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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."

In 1997 and 1998, the JPL engineers conducted dozens of low-altitude deployment tests of small (up to 17 feet in diameter) hot-air Montgolfiere balloons launched from Oregon. All the tests were successful. Three of the flights even demonstrated an altitude control system that used a radio-controlled gas vent in the top of the balloon to control its rising and falling. The idea was to show that Montgolfiere balloons could be safely navigated to within about 100 yards of a planetary surface, which is close enough to drop a sample collector to the ground.

From 1998 to 2002, the balloonists turned to high-altitude tests that better simulated deployment in the thin, cold Martian atmosphere. Some of the Mylar balloons used for these tests ended up ripping. But three of four small (up to 50 feet in diameter) polyethylene balloons deployed successfully in the stratosphere. When larger polyethylene balloons were tried, they failed shortly after deployment. Realizing that the larger balloons experienced greater stress during deployment, the engineers tried stronger balloons with a gentler deployment sequence.

That appears to have done the trick. Jones' most recent flight from Oregon, made in December, resulted in a successful deployment of a 66-foot Montgolfiere lasting one minute, until its parachute descended and collided with the slower-descending balloon. An 83-foot Montgolfiere is scheduled to be flown this summer, and this time it will be equipped with a gliding parachute to prevent the same outcome.

Armed with at least partial success, in 2002 Hall and Jones submitted competing proposals to NASA's Mars Scout program, which, beginning with next year's Phoenix lander, will send small missions focused on a few scientific objectives to the Red Planet at regular intervals. True to their preferences, Hall submitted a design for a balloon inflated by helium tanks, while Jones proposed a Montgolfiere balloon that fills with heated Martian air as it descends. This time they competed, but just as often they collaborate. NASA's planetary balloon program is not large enough for civil war.

Nor is Mars the best place to demonstrate a balloon's advantages. Because the atmosphere is thin, the designs most likely to succeed are large, thin-skinned balloons carrying small payloads. The payload falls faster in thin air, so deployment and inflation are quick and violent. That puts the balloon under a lot of stress, increasing the risk of a tear or a tangle. Still, because opportunities to fly in space are rare, when NASA first solicited ideas for Mars Scout missions in 2002, the aerostat program threw the two contenders into the ring.

Even by NASA standards, the competition was intense. Facing about 30 other proposals, neither balloon mission made the first cut. "The science was excellent but the technology was not yet ready," says Samad Hayati, manager of JPL's Mars Technology Program. The problems of entry and inflation were still unsolved, and NASA wanted a smaller navigation package. In addition, Hall says the proposal included too much detail on past French and Russian balloon projects, a mistake he will not make again. "Saying the Russians did it first does not carry a lot of weight at NASA," he deadpans.

The balloons also faced competition from a Mars airplane with similar scientific goals, called ARES. Says Hall: "Airplanes and balloons are close to direct competitors," since they both fill the niche between orbiters and surface rovers. ARES survived to the final round of proposals, only to lose to the more conventional Phoenix Mars Lander, which will go on the first Scout launch next year. The airplane's good showing led to additional funding and a higher profile that improves its chances for this year's competition. In contrast, no balloons are likely to be proposed for the 2012 Mars Scout opportunity. Hayati says that if the complexities of entry, deployment, and inflation can be solved for the thin Martian atmosphere, things might change. But for now, the aerostat teams are turning their attention to other worlds. Atmosphere required.

THE SCATTERED BUILDINGS OF the Jet Propulsion Laboratory stand against the San Gabriel Mountains outside Pasadena like a college campus tossed carelessly onto a hillside. New employees live and die by facility maps. After a while they learn to ignore the deer that routinely graze on the sculpted lawns. Harder to ignore are the occasional mountain lion tracks found outside the buildings or between cars in the parking lots.

Jeff Hall's aerostat office is tucked away in a nondescript building that at first blush resembles a trailer. Entering the lab doesn't dispel the impression-it looks like nothing more than a plain rectangular meeting room. But step through a side door and you find the heart of the place, a 1,500-square-foot workshop. Winches, workbenches, soldering equipment, and rolls of balloon material indicate the kind of hands-on work being done here. In one corner, past the sensor payloads and video processing equipment, an undergraduate student attaches straps to a rolled-up trial balloon.

Then there's the fully inflated, 36-foot-long blimp, lashed to a 60-foot table.

This, Hall explains, is the test bed for an aerobot designed to explore Saturn's largest moon, Titan. The Huygens probe's brief look at the cloud-covered moon last year (see "219 Minutes on Titan," Oct./Nov. 2005) whetted scientists' appetite for a more thorough investigation. And Titan's atmospheric density-about four to five times greater than Earth's-makes it better suited than Mars for exploration by lighter-than-air machines. "We're only underdogs on Mars," Hall says.

The test bed is slightly smaller than the actual, 50-foot Titan airship would be. Since 2001, it has flown nearly 20 times in the Mojave Desert, northeast of Pasadena, operated by a pilot on the ground or by the onboard computer. The main objective has been to test a basic autopilot system and a more sophisticated guidance system that determines the balloon's motion from video pictures of the terrain below.

A Titan aerobot will need to be able to react to its surroundings without help from Earth, since radio signals take 70 or 80 minutes to travel one way, and at times Saturn will block direct radio signals altogether. "You just can't joystick it from down here," Hall says.

The prototype has a gasoline engine to fight winds in Earth's atmosphere. The real one would likely have a nuclear power source that could keep the balloon conducting science investigations for as long as six months, and even longer if researchers can slow the rate of gas leakage or replenish the gas.

It also will have to operate in the hostile environment of Titan, where it rains methane and surface temperatures drop to -289 degrees Fahrenheit. Having designed machines for the space environment, Hall knows what happens if you dip a balloon in liquid nitrogen and throw it to the ground: It smashes into pieces. That's something the JPL engineers would like to avoid.

A 2002 small-business solicitation requesting solutions brought a response from the New Jersey-based Lamart Corporation, which usually makes high-performance sail material for America's Cup sailboats. The company offered a blend of their high-performance fiber and Mylar as a possible balloon material that could withstand Titan's deep freeze. For the past three years, the material has been torture-tested in cryogenic conditions and vacuum chambers. "Fabric for toughness and film for gas retention," Hall says, twisting and pulling a sample of the material in both hands.

Along with their blimp-like aerobots, the JPL engineers have been looking lately at conducting a Titan mission with Montgolfiere balloons, which have the advantage of being simpler. In collaboration with the balloon team at Wallops, they have signed another small business to develop an even more esoteric technology: a way to convert the smoggy atmosphere of Saturn's moon to gas for inflating a balloon. Lynntech Inc. of College Station, Texas, normally is involved in fuel cell research. Since Titan's atmosphere is three percent methane (CH4), hydrogen could be extracted from the gas and used to replace gas lost from the balloon. The trick is to make the converter lightweight, low-power, and reliable. JPL and Wallops are currently funding the creation of a full-scale prototype that weighs just 11 pounds and runs on only 10 watts of electrical power.

The balloonists have also set their sights on Venus, the closest planet with an atmosphere and the only one where a scientific balloon has already flown. Although spacecraft have skirted past, orbited, and landed on the planet since 1962-the most recent is Europe's Venus Express orbiter, which arrived in April-fundamental questions remain. The thick hazy atmosphere prevents high-resolution photography from orbit, so scientists have to rely on radar images to view the topography. And only a handful of photos of the surface exist, taken by short-lived Soviet landers in 1975 and 1982.

Scientists also want to bring Venusian rocks back to Earth, but launching a sample container directly from the surface, through the dense atmosphere, and into orbit is very tough. Balloons could save the day by lifting the sample-containing rocket to a higher altitude and launching it from there. First, though, a balloon has to be built that can handle the harsh Venusian atmosphere. The upper layers have thick, corrosive clouds, while the bottom layers, close to the surface, are a scorching 450 degrees . "It's really like two atmospheres," Hall explains. "One is Earth-like, except for the clouds of sulfuric acid."

Finding a single balloon material that can withstand both of these environments has proven difficult. Materials like Zylon can handle the heat. But at higher altitudes Zylon would be corroded by sulfuric acid. And Teflon, which could survive the acid, is brittle at the high temperatures down below. "Many people have looked at a single balloon [for both atmospheres]," says JPL's Viktor Kerzhanovich, who worked on the Soviet Venus balloon missions before coming to the United States. "As far as I know, none would be successful."

His solution, unveiled at an aerospace conference in Washington, D.C., last year, is a two-balloon system. The first would operate near the planet's surface and would look like a cylindrical bellows made of extremely thin sheets of stainless steel "or other suitable alloy," according to Kerzhanovich's paper. The term "metal balloon" might seem an oxymoron, like "jumbo shrimp," but when designing lighter-than-air vehicles for Venus, unconventional thinking is required.

In fact, a corrugated-metal cylinder leans in one corner of the JPL workshop. The bellows is flexible enough that it can be squished like an accordion for storage on the way to Venus, and tough enough that it could survive the clouds of acid. Filled with helium, the thin metal balloon would rise from the surface of Venus, taking photos or carrying a sample container, depending on the mission. When it got above the hot zone, some 10 miles up, it would release a second balloon, which would climb to higher altitudes.

The metal balloon concept isn't quite ready for prime time. For this year's Discovery mission competition, Hall, Kerzhanovich, and colleagues from JPL and the Universities of California, Michigan, and Wisconsin at Madison submitted a more conservative concept. It uses layers of balloons, one tucked inside the other, and operates only in the upper Venusian atmosphere. An entry vehicle would jettison a folded, 17-foot-diameter balloon, which would inflate under a parachute, detach, and begin floating in Venus' upper atmosphere, protected from the acid clouds by a layer of Teflon film. The balloon would last about a month.

The JPL team finished a prototype of this Venus balloon in February, just in time to make the April deadline for Discovery proposals. If the design is selected this fall, the team will get more money to refine their study. Then, if they get the green light for full funding next year-a long shot, admittedly-their projected launch date to Venus would be the autumn of 2013.

Along with the technical details, a scientific paper by Kerzhanovich's Discovery proposal team includes a stirring vision of what their invention might spawn. "In the not-too-distant future, aerial rovers directly descended from the Venus aerostat could be plying the skies above treacherous landscapes and inhospitable depths of a number of worlds across the Solar System.... Such an aerial vehicle funded on a Discovery-class budget would herald a new era in planetary exploration."

The aerobot researchers are well aware that they've got a lot of hard engineering to do first. And there's always that frustrating Catch-22 of the space business: You can't fly until you're proven, and you can't be proven until you fly. But don't cry for Jack Jones, Jeff Hall, and their crew, whether testing prototype airships in Oregon or over a tabletop in California. They'll tell you themselves: It may not be easy to be a balloonist in a world of rockets and wings, but it sure can be fun.

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