A Giant Bounce for Mankind

Heralding a new era in planetary exploration, a small spacecraft is set to make an air-bag-cushioned landing on Mars tomorrow, revealing new insights into a planet that has fascinated humans since the days of the Phoenicians.

Mars Pathfinder - with its computers, sensors, and microwave-sized robotic "geologist" named Sojourner - is designed to help researchers solve once and for all one of the great conundrums of modern science: Whether Mars at one time could have supported primitive life.

Set to touch down appropriately on the Fourth of July, the probe will test new techniques and technologies for future spacecraft. The landing in particular will be unusual. Just seconds before reaching the surface, a series of air bags will inflate and cocoon the probe, allowing it to make a pogo-stick landing. The craft may take a few 100-foot bounces before dribbling to rest.

"The air bags act like a superball," says Brian Muirhead, the project's flight-systems manager. They're "probably the things we're the most uncertain about."

Pathfinder's science objective is markedly different from that of the Viking program, which placed two landers on Mars in the mid-1970s and yielded more than 50,000 photos of the planet.

"Viking was designed to answer one fundamental question: Is there life on Mars?" says Matthew Golombek, a Mars Pathfinder project scientist. The answer teased from Viking's data, however, turned out to be: very unlikely. The Viking craft tested soil samples for telltale chemical signs of life. The tests failed to find any.

But the question of whether the planet once could have supported life was left open. To tackle that question "you have to ask: What kind of environment existed early on?" Dr. Golombek continues. The answers are found in a planet's geological record. That's what Mars Pathfinder is all about. "It's a rock mission," he says.

The landing site that mission planners selected is Ares Vallis, a expansive flood plain that planetary geologists believe was formed by the sudden released of an immense volume of water. The channel formed when a vast lake broke through a glacial dam and sent the equivalent of Lakes Erie and Ontario thundering into the region, flooding it in two weeks.

The Martian flood plain is thought to contain a wide variety of rock types, including rocks from nearby crater-pocked highlands. These older highland rocks are thought to represent two-thirds of Mars' crust, and so should give scientists clues about the planet's early environment. Highland rocks are thought to date from 3.6 to 4.6 billion years ago - about the age of the Martian meteorite purported to contain fossilized evidence of microbial life.

Despite the mission's high profile, the atmosphere in the tiny mission control center here at the Jet Propulsion Laboratory has been relaxed in the days leading up to tomorrow's landing. Next to one flight controller's workstation sits a copy of "Zen and the Art of Juggling."

At this point in the mission, the craft is flying autonomously. Its performance to date has been flawless, and a light work schedule in the final few days was designed to give members of the mission operation center's team a chance to rest up for what promises to be an intense period beginning this weekend, according to Jennifer Harris, Pathfinder flight director.

THE excitement begins tomorrow, 34 minutes before the lander's scheduled touchdown at 10 a.m. Pacific Daylight Time. The lander, encased in a protective shell and surrounded by large air bags, separates from its cruise stage, which kept Pathfinder on course during its flight from Earth.

Four minutes before touchdown, Pathfinder plunges into the thin Martian atmosphere and begins to glow like an incoming meteor as its heat shield reaches temperatures approaching 2,000 degrees F. About 8 seconds before landing and after its parachute deploys, a 19-foot-diameter cocoon made up of two dozen air bags inflates. Two seconds before touchdown, the entry system cuts the air-bag-enclosed Pathfinder free to bounce to a stop on the Martian surface - a process that could take several minutes.

Once it stops, the bags deflate, and Pathfinder opens its three petal-like solar panels, one of which carries Sojourner and the rover's off ramps.

The bounce phase is one aspect of the mission that still concerns Pathfinder planners. Although the air bags are made to withstand the wear and tear of a landing that would embarrass any self-respecting airline pilot, too little is known about the rocks at the landing site to say for sure the bags will hold. Nor have planners been able to practice using the bags the way they have with other Pathfinder technologies.

At a minimum, planners hope for a safe landing and some images of Mars. "There's always a risk associated with landing a spacecraft," Mr. Spear says. "If Mars Pathfinder succeeds, it will contribute significantly to future space missions." If it fails, "we will be bummed out, to say the least."

But noting the craft's "flawless" performance so far, the team remains upbeat.

"The mission has gone so smoothly," says mission manager Richard Cook. "We've got a lot of confidence in that craft."