Esa's Comet Hunting Rosetta Probe Wakes Up

Rosetta deploys Philae for the first ever landing on a comet in this artist's impression

In space, no one can hear your alarm clock.

At exactly 10am GMT on Monday morning, 807 million kilometres from Earth, an alarm clock went off. For the first time in two and a half years, a spacecraft stirred into life. Heaters switched on, warming up star trackers so that the spacecraft could find out where it was. Thrusters fired, stopping a slow spin that had kept its solar panels facing the Sun during it's long sleep. Six hours after the first alarm, enough systems were active for Rosetta to point at it's distant home and tell its creators that it was awake.

At 6.18 that evening, 48 minutes into the one hour window that Rosetta had to contact Earth, the message was received. A ten year voyage is about to come to a dramatic conclusion.

Launched in 2004, Rosetta's mission looks fairly straightforwards on paper. Fly out to Comet Churyumov-Gerasimenko, a four kilometre wide ball of ice and dust, go into orbit around it, and deploy a small lander called Philae to touchdown on the surface. Sounds simple enough.

(Comet Churyumov-Gerasimenko also rejoices in the the much more pronounceable but less impressive name of 67P)

This mission would however require one on the most complex journeys ever taken by a spacecraft, a six billion kilometre quest around the inner solar system in order to catch up with Churyumov-Gerasimenko.

Comets travel around the Sun in wildly different orbits to planets like the one we live on. Churyumov-Gerasimenko orbits the Sun once every six and a half years, sweeping out a huge ellipse between the mostly circular orbits of Earth and Jupiter. At closest approach it is three times closer to the Sun than when it reaches the edge of it's orbit. 

This means that, at any given point in its orbit, Churyumov-Gerasimenko is travelling at a very different speed to the planets around it, as well as any spacecraft that launch off those planets. Previous missions to comets, such as the rather spectacular Deep Impact, haven't had to worry about that, simply flying past the comets at tremendous speed in one brief encounter, desperately snapping pictures as they went.

However, for Rosetta to go into orbit around Churyumov-Gerasimenko it will have to be able to match speeds with the comet, slowing down enough to be captured by the comet's weak gravity. No rocket on Earth was (or is) capable of providing the change in velocity, or delta-V, needed to accomplish this feat. 

Instead Rosetta has spent the last ten years looping backwards and forwards between the inner solar system. Flying past first Earth, then Mars, then twice back around Earth, Rosetta has used the planets' gravity to sling it out into the orbit it needs to catch Churyumov-Gerasimenko at just the right moment. 

As Rosetta swung by Earth for the last time in 2009, it took this rather spectacular picture.

Speeding away from the inner solar system, flying past and imaging two asteroids on the way, Rosetta's orbit carried it out beyond the orbit of Jupiter. 800 million kilometres from the Sun, the spacecraft's two 14 meter long solar panels could no longer provide enough power to keep Rosetta alive. Shutting down everything except for the main computer and a few heaters, Rosetta went to sleep for two and a half years.  

Rosetta's awakening on Monday was both a technological and media triumph.Every major new website was running with the story. It made the front page of the Guardian and the Independent, #WakeUpRosetta trended on Twitter and Newsnight gave Jeremy Paxman an excuse to show off his scientific ignorance.

It's easy to see why. Esa could have talked about a preprogrammed reactivation sequence and establishing contact. Instead they described it as an alarm clock going off, as Rosetta waking up and calling home.

By talking about it in these very human terms, people have connected with the space mission far more than if it had just been seen as a technological marvel. Rosetta waking up has reminded people that our spacecraft aren't just computers and wires. They're us, an extension of our desire to explore, to investigate our home around us. Robots that we cast out into space to go where we can't go (yet).

Rosetta may be awake, but its real mission hasn't even started. The next few months will be spent testing the eleven scientific instruments on board, ready for when it finally catches up with Comet Churyumov-Gerasimenko in May.

Once there Rosetta will study the comet as it plunges towards the sun, watching it change from a dormant ball of gas and dust to one of the most active objects in the Solar System. By August it will have closed into within twenty five kilometres: close enough to enter orbit. Throughout the approach it will have to dodge eruptions of gas and rock as the comet heats up and begins to develop a tail. Any impact to Rosetta's delicate instruments or solar panels could prove fatal.  

Rosetta will map the comet in detail, providing a new understanding of the processes that take place on these tiny objects. It will sample the gas and dust streaming off the comet, studying it's chemistry and behaviour.

Churyumov-Gerasimenko represents a unique laboratory, a sample of the primordial building blocks of the planets untouched since the dawn of the solar system. By understanding this tiny world we will be able to shed light on how our world and everything around it came into being.

Perhaps the most interesting results will come when Rosetta samples the water that will start to pour off the comet, eventually creating a massive tail behind it. Our current ideas about how Earth was formed suggest that almost all of the water that life relies on was delivered by impacting comets (A theory backed up by some work in the area I'm studying for my PhD). Rosetta's discoveries at Churyumov-Gerasimenko could help see if that really was the case or not.

Rosetta's images of the comets will provide more than just pretty pictures. They will also be scouting the comet out for a suitable landing site. Sometime in November a small spacecraft called Philae will detach from Rosetta to attempt the most difficult part of the mission: The first spacecraft to land on a comet.

If all goes to plan, Philae will drift down from a height of around one kilometre, using a harpoon to drag itself onto the comet's surface. Churyumov-Gerasimenko's gravity is too small to hold it in place, so once down Philae will drill into the surface, clinging onto the side of the increasing volatile comet. There it will deploy nine experiments of it's own, capturing hopefully superb close up images and taking samples from under the surface. Philae is expected to last around a week, but could return data for many months.

If everything goes to plan, by the time the mission finishes in 2015 Rosetta and Philae will have revolutionised our knowledge of comets and answered some of the biggest gaps in our knowledge of how our solar system, our planet and everything alive on it came to exist. Not only that, but the images of a comet exploding into life as it reaches the heat of the inner solar system should be spectacular.

Comet ISON grows a spectacular tail. Soon Rosetta will show all of us how this happens from the inside.

Rosetta is now awake, hurtling towards it's distant target. The Rosetta stone revealed the secrets of ancient Egypt, but the spacecraft that it gives its name to is ready to unlock even greater mysteries.

New blogs will be posted on Twitter, which is also the best place to keep up to date with Rosetta's progress.