After a long 10-year journey, the Rosetta spacecraft arrived at comet 67P/Churyumov-Gerasimenko in early August this year and has been busy mapping the comet’s surface to choose a landing site. Rosetta is the first satellite to orbit a comet, and will soon be the first mission to place a lander on the surface of a comet. The Rosetta satellite and the lander Philae, will follow the comet as it approaches the Sun over the next year. Five potential landing sites are currently being assessed and ranked, and next week at the ESA Headquarters in Paris the primary landing site for Philae will be announced, with the landing currently scheduled for mid November.
The Rosetta mission is a joint ESA and NASA/JPL collaboration. The mission aims to keep Rosetta stay in close proximity to the icy nucleus of comet 67P/Churyumov-Gerasimenko as its orbit carries it into the warm, inner Solar System, where the comet will become more active. The lander, which contains ten instruments, will be able to study in detail the physical properties of the comet’s surface, measuring its chemical, mineralogical and isotopic composition. The spacecraft, which contains 11 of its own instruments, will orbit and map the comet as it gets closer to the Sun, investigating how the active comet interacts with the solar wind.
Rosetta’s journey began in March 2004 in French Guiana, when an Ariane 5 rocket launched the mission. During the 10-year journey to the comet, Rosetta has had its orbital energy boosted by four separate gravity assist flybys, three with the Earth and one with Mars. The satellite has crossed the asteroid belt twice, including a flyby of asteroid Steins in September 2008 and asteroid Lutetia in July 2010. It had almost 3 years of hibernation before being “woken” in January 2014 to prepare for the rendezvous sequence which began in May 2014, slowing the relative velocities between the satellite and the comet, allowing Rosetta to finally come alongside the comet in August 2014.
Comet 67P/Churyumov-Gerasimenko is a short period comet which orbits the Sun every 6.5 years. Its average distance from the Sun is about 3.5 AU, though its eccentric orbit (e=0.64) carries it between 1.2 AU, a little further from the Sun than the Earth, and 5.7 AU which is just beyond the orbit of Jupiter. Leading up to the August arrival, Rosetta revealed comet 67P/Churyumov-Gerasimenko’s strange shape: the irregular nucleus contained two distinct lobes, suggesting that it might be a contact comet, which form when two comets collide. The strange shape might also result from large amounts of ice evaporating from the surface during its close approach to the Sun, leaving behind the asymmetric shape we see today. (Follow this link for an animated sequence of comet views from Rosetta’s arrival.)
For the past month, Rosetta has been mapping the surface of the comet to learn about its structure and to select a landing site for Philea, while slowly manoeuvring closer to the surface. In November, Philea will be released from a height of about 1 km above the comet’s surface and unfold its 3 legs ready for a gentle touchdown at human walking speed. Philea will then immediately fire a harpoon to anchor itself to the surface to ensure it doesn’t escape from the comet’s extremely weak gravity. Then both Rosetta and Philea will follow comet 67P/Churyumov-Gerasimenko as it approaches the Sun over the next 12 months or so. The current mission has a nominal end date of December 2015, while the lifetime of the Lander will depend on the cometary environment.
For more information about the mission, see
- Rosetta, ESA website
- Rosetta, ESA Science & Technology website
- Rosetta, NASA JPL website
- The long trek, ESA description of Rosetta’s journey
- Rosetta arrival & orbit entry, ESA video, 6 August 2014
- Rosetta arrives at comet destination, NASA/JPL News Archive
- Rosetta: Landing Site Search Narrows, JPL Latest News
- Rosetta comet is darker than charcoal, NASA Science News, 5 Sept 2014
- Rosetta Lander, DLR website