Astronomers set sights on Mercury and Venus

Outline proposals for two audacious interplanetary missions to recover rock and soil samples from Mercury and Venus have been drawn up for the European Space Agency.

The studies, which were unveiled at the American Astronomical Society's planetary sciences meeting in Padua, Italy on Monday, will be put before the scientific community for further examination before any decision on whether to incorporate them into ESA's long-term planning is made. Both will rely on solving complex technological problems, though scientists are confident that solutions are within Europe's technical capabilities.

Fred Taylor, head of atmospheric, oceanic and planetary physics at the University of Oxford and a member of the study team, said: "The list of new technologies needed for both the Mercury and Venus missions is not all that daunting - most already exist and the rest should be straightforward to develop."

The expeditions aim to bring back samples from both planets for analysis in terrestrial laboratories. This will not only inform scientists about the nature of Mercury and Venus but also reveal new clues to the formation of the solar system.

If they gain the scientific and political support needed, the missions could be launched in 15 years. They would most probably follow other expeditions to return samples from Mars, comets and asteroids, which are all planned for the next decade.

Information on the two planets is sketchy, with only a handful of probes successfully landing on Venus and just one fly-by mission to Mercury, by Mariner 10 in the mid-1970s.

ESA's Mercury mission will be by far the most significant exploration of the solar system's innermost planet. A five-tonne craft, which includes a tonne of fuel, would take six years to make the round trip, returning about 10kg of material to Earth.

Mercury's proximity to the Sun heats the daytime surface to 700°C, while on the night-side the temperature falls to -150°C.

Fortunately, the planet's days are very long - almost 59 Earth days. So picking a site close to the planet's north pole could provide a reasonably cool spot with enough sunlight to provide solar power.

A lander would spend a week picking appropriate samples and then blast off to rendezvous with the orbiter, transfer material to a return vehicle and fly back to Earth.

The situation on Venus is far worse than it is on Mercury. Its atmosphere is heated to 400°C, and is crushingly dense and filled with clouds of concentrated sulphuric acid. And, unlike Mercury, this extreme environment cannot be avoided by a trip to the poles.

At the climax of a six-year mission, the Venus lander would have just one hour to drill into the surface and extract samples before the combination of heat, pressure and corrosive chemistry destroys the probe.

The scientists propose protecting the drilling rig with state-of-the-art insulating materials that possess a very high thermal capacity, slowing the rate at which the vulnerable electronics inside warm up.

Getting off Venus is another problem. It would be impossible to use conventional rockets since the atmosphere is too dense. But a helium balloon attached to the sample carrier could be inflated to float the package above the atmosphere, where a rocket would then fire to carry it back to the orbiter.

Like the Mercury mission, the material would then be transferred to a return vehicle for the trip back to Earth.