The Hubble Space Telescope will be aimed at the moon to detect dips in brightness during the 2012 transit of Venus.  In an interview explaining how the Space Telescope Science Institute (STScI) allocates time on the telescope, Dr. Matt Mountain, STScI Director, describes a clever, high risk project with potential for high return that was selected among the 1,000+ proposals.

Alfred Vidal-Madjar, CNRS, Institut d'Astrophysique de Paris, submitted the proposal entitled Venus observed as an extrasolar planet.

While astronomers can discern the atmosphere of big planets 150 light years away, they seek to detect the atmospheres of smaller earth-size planets as well.  To mimic looking at a small exoplanet, the Hubble Space Telescope will measure small changes in light reflected off the moon as Venus diminishes the sunlight slightly when the inner planet passes between the sun and earth on June 5-6, 2012. 

"We don't know if it will work, but it's worth a shot," Mountain said. "If it does work, we'll get an idea of what earth-size would look like...It will guide us in the future if we ever see dip like it; we're seeing a very small planet...It's quite a risky project, but the payoff would be quite remarkable, for we'd actually be able to measure the atmosphere of Venus using the Hubble Space Telescope."

Alfred Vidal-Madjar, CNRS, Institut d'Astrophysique de Paris, submitted the proposal entitled Venus observed as an extrasolar planet.  His abstract, below, appears at http://archive.stsci.edu/proposal_search.php?id=12537&mission=hst:

Venus observed as an extrasolar planet
HST Proposal 12537

Alfred Vidal-Madjar
CNRS, Institut d'Astrophysique de Paris

Cycle: 19
Category:
Proposal type: GO
Status:

ABSTRACT:
In a relatively near future, numerous transiting extrasolar planets will be discovered {gaseous giant planets, Earth-size planets and temperate Uranus in the form of "Ocean-planets"}. Space telescopes operating in the UV-optical-IR will allow the study of their atmospheres. We have to show if and how these observations will give access to the detection of atmospheric species, particularly when telluric planets will be observed, to demonstrate that life may be possible on one of them. For that purpose, we propose to use the unique event of the century, the Venus transit in 2012 {next Venus transits are in 2117 and 2125!}, to demonstrate the feasibility of these observations and show precisely what a Venus-like planet will look-like. To observe the Venus transit with similar conditions as extrasolar planets {no spatial resolution}, we propose to observe the solar light reflected on the Moon during the Venus transit on June 5-6 2012, lasting about 7h 40mn, i.e. about 4 HST orbits. A total of 5 HST orbits will allow us to obtain high S/N transit spectra and reference spectra to reveal the detectable atmospheric species with current space instrumentations. Similarly, in a companion proposal, we propose to observe the Earth transit on the Moon through the reflected light during a total Moon eclipse to directly compare the observed atmospheric signatures of Earth-like and Venus-like extrasolar planets.

Data for Program ID: 12537 (as of now)

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See also blog entries:

Hubble Practices Moon Shot
The Hubble Space Telescope dedicates several orbits to practicing for the June 5 transit of Venus as HST instruments target the lunar crater Hipparchus-C.

Hubble Telescope Participates
The Hubble Space Telescope will observe the moon during the 2012 transit of Venus to measure dips in its brightness.