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For the past four years JPL has worked on the idea of aerial exploration as well: aerobots, autonomous robotic balloons that could cover a much wider territory than any ground rover while producing much higher-resolution photography than satellites could. The idea isn’t novel; in 1985 the Soviets and French sent a survey balloon to Venus. It performed briefly but well, bobbing upward when it approached the hot Venutian surface and the gases in its bag expanded, then downward when it hit the cold stratosphere and those gases condensed.

But the aerobots JPL is designing (and for which it will launch a test bed early this year) are much more sophisticated. Rather than floating at a constant height, they will control their altitude through valves that can release or confine the gases that give them buoyancy. Thus, explains JPL aerobot systems engineer Aaron Bachelder, they will be able to hover for some time (perhaps an hour or so above Venus, because of the fierce heat, around 460 degrees Celsius on its surface). Then they’ll retreat to the stratosphere to cool off. “Snakes”-long, dangling flexible appendages-will protect against crashes by transferring weight from the aerobot to the ground if the balloon hovers too low. The aerobots will also contain streamlined sensing and scientific instruments for up-close study of the surface. And, JPL special projects manager Jim Cutts insists, they’ve actually been proven not to catch on rocks in earth trials by the French. He adds, taking a page from the nanorover play book, that his crew is designing aerobots light enough-around 22 pounds-to hitch a ride with other missions.

NASA was figuring on Venus as the first aerobot destination, since its heat-too great for ground rovers-makes that planet an optimal choice for the balloons. And Venus’s predictable winds also make it an easier place to plan routes for the devices than blustery Mars. But now that Pathfinder’s triumph has made Mars fashionable, Cutts also hopes to send an aerobot to Mars in 2003-perhaps riding along with that year’s rock-hunting rover.

Aerobots’ prospects don’t end with the two nearest planets. JPL has also sketched out aerobot missions to the Jovian moon Titan and the outer gas planets. Because those planets have much lighter atmospheres, light-gas balloons would not work on them in the way they’re expected to on solid planets. And so missions to Jupiter, Saturn, Uranus, and Neptune would rely on a different, venerable technology: hot-air balloons, heated by the planets’ own infrared radiation.

If the aerobots pan out, technology will have come full circle; balloons, the earliest form of airborne transport, will fly in the vanguard of planetary exploration. That’s just one more indication of the diversity in approaches NASA has been employing in the eight years since it stopped pinning its hopes for investigations on a single massive Mars rover.

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