The most difficult phase of the Rosetta mission is the final rendezvous with the fast-moving comet. After the braking manoeuvre in May 2014, the priority will be to edge closer to the nucleus.
Since this takes place before Rosetta's cameras have imaged the comet, accurate calculations of Comet 67P/Churyumov-Gerasimenko's orbit, based on ground-based observations, are essential.
Comet approach (January – May 2014)
The spacecraft is re-activated prior to the comet rendezvous manoeuvre, during which the thrusters fire for several hours to slow the relative drift rate of the spacecraft and comet to about 25 metres per second.
As Rosetta drifts towards the heart of the comet, the mission team will try to avoid any comet dust and achieve good comet illumination conditions. The first camera images will dramatically improve calculations of the comet’s position and orbit, as well as its size, shape and rotation.
The relative speeds of the spacecraft and comet will gradually be reduced, slowing to 2 metres per second after about 90 days.
Comet mapping and characterisation (August 2014)
Less than 200 kilometres from the nucleus, images from Rosetta show the comet’s spin-axis orientation, angular velocity, major landmarks and other basic characteristics.
Eventually, the spacecraft is inserted into orbit around the nucleus at a distance of about 25 kilometres. Their relative speed is now down to a few centimetres per second.
The orbiter starts to map the nucleus in great detail. Eventually, five potential landing sites are selected for close observation.
Landing on the comet (November 2014)
Once a suitable landing site is chosen, the lander is released from a height of about one kilometre. Touchdown takes place at walking speed — less than one metre per second.
Once it is anchored to the nucleus, the lander sends back high-resolution pictures and other information on the nature of the comet’s ices and organic crust.
The data are relayed to the orbiter, which stores them for transmission back to Earth at the next the period of contact with a ground station.
Around the Sun (November 2014 – December 2015)
The orbiter continues to orbit Comet 67P/Churyumov-Gerasimenko, observing what happens as the icy nucleus approaches the Sun and then travels away from it.
The mission ends in December 2015. Rosetta will once again pass close to Earth’s orbit, more than 4000 days after its adventure began.