Rosetta Spacecraft Set for Unprecedented Close Study of a Comet

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After 10 years and four billion miles, the European Space Agency’s Rosetta spacecraft will arrive at its destination on Wednesday for the first extended, close examination of a comet.

The last in a series of 10 thruster firings over the past few months will slow Rosetta to the pace of a person walking, about two miles per hour relative to the speed of its target, Comet 67P/Churyumov-Gerasimenko, at a distance of about 60 miles.

Photographs have already revealed a surprisingly irregular shape for the 2.5-mile-wide comet, possibly an amalgamation of two icy bodies or a result of uneven weathering during previous flybys. From a distance, the blurry blob initially looked somewhat like a rubber duck. As the details came into the focus, it now more resembles a knob of ginger flying through space.

Continuing a trend of anthropomorphizing, the Rosetta mission managers tweeted a photograph of the comet on Monday with the comment “Do you think I got #67P’s good side yesterday?”

Over the coming months, Rosetta and its comet, called C-G for short, will plunge together toward the sun.

“The key thing is we’re rendezvousing and escorting right in alongside the comet for an extended period, for over a year,” said Matthew Taylor, the mission’s project scientist.

They are 334 million miles from the sun (more than three times as far out as Earth), traveling at 34,400 miles per hour.

Comets, made of ice, dust and rock, are frozen leftovers from the formation of the solar system. Rosetta is named after the Rosetta Stone, the engraved block that was crucial in deciphering Egyptian hieroglyphics, and scientists hope that the spacecraft’s observations will offer important clues to how the solar system came together 4.5 billion years ago.

In June, the spacecraft measured the flow of water vapor streaming off the comet at a rate of about two cups a second, which would fill an Olympic-size swimming pool in about 100 days. As the comet accelerates toward the sun, its surface will warm, and the trickle will grow to a torrent of hundreds of pounds a second, forming the long tail characteristic of comets.

Measurements in July put the surface temperature at minus 94 degrees Fahrenheit, or minus 70 Celsius. That was warm enough to indicate that the surface was not exclusively ice and that some parts were dusty and darker, absorbing more heat from the sun.

Rosetta is carrying a small 62-pound lander, named Philae after the island in the Nile where the Rosetta Stone was found. In November, Philae is to leave the spacecraft, set down on the comet and harpoon itself to the surface. That will be the first time a spacecraft has gently landed on a comet. “It’s really going to get down and scratch the surface to get the most pristine material that we can from the surface of the comet,” Dr. Taylor said.

Designed to operate through 2015, Rosetta and Philae will make observations as the comet makes its nearest approach to the sun a little more than a year from now, at 115 million miles, still outside of the orbit of Earth. The comet will remain too dim to be seen by the naked eye.

Other missions to comets have made brief flybys, beginning with the International Sun-Earth Explorer-3 in September 1985. NASA’s Deep Impact slammed into a comet in 2005, and another mission, Stardust, collected particles of dust and returned them to Earth for study.

(ISEE-3 is back in the news, because the spacecraft, still largely working, will zip past Earth again next week.)

The Rosetta mission, costing 1.3 billion euros, or $1.7 billion, will provide a much longer, much closer look at one comet. Instead of taking a brief snapshot, Rosetta will observe the comet going from quiescent to active, and then will make before-and-after comparisons.

“We’ll observe how this occurs, how this activity is onset, how it fluctuates, really how a comet works over a long time period,” Dr. Taylor said. “That’s really the difference between this and anything that’s been done before.”

Launched in March 2004, it followed a circuitous route through the solar system, using flybys of the Earth and Mars to fling itself into the same orbital path as Comet C-G. In January, it successfully emerged from a long hibernation and began its final approach.

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Rosetta Spacecraft Set for Unprecedented Close Study of a Comet

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