SPACE — Spacecraft controllers have awakened Rosetta from hibernation to prepare for the spacecraft’s encounter with asteroid (2867) Steins on September 5th. ESA‘s comet chaser will study the relatively rare asteroid as it flies by on its way to comet 67/P Churyumov-Gerasimenko. Launched in March 2004, Rosetta will reach its final destination in 2014, after travelling a total of about 6,500 million km. The distance between the spacecraft and the Sun as it approaches the comet will be about 600 million or 4 AU (1 AU or 1 Astronomical Unit, which is equal to 150 million km, the mean distance between Earth and the Sun). Rosetta has swung by Earth twice and Mars once, performing gravity-assist manoeuvres, that gave it the necessary boost to continue on its journey. The third and last Earth swing-by is scheduled for November 2009. The spacecraft will also fly by two asteroids and study them on the way: (2867) Steins in September this year and (21) Lutetia in June 2010. As it closes in on (2867) Steins in September, Rosetta will have travelled about 3700 million km and will be 2.1 AU from the Sun. After its last planetary swing-by on 13 November last year, Rosetta headed towards the asteroid belt located between the orbits of Mars and Jupiter. On March 27th, 2008, the spacecraft switched to its near-Sun hibernation mode for a period of three months. During this phase, a few subsystems were put into a dormant state to optimise their lifetime (as this is only the beginning of the mission’s science phase).
Next stop, Steins. Rosetta will be closest to (2867) Steins at 20:37 CEST on September 5th, at a distance of 800 km. The spacecraft will zoom past at a relative speed of 8.6 km/s. In preparation for the fly-by, all the instruments will be checked and tested through the month of July. Spacecraft operators will conduct an optical navigation campaign: Steins will be tracked by the on-board cameras and the observations will be used to refine the knowledge of its orbit which has been derived only from ground-based measurements so far. (2867) Steins is a relatively rare type of asteroid. Based on ground-based observations, it has been classified as an E-type asteroid, composed mainly of silicates and basalts, but its properties are not known in detail. For these reasons, it has been selected as one of the two asteroids that Rosetta will study, from among those that were within reach of the mission. The knowledge gained from the measurements will add to our knowledge of the composition and evolution of E-type asteroids and will also supplement and help interpret future ground-based data on asteroids.
