Rosetta mission: Can you land on a comet?

The final countdown for the European Space Agency’s ambitious mission to land on a comet in deep space has begun.

After 10 years, and a journey of more than six billion kilometres, the Rosetta spacecraft is set to launch its fridge-sized Philae lander on to Comet 67P/Churyumov-Gerasimenko on 12 November.

If successful, Philae and Rosetta could be key to unlocking answers about the formation of the Solar System, the origins of water on Planet Earth and perhaps even life itself.

The challenge for the flight team operating Rosetta from back on Earth is to land Philae on a rotating, duck-shaped comet travelling through space at 18km/s (40,000mph).

Exactly where the lander will touch down was decided in September, after Rosetta caught up with the comet and started to orbit around it. Scientists and engineers identified Site J (now known as “Agilkia”) on the smaller “head” lobe as the best site for landing and subsequent experiments.

However, the one square kilometre site contains cliffs and crevices and has huge boulders – any of which could scupper a landing.

Agilkia has good lighting conditions, which for Philae means having some periods to recharge its solar-powered batteries and periods of darkness to cool its systems. Site C has been selected as a back-up.

Getting closer to the unusually shaped comet has given us its dimensions, but analysis has also revealed other details:

Comet’s rotation: 12.4 hours Mass: One trillion kg (or 10 billion tonnes) Density: 400kg per cubic metre (the same as some woods) Volume: 25 cubic km

Colour: Charcoal – based on its albedo, or the amount of incident light it reflects back into space.

Separation and landing

The landing is set for 12 November. Rosetta will release Philae at 08:35 GMT from a distance of 22.5km from the centre of 67P. An inaccuracy of a few millimetres per second in Rosetta’s orbit could result in Philae completely missing the comet.

The descent, monitored from Esa’s mission control in Darmstadt, Germany, is expected to last about seven hours.

Because the event is taking place 510 million km from Earth, communication between Rosetta and controllers takes 28 minutes and 20 seconds each way. As a consequence, confirmation of separation is not expected until about 09:03 GMT and of the landing until just after 16:00 GMT.

There will be no steering of the lander down to the comet’s surface – once released, it is on a path of its own.

“We need a certain amount of luck to end up in a nice spot,” Paolo Ferri, head of mission operations, said.

Although the date has been set, the Rosetta team will have to make a series of Go or No-Go decisions before the landing attempt on 12 November.

“If any of the decisions result in a No-Go, then we will have to abort and revise the timeline accordingly for another attempt, making sure that Rosetta is in a safe position to try again,” says Fred Jansen, Esa’s Rosetta mission manager.

1: Release from Rosetta

Rosetta will push the Philae lander away when the spacecraft is about 22.5km from the comet’s centre. Rosetta needs to release Philae at exactly the right place in time and space to be sure of putting the little robot on the correct path to the comet

2: Descent

The descent to the comet’s surface is expected to take about seven hours. On the way down, Philae will take pictures of the comet and start taking measurements of the environment around the comet

3: Comet activity

The comet activity on the day – throwing out gas and dust or even the splitting up of the comet itself – cannot be predicted. The descending robot will just have to cope with whatever is chucked at it

4: Landing zone

The chosen landing area is not perfectly flat, but most slopes are at an angle of less than 30 degrees. There are some boulders that could pose a problem if Philae hits them, however

5: Touchdown

When the lander hits the surface – at walking pace – footscrews will drill into the surface and harpoons will be used as anchors. A thruster on top of Philae will also gently push the robot into the surface to stop it bouncing off into space. If the surface is very soft, the screws may not secure the lander. If it is very hard, they may not penetrate it at all

Once on the surface, Philae can get to work. The lander will take a panoramic photo of its surroundings using its onboard micro-cameras. Next, about an hour after touchdown, the first sequence of surface science experiments will begin, and will last for 60 or so hours.

The Rosetta orbiter will continue to study the comet using its 11 science instruments – but it will also be relaying data from Philae’s instruments. Radio waves sent from Philae to Rosetta when the orbiter spacecraft is on the opposite side of the comet will help determine the structure of the comet’s interior.

Drills, ovens, cameras and sensors onboard Philae will analyse everything from the surface composition and temperature to the presence of amino acids – essential building blocks in the chemistry of life.

1: Cameras – Philae’s CIVA imaging system has cameras that will take panoramas of the comet’s surface terrain. The download-looking ROLIS system will spy the comet on descent, and take close-ups once landed

2: Nucleus probe – CONSERT – will use radio waves to probe the internal structure of the comet nucleus

3: Footscrews – Ice screws on the feet of Philae’s legs will drill down into the comet to secure the lander. Problems could arise if the surface is too hard or too soft

4: Sample drill – SD2 – Sample and Distribution Device – will drill more than 20cm into the surface, collect samples and deliver them to onboard laboratory equipment COSAC and PTOLEMY for analysis.

5: Harpoons – Immediately after touchdown, a harpoon will be fired to anchor Philae to the comet’s surface and prevent it bumping off because of the comet’s weak gravity

6: Surface probe – MUPUS – Sensors on the lander’s anchor, probe and exterior will measure the density, thermal and other properties of the surface and subsurface

Prof Ian Wright, of the Open University, is the principal investigator of the Ptolemy instrument. He says the Rosetta mission is already a success, whatever happens with the landing – which everyone on the project knows is a risky venture.

“As things stand, the orbiter will continue to shadow the comet until the end of next year. This will be an opportunity to observe how the body responds to its close passage to the Sun,” he said.

“The point though is not merely to watch the comet from a safe distance, but to get down on the ground and actually touch the object. For those of us who are used to handling and analysing samples in the lab it is the only way to study it. We realise that we may ultimately end up with nothing but that is the nature of exploration.”

What next?

After the initial science sequences, longer-term studies are planned, depending on how well Philae’s batteries are able to recharge. This could be affected by how much dust gathers on its solar panels.

As the mission continues and the comet journeys closer to the Sun, temperatures inside the lander will get so hot that its batteries and electronics will stop working. This could be around March 2015.

But even after Philae’s mission ends, Rosetta will continue its escort and remote analysis of the comet for a few more months.

August 2014: Rendezvous with comet – Rosetta reaches Comet 67P/Churyumov-Gerasimenko after a 10-year journey. The craft starts orbiting the comet and identifies suitable sites for the Philae lander.

November 2014: First Science Sequence – After landing on the comet, Philae’s first few days are spent running through a predetermined set of experiments.

December 2014: Long-term science – The team hopes Philae will continue working and recharging its batteries, to continue its observations despite its temperature constraints on the comet.

March 2015: Lander limits – Philae could be affected by increasing temperatures on the comet and may be at risk of layers of dust hampering the effectiveness of its solar panels.

August 2015: Perihelion – The comet reaches its closest position to the Sun. Rosetta will be measuring the level of activity as the icy object enters its most active phase.

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