The Transit of Venus – across the Moon!
NASA/ESA came up with a unique way to observe the recent transit of Venus using the Hubble space telescope. Hubble doesn’t have a solar filter as it was never designed to be a solar telescope so can’t be pointed at the Sun. However, as the Moon shines by reflecting sunlight from its surface Hubble was targeted on the Moon instead to observe this “second hand sunlight” including the tiny fraction of that light that passed through Venus’s atmosphere as it transited the Sun.
The point of the exercise was not to find out more about Venus’ atmosphere as that has already been extensively studied. Rather it was to use a local transit of an Earth-sized rocky planet as a convenient test bed. Venus and Earth have similar masses. The experiment will show whether the method used to analyse the bloated atmospheres of transiting giant exo-planets and super-Earths will be successful at detecting the atmosphere of a smaller Earth-sized planet. And as a sanity check whatever signature Hubble picked up can be checked against what we already know about Venus to see if it all makes sense. Analysing the atmospheres of Earth-sized exo-planets is tricky. Even spotting Venus’ atmosphere, which involves looking at just 1/100,000th of the sunlight that bounces back off the Moon, is tricky.
On June 5th-6th Hubble observed the Moon for 7 hours both before and after the transit to establish a good baseline and also for the full transit to maximise the chances of filtering out the signature of Venus’ atmosphere. The only problem was that 40 minutes out of each 96-minute Hubble orbit was lost as Earth obscured Hubble’s view of the Moon. As part of the preparations for the observation and to make sure that after each 40-minute blackout Hubble was still pointing at the correct location this test image was taken of Tycho.
Hubble used its Advanced Camera for Surveys, Wide Field Camera 3 and Space Telescope Imaging Spectrograph to view the transit in several wavelengths from ultraviolet to near-infrared and took both spectra and photographs.
Astrophotographer Thierry Legault took this image of the transit of Venus. Look closely – he also captured a transit of the Hubble telescope too!