<p>Continue reading the main story 67P/Churyumov-Gerasimenko<br />
Named after its 1969 discoverers Klim Churyumov and Svetlana Gerasimenko</p>
<p><u><b>Referred to as a “Jupiter class” comet that takes 6.45 years to orbit the Sun</b></u></p>
<p><u><b>Orbit takes it as close as 180 million km from the Sun, and as far as 840 million km</b></u></p>
<p><u><b>The icy core, or nucleus, is about 4km (2.5 mi) across and takes 12.4 hours to rotate</b></u><br />
The European Space Agency says it will be a one-shot opportunity.<br />
Rosetta and the comet are currently about 400 million km from Earth, making real-time radio control impossible.<br />
Instead, the process will have to be fully automated with commands uploaded several days in advance.<br />
The five sites on the “longlist” were selected at the end of a special meeting convened in Toulouse, France, this past weekend.<br />
Esa project managers were joined by attendees from the space agencies of France (Cnes) and Germany (DLR), which play key roles in the Philae effort.<br />
Instrument principal investigators on the washing machine-sized robot were also there to argue their preferences, as were the engineers, who could explain the technical possibilities.<br />
If one considers the comet to look like a rubber duck, then three of the chosen potentials (B, I and J) are on the head. Two are on the body (A and C). The dramatic neck region has been ruled out.<br />
The letter designation stems from an even longer list of 10 that was used to kick-off the whole selection process. The letter ordering carries no weight.<br />
A landing site needs to be relatively flat and free from boulders and fissures.</p>
<p><u><b>(A) interesting “body” site that will be attractive because it will afford a view of both lobes</b></u></p>
<p><u><b>(C) another body site that features depressions, cliffs, hills and smooth plains</b></u></p>
<p><u><b>(B) on the “head” and has a crater-like structure with a smooth bottom</b></u></p>
<p><u><b>(I) looks to have a lot of fresh material on its surfaces. Also on the head</b></u></p>
<p><u><b>(J) the third head location, with interesting features and good lighting conditions</b></u><br />
One key requirement has been the need to find places on the comet that experience something of a day/night cycle.<br />
This will give not only a better appreciation of the changing behaviour of 67P under all conditions, but will provide the lander with some important protection – from too much sun, which could lead to overheating, or too little light, which would make it difficult to charge the batteries.<br />
The engineers have also emphasised the need to find locations where Rosetta can deliver Philae at the right altitude and velocity, and maintain a communications link throughout the descent, which is likely to take several hours.<br />
The neck region with its dramatic gorge has been ruled out of contention The longlist will be reduced to a leading candidate and perhaps a couple of back-ups in mid-September.<br />
A final go/no-go decision on a target landing site is expected by mid-October.<br />
By then, Rosetta’s cameras and other instruments will have returned detailed data on the number one choice.<br />
Comet 67P has very little gravitational attraction – several hundred thousand times weaker than what Philae would experience at the Earth’s surface.<br />
For this reason, it will touch down at no more than <a href="https://www.youtube.com/watch?v=kn8a4YOW7ds">Funny Videos failA</a> a walking pace – about 1m/s.<br />
It will use harpoons and ice screws to try to hang on to the comet and avoid bouncing back into space.<br />
Getting Philae down safely to any of these sites is going to be an immense challenge Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos</p>
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