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Mars survey craft nears its destination

This week, the winged Mars Global Surveyor (MGS) puts on the brakes -- the aerobrakes, that is -- and Maria Zuber, professor of planetary science in the Department of Earth, Atmospheric and Planetary Sciences, revs up.

Professor Zuber will spend this week at NASA's Jet Propulsion Laboratory in Pasadena, CA, then fly to the Goddard Spacecraft Center to watch as MGS begins six months of aerobraking, a technique that "drags" the spacecraft into a sedate, polar, circular orbit around Mars similar to the way the wind drags on a hand that's stuck out a car window. Preceding the drag is a 25-minute "burn," which slows the spacecraft and allows the red planet's gravity to "capture" it. The initial orbit, which is highly elliptical, will take about 48 hours to complete. Professor Zuber will watch as the spacecraft gets through one orbit.

The general goal of the MGS mission, launched last November, is to provide complete, accurate mapping of Mars, with a view to precise future Mars landings, including those to search for signs of life.

Professor Zuber, a geophysicist whose research deals with the structure and evolution of the surfaces and interiors of the terrestrial planets, is deputy principal investigator on the science team for the Mars Orbiter Laser Altimeter, or MOLA, which the Global Surveyor is carrying. The role of MOLA is to gather data via lasers that will enable scientists to calculate the height of surface features on Mars.

MIT Professor Emeritus Gordon Pettengill, a co-investigator on the experiment, is leading the effort to determine cloud heights on Mars.

"Mars has a lot to tell us about the greenhouse effect and about climate change," said Professor Zuber. "And then there is the possibility of life. We don't know if Mars developed life, and if it did, why it failed. Understanding what happened on Mars may well have implications for understanding the origin of life."

Once MGS settles into its circular mapping orbit, 378 kilometers above the surface of Mars, it begins 687 days (1 Mars year) of data collection. Mars and Earth shared similar conditions billions of years ago, and a comparison of the planets will allow scientists to learn more about Earth's history and possibly its future. Mars Global Surveyor will make a global reconnaissance of Mars and watch for daily and seasonal changes. The collective data sets will give a picture of the way Mars works.

The instruments on board MGS will also do the "all-important global science survey that will dictate where future landers should be targeted to look for life. It will allow the best sites to be identified -- those most likely to have held past life," said Professor Zuber.

The science payload on board MGS includes a magnetometer, an electron reflectometer, the Mars orbital camera, a thermal emission spectometer, a Mars relay radio system and radio science investigation, which will measure changes in spacecraft radio signals.

MOLA, which resembles a pigeon-toed washing machine, will sample Mars' topography profile 10 times per second. The topography from MOLA combined with gravity from the radio science experiment will provide the internal density structure of the planet, including information on the nature of the core. This information is critical to understanding the heat lost from the planet over time, which controls the surface geology and affects the climate.

MOLA's method of mapping Mars is based in laser technology originally developed for the Strategic Defense Initiative or "star wars" program. With its 40-centimeter (15-inch) height precision, the instrument is the most precise laser ranging instrument that has been flown in space.

As the MGS spacecraft flies above hills, valleys, craters and other surface features, its altitude will constantly change. A combination of MOLA data with images from the camera will allow scientists to construct a detailed topographical atlas of the planet. Such maps will help in the understanding of the geological forces that shaped Mars, and may even be able to measure the waxing and waning of the polar caps over the course of the Martian year. It will also provide information about the structure of the atmosphere by measuring the heights of dense clouds such as are being observed by the Pathfinder mission currently taking measurements on the Martian surface.

The altitude determination process works by measuring the time that a pulse of light takes to leave the spacecraft, reflect off the ground and return to MOLA's collecting mirror. By multiplying the reflection time by the speed of light, scientists will be able to calculate Surveyor's altitude above the local terrain.

A version of this article appeared in MIT Tech Talk on September 10, 1997.

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