What a satellite can do, balloons can do cheaper.
- By T. A. Heppenheimer
- Air & Space magazine, January 2002
(Page 3 of 8)
Not only is a balloon cheap to launch, the culture of balloon science encourages thriftiness in the assembly of payloads as well. One researcher avoided paying $30,000 for a space-related video camera and used an ordinary $200 security camera, relying on bathtub caulking when he needed additional electrical insulation. A gamma-ray telescope built by a team at the California Institute of Technology in Pasadena used home-movie video cameras to store data during the flight. One group of scientists, needing to protect photomultiplier tubes from stray light, fashioned shields from beer cans.
At Raven Industries, a balloon manufacturer in Sulphur Springs, Texas, senior engineer Mike Smith points out that in the world of balloons, "you don't see clean rooms. The instruments are built by guys wearing T-shirts and jeans. Probably every NASA flight has plywood as part of its payload," rather than the titanium honeycomb or similarly exotic materials used in satellites.
According to Mike Zimmerman, Raven's chief of quality control, payloads can also be cheaper because balloons can carry odd shapes that would have to be folded to fit onto a rocket, then unfolded in space. With balloons, he says, "you can have solar panels sticking out. You don't have to withstand G-loads" or strong vibrations of a rocket launch. Designers avoid costly test programs, since they don't have to demonstrate that their instruments can withstand such forces. In fact, balloons have served as test platforms for instruments that were later space-rated and flown on satellites. The Compton Gamma Ray Observatory, on of NASA's premier astronomical satellites, is one that benefited from balloon tests. The satellite helped astrophysicists learn about violent events occurring near quasars, neutron stars, black holes, and supernovae, or exploding stars. Such events produce gamma rays and X-rays, which the orbiting observatory was able to detect. "Every instrument on the Compton was first developed on a balloon," says Jonathan Grindlay.
Yet for all their usefulness, balloons have an ongoing problem: They don't stay up very long. Most flights last between 12 and 24 hours. Those launched from Ft. Sumner are not allowed to cross the Colorado River, the state line of California, because they would pose hazards to air traffic if they descended near Los Angeles. Those launched from the National Scientific Balloon Facility's main base in Palestine, Texas, must come down before they cross the border of Mexico, only a few hundred miles away, because that country will not allow overflights. Ground controllers send a radio command to release the payload and its parachute if winds carry it toward the border. That action tears the balloon, which, venting helium, descends. Even balloons that fly in Australia, crossing the Outback and the Indian Ocean, stay up no more than a few days.
When borders aren't the problem, the duration of a balloon flight is limited by physics.
As a balloon rises, its helium expands. By the time it reaches its targeted altitude—in excess of 22 miles—the gas has expanded to fill the volume of the balloon, typically 40 million cubic feet. (The National Scientific Balloon Facility Web site notes that you could fit two Boeing 747s back to back inside the envelope.) The balloon has enough lift to rise still higher; however, it can expand no more and would burst if it continued to ascend. To prevent that, polyethylene tubes serve as vents, permitting surplus helium to escape. The excess lift vanishes, and the balloon flies near the desired altitude.
The night falls. The balloon cools, contracts in volume, and sinks. To prevent it from descending, ground controllers send a command to drop ballast. When the sunlight of the following morning warms it anew, it rises again—and because it has dropped ballast and therefore lost weight, it vents still more helium. Such cycles can continue only as long as the balloon has ballast. Once the ballast is gone, it descends for good.
If there were a way to interrupt the warming and cooling cycle, a balloon flight could last longer. One alternative is to travel to a place where the sun never sets. For the past 10 years, NASA's balloonists have launched from a base in Antarctica, where during the south polar summer the sun shines continuously. The balloons stay aloft for as long as two weeks, and several years ago, one of these longer flights produced headlines.