European Spacecraft Nears Rendezvous With Comet

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from The Wall Street Journal,

After Decadelong Journey, Rosetta Mission About 125 Miles From Comet.

An artist's rendering of the European Space Agency's Rosetta spacecraft.

After journeying for a decade and about four billion miles through the solar system, a European mission called Rosetta is tantalizingly close to becoming the first spacecraft in history to rendezvous with a comet.

The craft is currently about 125 miles from the comet and traveling at about a yard per second.

A final maneuver on Wednesday is expected to brake Rosetta to a much slower pace alongside the comet. By then, both craft and comet will be in the same orbit around the sun.

In November, the Rosetta scientists hope to pull off an even more ambitious trick: landing a probe on the comet’s surface, something that has never been done before.

Both the orbiting craft and the surface probe are then expected to provide a wealth of data, especially as the comet gets closer and closer to the sun.

As it does so, it will become increasingly active, start to lose material and then form a halo and twin tails.

“We’ll have an unprecedented look at how a comet works,” said Matt Taylor, project scientist at the Paris-based European Space Agency, or ESA, which runs the Rosetta mission.

Rosetta was originally intended to also bring back material to Earth for study, but that was deemed to be too complicated.

The craft was launched in 2004 and put into deep-space hibernation for 31 months, waking up in January for the last leg of its rendezvous with a comet called 67P/Churyumov-Gerasimenko.

The comet is roughly 2.2 miles by 2.5 miles in size and a daunting destination. It is likely composed of a fluffy mix of dust and ices.

Instead of being spherical, its shape resembles a rubber duck. Its gravitational field is weak and hard to predict.

When the probe lands on the surface, it must immediately fire an anchoring harpoon into the surface to prevent it from bouncing back into space.

Right now, the comet is largely dark and inactive. Recent measurements by ESA suggest that it has a dark, dusty crust.

But eventually, the Rosetta scientists will put their craft into orbit some 19 miles above the comet’s surface—and that will be a good place to witness some celestial fireworks.

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