‘A very risky event’: Rosetta prepares to land on a comet

An enormously successful space mission is about to become a spectacular one as the European Space Agency prepares to drop a mini-science lab on to the back of a comet.

The Rosetta mission has been under way for more than two decades: from agreement in 1993 through launch in 2004 to having a satellite in position above Comet 67P/Churyumov-Gerasimenko.

In the midst of this great human endeavour is an Irishman from Killucan, Co Westmeath, Laurence O'Rourke, who is the ESA lander systems engineer. He provides a link between the agency and the team that will be controlling the Philae lander. "People are very nervous; very excited but very nervous," he says as the countdown proceeds.

“We have been preparing for this for many years, but all the checks run so far say the lander is in very good condition.”

The overall goal is to reach the comet and place a lander on it as it hurtles towards the sun at 37km per second. The idea is to arrive while the comet is in a frozen “sleep” and relatively inactive, then watch and record what happens when the heat of the sun wakes it up to form a tail and fire off jets of gas, dust and vapour.

“The mission itself is not just about arriving or the lander; it is about following the evolution of a comet going around the sun,” says O’Rourke.

Rosetta finally coasted into place in August after a 10-year journey. Since then it has been running a battery of experiments to learn as much as it can about the comet. In six days' time Rosetta will release the Philae lander and we will – all going well – learn much more about these ancient bodies.

The lander measures one cubic metre and weighs about 100kg. During the critical phases of its final short 22km journey it will operate autonomously. Here is its timeline.

November 12

3.35am

Philae goes autonomous five hours before separation from Rosetta, working under its own battery power.

8.35am

Philae strikes out on its own 22km above the comet, detaching from the satellite and beginning a seven-hour freefall. It takes farewell pictures of Rosetta before getting clear and deploying its three legs, booms and an antenna.

Rosetta manoeuvres away so it can watch Philae travel down to the comet's surface. Ground controllers will not hear until 9.03am whether separation has been successful.

2.35pm

Philae begins preparing for arrival. It starts taking pictures of the approaching landing site, holding images in a buffer so that no matter what happens it will retain the last seven images just before a successful – or crash – landing. By

2.55pm

all will be ready.

3.35pm

At this point it is down to a bit of luck and the care taken by controllers in choosing a landing site. Philae is in freefall and cannot manoeuvre itself into a better position. If there is an obstacle below, it cannot avoid it. Equally, if it lands next to a boulder and is shaded from the sun, there is little it can do and its batteries may run out.

“It is a very risky event, the landing. We estimate there is a 70 per cent chance of success,” says O’Rourke. “You can plan everything and separate well but the flat landing area may have boulders the size of a house and they might kill the lander. It is basically out of our control. In the end it comes down to luck.”

If the landing site is clear then a hectic 15 seconds will follow the moment Philae’s feet make contact at 3.35pm. The lander will be moving at about 95cm per second, so a small thruster will fire to press the lander on to the surface to counteract bounce. At the same time, it will fire two harpoons to help anchor it tightly in place.

Each of the three legs has two feet, and between them are screws that will burrow into the surface and help hold down the feet. The anchoring is needed later when the comet heats up and jets start spouting from the surface.

“All of this happens in the first 15 seconds. This lander is an amazing thing when you think about it,” says O’ Rourke.

Once secured, the lander will check its systems are all working. It will also start taking panoramic images with its seven cameras and these will be uploaded to Rosetta.

Accelerometers in the harpoons will give information about the consistency of the comet – how hard or soft it is – and seismic detectors in each of the feet will gather details of the subsurface structure.

It will take about 60 hours to ensure the lander is safe and working and communicating with the satellite, says O'Rourke. Controllers at this point will issue a go for Philae to begin using its collection of experiments to gather data.

It will drill into the surface to collect material; this will be put in an onboard oven, which will heat it and identify what the comet is made of using a mass spectrometer. The lander will send a radar signal through the comet for collection on the other side by the satellite to map the comet’s interior.

Philae will use a robot arm to hammer pins into the surface to gather temperatures and will use blue LED light to capture true colour images of the comet when on its dark side as it rotates. “It is astounding if it works,” says O’Rourke.

Even if its batteries were to last forever, Philae’s days would be numbered. It will relay data to Rosetta until the comet gets closer to the sun. As it moves to within 225 million kilometres, the heat will be intense and Philae’s solar panels and sensitive instruments will get cooked long before the comet swings around to head back into the cold darkness of space.