Philae, now silent, was an 'incredible scientific success'

The European Space Agency's Philae lander has ended its brief, historic mission to visit the surface of a comet. The craft has given scientists an unprecedented look at the structure and composition of a comet's nucleus.

The 220-pound lander fell silent overnight just before losing communication with its mother ship, the comet orbiter Rosetta. In addition to its own science agenda at comet 67P/Churyumov–Gerasimenko, Rosetta served as a relay station for data Philae gathered. Regular communications blackouts between the two craft were expected during periods when Rosetta's orbit took it below the comet's horizon.

However, by the time Philae's batteries drained to a point where they no longer could support communications, the craft had successfully shipped Earthward all of the data it had gathered since landing on the comet on Wednesday.

Moreover, mission controllers were able to rotate the craft's body so that the largest of its solar arrays – installed along the craft's sides – would be exposed to what little sunlight the landing site allows to reach the craft.

Engineers overseeing the lander's systems say they hope that the intensity of the sunlight hitting the panels increases as the inbound comet nears its closest approach to the sun during the next nine months. But they also acknowledge that this is a long shot.

As a comet moves closer to the sun, it warms up to eject increasing amounts of gas and dust. In addition to forming a comet's tail, this material also gathers as an extended halo around the nucleus. Even as the sunlight grows more intense, the thickening halo would block some of the increase.

Still, “this machine performed magnificently under tough conditions, and we can be fully proud of the incredible scientific success Philae has delivered,” said Stephan Ulamec, the mission's lander manager, via the missions blog.

“The lander was able to transmit all science data gathered” during the all-important first – now likely final – agenda for the nine experiments on board, he noted. All nine were able to gather data – from images of the surface to the composition of material beneath the surface.

Members of the science team said that their first major “data dump” to colleagues will take place during presentations at the fall meeting of the American Geophysical Union, which is scheduled for  Dec. 15-19 in San Francisco.

Philae's final signals marked a jubilant end to a key element of ESA's $1.7-billion Rosetta mission, if for no other reason than the landing could have gone horribly wrong.

In the run-up to Philae's release from Rosetta, controllers learned that a thruster on top of the boxy lander was not working. The comet's gravity is weak; the thruster was designed to offset the recoil from two harpoons the lander was to have fired into the comet's surface on touchdown. These were science instruments, as well as anchors to keep the craft firmly on the surface of the nucleus, since they would provide information on the mechanical strength of the surface.

Upon touchdown, however, the harpoons failed to fire as well. This left the craft to bounce twice. The first bounce lasted nearly two hours. The craft touched down, then bounced again for another seven minutes, finally coming to rest on its three legs under the shadow of a cliff – with one “foot” apparently dangling over a precipice.

Yet even this unplanned hopping provided unexpected insights, mission managers suggested, because the bounces implied a harder surface than they had expected to encounter, given the images showing that the planned landing site was covered with what looked to be loose sediment.

Researchers still aren't sure where Philae settled. The Rosetta team is poring over images taken from orbit in hopes of spotting the lander on the surface.

But now, they have data that will keep them busy for years as they analyze it and incorporate it into the larger body of knowledge about comets – primitive objects from the dawn of the solar system that are widely thought to have contributed to Earth's inventory of water and of organic compounds that served as building blocks for the emergence of life.

As for ESA, despite the malfunction and misfire the mission so far represents a spectacular success. The agency has overseen a project that has sent the first craft to orbit a speeding – and still accelerating – comet and operate a lander from the comet's surface.

The last mission the agency launched to deliver its own orbiter-lander combination to another object in the solar system was Mars Express in 2003. When the pair reached the Red Planet, the orbiter made it to orbit and is still returning data today. The lander, Beagle 2, never phoned home and was declared lost.

As for Philae, it phoned home repeatedly and, comet alignment and coma permitting, might do so again, even if to say nothing more than: I'm still here.