The ULDB programs started in 1997, but materials available at the time were either too heavy or lacked the necessary durability. Then in 1999 and new synthetic fiber became available: Zylon.
For decades the ultimate in high-strength materials had been Kevlar, which is used in bulletproof vests. But for large ULDBs, Smith says, "Kevlar is not strong enough. Zylon has about four times the strength. The only thing that's stronger is carbon fiber." A short length of tendon looks like braided brown cord and can hold 3,200 pounds.
Even with this breakthrough, however, ULDBs have had their ups and downs. A June 2000 test flight of a small balloon went well. It stayed aloft for 30 hours, floating steadily at 93,000 feet, even at night. NASA balloon chief Henry Cathey was impressed by its performance. It even stayed up "when flying over a very cold thunderstorm at night, which tends to bring a balloon down in altitude," Cathey says. But during two tests in 2001 of full-size ULDBs, with inflated volumes of 18.4 million cubic feet, the envelopes sprang leaks.
NASA officials plan to continue ULDB flight tests in 2002. Although the balloons aren't ready to carry payloads, scientists are already lining up for places on the launch manifest. Cosmic ray detectors have a high priority on the flight list, and a larger version of Boomerang may also fly on ULDB.
"People are just waiting for verification that the ULDB will perform and will get those hundred-day flights," says Steve Smith. But even when the ULDB has proved that capability, Smith concedes another problem may interfere with the hundred-day span—a problem springing not from technology but from politics. ULDBs are to fly from Australia and to circle the world repeatedly, but winds will blow them slowly northward—towards countries that do not permit overflights. Diplomats must address these issues, if ULDBs are to remain aloft.
Will these long-duration balloons replace satellites? Astronomer Jonathan Grindlay says that "a 30-hour flight" using conventional balloon "is not competitive" with a spacecraft. However, "a hundred-day flight is very competitive." Indeed, plans already are afoot to us a ULDB itself as a satellite, in order to fly in the atmosphere of Mars. The idea is to deploy the Mars ULDB from a landing craft during the latter's decent. But let's say that on some future mission a need arises to launch a balloon from the Martian surface. There's a balloon facility we know of with an experienced launch team of very seasoned travelers...
THE FIRST STEP IN LAUNCHING a science balloon is to find a remote area with plenty of room in case the launch vehicle springs a leak. The main balloon base of the National Scientific Balloon Facility is near Palestine, Texas, in pine and farm country southeast of Dallas. The closest neighbors are prisons; a road sign warns motorists that hitchhikers may be escaping convicts.