This mottled landscape showing the impact crater Tycho is among the most violent-looking places on our Moon. Astronomers didn’t aim NASA’s Hubble Space Telescope to study Tycho, however. The image was taken in preparation to observe the transit of Venus across the Sun’s face on June 5-6.
Hubble cannot look at the Sun directly, so astronomers are planning to point the telescope at the Earth’s moon, using it as a mirror to capture reflected sunlight and isolate the small fraction of the light that passes through Venus’s atmosphere. Imprinted on that small amount of light are the fingerprints of the planet’s atmospheric make-up.
These observations will mimic a technique that is already being used to sample the atmospheres of giant planets outside our solar system passing in front of their stars. In the case of the Venus transit observations, astronomers already know the chemical make-up of Venus’s atmosphere, and that it does not show signs of life on the planet.
But the Venus transit will be used to test whether this technique will have a chance of detecting the very faint fingerprints of an Earth-like planet, even one that might be habitable for life, outside our solar system that similarly transits its own star. Venus is an excellent proxy because it is similar in size and mass to our planet.
Hubble will need to be locked onto the same location on the Moon for more than seven hours, the transit’s duration. For roughly 40 minutes of each 96-minute orbit of Hubble around the Earth, the Earth occults Hubble’s view of the Moon. So, during the test observations, the astronomers wanted to make sure they could point Hubble to precisely the same target area.
This is the last time this century sky watchers can view Venus passing in front of the Sun. The next transit won’t happen until 2117. Venus transits occur in pairs, separated by eight years. The last event was witnessed in 2004.
Via NASA’s HubbleSite.