A BALLOON drifts gently on the breeze skirting the edge of a spectacular mountain. A camera snaps pictures of the scene below. It sounds idyllic, but this is no champagne-soaked sightseeing tour over green fields and vineyards. In fact there are no signs of life beneath the balloon at all, just rock. And the temperature is 460 °C.
Space scientists are increasingly looking to balloons as an effective way to explore other planets, even those with extremely hostile environments. Balloons seem curiously low-tech compared with the robotic rovers doing ground-breaking work on Mars, or rocket-driven aircraft. But a number of research teams claim balloons have advantages that wheeled rovers and the proposed aircraft cannot match.
“We really like balloons because the buoyancy force keeps you in the air. You don’t have to keep a propeller going,” says Jeffery Hall, an engineer at NASA’s Jet Propulsion Laboratory. Balloons can also cover far more ground than wheeled rovers, he says. “On Mars we have had considerable success with rovers. But they tend to be limited in range to a few kilometres per year.”
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Next month, a UK team led by David Barnes from the University of Wales, Aberystwyth, will demonstrate a 5-metre-long prototype of a helium-filled airship at the European Space Agency’s test facility in Noordwijk in the Netherlands. They hope within 10 years to see the Aberystwyth Lighter than Air Intelligent Robot travelling thousands of kilometres through the Martian atmosphere and taking high-resolution images of the planet’s surface.
NASA and ESA are considering a number of different balloon missions, from self-propelled helium balloons venturing under the thick cloud cover of Venus, to solar-heated balloons rising and dipping as they are alternately warmed and cooled over the course of the Martian day and night. NASA is also evaluating proposals for an airship to explore Titan, Saturn’s largest moon (żěè¶ĚĘÓƵ, 14 January, p 40).
The idea has been tried before. Back in 1985 the Soviet Union launched two balloons designed to record temperature, pressure and wind data in the Venusian atmosphere. One balloon failed after less than an hour, but the other kept transmitting until its batteries ran down two days later.
Yet despite this moderate success, no one has attempted a balloon mission since. That is odd, considering the advantages. Thanks to their altitude balloons are good at collecting atmospheric data, and they can view large areas of the planet’s surface at one time. They also fly lower than orbiting satellites, and so can take higher-resolution images of the surface – possibly down to 1 centimetre per pixel, compared with 20 centimetres per pixel for satellites. And they can continue to float around the planet for months or even years after being deployed.
The folded and packed balloon would enter the planet’s atmosphere in the same way as a conventional lander, first skimming off the atmosphere to reduce speed, and then descending behind a metal heat shield before releasing a parachute. While still aloft, the balloon would inflate using pressurised helium or hydrogen gas released from a tank. After this, the simplest option is to allow the balloon to drift, taking measurements and photographs as it floats along.
Power steering
But a balloon explorer would be even more useful if it could be steered towards targets of interest. To this end, Global Aerospace of Altadena in California is working on a NASA contract to develop a balloon guidance system.
The unpowered balloon would float about 10 kilometres above the surface, trailing a 9-kilometre tether made of a strong plastic called Zylon. A steering vane would be attached to the tether. With the balloon pushed along by high-altitude winds, the vane would drag behind, generating lift. The vane’s wings could be moved from side to side by solar-powered, radio-controlled actuators to adjust the balloon’s direction of travel. Although the pull would be relatively small, it would be enough to steer the balloon around mountains it might otherwise crash into, and to parts of the planet scientists want to study.
Alexey Pankine, project scientist at Global Aerospace, says a balloon could drop small research stations onto the planet’s surface, or retrieve samples collected by surface rovers at different locations. To do this the samples could be loaded into canisters fitted with a tether, which the balloon would snag as it passed overhead. The balloon could gather samples from all over the planet and take them back to a single return vehicle.
An alternative method of steering a balloon could be with propellers hanging from its gondola. For a mission to Mars these could be simple solar-powered propellers. But on Venus or Titan, where the heavy cloud cover would make solar power impossible, the propellers could be powered by a radioisotope thermal generator of the type now used on interplanetary probes. These generate electricity from the heat released by the decay of radioactive material.
For most missions the material used to build the balloons themselves would have to be both light and tough, such as polythene, Mylar, or composites of these and other plastics. The main threat to the balloon would be during the relatively rough deployment, says Hall. After that, it would be in a fairly benign environment, carried along with the wind and suffering little stress.
However, a mission to Venus would be less straightforward. The Venusian atmosphere contains sulphuric acid, which would gradually eat away at the balloon’s fabric. This means the balloon would have to be coated with a resistant material such as Teflon. To add to the mission designers’ troubles, the temperature close to the surface of Venus is about 460 °C, high enough to melt most plastics.
“Venus has such a dense atmosphere that you can actually fly a metal balloon”
One possible solution would be to use a metal balloon, says Hall. “Venus has such a dense atmosphere you can actually float a metal balloon. You can carry scientific instruments and have a viable science mission.”
Floating steel
A “bellows balloon”, made of stainless steel, would look a little like an accordion on its side, expanding vertically as it filled with hydrogen. The Venusian atmosphere is so dense at the surface – 50 times what it is on Earth – that a 42-kilogram metal bellows filled with 5 cubic metres of hydrogen at close to ambient pressure would have enough buoyancy to lift a 100-kilogram payload.
Although none of the balloon missions being developed for NASA and ESA has yet been given the go-ahead to launch, Hall believes at least some stand a good chance. The case for balloon missions to Venus and Titan is especially compelling, he says, since cloud cover prevents probes taking high-resolution images of their surface from space.
But despite the interest of the space agencies, the balloon mission that appears closest to launch is a private one, sponsored by the Mars Society of Germany. The society is planning to launch a balloon on a 2009 mission to Mars by AMSAT Deutschland, the German amateur satellite organisation, says Hannes Griebel, the engineer designing the mission.
The 15-metre-diameter helium balloon will carry a high-resolution camera and atmospheric sensors in a gondola attached to its base. After entering the planet’s atmosphere, the balloon will float for just an hour, slowly descending to the surface and taking pictures and atmospheric readings on its way. “I’m pretty confident this can be done,” Griebel says.
Blimps on guard
Near-space blimps capable of monitoring the Earth from 65,000 feet are attracting increasing interest from the US military.
Under a $150 million contract with the Missile Defense Agency, Lockheed Martin is developing a high-altitude airship that is expected to be floating in the upper stratosphere by 2009. Serving as a type of low-orbit, low-cost satellite, the craft will be used for communication and surveillance. Twelve of them could provide comprehensive missile detection along US coastal borders, says project manager Ron Browning.
The 130-metre-long airship, the preliminary design of which is nearing completion, will be wider than conventional blimps. This is to add more lift in an atmosphere one-twentieth the density of that at sea level. Four rotors will maintain its position, while heating elements will help stabilise the helium’s density during the 40 °C variation in temperature between night and day.
It will be powered by lithium ion batteries, which will be recharged by solar cells woven into the top of the craft. To withstand the hostile conditions, the airship will be made of a spun liquid-crystal polymer called Vectran, which has a melting point of 330 °C. The craft will be laminated with several layers of polymer to help prevent helium escaping.
A team at the Southwest Research Institute in San Antonio, Texas, is also developing a near-space blimp for the US Missile Defense Agency. The HiSentinel stratospheric airship, which has ascended to 74,000 feet in demonstrations, is powered by solar cells. To protect the vulnerable solar arrays from ozone, extreme temperatures and ultraviolet radiation, they are mounted inside the blimp’s translucent hull. A swivel mount allows the cells to position themselves to receive the maximum amount of sunlight at all times.
Stu Hutson
