BULL'S-EYE ON MARS

By Jeffrey Kluger

For the past two months, Mars has been a lonely place. Despite the kudos that the Pathfinder lander and its sturdy rover have received from Earth, the spacecraft have operated largely alone, trying to study all of Mars without moving beyond a small patch of riverbed real estate.

Last week reinforcements showed up when one of Pathfinder's robotic littermates--the Global Surveyor--arrived at the planet and swung into orbit. Though the landers are likely to wink out within months, the Surveyor orbiter will be studying the planet's surface and atmosphere for years to come. When it's done, NASA should have a better picture than it's ever had of Mars' chemistry, meteorology and perhaps even biology. "We want to understand the evolution of water in that atmosphere," says Arden Albee, Surveyor project scientist, "and whether life could possibly have existed on Mars."

Just getting to the Red Planet was a signal accomplishment for Surveyor. Launching a spacecraft from Earth to Mars, says NASA, is like firing a baseball from California to New York and hitting a particular window in the Empire State Building. Having managed this navigational tour de force, Surveyor then had to enter Mars orbit--a maneuver that carried its own risks.

Before it could slow itself sufficiently to be captured by Mars' gravity, the ship had to pressurize its fuel lines and fire its retrorockets. The last time NASA tried to orbit Mars was in 1993, and that mission ended during pressurization when a fuel line hemorrhaged, sending the billion-dollar Mars Observer probe spinning into the void. Last Tuesday was Surveyor's turn to prime its lines, and despite some well-bitten nails in Mission Control, this time things went perfectly. "To see this event pass successfully is a great relief," said project manager Glenn Cunningham after the pressurization was done. Two days later, the ship fired its engine and entered orbit.

For the time being, Surveyor won't do much at Mars but orbit. The ship's path around the planet is elliptical, with a low point of 155 miles and a high point of 35,000. Each time Surveyor barnstorms Mars on its close approach, however, it drags a solar panel through the atmosphere in a process called aerobraking. A few months of this cosmic paddling will refine the orbit so that by early next year, the ship will inscribe a near perfect 235-mile circle. Then it will switch on its instruments.

When it does, it will find a lot to study. While satellites usually travel horizontally around a planet's equator, Surveyor orbits vertically, flying over both Martian poles. If there is organic chemistry on Mars, it will probably be in a wet, carbon-rich region, and the ice caps--made largely of water and carbon dioxide--fill that bill nicely.

If all goes well, Surveyor will operate into the year 2000. Even before then, NASA's next Mars ships--a lander and an orbiter set to launch in 1998--should have arrived and begun their own surveys. "We're here for a long visit," said Cunningham. "We're here to stay, essentially." It will be a while before Mars is lonely again.

--By Jeffrey Kluger. With reporting by David Bjerklie/New York

With reporting by David Bjerklie/New York