Global Surveyor arrives at Mars.

Last week, researchers who had spent a decade building -detectors for the errant Mars Observer spacecraft finally scored a success. Duplicates of five of the seven instruments lost when that satellite vanished just 3 days before it was to have arrived at Mars in 1993 are revolving around the planet on the Mars Global Surveyor. On Sept. 11, Surveyor became the first U.S. craft to orbit the Red Planet in 21 years.

Designed to monitor the Martian climate and map the planet's surface at an unprecedented resolution of 1.4 meters, the craft faces a major hurdle before it even begins its 2-year mapping mission in mid-March. Surveyor must transform its initial, elliptical orbit, with a high point of 56,000 kilometers and a low point of 250 km, into a circle with an altitude of 378 km. To accomplish this feat, the craft is dipping into Mars' upper atmosphere, using air resistance to drop its height and speed over 4 months.

That maneuver, known as aerobraking, has been performed only once before, at the end of Magellan's mission to Venus, and it requires careful planning. If aerobraking occurs too rapidly, the craft could heat up dangerously. "Nobody really knows what to expect in terms of the actual [atmospheric] densities the craft will encounter," says Bruce M. Jakosky, a Surveyor, researcher at the University of Colorado at Boulder. "These are things that have never been measured on Mars, so it's going to be a risky business."

Surveyor is the second in an armada of nine Mars-bound craft scheduled to be launched by NASA every 2 years through 2005. It will act as a scout for future landers, identifying sites whose mineral content or eroded terrain suggests that water once coursed through them.

The crafts thermal emission spectrometer is expected to examine the composition of the Martian surface; it is already providing data on the temperature of the atmosphere during aerobraking.

This week, Surveyor's magnetometer reported that Mars possesses a weak magnetic field, one eight-hundredth that of Earth's surface, says Mario H. Acuna of NASA's Goddard Space Flight Center in Greenbelt, Md. If the field were much stronger in the past, it could have shielded living material from the solar wind and cosmic rays, he adds.

In March, when Surveyor is slated to begin orbiting Mars once every 2 hours, its camera will record detailed black-and-white images as well as global color panoramas similar to weather maps of Earth. A laser altimeter will measure the heights of mountains and depths of canyons.