The European Space Agency’s (ESA) pioneering Rosetta mission to land on a comet has been wildly successful, but now it appears that part of its aim, the exploration of the surface by the little Philae lander, may have been cut short.
Problems with the landing mean Philae is now out of power in a shady area, unable to tap into enough sunlight on its solar panels to recharge its batteries. But even with this setback, the mission is far from over, and has already brought us fascinating new science.
The mission is to study the comet at close range for potential insights into the origins of life on Earth. While comets originate far in the edges of the solar system, it’s thought these small icy bodies may have been responsible for bringing water and other volatile elements to a young, dry Earth in a time when the solar system was far more chaotic than it is today.
After 10 years in space, five loops around the sun and four gravitational slingshots around planets, ESA’s Rosetta spacecraft caught up with Comet 67P/Churyumov-Gerasimenko in August and manoeuvred into orbit to land a probe on it – a solar system first.
Last week, on November 12, Rosetta released Philae from 22.5km above the comet’s surface. Since the comet’s gravitational pull is tiny (rendering the 100kg probe about as heavy as a piece of paper), the lander was equipped with two comet-piercing harpoons, a thruster to push it onto the surface and ice screws on each of its three feet. The plan was for the comet to touch down, dig in, and hold on.
The bouncy landing
When Philae landed within only 15cm of its target after a suspenseful seven-hour descent, it signalled a major triumph and an incredible feat of science and engineering. The mission leaders, along with those of us cheering Philae on from home, were elated, and rightfully so.
But, unfortunately, we soon learnt that not everything went according to plan. The thruster failed and the harpoons didn’t fire, rendering Philae unable to stick its landing.
It bounced. The first bounce sent the lander flying for nearly two hours as the comet rotated underneath it. It took another short hop before it finally settled a kilometre away.
Once the dust settled and the data arrived, it became clear that instead of anchoring down on the smooth plane of the designated landing site (named Agilkia by public vote), Philae had ended up wedged in a rocky outcropping, with two feet on the ground and one reaching out into space.
The shadow problem
The awkward pose was less of a problem than the fact that the lander was now up against solid comet on three sides and therefore in near complete shadow — bad news for a solar-powered robot.
To the mission commanders at ESA, this setback meant their investigations were now governed by a countdown clock. The lander had a full battery upon release, meaning it could potentially study the surface for as much as 72 hours before it would power down.
The MUlti-PUrpose Sensors for Surface and Sub-Surface Science (MUPUS) instrument got to work measuring the comet’s temperature and attempting to hammer away at the surface. The lander’s drill managed to get 25cm into the comet to take samples, though the full analysis would take more battery than was available.
After considering all possible manoeuvres that the built-to-be-stationary lander might attempt to improve its position, the team even sent the lander a “lift and turn” command that managed to tilt the solar panels by 35 degrees — enough to give them just a little more time.
So to sleep … for now
After about 66 hours, the control room watched the voltage meter drop until the communication link could no longer be maintained. Philae had gone into sleep mode, having achieved an impressive 80% of its first-stage science goals.
The data from Philae are still being analysed and are sure to be immensely valuable. Even the unfortunate bounce might indicate that the surface of the comet is far stiffer than anticipated — not so much a “dirty snowball” as comets are generally envisioned, but a conglomeration of loosely packed clumps of hard ice.
As the mission stands now, despite Philae’s misfortune, ESA considers the landing to have been a success. But there is also some small hope that Philae might come back to life.
One of the main goals of the Rosetta mission is to observe how the comet changes when it approaches perihelion — the closest point to the sun in its highly elliptical orbit.
At the moment, the comet is 3 Astronomical Units from the sun (Earth is at 1 AU), but perihelion in August 2015 it will be only 1.2 AU. This means a huge boost in solar power, and (we hope!) it could be enough to give Philae a second lease at life.
Meanwhile, we can continue to enjoy the breathtaking images provided by Rosetta and the few close-ups Philae took before shutdown.
Rosetta has already done some fantastic science, including sampling the comet’s “perfume” and listening to it “sing” as its magnetic field fluctuates. And it will only get more interesting as the solar radiation heats and reshapes the comet’s surface.