Heading toward its first target-asteroid, (2867) Steins, ESA's Rosetta spacecraft is using its cameras to visually track the asteroid, and eventually determine its orbit with more accuracy. Rosetta started the optical navigation campaign this week on August 4, at a distance of about 24 million km from Steins; the campaign will continue until September 4, when the spacecraft will be approximately 950,000 km (570,000 miles) from the asteroid. "The orbit of Steins, with which Rosetta will rendezvous on September 5, closing to a distance of 800 km (497 miles), is only known thanks to ground observations, but not yet with the accuracy we would like for the close fly-by," said Gerhard Schwehm, Rosetta Mission Manager based at ESA's European Space Astronomy Centre (ESAC), near Madrid, Spain. “We will be able to use the first data set for the trajectory correction maneuver planned for mid-August.”

The purpose of the tracking campaign is to reduce the error learning about Steins' orbit from about 100 km to only within 2 km (in the direction perpendicular to the flight direction of the asteroid, called 'cross-track'), so as to allow Rosetta an optimal approach to this celestial body. Both Rosetta's navigation cameras and the OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) imaging system will be used to track Steins. "For the first three weeks of the campaign, however, only the powerful eyes of OSIRIS will actually be able to spot the asteroid, which will look only like a dot in the sky," said Andrea Accomazzo, Rosetta Spacecraft Operations Manager at ESA's European Space Operations Centre (ESOC), Darmstadt, Germany.

  Navigation cameras are located on the spacecraft's 'top', as seen in the diagram. Eleven days before the closest approach, as the distance with Steins decreases, the two Rosetta navigation cameras will be able to see and track the asteroid, too. As Rosetta's distance from Steins decreases, the precision of the measurements for Steins' orbit will increase even further, providing the best possible trajectory corrections later on before closest approach, especially in early September.

   Rita Schulz, Rosetta Project Scientist based at ESA's European Space Research and Technology Centre (ESTEC), Noordwijk, the Netherlands, explained that this is the first time in the Rosetta mission that the OSIRIS scientific instrument is being used for tracking purposes. In addition, OSIRIS will obtain 'light curves' of Steins. Light curves reveal how the asteroid brightness varies with time, provide us with additional preparatory information about the asteroid, such as better knowledge of its shape and rotation characteristics.