Watching the transit of Venus? from Saturn

Earth may have a front-row seat for this week's Venus transit, but it's the last such show for more than a century. Way back in the outer solar system, however, Venus transits are more frequent, and this year astronomers will try to glimpse the spectacle from Saturn using the Cassini spacecraft.

On 21 December, Saturn, Venus and the sun will be lined up and the Cassini spacecraft will attempt to catch the event.

It will be a much bigger challenge than spotting the transit from Earth. At Cassini's distance, the sun looks like a bright speck of light, so the craft will not be able to resolve the silhouette of Venus's disc. Instead it will detect a very slight dimming ? less than 0.01 per cent ? of the sun's light when Venus passes in front of it.

Cassini will attempt to measure the spectral signature of gases in Venus's atmosphere, something that has only been done for a handful of transiting exoplanets. Astronomers can then compare the data collected by Cassini with the known composition of Venus's atmosphere, which has been measured with great precision by orbiters and landers. This comparison can then be used to interpret studies of the atmospheres of transiting extrasolar planets.

Test case

"This is mostly a test of observing extrasolar planet transits, using a known planet ? Venus ? for the test case," says Phillip Nicholson of Cornell University in Ithaca, New York.

Such extrasolar measurements are exceedingly difficult because the background star's light tends to swamp the feeble signal of light filtering through the planet's atmosphere. Still, the pay-off for probing exo-atmospheres stands to be huge. "You want to know if a planet can support life," says Wes Traub, head of NASA's Exoplanet Exploration Program.

Cassini does not provide the only hope of observing a Venus transit from the outer solar system. Astronomers have applied to use the Hubble Space Telescope to observe a Venus transit in sunlight reflected off Jupiter on 20 September. A decision on whether it will go ahead is expected to be announced later this month.

If it is approved, the team hopes to catch an Earth transit in sunlight reflected from Jupiter in 2014. That way, the spectral signatures of two dramatically different worlds ? one hellish and one brimming with life ? can be compared.

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