Rosetta sets Steins in sights
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: August 5, 2008

ESA’s Rosetta spacecraft has begun its survey of asteroid (2867) Steins at a distance of 24 million kilometres, in preparation for a much closer encounter next month.

ESA's Rosetta spacecraft has asteroid (2867) Steins firmly in its sights as it prepares for a close-up view of its surface on September 5. Image: ESA.

"The orbit of Steins, with which Rosetta will rendezvous on 5 September, closing to a distance of 800 kilometres, is only known thanks to ground observations, but not yet with the accuracy we would like for the close fly-by," says Gerhard Schwehm, Rosetta Mission Manager. 

The aim of the current tracking program, therefore, is to determine the asteroid’s orbit with much greater accuracy than presently known, with the aim to reduce the current ‘error’ margin of 100 kilometres down to just 2 kilometres (in the direction perpendicular to the flight direction of the asteroid, to allow Rosetta the best possible approach.

"As Rosetta's distance from Steins decreases, the precision of the measurements for Steins' orbit will increase even further, allowing us the best possible trajectory corrections later on before closest approach, especially in early September," says Sylvain Lodiot, one of the Rosetta Flight Control Team members.

Rosetta’s navigation cameras and the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) will be used to track Stein, but for the next three weeks only OSIRIS will be able to spot the asteroid, appearing simply as a dot in the sky. During this time Rosetta will image Steins twice a week, and from 25 August it will take images daily until 4 September, at which point the orbital information gathered will be used to adjust Rosetta’s orbit for the 5 September flyby.

The OSIRIS instrument will keep track of brightness variations as Steins rotates. This light curve shows a variation of about 23 percent from maximum to minimum brightness, measured over one day. Image: Stefano Mottola (DLR), OSIRIS team.

This is the first time in the Rosetta mission that the OSIRIS instrument is being used for tracking purposes, but it will also perform its more familiar task of obtaining light curves to learn about how Stein’s brightness varies with time, providing vital information about its shape and rotation characteristics.

The current optical and navigation campaign follows a month-long check-up of Rosetta’s scientific instrumentation (read our report Rosetta wakes from hibernation for asteroid flyby), verifying the spacecraft’s readiness for the up-coming close encounter.