A European Space Agency probe, the first spacecraft to orbit a comet and the first to deploy a lander on it. Launched on 2 March 2004, Rosetta gained speed on its way to the comet by making three fly-bys of the Earth and one of Mars. It also passed two asteroids: one called Steins in September 2008, and another called Lutetia in July 2010. Rosetta encountered comet 67P/Churyumov–Gerasimenko in 2014 and went into orbit around its elliptical nucleus, which is estimated to measure 5 × 3 km. Rosetta encountered the comet at a distance of around 600 million km from the Sun, four times the distance of the Earth from the Sun.

After mapping the comet, Rosetta released a lander, named Philae, on 12 November 2014, which made the first-ever landing on a comet. A large amount of data were transmitted in its first few days on the comet, but then it went into hibernation, as its final landing site received too little sunlight to recharge its battery. As the comet moved closer to the Sun in June and July 2015, Philae briefly began transmitting again. Less than a month before the end of the mission, Rosetta’s high-resolution camera revealed the 1-metre-wide Philae lander wedged into a dark crack on the comet’s surface, explaining why establishing communications was so difficult following its landing. Philae’s data transmissions revealed that the surface is covered in ice with a covering of dust, and that molecules containing carbon have been identified.

Overall, the results delivered by Rosetta so far paint comets as ancient leftovers of early Solar System formation rather than fragments of collisions between larger bodies later on, giving an unparalleled insight into what the building blocks of the planets may have looked like 4.6 billion years ago.

Rosetta takes its name from the Rosetta Stone, an engraved stone that provided the key to deciphering Egyptian hieroglyphs.

Rosetta's landing site to scale. In order to give a feeling of scale, this artist’s impression of the spacecraft is superimposed on an actual wide-angle camera image of the region of the surface of comet 67P/Curyumov–Gerasimenko on which it landed on 30 September 2016. Also marked on the image are the approximate locations of the final two images taken by the spacecraft from around 20 m altitude. The cross indicates the estimated centre of touchdown of Rosetta. The background image measures about 55 m across, while the final image is about 1 m across. For comparison, Rosetta measures 32 m from tip to tip, and its solar panels are a little more than 2 m high each.

Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; spacecraft: ESA/ATG medialab

On 2 September 2016, less than a month before the end of the Rosetta mission, the spacecraft's high-resolution camera revealed the Philae lander wedged in a dark crack on comet 67P/Churyumov–Gerasimenko. This image shows the lander from a distance of 2.7 km. The image scale is about 5 cm/pixel.

Credit: Main image and lander inset: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA