Steins, the first nominal scientific target of Rosetta, is known to be a relatively rare type of asteroid (E-type) about 5 km across, but its properties were not yet known in detail before Rosetta’s encounter. Rosetta pre-imaged Steins on 10 March 2006 from a large distance, and obtained a few preliminary data about rotation and shape of the asteroid. The Steins fly-by campaign in 2008 was intended to provide a detailed description of this body and of its near environment from a closer distance.

Credits: ESA, animation by C.Carreau

Steins was Rosetta’s first nominal scientific target. The spacecraft encountered the asteroid in the course of its first incursion into the main asteroid belt located between the orbits of Mars and Jupiter, while on its way to Comet 67P/Churyumov-Gerasimenko.

The study of asteroids is extremely important because they represent samples of Solar System material at different stages of evolution – key to understanding the origin of our own planet and of our planetary neighbourhood.

Credits: ESA, image by C.Carreau

Rosetta’s 11-year expedition began in March 2004, with an Ariane 5 launch from Kourou in French Guiana, and the spacecraft was then sent towards the outer Solar System. The long journey includes three gravity assists at Earth (2004, 2007, 2009), one at Mars (2007), and two asteroid encounters: (2867) Steins (2008) and (21) Lutetia (2010).

After the third Earth-gravity assist and a large deep-space manoeuvre, the spacecraft will go into hibernation (July 2011 – January 2014). During this period, Rosetta will record its maximum distances from the Sun (about 800 million kilometres) and Earth (about 1 thousand million kilometres). The spacecraft will be reactivated prior to the comet-rendezvous manoeuvre, during which the thrusters will fire for several hours to slow the relative drift rate between the spacecraft and comet to about 25 m/s.

Credits: ESA, image by AOES Medialab

Selected in November 1993 as a cornerstone mission of ESA's long-term science programme, the Rosetta probe was launched by an Ariane 5 on 2 March 2004, on an 11-year journey to the comet 67P/Churyumov-Gerasimenko.

Built by EADS Astrium, the Rosetta probe consists of a 3,065-kg spacecraft (1,578-kg dry mass) designed to enter orbit around the comet's nucleus in August 2014 after a series of gravity assist manoeuvres to gain enough orbital energy, with three swing-bys at Earth (March 2005, November 2007 and November 2009) and one at Mars (February 2007). En route to the comet, the probe will flyby the asteroids 2867 Steins (September 2008) and 21 Lutetia (July 2010).

The spacecraft carries 11 science instruments to probe the comet's nucleus and map its surface in fine detail. It will also land a package of instruments (the Philae Lander) to study some of the most primitive, unprocessed material in the Solar System. The mission will provide clues to the physical and chemical processes at work during the formation of planets, beginning 4.6 billion years ago.

Credits: ESA - J. Huart