Astounding: We're about to stick a probe in orbit of a COMET

Rosetta craft screams 6 billion km through space to reach its cosmic prey

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Pic The European Space Agency will be on tenterhooks tonight as the Rosetta space probe reaches its final destination and begins the burn sequence that will put a man-made spacecraft in orbit around a comet for the first time.

After a much-delayed launch in 2004, Rosetta has spent the last ten years chasing through the solar system after comet 67P/Churyumov-Gerasimenko. The 2.8m-by-2.1m-by-2.0m probe is now within 300km of its target and on Wednesday will begin the final six-minute, 26-second rocket burn that will put it into a looping orbit.

If successful, it will be a fantastic achievement for the ESA. The orbital mechanics required to catch and circle its target alone have taken years to formulate and massive amounts of computer processing to check.

So far, Rosetta has used the gravity of Earth and Mars to pick up speed, traveled past the asteroid belt, made it almost to Jupiter, and is now approaching the dusty ice-ball that is 67P.

Rosetta has used its encounters with Earth and Mars to boost it up to speeds of over 800 meters per second using virtually no extra fuel. The Mars maneuver was particularly tricky, taking Rosetta to within 250km of the Martian surface and stretching the solar-powered probe's battery to the limit as it moved into the Red Planet's shadow.

While 67P is Rosetta's final target, the probe hasn't been slacking on the scientific front. In 2008 it sent back the first pictures of asteroid 2867 Šteins from 800 kilometers away, and it scraped by the 100-kilometer-wide 21 Lutetia asteroid in 2010, sending back over 400 pictures of the wandering space rock.

Comet 67P/Churyumov-Gerasimenko

Comet 67P/Churyumov-Gerasimenko, as photographed by Rosetta from 1,000km away

After its epic trip, the spacecraft's main mission begins. Once a stable orbit has been achieved, Rosetta will start using its 12 scientific instruments to investigate the rapidly spinning comet.

67P is an unusual comet, based on what little information we have about these traveling snowballs. For a start, it looks like two bodies that have fused together gently enough to have avoided shattering.

Early temperature readings have shown that 67P is covered with a thick layer of dust that it has picked up in its travels through the Solar System. As the comet approaches the Sun, this dust will be ejected along with volatiles from the comet's core, which will form a coma around the comet that will be visible to space telescopes.

After mapping out the comet and finding a flat spot, Rosetta will deploy its Philae lander. Philae will power towards the comet at a relatively leisurely one meter per second and fire two harpoons into its surface, then draw itself down to make mankind's first landing on a cometary body.

Philae will then scan the material that makes up the comet using its nine instruments. These include radar and spectrometers, and the lander will also drill 20cm into 67P's surface to take samples of the raw material that lies beneath its dusty shell.

Philae has enough battery power for around 60 hours of observations, but it is also equipped with solar cells that could keep it going for months. It will send its information back to Rosetta, which will forward it on to Earth's ground stations.

In all, the ESA scientists are expecting to get around 19 months of good science from Rosetta and Philae as 67P falls towards the Sun and swings back out into space.

That last part is slightly speculative – since we know so little about 67P, we can't be sure it won't suffer comet ISON's fate and burn out completely.

But hopefully that won't happen, and Rosetta and its lander will continue sending back data until their batteries are drained and the probes are too far from the Sun for a recharge. After that, they will remain a wandering testament to mankind's scientific exploration – possibly long after humanity has shuffled off this mortal coil. ®

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