Credits: ESA/AOES Medialab

The European Space Agency (ESA) announced that on 14 August, the Rosetta spacecraft performed a successful trajectory correction maneuver using data obtained from ESA's first-ever optical tracking of minor planet (asteroid) Steins. Images from the spacecraft's cameras were used to calculate the asteroid's location and optimize its trajectory for fly-by next month.

Rosetta's first major correction maneuver in the approach phase took place while the spacecraft was just less than 17 million km from Steins. The spacecraft’s thrusters burned continuously for approximately 2 minutes, starting at 13:42 CEST (Central European Summer Time). This yielded a change in Rosetta’s speed relative to Steins of 12.8 cm/sec.
However, this tiny change in speed will be enough to correct the spacecraft's 5 September Steins fly-by distance by 250 km. Prior to this maneuver, the spacecraft’s trajectory would have brought it too close to the minor planet at the time of closest approach; to be compatible with spacecraft performance, Rosetta must be no less than 800 km from Steins.

Rosetta's OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) scientific camera and the on-board star-tracker navigational cameras began twice-weekly imaging periods on 4 August, recording 52 images of Steins. Within the coming few weeks, optical tracking of Steins will intensify, with imaging slots daily between 25 August and 4 September, when the asteroid will be much closer.

"The closer we get to Steins, the more accurate our knowledge of the asteroid's position relative to Rosetta will be," said Trevor Morley, leader of the Rosetta Flight Dynamics Orbit team. "Thanks to Rosetta's cameras, we will obtain increasingly precise measurements that will allow us to adjust again, if necessary, Rosetta's orbit for an optimal asteroid encounter."

Rosetta's optical tracking campaign makes history for the ESA’s spacecraft operations. "It is the first time ever that we have used optical instrumentation on board a scientific spacecraft for navigation, as opposed to the usual techniques based on analyzing radio signals," said Morley. "The exceptional results are really encouraging and we look forward to using this technique again in the future, if and when it is possible."

Further Reading

ESA Press Release
http://www.esa.int/esaCP/SEMC9R6UWJF_index_0.html
The Asteroids
http://www.nineplanets.org/asteroids.html

Aymen Mohamed Ibrahem
Senior Astronomy Specialist