The European Space Agency's Rosetta spacecraft started using its Optical, Spectroscopic, and Infrared Remote Imaging System (Osiris) to track the Steins asteroid on 4 August to more accurately determine its trajectory for a safe 800km (496 miles) rendezvous in September.

Now about 24 million km from Steins, the use of Osiris alone, for the first time, continues until 11 days from the rendezvous, when optical cameras will be used to close the distance to 950,000km by 4 September and the day after to 800km. Using Osiris and the cameras, the orbit accuracy will improve from 100km to 2km for an optimal approach.

"For the first three weeks of the campaign only Osiris will be able to spot the asteroid," says ESA's Rosetta spacecraft operations manager Andrea Accomazzo. The cameras will be used 11 days before rendezvous.

For those weeks Rosetta will image Steins twice a week and from 25 August it will take images daily until 4 September.

Source: Flight International