About 6 weeks ago, an 85-second message from the Philae lander put its project manager, Stephan Ulamec, in a justifiably exultant mood. After 7 months of silence, the lander was alive and well on the surface of comet 67P/Churyumov-Gerasimenko. But Ulamec's mood has darkened since then, as radio links with the lander have become short and spotty, then faded out. The team has not heard from Philae since 9 July. “It's a bit frustrating to have an apparently working lander on a comet surface and not being able to communicate with it,” says Ulamec, of the German Aerospace Center in Cologne.
But even if little Philae's working life is over, its short career yielded solid returns. On 12 November 2014, the orbiting mother ship Rosetta dropped Philae to the surface. The harpoons and retrorockets meant to hold it there did not work, but the lander did not fly off into space. Instead, it bounced and came to an awkward stop on its side in the shadow of a cliff. It had less than 3 days to perform science before its batteries were exhausted and it went into hibernation. This week in Science, the lander's science team publishes the fruit of those 3 days: a suite of seven papers that describe the mechanical, compositional, and textural properties of the comet surface and its interior.
Philae's investigators had hoped to build on those findings. After Philae woke up on 13 June, it was in contact with Rosetta six more times over 10 days. It sent encouraging “housekeeping” data: The lander was warm and its solar panels were receiving sunlight, which will increase up until 13 August, when 67P reaches its closest point to the sun. Engineers lowered the altitude of Rosetta's orbits in hopes of improving the radio link.
But since 24 June, the team has heard from the lander just once—on 9 July. Ulamec says one of Philae's two receivers is dead, and one of its two transmitters may also be on the fritz. Flight engineers are reluctant to send Rosetta too close to the comet, which is belching an expansive halo of dust as the sun warms ices in its interior, causing them to sublimate into jets of gas. Too much bright, reflective dust can mimic the stars that Rosetta relies on for navigation, overwhelming the spacecraft's star trackers and causing it to reboot—a process that wastes days of valuable scientific time and risks leaving Rosetta stranded in an orientation in which it cannot receive commands from Earth.
On 11 July, 2 days after the last contact, the craft had star tracker problems at about 165 kilometers above the comet. Engineers have since pulled back to safer orbits 190 to 210 kilometers above the surface. Moreover, during August, the orbiter will spend more time exploring the comet's southern terrains, which are now illuminated—but that will mean fewer passes over Philae, which sits in the north.
The lander team can console itself with what it did find. Two mass spectrometer experiments designed to ingest and analyze samples never received material drilled from the subsurface, as planned. But scientists working on the experiments think they did capture material that accidentally fell in after the lander's first bounce kicked up dust. The instruments detected organic compounds similar to those that astronomers have spotted in the dust and gas of other comets' comas. But they found no sulfur-bearing compounds—a surprise given that the Rosetta orbiter has remotely detected such compounds just above the surface. And one of the two experiments found four compounds never before detected on comets.
An experiment that analyzed radio waves traveling through the body of the comet when the orbiter and lander were on opposite sides of it found that 75% to 85% of 67P's interior is nothing but void space. If comets are icy dirtballs instead of the dirty snowballs scientists once pictured, those dirtballs must be loosely packed. Valerie Ciarletti, a member of the radio experiment team, says the interior was also strikingly uniform. That's odd, she says, because the orbiter has detected strong variations in both the abundance and composition of ices outgassing from the surface. Some areas are rich in carbon monoxide, for instance, whereas others are enriched in water.
The uniformity also raises doubts about theories that comets accreted layer by layer in the early solar system, says Jessica Sunshine, a comet researcher at the University of Maryland, College Park, who is not on the mission. However, the radio experiment fell short in one of its key tasks: explaining whether the two lobes of 67P began as separate cometesimals that stuck together, or whether they formed from a uniform body that eroded away preferentially around its neck. The experiment had time only for the lobe where Philae came to rest—the “head” of the duck-shaped comet. “It's really a pity that we were not able to sound both of them,” says Ciarletti, a planetary scientist at LATMOS-IPSL, a research institute near Paris.
Philae even sent pictures from its 360° camera, CIVA. Jean-Pierre Bibring, who led the camera team and is one of Philae's two lead scientists, says he was surprised to see very little ice. Instead, he found grains of dust that were far bigger than expected, stuck together to form meter-sized blocks resembling conglomerate rocks on Earth. “The cement of a comet is probably not ice,” says Bibring, a planetary scientist at the Institute of Space Astrophysics in Orsay, France. Instead, it may be the stickiness of the dust grains themselves—complex organic material from the presolar nebula in which the comet formed more than 4.5 billion years ago.
Sunshine isn't convinced yet. She says the surface materials may not be pristine—they may have been altered by ultraviolet light. “We're still dealing with the skin-deep problem,” she says. For her, one of the surprising results came from an analysis of the lander's initial bounce. Sunshine says most scientists expected comets to be “fluffy,” but Philae's trajectory showed that, underneath about 20 centimeters of soft dust, the comet has a thin, relatively strong layer, perhaps formed as ice sublimed and left behind dust that “sintered” together. The soft outer layer may consist of dust that blew off and settled back to the surface. Computer simulations work showed that larger infalling objects can splash dust across the surface, covering its features like drifting snow—but with no need to invoke any cometary “wind.” “Material is moving around this thing,” Sunshine says. “That's sedimentary geology.”
Layers of settling dust could eventually become a problem for Philae's solar panels. For now, light levels look good, and Ulamec says that, since the 13 June contact, they have increased beyond what would be expected with the change in seasons—an indication the lander has shifted to a more favorable angle (although the tilt may have also made its radio connections worse). He is looking to September and October as the last chance to get the lander doing science again before the comet leaves the inner solar system and sunlight dims too much. “We of course keep trying,” he says. “The lander surprises us again and again.”