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.