A spacecraft's long comet chase into space


How Rosetta used the gravity of two planets to slingshot itself in pursuit of Comet 67P.


An artist’s impression of the complex flight path that saw Rosetta flung into deep space before heading back to track Comet 67P, and close in to launch the lander. – ESA–C. Carreau

Putting Rosetta into hibernation may have given ESA staff a bad case of nerves but the decision was made for good reasons, say engineers. In order to track Comet 67P as it heads toward the Sun, the probe had to fly deep into space before heading back just behind the comet.

Mission controllers achieved this orbit by making three close fly-bys of Earth and one of Mars, flinging Rosetta so far from the Sun that its solar panels could harvest little energy. As a result, the spacecraft was put in a deep sleep to save power.

But now it is arcing back towards the Sun with more power becoming available for its instruments every day. By May it will be ready to fire its thrusters and move in on Comet 67P. By September it will have reached its target and will then follow the comet as it makes its closest approach to the Sun.

“With Rosetta, we will track the evolution of a comet on a daily basis and for over a year,” says Matt Taylor, a Rosetta project scientist.

It will not be easy. Comet 67P is only 2.5 miles in diameter, too small to have a meaningful gravitational field. As a result, Rosetta will have to make constant use of its thrusters to keep in position near the comet – half the probe’s launch weight had to be fuel, for this reason – while its lander Philae will have to use harpoons to bolt itself to Comet 67P.

In addition, vapour and gas pouring from the comet will slam into Rosetta’s solar panels like winds whipping into sails. But the probe can’t be flown directly by controllers on Earth. The spaceship will be hundreds of millions of miles from home and signals will take tens of minutes to be transmitted to and from ESA control, which means most of the probe’s operations will have to be controlled semi-autonomously by on-board computers.

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