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."