For such a small craft, the Mars Polar Lander is carrying an unexpectedly weighty cargo as it streaks toward a risky touchdown Friday near the Martian south pole. On board are a pair of basketball-size probes designed to shoot separately into the planet's surface, cameras, a weather station, a robotic digging arm, a mini-lab for analyzing soil samples, and the first microphone ever sent to another world.

Also riding along are the Mars team's hopes for a comeback after the loss of a sister craft, the Climate Orbiter, almost 10 weeks ago as it arrived at the Red Planet.

Mars is notoriously unforgiving. Of 25 U.S. and Russian missions launched toward Mars since 1962, 11 have failed outright and four were incomplete. The need to carry out a landing--the fourth by a U.S. craft on Mars--so soon after a stunning failure has created what scientists describe as "very serious resolve" among members of the team based at the Jet Propulsion Laboratory in Pasadena, Calif., which manages the missions for NASA.

"Obviously the team was extremely disappointed by the loss," said lander project manager Richard Cook. But "in a sense, having [the landing] coming up in less than two months is really the best thing for them. Everybody [is] . . . focusing on what needs to be done."

The lost orbiter and arriving lander, a $327.5 million package, collectively represent the second installment in NASA's long-term program of "cheaper, smaller, faster" robotic exploration of Mars. Although the lander is not designed to look for signs of life, the search for water--considered key to the development of life--is the overriding theme of all Mars missions. The series began with the 1996 launches of the Mars Pathfinder lander, with its puppyish rover (now defunct), and the Global Surveyor (still churning out data in Martian orbit).

But unlike Pathfinder, which tried out experimental technologies such as an air-bag cocoon for bouncing onto the surface, this lander is using the same basic controlled-descent system as the successful Viking craft of the 1970s. At the same time, said chief lander scientist David Paige of the University of California at Los Angeles, this mission has a much more ambitious science agenda than did the Pathfinder. "I'm most concerned about being ready to deal with all the results that are going to come in and the large amount of work that's involved."

If all goes well, he said, the lander will be the first craft ever to land in a Martian polar region, the first to take comprehensive measurements on the behavior of water on the Martian surface, the first serious effort to prospect for ice below the surface and the first to take close-up images of the rusty Martian soil. A camera mounted on the 6 1/2-foot robot arm will zoom in on samples as they are scooped up, showing details as small as the smallest thing you can see under a magnifying glass, Paige said.

On Friday, the spacecraft is to hit the atmosphere at a velocity of more than 15,000 mph, where the heat from friction will subject it to temperatures as high as 3,000 degrees Fahrenheit. At about 3 p.m. EST, having slowed to about 5 mph, the 3 1/2-foot-tall lander is supposed to settle gently on the surface. (The first post-landing signal from Mars is expected on Earth at 3:37 p.m.)

The landing site, announced in late August, is a zone of what appear to be gentle, rolling plains at a latitude of about 76 degrees south, near the ice cap. Mission planners said they are hoping to settle down on bare soil, where the seasonal carbon dioxide frosts have dissipated with the coming of the southern spring. This is the only mission planned that will study Mars so far from the equator. "We believe this layered terrain is a record of climate changes on Mars and, in a sense, digging into its surface will be like reading tree rings or layers in an ice core," said project scientist Richard Zurek of JPL.

During the 90-day mission, the robot arm is to trench into the surface, scoop up samples and dump them into small toaster-oven devices that will heat them so that any carbon dioxide or water can be detected in the beam of a laser. The scientists hope to find clues to the seasonal cycles of water, carbon dioxide and dust movement.

"It's a laborious process," Paige said. "We spend about a day poking around on the surface, getting the sample, putting it in the oven. We cook it for about two days." Each of the eight sample chambers can accommodate only 1/300th of an ounce of material.

The destroyed orbiter was to have served, among other things, as a communications relay for the lander. As a result, transmissions between the machines on Mars and their handlers on Earth will be more complicated and slower than planned. Initially, the two subsurface penetrators will use the orbiting Surveyor craft to relay transmissions, while the lander itself transmits directly from the surface.

The penetrators--named Amundsen and Scott in honor of the first explorers to reach Earth's South Pole--are piggybacking aboard the mother ship, which is to release them on their high-speed plunges just 10 minutes before its own gentle touchdown.

The main job for Amundsen and Scott is to test innovative technologies for future "microlanders." Built to survive a sudden stop at 400 mph, with forces up to 60,000 times that of Earth gravity, the five-pound projectiles carry instruments that can draw in a tiny soil sample for deposit in their own miniature ovens, where small lasers will search for signs of subsurface water ice. Engineers intend the impact to bury the probes about three feet deep--the geological equivalent of perhaps 100,000 years back in time--leaving their tail-ends sticking above the surface to transmit data.

Also aboard the landing craft is a lidar instrument (a cousin to radar) designed to study ice and dust hazes in the lower Martian atmosphere. The first Russian experiment to be flown on a U.S. interplanetary mission, it also contains the first microphone ever sent to capture the sounds of an alien planet.

The 1.8-ounce "bug," funded privately by the California-based Planetary Society, is of a type used in hearing aids. Despite Mars's extremely thin atmosphere, its sponsors expect the microphone to pick up the whirring of the spacecraft gears, the crunching of the robotic trowel and, with luck, the sighs of the Martian winds.

CAPTION: Landing on Mars (This graphic was not available)