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Blimps and balloons are routinely used on Earth for aerial surveillance or to conduct research in hard-to-reach places. Similar unmanned flying vehicles, or aerobots, could open up new vistas in planetary exploration, providing an inexpensive way to observe large swaths of alien terrain without ever touching the ground.
Rovers that can cover a few kilometers have boosted our knowledge of the moon and Mars, taking up-close images of the surface and analyzing rocks and soil. A downside to rovers is not only their slow travel speeds and limited mileage, but their tendency to get stuck in sand traps or other treacherous terrain.
Aerobots would be able to smoothly glide over regions that rovers view with trepidation, covering thousands of kilometers by floating in the atmosphere of planets and moons and providing a closer look at volcanoes and lakes than is possible with orbiting spacecraft.
"A balloon is a platform to carry scientific instruments to places you couldn't get to otherwise, a way of covering a relatively large distance in a very short amount of time," said Jeffery Hall, who is leading the planetary balloon effort at NASA's Jet Propulsion Laboratory in Pasadena, CA.
Aerobots could be as simple or sophisticated as a mission demands, Hall said. On the one end they could simply sail with the winds at a constant altitude, taking pictures and sampling the atmosphere's chemistry. "At the other end you could imagine a blimp-like vehicle with a power plant, propellers and propulsion system with smarts so that it could fly itself in a highly autonomous fashion," he said.
The Soviet Union's 1985 Vega mission to Venus has been the only successful planetary balloon exploration to date. That mission sent two balloons, each 3.5 meters in diameter with a 7-kilogram instrument pod hanging below. Each balloon operated on battery power for nearly two days (46 hours), floating 54 kilometers above the planet's surface.
Hall and his colleagues are developing and testing much more sophisticated balloon technology that could be used to explore Venus, Mars and Saturn's moon Titan. Each of these destinations has its own unique challenge.
Titan's cryogenic environment, with a surface temperature of negative 180 degrees Celsius (minus 292 F), would need a balloon made of a material that does not become brittle when exposed to extreme cold. The surface temperature on Venus is hot enough to melt lead and while the planet's upper atmosphere boasts benign, Earth-like temperatures, it is shrouded with sulfuric acid-laden clouds. Meanwhile, the low atmospheric density on Mars makes flying balloons a challenge, requiring very large, lightweight balloons that can carry small payloads.
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