Interesting plans are afoot, to use the Moon’s surface as a mirror for observation:
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 makeup.
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 makeup 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.
The astronomers will use an arsenal of Hubble instruments, the Advanced Camera for Surveys, Wide Field Camera 3, and Space Telescope Imaging Spectrograph, to view the transit in a range of wavelengths, from ultraviolet to near-infrared light. During the transit, Hubble will snap images and perform spectroscopy, dividing the sunlight into its constituent colors, which could yield information about the makeup of Venus’s atmosphere.
Pretty fascinating stuff!