Mars, Venus, Titan - wherever there's air, we can explore by balloon.
- By Joe Pappalardo
- Air & Space magazine, July 2006
(Page 2 of 5)
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.