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Ballons may someday collect samples from the surface of Saturn's Titan moon. (NASA)

Floaters

Mars, Venus, Titan - wherever there's air, we can explore by balloon.

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At a quiet airstrip near Tillamook, Oregon, 10 people in comfortable running shoes take their positions along an 82-foot tether secured to the front of an SUV. At the far end of the line, a giant helium balloon bobs like a translucent white jellyfish. As they stand holding the connected components of an instrument payload attached to the tether, some of the runners wait for a signal from Jet Propulsion Laboratory engineer Jack Jones, who's standing by the SUV, hand on the release mechanism, peering at the teardrop-shaped balloon. For this test, being conducted in December 2005, the payload is bound for the stratosphere. But Jones' interests range much farther. Inside one of the payload canisters is another, smaller balloon similar to the ones he'd someday like to fly over Mars.

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At around noon, Jones releases the helium tow balloon and the scramble begins. To ensure a smooth sendoff, the members of the ground crew have to stay under the drifting balloon. That means running, components in arms, and letting go only when they're directly underneath it. One by one, the pieces take flight: a parachute, the stowed Martian balloon-a Montgolfiere hot-air type, named for the French brothers who pioneered the technology in 1782-and a sensor package with guidance system, radio transmitter, and video camera. At an altitude of 22 miles, where the thin air resembles the Martian atmosphere, the scale-model Martian balloon is supposed to separate with its own parachute, cut away from the parachute, inflate, and float to the ground. But within a minute or so of initial separation it gets tangled in the parachute, and after a couple stressful minutes, it tears apart.

Later, Jones, usually gregarious and prone to bouts of rumbling laughter, makes a wounded-animal noise as he recalls the scene: "When you drop five kilograms of payload 30 meters [from the tow balloon], it's a big tug on the balloon. It's kind of like popping a paper bag." Looking on the bright side, this was the largest hot-air balloon ever to survive its deployment in the stratosphere. So in strict engineering terms, it hasn't been a total disaster, and another test is planned for this year. But setbacks like this have already cost Jones and his fellow balloonists the chance to be considered for NASA's 2012 Mars Scout mission, a new, economy class of Martian expeditions using small landers or aerial vehicles.

Luckily, the solar system is big and other places beckon. The team is also designing a balloon mission for Venus, which they have submitted to NASA's slightly pricier Discovery program. And the aerostat office at JPL is hard at work on concepts for a balloon mission to Saturn's moon Titan.

The idea of balloons flying low and slow over alien landscapes appeals to researchers like Wolfgang Fink, a physicist and visiting associate at the California Institute of Technology who co-authored a report last year in the journal Planetary and Space Science arguing for a mix of techniques for exploring other worlds. "We're not trying to take anything away from the successful landings on Mars, Venus, and Titan, nor the orbital-based successes," Fink says. "We're looking at a new way to cover lots of distance."

Planetary balloons can travel farther than rovers and get closer to the surface than orbiters, sniffing the atmosphere or taking pictures and temperature readings as they go. For JPL's engineers, their value as research tools has always been obvious. The technology, not the rationale behind it, is what needs shoring up.

IN 1995, WHILE ATTENDING a conference in Japan, Jim Cutts made a side trip. As JPL's advanced projects manager, he jumped at the chance to go to Osaka to observe the manufacture of a new material, Zylon, one of the strongest synthetic fibers in the world. Used for bulletproof vests, its high tensile strength and heat resistance made it perfect for balloons that have to survive the harsh conditions on Venus.

At the time of Cutts' trip, interest in planetary balloons was at an ebb. The Russian Vega probes that visited Venus in 1985 had carried French-designed helium balloons that spent two days studying the atmosphere as they floated. But it was downhill from there. Another French-Russian aerostat, this one for Mars, was delayed twice for money problems, then removed from a 1996 Mars-bound probe, which ended up crashing to Earth shortly after launch anyway.

By then the United States had already picked up the torch. In 1993, JPL researchers including Cutts, who today serves as the lab's Chief Technologist for the Solar System Exploration Directorate, and Jack Jones had scanned the list of planets and moons with atmospheres and begun working on designs for balloons to use for Venus, Mars, and Titan. By 1997 the lab had a Mars Balloon Validation program, which aimed to prove that aerial inflation of a balloon over Mars was possible. It would take more than just filling a sphere with gas and setting it adrift. The whole EDI-entry-descent-inflation-sequence had to be carefully worked out, from high-speed entry into the Martian atmosphere to separation from the incoming spacecraft to the timing and method of inflation.

"On some level, balloons are very simple devices," says Jeff Hall, JPL's lead engineer for interplanetary lighter-than-air missions. "On other planets, with different environments, it gets difficult quickly."

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