But now that Sojourner has paved the way, other planetary explorers can realize the potential it only hinted at. Rocky 7-outfitted with a robot arm and camera mast as well as a Sojourner-style chassis-is going through its paces at dry Lavic Lake in the Mojave Desert. The lakebed is an especially apt Mars analog, thanks to the aviators at the nearby Twentynine Palms Marine base; their bombing practice has left it pocked with craters, approximating the thousands that asteroids have nicked in Mars’s surface.
The long-distance hiking that Rocky 7 is practicing under autonomous operation and with improved sensory and navigation systems will be critical for future Mars rovers. Sojourner crawled only a few score meters under the watchful, position-checking electronic eye of its Pathfinder mother ship. But Rocky 8, the rover-apparent for the 2001 Mars mission and its 2003 successor, will have to cover many miles, probably on rougher, older highland terrain where traces of ancient life may more likely be found. NASA wants these rovers to cache interesting samples that yet a third machine-a brawnier, more specialized “retrieval rover”-is slated to pick up in 2005 with the aid of electronic beacons left with the caches. That rover might have to haul the samples still more miles to the lander for return to Earth and the close examination that may finally settle the question of life on Mars.
Covering more ground is just one of many challenges to come for the rover makers. Others revolve around more restricted budgets. Steve Saunders, JPL’s chief project scientist for the Mars 2001 mission, notes that future vehicles will have to rely on less human involvement. While the Pathfinder mission occupied up to 10 people several months after the landing with matters such as control and troubleshooting, the Mars 2001 budget allows for an operations team of about 4. The next rovers also must cost less, ride on smaller rockets, do more science (NASA is still hashing out exactly what) and run 3 times longer than Sojourner. And aside from monetary factors, they must survive an even wider range of temperatures than Sojourner did (experts figure the Martian cold finally silenced the Pathfinder mission).
Much of the hope for meeting these challenges rests on the new graphite composites that JPL’s mechanical systems division is creating. Using consistent composites throughout a rover could reduce the destructive differential cooling and contracting that now occurs when various metals are used, making the machine less vulnerable to temperature changes. And composites could shave more weight off the final payload, reducing costs. Already, a prototype “Lightweight Survivable Rover” for the ‘05 mission weighs only 15 pounds-two-thirds as much as Sojourner-while stretching more than one-and-a-half times as long and wide and standing nearly twice as high, a foot off the ground.
Further, this prototype’s wheels collapse to a third of their extended volume and thus can pack into a smaller flight capsule, says Paul Schenker, the division’s research and development leader. “We’re extending that idea to the whole rover frame,” he adds, vowing to make the sample-retrieval rover “truly collapsible” and hence even cheaper to send.
The greatest progress by Schenker’s team has been in building arms from the lightweight composites. One arm, all composite down to its motor, weighs only about eight pounds but can lengthen to about six feet, dig a trench, lift and deposit samples, and sling a microcamera. Another, weighing two pounds, can lift several times its own weight, in part due to ultrasonic motors (so called because they whir at inaudible frequencies). Such motors maximize torque-hence traction and leverage-at very low speeds, which is just what you want for extraterrestrial uses, where high speeds increase the risk of accidents and require more information processing. Moreover, low-speed ultrasonic motors don’t require gearboxes as conventional motors do to reduce their rotations to useful speeds. Eliminating the gearbox eliminates more weight; again, less is more.
By the time the ultralight rovers with boarding-house reaches are ready to fly, an even more dramatic essay in rover miniaturization may have proven itself in the first-ever landing on an asteroid. Again, necessity, in the form of payload restrictions, is the mother of design. In September 2003, the Japanese space mission known as Muses-C is due to land on the half-mile-wide, earth-crossing asteroid Nereus. The plan is to touch down at three sites (lifting off and landing again requires little power in an asteroid’s low gravity), collect samples, and dispatch these to earth by January 2006 using parachutes dropped from space flights. But first Muses-C should drop off an American passenger-a rover.