Gaia spies, with its gigapixel eye, something beginning with “P”. The European Space Agency launched its Gaia space telescope in December 2013 to map the locations of a billion stars in the Milky Way, but it can also spot objects a little closer to home. Between September 2014 and December 2015 it made nine observations of Pluto and its moon Charon, which have served as a kind of eye test for the probe.

When you take an eye examination, your optometrist asks you to distinguish between increasingly small letters on a chart. The test is designed to determine the resolution of your vision. Having 20/20 vision means you can distinguish two distant objects separated by one sixtieth of a degree, otherwise known as an arcminute. Any closer, and the objects blur together, appearing as one.

Gaia can do the same trick with Pluto and Charon, though it is a little more difficult because both bodies are moving with respect to each other and the spacecraft, slightly changing their angular separation. But Gaia has the advantage of being in orbit. Observations from Earth’s surface can be thrown off by the shimmer of the atmosphere, which can slightly bend light and make it harder to distinguish distant objects.


A good warm-up

In seven of the observations, Gaia was able to make out both the dwarf planet and its moon, but only saw a single point of light in the other two.

From these measurements, Gaia team member François Mignard of the Côte d’Azur Observatory in Nice, France, was able to calculate that Gaia’s resolving power is between 0.23 and 0.36 arcseconds – around 200 times better than 20/20 vision.

Mignard says this value should improve once the Gaia team starts detailed analysis of data from binary stars, which also orbit each other. But it’s a good warm-up challenge for this particular test.

“Pluto and Charon have a symbolic value of a source not easy to resolve from the Earth,” he says. “These observations with Gaia epitomise the gain in spatial resolution we get when go outside the atmosphere.”

Gaia will also help pinpoint the orbit of Pluto, eventually bringing down errors in its measurement from 2000 kilometres to around 50 kilometres. Improvements will partly come from tracking Pluto itself, but also from clearing up the positions of the stars behind it, which should benefit all astronomical measurements, says Mignard. “This will be a lasting legacy of Gaia.”

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