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Mars Global Surveyor in Orbit Around the Planet

On September 11, 1997, two months after Pathfinder arrived on Mars, another spacecraft entered the planet's atmosphere and began a complex mission expected to last at least five years. The Mars Global Surveyor (MGS) was launched on November 7, 1996, a month before Pathfinder. Because it was traveling on a different trajectory, its voyage to the Red Planet took 10 months compared with 7 months for Pathfinder. MGS was the first spacecraft to be launched in a planned 10-year series of Mars exploratory missions. Subsequent launches are expected to take place at 26-month intervals through the year 2005.

The approaching MGS skimmed the upper atmosphere of Mars and the resulting friction captured the spacecraft into a highly elliptical orbit around the planet. The use of atmospheric drag to change the orbit of a spacecraft without expending rocket propellant is called aerobraking. The initial orbit after capture around Mars was about 200 kilometers (120 miles) above the planet's surface at the near end but over 50,000 kilometers (31,000 miles) away at the far end.

The original plan had called for the spacecraft to spend the next four months in a sequence of aerobraking maneuvers until its orbit became a nearly circular path crossing over the north and south poles of Mars at low altitude. (With each dip into the atmosphere at the low point of the orbit, the satellite loses more energy to friction, which lowers the high part of the orbit.) However, one of the spacecraft's two solar panels was slightly bent by the feeble atmospheric pressure encountered during the initial dip into the atmosphere. To avoid serious damage to this crucial component, mission planners decided to carry out the rest of the aerobraking maneuvers by dipping into the atmosphere at a higher altitude, where there is less air and lower pressure. Unfortunately, this will set back the schedule and the spacecraft orbit will not be in the proper orientation for mapping Mars until March 1999.

Layered Cliff Face,
Mariner Valley

However, the MGS satellite has already provided some stunning images of Mars, taken during its aerobraking dips at the low point of its orbit. Some of these images show detail 50 times better than the previous best orbital images from the Viking spacecraft. Individual rocks the size of a small automobile can be seen. In a portion of Mariner Valley, the MGS camera photographed a triangular sheer mountain cliff over 1 km (3200 feet) tall, revealing some 80 layers of alternating brightness and thickness ranging from 5 to 50 meters (16 to 160 feet). These features are seen in several other outcrops and, together, indicate that amost the entire great depth of the Mariner Valley is layered. Nothing quite like this was ever seen or suspected on Mars before.

In another part of Mariner Valley, the orbiting Surveyor took this picture of layered rock formations resembling the Grand Canyon. The image is about 10 km across.

The Mission

When MGS achieves its final orbit, about 440 kilometers (248 miles) above the planet, it will begin collecting and transmitting a wealth of data about surface features (topography), weather and climate, atmosphere, and more. MGS will circle the planet once every 20 hours.

The mission objectives are:

to enhance understanding of Martian geology and climate
to monitor Martian global weather
to monitor surface features, polar caps, atmospheric dust, and the seasonal cycle of clouds
to help planning for future missions, especially landing site selection
to provide a communications relay to the United States and other nations

The Tools

To accomplish these ends, MGS is carrying an impressive array of equipment, including:

Mars Orbital Camera (MOC), which is equipped with specialized lenses. One will provide a wide-angle view of the entire planet; another will be able to capture high resolution close-ups of objects as small as 1.5 meters (5 feet) across.

Thermal Emission Spectrometer (TES), which measures heat given off by various things. It will be used to study the properties of Martian rocks, soil, ice, dust, and clouds.

Mars Orbital Laser Altimeter (MOLA), which will help scientists determine the elevation (height and depth) of mountains, valleys, and other features on Mars. It does this by measuring how long it takes for a laser beam to reach the target and then be reflected back to MOLA. Many measurements will be taken to compile a detailed topographic map of Mars.

Other instruments will be used to conduct radio science investigations designed to gather information about the Martian gravity field and atmosphere. A magnetometer will study the magnetic field of Mars. This instrument has already provided hints that Mars has a patchwork magnetic field which varies in direction from place to place. Thus, a compass would not be very useful to find your way around on Mars! Finally, a sensitive antenna will route signals back and forth to Earth from later surface landers and rovers.

Throughout its life, MGS will be transmitting data and visual images back to Earth, which will be accessible to the scientific community and the general public on the World Wide Web.

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