Spacecraft Tracks Asteroid For Rendezvous

A European probe is tracking a rocky asteroid, where it will make a pit stop next month on the way to its main comet target.

The European Space Agency?s (ESA) Rosetta probe, which launched in March 2004, is on a 10-year mission to hunt down the distant comet 67/P Churyumov-Gerasimenko. But on the way, the spacecraft is due to fly by two asteroids, the first of which is Steins.

This week, Rosetta began using its cameras to visually track the asteroid, in order to measure its orbit accurately enough to steer in for a close flyby.

"The orbit of Steins, with which Rosetta will rendezvous on 5 September, closing to a distance of 800 km [500 miles], is only known thanks to ground observations, but not yet with the accuracy we would like for the close flyby," said Gerhard Schwehm, Rosetta mission manager at the ESA's European Space Astronomy Centre near Madrid, Spain.

For the next three weeks, Rosetta is programmed to photograph Steins twice a week. As it gets closer it should begin observing the asteroid daily.

"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," said Sylvain Lodiot, from the Rosetta Flight Control Team at the European Space Operations Centre in Darmstadt, Germany.

As the spacecraft nears Steins, it will also measure how the asteroid's brightness varies over time, which will reveal more information about its shape and rotation.

By studying the asteroid, researchers hope to uncover secrets of its composition, rotation and how it interacts with the charged particles streaming off the sun, called the solar wind.

Rosetta previously swung by Earth in November 2007 and March 2005, and passed Mars in February 2007. The maneuvers gave the spacecraft gravitational boosts, as well as good photo-ops. Rosetta plans to make a third and last swing by Earth in November 2009. The spacecraft is also slated to fly by the asteroid Lutetia in June 2010.

The mission is the first designed to orbit and land on a comet. When it finally arrives at 67/P Churyumov-Gerasimenko, slated for 2014, Rosetta plans to map the comet's surface to select a touchdown site for its lander, Philae. The vehicle is due to operate from the surface for at least a week, sending back data and high resolution pictures of the icy comet as it nears the sun.

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