Astronomers are planning to use the Moon as a mirror to capture reflected sunlight and isolate the small fraction of the light that passes through Venus's atmosphere. Imprinted on that light are the fingerprints of the planet's atmospheric makeup. Illustration: NASA, ESA, and A. Feild (STScI)

This June, many people on Earth will bear witness to the spectacular event that is the transit of Venus across the face of the Sun. The Hubble Space Telescope will be one of the numerous telescopes used to monitor the event - but it will do so using the Moon as a mirror.

Telescopes on Earth can be used to view the transit of Venus directly, provided that a proper solar filter is used. However, Hubble was not designed to look straight at the Sun, and lacks the advantage of a solar filter. This is why astronomers are planning to look at the Moon instead.

The Moon is only visible to us because it reflects light from the Sun. On 5/6 June, some of this sunlight will be passing through the atmosphere of Venus before bouncing off the lunar landscape. Details about the atmospheric composition of Venus will be nestled within this reflected light, allowing scientists a unique way of observing Venus' atmosphere.

The opaque atmosphere of Venus has already been studied intensely using instruments more suited to the job, such as ESA's Venus Express spacecraft. The purpose of the Hubble observations is not to gather information about Venus, but to see how precisely this indirect method can pick out details of the atmosphere.

Many exoplanets have been seen transiting other stars in the Galaxy, and astronomers are eager to know if a small planet's atmospheric signature could be liberated from the stellar light. This has already been performed for giant exoplanets that have bloated atmospheres, but as Venus is similar in mass and size to Earth it will be interesting to see what Hubble's limitations are for terrestrial planets.

Venus' reflection on the Moon will be miniscule, comprising only 1/100,000th of the overall light, meaning that a long observation is needed. Hubble will observe the Moon before, during, and after the transit for a total of seven hours. Observing the Moon before and after the transit is necessary to get a baseline for comparison with the transit data.

Although Hubble will be pointed towards the Moon continuously for the seven hours, 40 minutes out of each 96 minute Hubble orbit will be lost as the Earth obscures the Moon from the telescope's view. It is thus extremely important that after each 40 minute period, Hubble is still pointing at the correct location. The rarity of the Venus transit meant that astronomers needed to perform test observations of the Moon to ensure that this observation method is feasible.

Hubble’s test observations of the Moon included this image of the Tycho crater. Image: NASA/ESA/D. Ehrenreich.

On 11 January 2012, Hubble captured an image of the 80 kilometre wide Tycho crater on the Moon and the entire image depicts an area 700 kilometres across. Rays are shown emanating from the crater that were caused by ejected material after an asteroid smashed into the Moon 100 million years ago. The test image was taken using the Advanced Camera for Surveys, but during the Venus transit the Wide Field Camera 3, and Space Telescope Imaging Spectrograph will also be utilised.

The transit of Venus will occur on 5 or 6 June, depending on location, and the next transit won't occur until 2117. You can read more about the transit of Venus in the June issue of Astronomy Now magazine.