Rosetta: 'Rock and roll' space mission crashes to an end

Rosetta awoken: why is unique space mission so significant?

20 January

EUROPE'S Rosetta spacecraft is being "woken up" today in preparation for an unprecedented mission to land on the surface of a comet. It is a key milestone in the decade-long quest to learn more about the 67P/Churyumov–Gerasimenko comet and the origin of the solar system.

So what exactly is Rosetta?

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The Rosetta spacecraft consists of two main parts: a space probe orbiter and a small three-legged robot lander called Philae. It has been chasing comet 67P/Churyumov–Gerasimenko since it was launched in 2004 but has taken quite a circuitous route. It has made three flybys of Earth and one of Mars, using the planets' gravitational pulls to build up enough speed and get on a trajectory towards the comet. It has also encountered the asteroids Steins in 2008 and Lutetia in 2010 along the way.

Where is Rosetta now?

The reactivation of Rosetta is happening some 800 million kilometres from Earth, out near the orbit of the planet Jupiter. It is believed to be nine million km away from 67P/Churyumov–Gerasimenko.

Why was it 'sleeping'?

The spacecraft, which operates on solar energy, was placed into deep space hibernation on 8 June 2011 to save energy. After crossing the asteroid belt between Mars and Jupiter it cruised too far from the Sun to keep going. During its hibernation, its solar panels have been reoriented to face the Sun. Now, 31 months after it was shut down, Rosetta's orbit has brought it back to within 673 million kilometres of the Sun, and there is finally enough solar energy to power the spacecraft fully again.

What happens now?

Rosetta's on-board alarm clock – described by the European Space Agency as the "most important alarm clock in the solar system" – was due to go off at 10am GMT today. It will take the spacecraft about seven hours to warm up its star-tracking navigation gear, fire up rocket thrusters to slow its spin, turn on its transmitter and beam a message back to Earth. Controllers at the ESA are hoping to receive the message at between 5.30pm and 6.30pm today. Rosetta is then due to "rendezvous" with Comet 67P/Churyumov-Gerasimenko in August. Philae, the small robot piggybacking on the spacecraft, is due to land on the comet in November.

Why is Rosetta so significant?

If the spacecraft achieves its aims, it will become the first space mission to orbit a comet, land on a comet's surface and follow a comet as it moves around the Sun. By understanding the make-up of the comet, scientists hope to learn more about the solar system itself. Jean-Jacques Dordain, ESA director general, tells the BBC: "Rosetta is a unique mission - unique technologically, unique scientifically, and unique philosophically because comets may be at the origin of who we are."