Perfecting the art of balloon science6 min read . Updated: 16 Jun 2010, 09:45 PM IST
Perfecting the art of balloon science
Perfecting the art of balloon science
Hyderabad: To construct a scientific balloon, first build a really, really long table.
The longest table at the National Balloon Facility (NBF) measures 178m; it would take Usain Bolt a quarter of a minute to run from one of its ends to the other. Covered in spotless white plastic, the table is the most arresting occupant of a long, slender room. This is the balloon fabrication unit, which makes NBF the only facility in the world to design, manufacture and launch its own balloons, tugging along a raft of science experiments as payload.
On this table, the seams of a balloon can be sealed as effectively as its fate. Long stretches of polyethylene film, often as thin as 6 micron—six thousandths of a millimetre, so thin that even sweat can ruin it—are heated and joined to each other. Too much heat, and the film will burn; one careless move, and the film will tear. The careful fabrication of a balloon may require 90 days and cost up to Rs50 lakh.
The final, pear-shaped envelope of polyethylene, when filled with hydrogen, can be enormous. “The largest balloons we’ve launched," says S. Sreenivasan, the scientist-in-charge at NBF, “have had volumes of 26 million cubic feet." NBF balloons have sent up experiments from India as well as from the US, Russia, Japan and Italy, all eager to take advantage of the low magnetic interference this close to the equator.
Now, in a literal instance of pushing the envelope, NBF is developing a balloon that can regularly take payloads more than 50km above the earth’s surface—very close to, if not over, the 53km altitude record achieved precisely once by a team in Japan. It’s harder than it sounds. It requires polyethylene so thin as to be virtually non-existent, and it is a significant leap from NBF’s present high of 42-43km. If these balloons succeed, they could replace the traditional atmospheric and weather-sounding rockets used by the Indian Space Research Organisation (Isro).
The history of scientific ballooning is, in a way, as old as modern ballooning itself: In 1783, in one of their very first hot-air balloon flights, the Montgolfier brothers took along a sheep to test the effects of altitude on animal physiology. (The sheep returned unharmed.) But the lifetime of NBF, now beginning its 40th year under the stewardship of the Tata Institute of Fundamental Research (TIFR), has coincided nearly exactly with that of the artificial satellite, a fearsome competitor. The satellite can arm itself with far more sensors, and it can stay off the ground for many months, rather than the balloon’s few hours.
“The scientific balloon is often called the poor man’s satellite," Sreenivasan says, almost as if he were personally hurt by that description. But it isn’t strictly true. Scientific balloons have kept themselves busy by collecting the sort of fine-grained data about the lower atmosphere that satellites, floating far above the world, are hard-pressed to provide.
The 50km balloon, as NBF’s scientists refer to it, is a part of that effort to remain relevant. “We started designing it," says R.K. Manchanda, chairman of the NBF board, “when somebody at Isro told me, a few years ago: ‘Manchanda, why don’t you give us something for 50km?’" A 76m-long table is already being built to shape and seal the special 3.8-micron-thin polyethylene, and Manchanda expects to launch the balloon later this year, during the first of the narrow, two-month launch windows that swing open in November-December and March-April.
How useful a 50km balloon would be beyond Isro is unclear. A few years ago, S. Shivaji, a microbiologist at Hyderabad’s Centre for Cellular and Molecular Biology, worked with scientists from Isro and TIFR to send up an NBF balloon to collect air samples from a height of 41km. From these samples, Shivaji’s team isolated three wholly new species of bacteria. But collecting air even at 41km “was hard enough, because the air is so rare at that altitude," he says. “At 50km, it’ll be even rarer."
Manchanda knows that such atmospheric experiments will not find much use for a 50km balloon, but off the top of his head, he can name at least two other research fields that would. “Usually, when you don’t have a tool, you don’t think of the possibilities," he says. “When the balloon is there, scientists will come up with experiments for it."
NBF has a history of pushing against—and successfully circumventing—the limits of balloon physics. When TIFR scientists such as Homi Bhabha first began sending up balloons from the Osmania University grounds in the late 1950s to study cosmic rays, and even after NBF was established in 1969, a peculiar problem persisted. At minus 65 degrees Celsius, polyethylene becomes brittle and cracks, so balloons were unable to cross the tropopause, a point between the first two atmospheric layers where temperatures plunge as low as minus 90 degrees Celsius.
Paradoxically—and unfortunately for Hyderabad—the problem is most acute above warm latitudes, where the tropopause is at its coldest. A US scientist named Robert Kubara, writing up a report of launches from Hyderabad in 1964, moaned that the city’s latitude “further aggravate(d) the already difficult problem of tropopause penetration".
In the US, the problem was resolved with the development of a special polyethylene known as Stratofilm. In Hyderabad, scientists jury-rigged a solution by coating their polyethylene with carbon black, which drew heat from the sun and kept the film warm. “But these balloons would sometimes absorb too much heat, so they would go up very fast and burst, or the film would melt," Sreenivasan, who joined NBF in 1971, recalls.
Only in the mid-1970s did NBF begin using imported thin-film polyethylene. The last import of film, says B. Suneel Kumar, a scientific officer in the balloon fabrication unit, came into NBF in 1998. “After that, we started importing only the resin," he says, “and we found a factory in Daman to make the film."
As a large, composite photograph hanging behind Sreenivasan’s desk shows, the launch of a scientific balloon can be a spectacular sight. It often takes place early in the morning, so the light of dawn and of strobe lamps filter muddily through the gauze-like balloon as it fills with gas. “The freed balloon rose in slow motion, with a mighty flapping of plastic," science journalist Anil Ananthaswamy wrote in his recent book The Edge of Physics, after observing a launch in Antarctica. “It looked like a giant translucent jellyfish."
NBF conducts six-seven such launches every year, down from 10-12 a decade ago. The demand for scientific balloons has been dipping in favour of satellites, and faster abroad than in India. A 2005 strategic road map by the Scientific Balloon Planning Team of the National Aeronautics and Space Administration (Nasa) admitted that a “more reliable means of support for balloon missions is needed".
Manchanda acknowledges this challenge. In fact, when he took charge of NBF in 2001, he thought “it would die unless we did something drastically different. Balloon work has never been fashionable". His vision of ballooning’s future echoes Nasa’s. He talks of long-duration flights, lasting three-four days, and of super-pressure balloons that stay afloat for weeks.
Sreenivasan thinks of further practical applications: sending up a balloon as a temporary, rapidly functional telecom tower, for instance. Recently, NBF and Tata Teleservices Ltd demonstrated that a scaled-down version of a cellphone tower, flying at a height of 400m, had an operational range of 6,000 sq. km—an experiment that holds glimmers of promise for rural telephony.
But altitude-wise, Manchanda says, ballooning is nearing a ceiling, a limit that even NBF cannot escape. “Significantly above 50km, really, you’re better off making observations with a satellite, and I can’t think of too many experiments that require further height," he says. “It becomes physically very difficult, very expensive, to use balloons, so finally you have to give up. You’re checkmated. This is the endgame."