PASADENA, Calif. – Bruised by the recent loss of the $165 million Mars Polar Lander, NASA is studying a new class of smaller, cheaper and more robust spacecraft. These probes could land on Mars to reconnoiter terrain that larger missions could later study in detail.

Called appropriately enough "Scouts," two of the 220-pound (100-kilogram) spacecraft could venture to Mars as soon as 2003, said Barry Goldstein, who is leading the study at NASA’s Jet Propulsion Laboratory (JPL).

The tenacious mini-landers would bounce down on the surface of Mars swaddled in airbags, like 1997’s Pathfinder mission. Engineers are also looking at making unprecedented use of a large, carbon dioxide-filled balloon to help slow each spacecraft as it drops through the martian atmosphere.

The folded spacecraft could withstand a landing impact 20 times the force of gravity on Earth. That would allow them to land amid extremely rugged terrain -- like within the depths of the Valles

Marineris -- which poses the greatest threat to spacecraft from an engineering standpoint. However, from a scientific one such terrain is often the most interesting.

"Let’s go somewhere where we would be nervous about landing a multi-hundred-million spacecraft," Goldstein said of the Scouts, which would have what he called "hazard bravado."

Since the loss of the Polar Lander, as well as the Climate Orbiter satellite and $28 million Deep Space 2 microprobes, NASA has undertaken a radical reassessment of its ambitious campaign to explore Mars.

As part of that effort, Dan McCleese, the chief scientist in JPL’s Mars Exploration Directorate, said that the agency must pare down the fleet of spacecraft it intends to use to probe Mars.

"We’ve got to get the product line down to a smaller number of vehicles," McCleese said at a recent conference at NASA’s Ames Research Center.

The laundry list of possible Mars spacecraft includes everything from airplanes to balloons to mobile spacecraft that could scoot about like rovers after landing.

"It’s a confusing mix right now," Goldstein said.

The Scout, if given the green light, could represent a basic platform that could later be modified for multiple purposes.

For now, NASA would want to gear the Scout to accomplish a narrow set of tasks at locations across Mars that vary in latitude, elevation and surface roughness:

• Use descent cameras for "ground truthing," imaging Mars at a resolution between that available from spacecraft in orbit, like the Global Surveyor, and those on the surface. For example, the Global Surveyor cannot spy any features smaller than several yards (meters) across on Mars, including in the region where the Polar Lander crashed on December 3, 1999.

• Use a mast-mounted stereo camera to view the spacecraft’s surroundings once it has landed. That would allow scientists to characterize and assess the area and decide whether it merits a visit by a larger lander to do more sophisticated analysis and perhaps even gather samples for return to Earth.

Lesser goals would be to:

• Perform infrared spectroscopy of the ground around a landed Scout to further examine the makeup of the rocks and the scientific interest of the site, and;

• Map the martian atmospheric structure during entry, descent and landing.

Like the Beagle 2 lander the British hope to send to Mars in 2003, the Scout uses a compact clamshell design.

"We ripped it off," Goldstein said.

Two Scouts could piggyback to Mars aboard a larger lander or orbiter as soon as 2003, perhaps even in the company of an even smaller orbiter to boost the communications link with Earth. Rather than communicate directly with Earth, the Scouts would use satellites in martian orbit as relays.

The Scouts would enter the martian atmosphere either directly, like Polar Lander, or from orbit, as did the twin Viking landers in 1976.

Once in the atmosphere, a parachute would slow the spacecraft. Goldstein said JPL engineers are also studying the feasibility of using a giant balloon to further slow the Scouts. The positive-buoyancy balloon, once released from the spacecraft, could then float off to do reconnaissance work with its own set of cameras.

"This has not been proven. We’ve never done this on Mars," Goldstein cautioned.

Once on the surface, the Scout would unfold like a four-petaled blossom just shy of 5 feet (1.5 meters) across. Three of the petals would hold solar arrays to generate electricity; the fourth a stereo camera or robotic arm. The mission would last a minimum of one week.

"We’re trying to open up options is the message," Goldstein said of the Scout study.

The study, begun just two months ago, should be completed by June.