European Spacecraft Nears Rendezvous With Comet

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European Spacecraft Nears Rendezvous With Comet
After Decadelong Journey, Rosetta Mission About 125 Miles From Comet
The Wall Street Journal
By Gautam Naik  Aug. 4, 2014 7:09 p.m. ET



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



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.

Comets are among the most primitive objects in the solar system.

Their chemical composition has stayed largely unchanged since they formed more than four billion years ago.

A close look at a comet can provide valuable clues about how the solar system formed and evolved, and whether comets, which carry complex organic molecules, may have helped to kick-start life on Earth.



European Space Agency technicians celebrated in January after receiving Rosetta's wake-up signal in the control room in Darmstadt, Germany. ESA/Associated Press


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.

"Navigating and orbiting such a complex gravitational field will always be fraught with problems," said Daniel Brown, astronomer at Nottingham Trent University in England, who isn't involved in the Rosetta mission.



This image was taken on Sunday from a distance of about 300 kilometers from comet 67P/Churyumov-Gerasimenko. European Space Agency


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.

As the comet flies closer to the sun, its icy material will break off in the form of gas jets. The comet will get progressively brighter and form two tails, one from dust particles and the other from ionized gas.

"Tens to hundreds of tons of material will fly off," said Dr. Taylor. For the first time in history, "we'll have a ringside view to see this."


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