Rosetta, the Comet-Chasing Satellite, Passes Earth and Gets a Boost

On November 13 the European Space Agency's (ESA's) Rosetta satellite will make a swing-by of Earth for an assist into its next phase and received a 42-second trajectory correction on October 18 in order to more accurately approach Earth.

Rosetta is what the ESA calls its comet-chasing satellite. Rosetta's mission is to engage in a long-term and close-up study of comet 64P Churyumov-Gerasimenko and on the way there it will enter the main-asteroid belt and pass Asteroid Steins.

The main-asteroid belt has an orbit around the Sun that is situated between the orbits of Mars and Jupiter. Asteroid Steins is on the small side for an asteroid but still its proximity to Rosetta will provide a unique opportunity for investigation of the general properties of asteroids. Rosetta is expected to pass Steins on September 5 of 2008.

Rosetta's journey to comet 64P Churyumov-Gerasimenko is a complex one. All in all, Rosetta will receive four swing-by gravitational assists, besides passing within investigating distance of two asteroids, Steins in 2008 and Lutetia in July of 2010. A swing-by is a partial orbit around a planet intended to allow a satellite to employ the planet's gravitational pull to implement a change in the satellite's direction. This is beneficial because in a swing-by the force of gravity takes the place of the force of fuel, which permits a trajectory correcting maneuver that is also a fuel-saving assist that puts the satellite out of the planet's orbit and back into space.

The first swing-by was an Earth assist, the second was an assist from Mars' gravitational force. The third is the anticipated Earth swing-by expected on November 13. The fourth and final one will be in 2009 following its encounter with Steins, after which Rosetta will pass by the second asteroid, Lutetia, on its way to comet 64P Churyumov-Gerasimenko.

Once there, Rosetta will enter a two-year orbit around comet 64P Churyumov-Gerasimenko. At the beginning of this period, a small lander called Philae will take an excursion to the surface and conduct a host of experiments that will result in a complete study of the comet. This is the first of any such endeavor. Of additional importance is the fact that while Rosetta is orbiting comet 64P Churyumov-Gerasimenko, the comet will be traversing its own orbit headed to the Sun.

The ESA trajectory correction maneuver proceeded automatically, as Rosetta's antenna were pointed away from the ground-based radio commands. A second maneuver may take place on November 1 if the rocket thruster burn boosts following the first correction should be inadequate and a greater correction needed.

"Thruster burn boosts accuracy of Rosetta's Earth approach," European Space Agency.