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Rosetta's comet

ESA's Rosetta spacecraft will wake up today, 20 January, after 31 months in deep space hibernation, to finally close in on comet 67P/Churyumov-Gerasimenko (67P/CG).

This image shows the most recent observations of the 4-kilometre diameter comet, taken on 5 October 2013 by ESO's Very Large Telescope (VLT) when the comet was around 500 million kilometres away — before it passed behind the Sun and out of view from Earth's perspective.

For this image, a long series of observations was processed to reveal both the comet without the background stars (on the left panel), and the star field with the track of the comet marked (on the right). Viewed against a crowded star field towards the centre of the Milky Way, the comet was still so far from the Sun that the icy nucleus was not releasing any gas or dust, and appears as a simple spot. As it approaches the Sun, its surface will heat up and its ices will sublimate, dragging dust out to form a tail.

The observation marks the start of a close collaboration between ESA and ESO to monitor the comet from the ground during Rosetta's encounter with 67P/CG later this year. Rosetta was launched in 2004 and aims to explore the surface of the comet, deploying a lander down onto 67P/CG to see what its surface is like [1].

The comet is on a 6.5-year orbit around the Sun and is currently out towards the orbit of Jupiter. It will be closest to the Sun — roughly between the orbits of Earth and Mars — in August 2015. This image suggests that the comet is not yet active, so scientists will be keen to check in on the comet again in February, when it is next observable by the VLT, and much closer to the Sun.

In the meantime, the observations carried out in October have been used to confirm the comet's orbit ahead of the major rendezvous manoeuvre planned for Rosetta in May, to line it up for orbiting 67P/CG in August. Further calculations will be made once Rosetta sights the comet in its own imaging system.

Notes

[1] Since its launch, Rosetta has travelled around the Sun five times, picking up speed and aligning itself with its final destination. For the coldest leg of the mission, as Rosetta ventured out beyond the orbit of Jupiter, the spacecraft was put into deep-space hibernation. 67P/CG is on a reasonably stable and well-known orbit, meaning that calculations for Rosetta's trajectory could be made far in advance of the spacecraft's launch, and it is far enough away from the Sun to make it a safe target.