NASA uses moon as mirror to watch Venus transit sun

Just like human eyes, the Hubble Space Telescope can’t observe the sun directly – so NASA scientists plan to study Venus’s solar transit in June by using the moon as a mirror.

As Venus passes across the face of the sun on June 5 and 6, the Hubble team will point the telescope at Earth’s moon. By using it as a mirror, they can 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.

It’s the same way astronomers sample the atmospheres of giant planets outside our solar system as they pass in front of their stars.

While we already know the chemical makeup of Venus’s atmosphere, and that it shows no signs of life on the planet, the observations will help test whether this technique has a chance of detecting the very faint fingerprints of an Earth-like planet outside our solar system.

The astronomers will use 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 capture images and carry out spectroscopy to try and detect information about the makeup of Venus’s atmosphere.

Hubble will observe the Moon for seven hours, before, during, and after the transit so the astronomers can compare the data. The team needs this long observation to detect the extremely faint spectral signatures – only 1/100,000th of the sunlight will filter through Venus’s atmosphere and be reflected by the moon.

It’s the last chance this century to see Venus passing in front of the sun, with the next transit due in 2117.