 missions are managed by the Jet Propulsion Laboratory for NASA's Office of Space Science, Washington, DC. Lockheed Martin Astronautics Inc., Denver, CO, is the agency's industrial partner for development and operation of the orbiter and lander spacecraft. JPL designed and built the Deep Space 2 microprobes. JPL is a division of the California Institute of Technology, Pasadena, CA.