The ESA Rosetta team has chosen the ‘head’ of the comet as the spot where their lander Philae will touch down on 11th November. The spot, known as landing site J, was picked from a shortlist of five possible locations as it offered the best chance of success, both in technical and scientific terms. The area is on the top of the smaller of the two lobes of the comet – a head and body-type structure which has lent the comet the nickname ‘rubber ducky’. The area of J is irregularly shaped, and about 4 kilometres across at its widest point.

The news was announced at the European Space Agency’s Paris headquarters and marks another milestone in the pioneering Rosetta mission, which last month became the first ever spacecraft to catch up with a comet and then fly alongside.

If all goes to plan then Philae will drop off the bottom of Rosetta and drift down to the comet surface at walking pace, using harpoon-like devices to anchor itself to the dark and dusty comet, known as 67P/Churyumov-Gerasimenko. The hope is that the lander will be able to take photos and measurements in-situ on the comet for weeks or even months, offering a chance to see how it evolves as it heats up on its journey towards the Sun from the outer solar system.

Philae has already been limbering up for the formidable task ahead of it by taking a ‘selfie’ of Rosetta’s solar panels with the comet in the background. It was part of a check that verified the 10 instruments on board are indeed ready for action when D-Day comes.

The choice of landing spot was made at the weekend at a meeting in Toulouse, France, after an intense period of discussion inside the Rosetta team, with scientists and flight engineers hammering out the pros and cons of different locations. Their goal was to find the ‘Goldilocks’ location, not too warm, not too cold, and not too covered in boulders and ravines to risk Philae plunging into a dark crevice, never to be heard from again. In the end, the decision to go with Site J was unanimous apparently. Meanwhile another backup site, called Site C, is located on the ‘body’ of the comet.

Mapping a comet

Like the explorers of the ‘New World’ 500 years ago, the Rosetta scientists are mapping everything as they go. Since meeting up with the comet in early August the flight directors have moved the spacecraft from what they call the ‘Close Approach’ phase – basically flying in a triangular orbit right in front of the comet to check out how mean this ducky is likely to get – to the ‘Global Mapping’ phase, which means going down to 30 kilometres or less, enough for the spacecraft to be influenced by the gravity of the comet.

Exactly how low Rosetta will fly in the next few weeks before releasing Philae is still open to question – it could stay at 30kms, move to 20kms, or even 10kms. The operations team at ESA’s space operations centre in Darmstadt, Germany will make a call on whether they consider it too hazardous to Rosetta to go closer, depending on how exposed it becomes to gas and dust from the comet. As Rosetta Flight Director Andrea Accomazzo put it in an interview with Euronews: “We are learning to fly around a comet while flying around a comet, so it’s a sort of self-referencing problem that we have to solve”.

Rosetta’s science bonanza

Comets are interesting because they’re some of the most primitive bodies in our solar system, and so offer us a chance to look back 4.5 billion years to the formation of the planets and to see what kinds of materials were circulating in space at that time.

Rosetta is already delivering a bonanza of science for the comet community in Europe, with details of some of the curious discoveries the spacecraft has already made now beginning to filter through.

The spacecraft’s ‘nose’, an instrument called ROSINA, has sniffed out some of the expected molecules in the comet’s coma, such as water, carbon monoxide, and carbon dioxide. However the ratio between those three appears to vary depending on where in the coma Rosetta is, something which the science team hadn’t been expecting.

They’re now rubbing their hands together with glee as the comet begins to get more active on its way towards the Sun next year, because they will be able to measure the ratio of hydrogen to deuterium in the water coming off the comet. They’ll then want to compare that to the constant ratio found in Earth’s oceans, in a bid to work out whether the water on Earth came from Kuiper belt comets like 67P during the earliest epoch of our solar system.

Since we really don’t know where all the water on our jolly little blue planet came from, that could be a major science breakthrough for Rosetta.

Whether the lander actually succeeds in touching down on 67P successfully or not, Rosetta will carry on following the comet right up to its closest approach to the Sun in August next year, a zone around the orbital path of Mars. At that time it should have plumes of gas and heated material shooting up from beneath the surface, revealing all kinds of juicy facts about what’s really inside this rocky, cratered and coal-black relic.

Then the spacecraft may well live up to its name: Rosetta, for the Rosetta stone, the key that unlocked the secrets of ancient Egypt.

Rosetta’s landing site choices



Philae’s descent and science on the surface

