CRUSHING atmospheric pressures, fierce winds, baking temperatures and acidic clouds have quickly destroyed every probe or lander ever sent to Venus. So the prospect of emulating the spectacular success of NASA鈥檚 Mars rovers Spirit and Opportunity on Venus might seem bleak. But there is hope. Space scientists in the US believe a solar-powered aircraft could explore the atmosphere of the second rock from the sun, and carry a flying 鈥渂rain鈥� to control a toughened rover on the ground.
Writing in the latest edition of the journal Acta Astronautica (vol 56, p 750), a team led by Geoffrey Landis of NASA鈥檚 Glenn Research Center in Ohio says that an autonomous solar-powered aircraft could cruise between different altitudes and locations in Venus鈥檚 wild atmosphere, making measurements and radar-imaging the surface at 10 times the resolution possible with an orbiting craft. They say this would provide far better data than the Soviet and US probes of the 1970s and 1980s, which were only able to make atmospheric measurements for a short time as they descended to their doom in the planet鈥檚 violent, corrosive winds.
But the planet鈥檚 dense atmosphere is ideal for a flying craft. A wing鈥檚 lift depends directly on the density of the atmosphere and the atmospheric pressure on Venus is about 90 times that of Earth. After being released by an orbiter, the craft鈥檚 origami-like wings would unfurl from an 鈥渁eroshell鈥� (see Graphic). Solar panels on the craft鈥檚 surface could absorb large amounts of the intense solar energy, powering motors to allow the craft to fly continuously. And the planet鈥檚 slow rotation, with one day and night on Venus taking 117 Earth days, means a solar flyer could stay on the daylight side indefinitely.
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NASA is particularly interested in studying a fast-moving cloud band that stretches around the planet at an altitude of 50 to 75 kilometres. This band is an enigma. Amazingly, it spins 60 times faster than Venus itself, taking only four Earth days to circumnavigate the planet. 鈥淲e really want to know how solar energy moves that upper atmosphere so very fast,鈥� Landis says. By cruising between the cloud base and cloud tops, where the temperature is a moderate 100 掳C, a solar flyer could help scientists find out what makes that cloud band tick.
鈥淎 solar flyer could find out how a band of cloud can spin round Venus 60 times faster than the planet itself鈥�
But down on the ground, where temperatures on the planet鈥檚 day-side reach around 450 掳C, a rover undertaking geological and imaging work would not last long. 鈥淲e think we can get electrical things like motors and transistors to work at those temperatures, but not the microelectronics for computers,鈥� says Landis, who is also part of the science team running the Spirit and Opportunity missions.
His answer is to land a relatively dumb rover on the surface 鈥� which would be heat-proof, acid-proof and pressure-proof. The microchips that control the rover鈥檚 motion, communication and imaging would be housed on the solar-powered flyer 50 kilometres above. 鈥淲ith no vulnerable on-board computer, we might then be able to duplicate the Spirit and Opportunity missions,鈥� Landis says. The down side would be the delay while the flying computer relays data to or from the dumb rover via a radio link.
Funding for such ambitious schemes is uncertain, with NASA having only recently acquired new administrator Michael Griffin. However, others welcome the idea of using an aircraft for atmospheric analysis on Venus. 鈥淎n in situ mission in the clouds of Venus would be very welcome,鈥� says Colin Wilson of the University of Oxford, an expert in atmospheric dynamics with the European Space Agency鈥檚 Venus Express mission, which launches in October.
Venus Express is a duplicate of ESA鈥檚 Mars Express orbiter, which has sent back stunning images of the Martian surface. The new orbiter will use infrared instruments to study the deep atmosphere of Venus. 鈥淏ut we won鈥檛 know exactly what鈥檚 going on in the clouds without in situ measurements,鈥� says Wilson.
Journal reference: Acta Astronautica (vol 56, p 750)
