Philae, the European Space Agency's little comet lander that could, has shown the world that a third landing – however unintended – is a charm.
The craft is healthy and is returning data from the surface of comet 67P/Churyumov–Gerasimenko after sending mission scientists and controllers on an emotional roller coaster ride – one in which two key systems designed to help the craft anchor itself to the comet failed.
Instead, the craft touched the surface after a seven-hour saunter from its mother ship, ESA's Rosetta comet orbiter, and bounced twice. It finally settled in a spot where one of the lander's three feet appears to be standing on the edge of a precipice. Although it is unanchored, it's upright and has enough power to perform a programmed sequence of experiments that form the core of the lander's science objectives.
Still, the science team is seeing if it needs to juggle the sequence. For instance, the lander lacks a firm anchor to the gravitationally weak comet core, so any experiment that would result in force being applied to the lander might have to wait until the end of the sequence. That way the craft can return as much data as possible from the rest of the instrument package before planners run the most mechanically intense experiments and risk inadvertently launching the craft back into space or sending it tumbling and bouncing along the surface.
Moreover, images and telemetry coming in from the lander overnight suggest that it landed in a rock-shaded spot that will expose its solar panels to far less sunlight than the panels would have received at the original landing site.
“We're receiving about one and a half hours of sunlight with respect to the six or seven we were aiming for,” said Kuhn Gerts, one of the specialists keeping track of Philae's systems, during a briefing Thursday.
The low-light condition could prevent Philae from executing much beyond its initial 60-hour science plan, which relies on a pair of batteries in the lander to power the nine experiments onboard. The panels are designed to recharge the batteries.
Yet even accomplishing the 60-hour program would yield a bonanza of new information about comets – rubble left over from the dawn of the solar system. These objects are thought to be among the construction leftovers that delivered water and organic compounds to Earth, laying the foundation for life to emerge on Earth.
“What's really impressive is not the degree of failure that we encountered but the degree of success we have here,” said Jean-Pierre Bibring, an astrophysicist at the Institute of Space Astrophysics in Orsay, France, and Philae's lead scientist. “It's amazing where we are. We landed. We've analyzed a lot of things already. Tomorrow three more instruments will be on.... It's gorgeous where we are.”
Ironically, Philae's sequence of bounces appears to have presented one of the first scientific surprises.
The craft touched down on 67P/Churyumov–Gerasimenko at 10:34 A.M. EST Wednesday. Mission controllers quickly announced that the harpoon system – the craft's ultimate anchor to the low-density nucleus – had activated. So had several science instruments. But the data coming back from the instruments indicated that the craft was moving again. No harpoon had launched from the lander into the nucleus.
Harpoon deployment would have been a challenge anyway because a thruster atop Philae that was to have counteracted the recoil from harpoon deployment wasn't working. Controllers learned that prior to Rosetta releasing the craft, but they opted to proceed anyway.
Mission managers say that Philae's first leisurely bounce – with a hang time of just under two hours – reached nearly 3,300 feet above the nucleus before it returned to the surface. The second and final bounce lasted just over seven minutes.
Although Rosetta has yet to spot the lander on the surface, mission officials say they suspect Philae landed just over a half a mile from its initial target. Indeed, the first landing brought the craft dead center of its landing site, a bare patch atop the smaller of the two lobes that make up the comet's rubber-ducky profile, mission officials said.
The nucleus is not very dense, leading researchers to think that its surface was relatively loose and porous, with no strong crust, explained Dr. Bibring. Data on the dust around the comet as well as images suggested that the dust-covered landing site would likely have a low-density, porous surface. Such a surface might still allow for a bounce, he suggested.
Still, maintains Stephan Ulamec, Philae's lander manager, “the rebounds of the lander are indications of a higher strength material. That was a surprise to us.”
The idea that the landing site hosts a highly porous surface built of layers of dust the comet's gravity reclaims from its halo of dust and gas, that fails to explain the rebound, he said.
The uncertainty over surface conditions is reminiscent of concerns early in the US Apollo program that the first craft to land on the moon might sink deep into layers of dust that had built up over billions of years.
Philae is the up-close-and-personal portion of ESA's $1.7-billion Rosetta mission. Launched in 2004, the Rosetta orbiter, bearing Philae, reached comet 67P/Churyumov–Gerasimenko in August and began to orbit the snowy dirt ball. It will provide a global analysis of the comet and the dust and gas it emits as it makes its closest approach to the sun next year. Philae provides detailed measurements from the surface. The two already are working together via radio links to build what in effect is an three-dimensional CT scan of the nucleus's interior.