The Rosetta Mission was a 10-year long project by the European Space Agency (ESA) to place a lander on a comet. It succeeded with the placement of the Philae lander on the Comet 67P/Churyumov-Gerasimenko (AKA Chury) on November 12, 2014.
The mission was originally aiming for a much smaller comet, 46P/Wirtanen1, but difficulty with the Ariane 5 engine postponed liftoff from January 2003 to March 2004, causing Rosetta to miss the launch window for Wirtanen. Chury was identified as a good backup target, and the mission was back on.
The goal of the mission was to find clues about the formation of the Solar System. Chury's composition should reflect the composition of the pre-solar nebula out of which the Solar System formed over 4.6 billion years ago. The ESA will also be looking at the isotopic profile of the cometary ice to see what percent of the water in Earth's oceans may have come from early comet impact, and will check for organic molecules that may have affected abiogenesis.
The media make a point of telling us that Rosetta traveled 6.4 billion kilometres (42.78 AU) to reach Chury, but this is potentially misleading. Chury is not nearly this far out2, and the distance traveled includes four trips around the sun, with three gravity assists from Earth (in 2005, 2007 and 2009) and one from Mars (2007). But any way you spin it, ten years of weaving through the inner solar system to hit a target the size of Cruithne is impressive.
Rosetta spent about nine months gathering data on the comet through photographs and sampling the various off-gassings. Finally, it moved within 10 km of the comet and released the Philae lander3, a clunky cube about the size of a washing machine, which was intended to land gently on the comet and take some serious rock samples and high-resolution photos. Chury is an odd shape, often compared to a rubber duck, consisting of a larger and a smaller lobe connected by a thick 'neck'. Philae was targeted to land on a large open area on the smaller lobe.4
In order to land on the comet in microgravity, Philae was supposed to rely heavily on a harpoon anchor system, but this failed to fire. The lander bounced off the surface, arching a full kilometer up and a kilometer to the side before coming back to the surface, bouncing lightly once more, and finally landing in shadow and at an odd angle. All systems continued to work properly, but Philae is solar-powered, and will run out of power shortly if it can't be moved. Fortunately, the drills were still able to take subsurface samples despite the tilt of the lander, and all of the planned initial experiments were completed successfully.
The lander has run out of power, but the solar panels have been re-positioned, and it is possible that it may be able to power back up at some point in the future. There is not any great expectation in this area, however.
In the meantime, here is the ESA's photo gallery.
1. Wirtanen is much smaller than Churyumov-Gerasimenko, with an estimated diameter of 1.2 kilometres v. Chury's 4.1. We know comparatively little about it, but it will be coming quite close to Earth in December of 2018, passing within a mere 0.0777 AU (Chury will only come within 3.3 AU of Earth, but we will only have to wait until 2015). We may yet pay Wirtanen a visit; NASA's proposed Comet Hopper mission would like to make comet-fall in the early 2020s, if funding comes through. It doesn't seem very likely at this point.
2. Aphelion of 5.6829 AU; Perihelion of 1.2432 AU. As the crow flies, Rosetta reached a max distance of 6.68 AU from the Earth (a point reached in 2012), before its orbit moved back in to align with Chury's.
3. Named after an island in the Nile region of Egypt, the place of discovery for the Rosetta stone.
4. The original landing site, originally known as Site J, was renamed Agilkia as the result of an international naming competition. Ironically, it is named for Agilkia Island, the island that many buildings, including the famous Temple of Isis, were moved to when Philae flooded during the building of the Aswan dams in the 1960s. This would be a very appropriate name for an unfortunate, unplanned landing site, but odds are that the new landing site will get some other title.