Since launch in 2004, ESA's Rosetta mission has been chasing down comet 67P/Churyumov-Gerasimenko. The comet is a regular visitor to the inner Solar System, orbiting the Sun once every 6.5 years between the orbits of Jupiter and Earth.
Like all comets, Churyumov-Gerasimenko is named after its discoverers. It was first observed in 1969, when several astronomers from Kiev visited the Alma-Ata Astrophysical Institute in Kazakhstan to conduct a survey of comets.
On 20 September, Klim Churyumov was examining a photograph of comet 32P/Comas Solá, taken by Svetlana Gerasimenko, when he noticed another comet-like object. After returning to Kiev, he studied the plate very carefully and eventually realised that they had indeed discovered a new comet.
The comet has now been observed from Earth on seven approaches to the Sun: in 1969, 1976, 1982, 1989, 1996, 2002 and 2009. It was also imaged by the Hubble Space Telescope in 2003, which allowed estimates of its size and shape to be made – an irregular object roughly 3 x 5 km across.
Most of the time, however, its faint image is drowned in a sea of stars, making observations with Earth-based telescopes extremely difficult.
However, during its short-lived excursions to the inner Solar System, the warmth of the Sun causes ices on its surface to evaporate and jets of gas to blast dust grains into the surrounding space.
Although this enveloping ‘coma’ of dust and gas increases 67P/Churyumov-Gerasimenko’s brightness, it also completely hides the comet’s nucleus to ground-based observers.
Rosetta's task is to rendezvous with the comet while it still lingers in the cold regions of the Solar System, and to deploy a lander to reveal in close-up detail exactly what the surface looks like.
Observations indicate that, if the activity of 67P is consistent from orbit to orbit, then Rosetta may return images of an active nucleus when it rendezvous with the comet when it is about 3.5 AU from the Sun.
Over an entire year, as it approaches the Sun, Rosetta will orbit the comet, mapping its surface and studying changes in its activity.
As its ices evaporate, instruments on board the orbiter will study the dust and gas particles that surround the comet and trail behind it as streaming tails, as well as their interaction with the solar wind.
Rosetta will also help identify which regions of the nucleus are more active than others. As is the case with most comets, activity is not evenly distributed on the surface of the nucleus and the coma of 67P is fed by several dust jets – at least three prominent active areas were identified during the 2009 apparition. If the comet behaves as in 2003 and 2009, the main jets should become visible a month before perihelion, i.e. mid-July 2015.
To ensure the spacecraft is kept safe during times of high activity, its orbit around the comet will be adjusted accordingly.