From Earth's perspective, on June 5 and 6, Venus will pass across the face of the Sun. By observing the tiny fraction of sunlight that passes through Venus's atmosphere using the Hubble Space Telescope, it is hoped that the planet's atmospheric makeup can be determined. Though we already know the nature of Venus's atmosphere, it is hoped the event will help astronomers hone techniques, already in use, that may one day help to identify Earth-like planets in far-away solar systems. The catch? Hubble cannot observe the Sun directly. Instead it will look at the Moon to observe reflected light.

Hubble's sensitive instruments were designed for the purpose of looking deep into space where traces of visible light (and other bands of the electromagnetic spectrum) are faint. This allows it to capture images over tremendous distances - images such as the Hubble Ultra-Deep Field: a composite of images capturing visible light looking through and beyond the Fornax constellation. It's thought that the image captures some 10,000 galaxies and, because of the time it takes for light from such distant objects to reach Earth, looks 13 billion years into the past.

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Such sensitive instruments would be completely frazzled by the brilliant light and intense heat emitted by the Sun, and for this reason Hubble always points away from our stellar neighbor. And because Venus and Mercury are in broadly the same direction as the Sun from Earth's point of view, they cannot be observed directly by Hubble either. A mere 1/100,000th of the sunlight emitted will pass through the atmosphere of Venus and be reflected by the moon.

Hubble will use its Advanced Camera for Surveys, Wide Field Camera 3, and Space Telescope Imaging Spectograph to view a range of radiation from ultraviolet to near-infrared light. To gather information about Venus's atmosphere, Hubble will carry out spectroscopy, breaking sunlight into its constituent colors. Because different chemical molecules in Venus's atmosphere absorb and emit different frequencies of radiation, it should be possible to determine what kinds of molecules are present.

The transit of Venus across the Sun will take a mere seven hours, but Hubble will be blind to proceedings for nearly half of that time. The Hubble Space Telescope is a satellite that takes 96 minutes to orbit the Earth. For approximately 40 of those minutes, Earth itself will block Hubble's view of the Moon.

Because of this narrow window, astronomers have already determined which part of the moon's surface to focus upon. A dramatic image captured on January 11 this year shows the location in question, around impact crater Tycho.

Though Venus transits occur in pairs, eight years apart (as worked out by Jeremiah Horrocks in time for the 1639 transit), this is the second transit of such a pair (the first occurring in 2004). Another won't occur until 2117.

Source: Hubble site

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