An illustration of Philae landing on the comet.
Comets are remnants of the birth of our Solar System four and a half billion years ago and are therefore important witnesses from that time. Most comets can be found in a spherical cloud that extends to the border of our Solar System. The influence of the Sun is so weak there that the comets there have hardly changed. If the orbit of a comet is perturbed, it can be diverted in the direction of the inner Solar System. Within Jupiter's orbit, the temperatures and the pressure of the solar wind are high enough that the comets become active and their coma and characteristic tail are formed.
Comets could have played an important role in the formation of life, as they contain complex organic molecules. The European Space Agency's (ESA) Rosetta mission is currently the most ambitious project for the investigation of comets. The probe was launched in March 2004 and will reach the comet 67 P/Churyumov-Gerasimenko, then located between the orbits of Mars and Jupiter, in May 2014 – before it becomes active. The probe will initially enter into an orbit around the comet in order to carry out the first measurements and to search for a landing site. Then the landing vehicle, Philae, will separate itself from the mother ship and land on the comet – this has never before been attempted in the history of space travel. After landing, Rosetta and Philae will accompany the comet on its several-month journey to the point in its orbit closest to the Sun. Philae was developed by an international consortium under the direction of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR).