By May 2014, Rosetta will be just 934,000 kilometres from the comet 67P/Churyumov-Gerasimenko . It will then spend several weeks conducting a rendezvous manoeuvre. In August 2014, a mere 100 kilometres from the comet, the orbiter will begin mapping its target to locate a suitable landing site for Philae. The actual touchdown point will be defined in October 2014. Philae is scheduled to land on the comet 11 November 2014 – a first in the history of space flight.
Philae will anchor itself to the comet
Comets are relatively small objects with small gravitational fields. So the problem is not that it will crash as it lands – even though it weighs 100 kilograms on Earth, this is only equivalent to a few grams on the four-kilometre-wide comet. The difficulty lies in ensuring that the Philae landing craft does not simply bounce back off the comet's surface. As soon as the landing craft makes contact with the comet, it will fire two harpoons that will anchor it to the surface. Philae has been designed in such a way that in the event of any breakdown in almost all important functions, one or more redundant solutions exist. So Philae has two harpoons and a small thruster that is ignited upon landing to counteract any possible rebound.
It is very probable that the comet will already be active upon touchdown, and that it will emit gas and dust, further complicating the situation. The landing will be entirely automatic, as Earth-based control is not possible due to the immense distance and the consequent time delay of around 30 minutes it takes for the signals to travel at the speed of light.