BOSTON — A 10-foot cluster of spare parts and high hopes is silently hurtling toward a Thursday rendezvous with Mars.
Its arrival will kick off a 10-year effort to explore a bright planet in the night sky that has captured the human imagination for millennia.
Unlike the high-profile Pathfinder mission, there's no petaled lander, flashy robotic rover, or cutely named rocks. Instead, the one-ton Mars Global Surveyor will spend 22-1/2 months in orbit training its sensors on the fourth rock from the sun.
The steady gaze of those sensors, researchers say, has the potential to revolutionize mankind's understanding of the solar system's most Earth-like body - and one of its lead candidates for having harbored life.
The Surveyor's arrival follows closely on the heels of the National Aeronautics and Space Administration's Mars Pathfinder mission. Although Pathfinder has beamed back stunning 3-D photos and useful data, it remains largely an engineering demonstration effort and the scientific observations have been highly local.
By contrast, during its primary mission, NASA's Mars Global Surveyor will return "more data than we've ever taken from Mars before," says Glenn Cunningham, project manager of the mission at Caltech's Jet Propulsion Laboratory in Pasadena, Calif.
"I can't imagine that this won't give us a completely new planet," says Bruce Jakosky, a member of the Mars Global Surveyor research team and an associate professor in the geological sciences department at the University of Colorado in Boulder.
Sweaty palms this week
Yet before it delivers, the mission still faces some tense moments. Today, for example, controllers are scheduled to pressurize the orbiter's fuel tanks so its propulsion system will get the steady flow of fuel it needs to maneuver the spacecraft into its initial series of orbits.
"That'll give us some sweaty palms," Mr. Cunningham says, noting that the pressurization procedure destroyed NASA's billion-dollar Mars Observer craft in 1993, only days before it was to begin its study of Mars.
To help hold Surveyor's mission costs to a relatively trim $155 million, designers built the craft around spare instruments from the Mars Observer.
Indeed, not since the two Viking missions launched in 1975 will Mars have fallen under the scrutiny of such high-tech eyes in the sky. The Global Surveyor's camera system, combined with a laser altimeter, will allow Earth-based researchers to compile the most detailed maps yet of the Martian surface.
Using the camera, the orbiter, launched last November, also will serve as a Martian-weather satellite, while other instruments will act as electronic prospectors, identifying and mapping minerals across the vast expanses of Mars' surface. The orbiter is expected to help unravel mysteries about the planet's interior. And it will help researchers track the sources and destinations of Mars' supply of water - a key ingredient for life.
Even after it achieves its main goals, the orbiter will serve as a radio-relay station for a Mars Surveyor Program lander, scheduled for launch in 1998.
Applying the aerobrakes
When Surveyor arrives, controllers will use a novel fuel-saving technique for easing the spacecraft from its initial 45-day orbit to the two-hour "mapping" orbit from which its instruments will operate. They will take advantage of friction between the planet's atmosphere and the spacecraft's extended solar panels to slow the orbiter.
Known as aerobraking, the technique worked with the Magellan orbiter, which used radar to map the surface of cloud-shrouded Venus.
But Magellan's controllers tested the approach only at the end of the craft's mission. The concern: Miscalculations could bring a spacecraft in too steeply, causing it to burn up. Or, if the angle is too shallow, it could skip off the atmosphere, like a stone thrown along the surface of a pond, and head back into space.
Mars Global Surveyor will be the first mission to use aerobraking as its primary method for easing a spacecraft into a lower orbit, Cunningham says, noting that the braking process won't be finished until March 15, 1998.
The mapping orbit itself represents a delicate dance to ensure that the Martian surface always appears to the spacecraft's camera to be in afternoon sunlight. Shadows cast in the "perpetual afternoon" will help surface features such as mountains, hills, and canyons stand out.
As an insurance measure, Cunningham says, controllers will activate the spacecraft's science instruments during its first pass by Mars. That way, researchers can be assured of getting some science out of the mission should something go wrong.