High flyer

 作者:和邬薪     |      日期:2019-03-07 10:02:05
By Rachel Nowak, Melbourne An experimental balloon designed to float up to 40 kilometres above the Earth for 100 days is ready for launch at Alice Springs in Australia. If the new design works, the Ultra-Long Duration Balloon could do the work of some research satellites at a fraction of the cost. NASA routinely uses high altitude balloons to collect data from outer space. But these balloons are not sealed and rise during the heat of the day and fall at night. Loss of gas means they also drift downwards, limiting the observation time to two weeks or less. Astronomers need much longer observation times and a constant altitude to pick up faint signals or observe rare events. “The beauty of this new technology is that if it really works it will offer the ability to do satellite-like experiments on balloons at a fraction of the cost,” says Louis Barbier of NASA Goddard Space Flight Center near Washington DC. If the launch is a success, Barbier will take ultraviolet light recordings during the flight. Paul Hink, at Washington University in St Louis, hopes a ULDB flight in 2002 will fly his team’s cosmic ray detector for over three months. “This represents an order of magnitude increase in duration,” he says. The detector will collect ultra-heavy cosmic rays, including some types never detected before. But the equipment weighs 450 kilograms, making the cost of a space launch “prohibitive”, says Hink. He estimates the cost of a ULBD flight for a particular payload could be as little as one-tenth of the bill of a space deployment. The trick to keeping the balloon at constant altitude is to keep its volume constant. ULDB Vehicle Manager Henry Cathey of NASA’s Wallops Flight Facility in Virginia and his team believe that they can achieve this by designing the ULDB so that it will not leak even though the balloon is super-pressurised. The key, says Cathey, is the balloon’s unique pumpkin shape, and its fabric. This is a specially-developed polyethylene film which is extremely strong but as fine as cling-film. However, even this material would not be strong enough if the balloon was designed along conventional lines. So the designers added external tendons that extend vertically, dividing the skin into segments like a pumpkin. The tendons increase the curvature of the balloon’s skin and mean the material can withstand greater pressures. The tendons are made of Zylon, a material used in bullet-proof vests. “People think that Kevlar or Spectra fibres are strong, but this material is stronger, much stronger than high-tensile steel,” says Cathey. During the test flight, the ULDB will stay up for two weeks at an altitude of 34 kilometres. During that time the ULDB team is hoping the balloon will circumnavigate the world. Alice Springs was chosen as the launch site in part because the prevailing winds in the upper stratosphere will carry the balloon over land for only one quarter of its journey. This reduces the risk to people if the 2700 kilogram craft were to crash. Also, surface conditions are usually calm. This is important because the balloon and payload could be damaged if the wind speed is above 10 knots. Unusually high rainfalls flooded the launch site earlier this month and delayed the launch. “A balloon launch is very quiet, almost in slow motion, but very large forces are involved,” notes Cathey. More at: