ON NOVEMBER 12th the European Space Agency announced, with a mixture of relief and triumph, that Philae, a robotic probe, had successfuly landed on its target, a 4km-wide comet called 67P/Churyumov-Gerasimenko. It was a stunning technical accomplishment. But as the minutes and hours passed, it became clear that things had not gone entirely according to plan. The Economist's print edition closed before much was known, but a general idea of what went wrong is beginning to emerge. The probe has landed successfully, but it is in the wrong position, short of power, and may not be securely anchored to the comet.
Landing on a comet is tricky, even by robotic spacecraft standards. Unlike planets, comets lack both an atmosphere, which can be used to slow a space probe's descent, and strong gravity, which sticks a landed probe to the surface. Because comet 67P is so tiny, its gravity is feeble. Anything lifting off from its surface at a speed greater than about one meter per second will escape entirely and go careering off into space. It was vital, then, that Philae make a gentle landing, and have some means of staying put once it was down. That, it has now emerged, did not happen, thanks to what could only be called hard luck.
The probe's designers equipped their craft with several solutions to anchor itself at the landing site. A small rocket was designed to fire on touchdown to push the craft downwards and prevent it from rebounding off the surface; that failed before Philae separated from its mothership, Rosetta. ESA's mission controllers decided to go ahead anyway, relying on a pair of harpoons designed to fire into the comet's surface and anchor Philae in place. In the event, those failed to deploy during the landing.
To complicate matters further, the comet appears to be made of stiffer stuff than anticipated. When Philae hit the surface, its flexible legs absorbed some of the impact energy-but not enough to prevent a bounce. And because 67P's gravity is so feeble, even a small jolt was enough to send the probe rebounding hundreds of metres back into space. Over the course of nearly two hours, the comet's gravity slowed Philae's escape and drew it back, whereupon it made a second bounce. Seven minutes later, it finally came to rest.
While the probe was briefly back in space, the comet continued to rotate beneath it, so that when Philae made its third and final touchdown, it was far from the chosen landing site (the ESA is still not sure of the exact location). Worse still, the probe is lying at an awkward angle, in the shadow of a cliff, with two of its feet on solid comet and one akimbo. That means its solar panels are illuminated for just an hour and a half of every day. If the sun cannot recharge the craft's secondary battery, mission controllers will have to think hard about how to use what is left in the primary. One option might be to use the craft's legs to hop it to a sunnier spot.
All of this has provided a gripping drama for a watching public. But the scientists behind the mission have repeatedly said that whatever the outcome, Philae is already doing what it was designed to do. Its science programme is designed to be finishable within two days. Eight of its ten instruments are working, it is in contact with Rosetta and it has from the start been furiously collecting data (it was by analysing the strength of the comet's magnetic field that ESA was able to infer the bounces). However, with such a loose grip, it may well be unable to use a drill designed to take samples from the comet's surface, for fear that the torque would simply send the probe spinning.
While it is surely the part of the Rosetta mission showcasing the most derring-do, Philae is just one piece of the puzzle. Rosetta will continue to orbit and monitor the comet as it nears the sun, getting closest in August 2015. The success of the first-ever landing on a comet must, at this stage, be considered a qualified one. But the mission's scientists and engineers, exhausted though they are after two days' continuous work, are still smiling.
Entities 0 Name: Rosetta Count: 4 1 Name: European Space Agency Count: 1 2 Name: ESA Count: 1 3 Name: Philae Count: 1 Related 0 Url: http://ift.tt/1ubKqBI Title: The Rosetta comet landing has made history Description: After 10 years of hard work and one nerve-wracking night, the Rosetta mission has made history by landing on the surface of a comet. The lander Philae was confirmed to touch down on the surface of the comet more than 300 million miles away at 11:03 a.m. Eastern.
Post a Comment for "Cometary science - The Economist"