Rosetta’s Philae becomes the first to touchdown a comet

SPACE exploration is a serious scientific business. But ever since the beginning of the Space Age in the 1950s, it has been accompanied by a hefty dose of glitz and PR. Two years ago, Earthlings watched with bated breath as a one-tonne, nuclear-powered, laser-armed robot rover fizzed through the Martian atmosphere, before being deposited gently on the surface by a rocket-powered “skycrane”. The distance between Mars and Earth meant that the mission’s controllers had to wait seven agonising minutes to find out whether the rover had survived the journey. Their fingernail biting was broadcast live by NASA. When news arrived of a successful landing, they whooped, hugged and lit cigars.

Now, it is the turn of the European Space Agency (ESA) to put on a show. In August, after ten years blazing a circuitous trail through the solar system, including three fly-bys of Earth, one of Mars, two trips through the asteroid belt and a two-and-a-half year hibernation in the chilly void beyond Jupiter, its spacecraft, Rosetta, caught up with a 4km-wide comet called 67P/Churyumov-Gerasimenko. It will spend a year in orbit around this, hitching a ride as it plunges towards the sun.

And Rosetta is not alone. At 08.35 GMT on November 12th, the craft launched Philae, a boxy lander the size of a washing machine, which then began a careful, nerve-jangling descent towards the comet’s surface. Seven and a half hours later, Philae reported that it was down—but perhaps not safely.

Philae’s landing marks the first time people—or, rather, their robotic representatives have made a soft landing on an astronomical body other than a planet or a moon. It raises the number of bodies on which a landing has been accomplished to five (the others are Earth’s Moon, Venus, Mars and Titan, a satellite of Saturn).

Engineers are reasonably comfortable with sending probes to big targets like planets and moons, which have useful features like an atmosphere to slow their descent, and gravity strong enough to ensure that once a probe reaches the surface, it stays there. Comets have neither, and that makes things difficult. Philae was nudged gently away from Rosetta at a speed of about 80 centimetres a second, and left to drift towards the comet under the influence of that object’s feeble gravity.

Ensuring the probe landed on the relatively smooth area of the comet chosen by ESA’s scientists was tricky. Philae had no ability to control its descent, which meant that everything depended on the accuracy of Rosetta’s initial shove. In the event, the probe landed almost exactly on target.

Once it reached the surface, though, things went less smoothly. The comet’s gravity is so low that even a slight bounce could have sent the probe careering back into space. A pair of harpoons, designed to anchor it in place, failed to fire. Philae’s mission controllers do not know why. In the absence of the harpoons, they think the lander’s flexible legs managed to absorb most of the impact energy, but not enough to prevent a small bounce.

Philae, in other words, may have landed twice. Even without the harpoons, a system of screws in the base of its legs may offer it some purchase. But it was unclear whether these had been deployed, and therefore (as The Economist went to press) how securely—or otherwise—the probe was attached to the comet. ESA would say only that the mission was a “success”, and that plenty of science data had been collected already.

That is good, for the mission is more than just an exercise in risky deep-space engineering. It is designed to drill into the comet, taking samples of its crust and attempting to work out exactly what it is made of. Is it, for instance, mostly rock and ice with a few pockets of gas, or is it a loose pile of rubble held together by gravity?

Space agencies have sent probes to comets before, but not like this. In 1986, NASA’s Ice mission flew through the tail of Halley’s comet. In 2005, the agency’s Deep Impact spacecraft fired a massive copper block at comet Temple 1. But none before now has landed.

“We are the first to do this, and that will stay forever,” said Jean Jacques Dordain, director general of the ESA.

Matt Taylor, a Rosetta project scientist, who had selected an extremely colourful shirt for the event, revealed an impressive – and brave – tattoo of the lander on the comet’s surface.

Researchers are interested in comets because they are space-going fossils—the builder’s rubble left over from the solar system’s construction 4.6 billion years ago. That means they should contain information about how the solar system came together. One theory, for instance, holds that cometary impacts seeded the newly formed Earth with much of its water. Another suggests that comets are the source of the carbon-based molecules that were the building blocks of the first life. Instruments aboard Rosetta and Philae should be able to address those questions. After ten years, the nail-biting engineering is over, and it is time for the science to begin.

Source Credits: The Economist, The Guardian

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