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The spacecraft has been in hibernation for more than two years now to conserve energy. ESA has been using the time to solve two glitches that could jeopardize the mission: a problem with two of the four ’reaction wheels’ used to turn the spacecraft, and a small leak of helium that could affect the thrusters which are vital for its final manoeuvrs.

“This is making our life more difficult,” Ferri acknowledges. But engineers have found workarounds.

The wakeup call for Rosetta is due to happen at 5 a.m. EST on Jan. 20. If all goes according to plan, the probe’s instruments will slowly switch themselves back on, though scientists will have a tense few hours of waiting before they can be sure the spacecraft is in full working order.

The following months will involve a gradual approach of 67P, as Rosetta gently slows down and eventually draws up alongside the comet in what Ferri likened to two planes flying in formation. This will give the mission team time to take plenty of pictures of the comet, which is about four kilometres in diameter, and identify a good landing site.

At the moment scientists are divided over whether to pick a site close to the comet’s three active regions — where matter appears to be released into space — or to stay away from them to prevent dust from harming the lander’s sensitive instruments.

The lander — called Philae — will drop onto the surface of 67P and latch on using a harpoon, to prevent it from drifting off into space due to the comet’s weak gravity. Using drills, it will dig up samples and analyze them using on-board instruments.

Researchers hope to gain fascinating insights from the results, because comets have remained largely unchanged since the beginning of the universe.

“This time capsule’s been locked away for 4.6 billion years,” said ESA’s director of science Mark McCaughrean.

One key question scientists hope the mission can help answer is whether comets are responsible for the water on Earth, he said.