The Telegraph
Thursday , August 7 , 2014
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Spacecraft pulls alongside comet
- After 10 years, Rosetta ready for the first close examination of a space rock

Aug. 6: After 10 years and a journey of four billion miles, the European Space Agency’s Rosetta spacecraft arrived at its destination today for the first extended, close examination of a comet.

A six-minute thruster firing beginning at 10am GMT, the last in a series of 10 over the past few months, slowed Rosetta to the pace of a person walking, about two miles per hour relative to the speed of its target, Comet 67P/Churyumov-Gerasimenk.

Over the coming months, Rosetta and its comet, called C-G for short, will plunge together towards the sun. In November, a small 220-pound lander 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,” said Matthew Taylor, the mission’s project scientist.

At this point, the comet and its shadowing spacecraft are more than 330 million miles from the sun (more than three times as far out as Earth), travelling at 35,000 miles per hour. With the firing of the final thruster, Rosetta was a mere 60 miles from the comet’s surface. “The key thing is we’re rendezvousing and escorting right in alongside the comet for an extended period, for over a year,” Taylor said.

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. (Rosetta’s lander, Philae, is named after the island in the Nile river where the Rosetta Stone was found.)

Photographs have 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 trips to the inner solar system.

From a distance, the blurry blob initially looked somewhat like a rubber duck. As the details came into focus, it began to bear a closer resemblance to a knob of ginger flying through space.

In June, the spacecraft measured the flow of water vapour 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 towards the sun, its surface will warm, and the trickle will grow to a torrent of hundreds of pounds a second, forming the long tail that is characteristic of comets.

Measurements in July put the surface temperature at minus 94 degrees Fahrenheit, or minus 70 degrees 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.

Designed to operate till 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 the orbit of Earth. The comet will remain too dim to be seen by the naked eye.

The $1.7 billion Rosetta mission will provide a much longer, much closer look at one comet.

Instead of taking a brief snapshot, Rosetta will observe as the comet goes from a quiescent ball of ice and rock to an active comet spewing out dust and gas and then 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,” Taylor said.