What if you had to design a desktop computer that could still function after hitting a truck at 400 miles an hour?

That was pretty much the assignment that engineer Sarah Gavit, 37, took on about four years ago. But in this case, the device is actually the first surface penetrator NASA has ever sent to another world, and the "truck" is the planet Mars.

Gavit manages the team at the Jet Propulsion Laboratory here that developed the two 14-inch probes riding piggyback aboard the Mars Polar Lander. The main purpose of the $30 million penetrator project, called Deep Space 2, is to test a number of new technologies and techniques for what could be low-cost swarms of similar microsensors bombarding the planet in the next century in search of knowledge, especially clues to the likelihood of past or present Martian life.

About 10 minutes before the Polar Lander touches down at 3 p.m. Friday (EST), if all goes well, the projectiles will dive off the mother ship without parachutes and slam into the planet's hide, experiencing deceleration forces up to 60,000 times that of Earth's gravity; or, in other words, a really sudden stop.

Each probe's tiny chip brain, electronics, connectors and wiring, batteries, a power drill and other micro-equipment are built to keep on ticking despite the licking. Some systems must also withstand temperatures as low as minus 184 degrees Fahrenheit in the soil where the bullet-shaped head presumably will bury itself to a depth of about three feet. There, its drill is to auger tiny soil samples into a cup, where they will be analyzed for signs of water ice.

"When we started out," Gavit said, "we really did not know how to do this." The military was known to have used penetrators to monitor enemy troop movements in Vietnam and submarines in the Arctic, she said, but the requirements were different and besides, "they didn't share. We had to reinvent it."

In pursuit of the right materials and designs, the team adopted measures that sound as much like a David Letterman stunt as big-time research and development. They dropped test models from planes and helicopters, and shot them many dozens of times from huge guns in the New Mexico desert, at an Air Force base in Florida and even at a supersonic wind tunnel in Kalingrad, Russia, Cold War facilities more accustomed to dealing with intercontinental ballistic missile techniques.

The probes not only were required to survive the fierce impact in working order but they also had to land in a certain position. And, of course, they had to be lightweight and tiny.

"We broke a lot of things, at first," Gavit said. The researchers once enlisted a skydiving outfit in the Mojave Desert, getting the divers to strap the missiles to their legs and release them. Later, they designed a bomb rack, of sorts, and dropped the devices like warheads.

"It takes a lot out of you," said Gavit, who also worked on the Magellan mission to Venus. "But it's an engineer's dream to do something like this . . . something that's never been done."

About 50 percent of the test objects went missing. "When you throw 'em out of a plane from 10,000 feet, just try and find them," Gavit said. "I personally have spent whole days walking the desert" with a line of people doing a sweep, looking for micromissiles. The team tried smoke signals and streamers but nothing yet existed that could survive the impact.

Eventually the engineers moved to shorter, squatter shapes and configurations, finally adopting the principle of the shuttlecock, or birdie, used in badminton: the probe's own weight distribution causes it to align properly, even if it starts tumbling.

The probes are supposed to hit the south polar region about 60 miles from where the mother ship is to land more sedately moments later. Each basketball-sized aerodynamic outer shell has to shatter instantly at impact so that the device inside can separate into two parts. The 4.2-inch-long bullet-head will remain connected by a tiny flexible cable to the hind part, whose "lawn dart" aspect should keep it on the surface to relay data back to Earth.

"The biggest risk to the Deep Space 2 probes is Mars itself," Gavit said, noting the mysteries of the polar region. "We're not sure what we're getting into."