When Joseph Montgolfier made a taffeta envelope rise to the ceiling of his Lyon apartment he could never have dreamt where his discovery would ultimately lead. Because now, scientists at NASA’s Goddard Space Flight Centre are working on balloons which will fly in the earth’s upper atmosphere and maybe eventually explore mars.
In December 2001 a huge, pumpkin-shaped balloon will take off from somewhere in either Australia or New Zealand and float to the outer edges of the atmosphere where it will stay for about a year and relay data via global satellites to scientists back on Earth. The main payload of this first mission will be the University of Washington’s Trans-Iron Galactic Element Recorder (TIGER), which will measure the amounts of elements in galactic cosmic rays. The designers therefore had to build a balloon to support a payload of 2200lb and deliver 800W of continuous power to the instrument.
Because of their relatively low cost, balloons have been used for research for quite some time, but their usefulness has always been tempered by how long they can stay up for.
The balloon works according to a `super-pressure’ principle. Unlike the majority of scientific balloons, super-pressure balloons are inflated and then sealed, and because there’s no escape for the gas, the sun’s heat causes the internal pressure to rise until it exceeds the outside pressure. At night, when the gas cools, the pressure drops, but if enough gas has been put into the balloon, the differential cannot drop below zero. Thus, the balloon can remain at a stable altitude without having to drop ballast.
By contrast, the length of time traditional ‘zero pressure’ balloons can stay up is largely limited by the amount of ballast they can carry. These balloons maintain a zero pressure differential with the atmosphere because they are vented and gas can escape. However, in darkness the gas cools, the balloon contracts and begins to descend. To maintain altitude, ballast is dropped until it runs out, and then the balloon descends.
Thus, for long flights the super-pressure idea is the most effective. However, to withstand the rigours of long operations and the harsh conditions in the upper reaches of the atmosphere, the researchers had to come up with a reliable material from which to build the balloon.
The solution is a composite fabric consisting of three bonded layers: a strong polyester fabric, a polyester film to prevent helium molecules from leaking out, and a polyethylene film to further contain the gas and provide added strength. This material has a density of 55 grams per square meter and a yield strength of 2,600 Newtons per meter.
The designers have also plumped for an overall change of shape, opting for a pumpkin-shaped design, where the tendons that bind sections of the material together carry much of the load. This reduces the material strength requirements to only 600 Newtons per meter, well below the yield strength of the composite.
NASA’s scientists believe that the lessons learned from the project might one day be used to help explore other worlds. Indeed, many of the planets in our solar system have an atmosphere that could support balloon flight.
It’s thought that Mars may provide the first opportunity, where the atmosphere at sea level is about the same as that at Earth’s stratosphere. However teams from NASA and the European Space Agency are also looking into a mission to bring a sample from Venus’s surface back to Earth using balloons to lift rocks and soil to an altitude where the planet’s atmosphere is thin enough for them then to launch rockets toward other spacecraft that would retrieve the contents and return to Earth.
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