After more than a decade of careful planning and hours of nail-biting tension, this morning an emissary from Earth made history’s first soft landing on a comet. The European Space Agency’s dishwasher-size Philae lander touched down on the craggy surface of Comet 67P/Churyumov–Gerasimenko shortly after 10:30 A.M. Eastern time, after being released seven hours earlier from its mother ship, the Rosetta orbiter.
“We are there. We are sitting on the surface. Philae is talking to us,” said Philae lander manager Stephan Ulamec during a live Webcast of the landing. “We are on the comet.” The lander seems to be fully operational, ready to begin its unprecedented close-up exploration of the icy comet. The lander’s suite of instruments notably includes a drill for accessing material from the comet’s subsurface as well as a small chemistry lab to analyze the retrieved samples. Data from the lander, it is hoped, will reveal more about the solar system’s earliest history, and potentially help scientists piece together a fuller picture of how water-rich comets and asteroids may have delivered oceans to the infant Earth.
Rosetta had made history itself in August when it arrived at Churyumov–Gerasimenko, becoming the first spacecraft ever to orbit a comet. It had launched in March 2004, swinging by Earth three times to gradually build up speed to reach its quarry.
Mission planners had expected the comet to be round and oblong; instead they found it resembling a toy rubber duck, made of two cratered globules—one large, one small—sutured together by a tapered neck of icy debris. That strange shape meant the comet possessed a more complicated gravitational field, which combined with the comet’s 12.4-hour rotational period and erupting plumes of gas and dust to make Philae’s landing a challenging feat of celestial navigation.
As Rosetta mapped the comet from orbit, researchers studied a plethora of potential landing sites for Philae. They quickly homed in on one in particular, a relatively flat one-square-kilometer region dubbed “Agilkia.” Perched atop the “head” of the rubber-duck shaped comet, Agilkia is a relatively flat region that struck a balance between delivering good science and ensuring safe landing and surface operations. Because of its smooth topography, Agilkia is largely free of dark shadows when illuminated, providing more sunlight for Philae’s solar panels. Near but not perilously close to sites of erupting gas and dust, the site also offered the chance for Philae to study pristine material from those jets while also radio-mapping the comet’s nucleus.
The tension surrounding the landing began to build in the predawn hours of November 12, when Rosetta released Philae from its clutches. The tiny probe’s descent would be entirely unpowered, relying solely on the comet’s weak gravitational pull to draw it to its site. The orchestration had to be impeccable, even a minuscule error in timing or trajectory would be magnified during the lander’s long free fall, putting it in the wrong region or, worse yet, making it miss the whirling comet entirely. It also had to occur almost entirely autonomously, as radio signals between the spacecraft and ground controllers took almost a half hour to cross the more than 480 million kilometers separating the comet from Earth.
If communications were lost and not reestablished between Philae and Rosetta during the descent, the mission would be in dire jeopardy—a possibility all the watching crew felt a couple of hours after Philae’s deployment, when it checked in with its mother ship late after an agonizing delay of several minutes.
Once Philae touched down, the risks would only grow; Agilkia was not perfectly flat and if the lander came to rest on a too-steep slope or a boulder, it would topple over irrecoverably. Alternatively, if its descent took it to a very firm landing site, Philae could bounce off the surface in the weak gravitational field, either falling away into space or tumbling haphazardly over the comet’s icy crags. To prevent this Philae was set to fire a stabilizing low-power nitrogen thruster and to deploy harpoons and ice-screws to secure itself to the comet. During preparations for the landing a seal failed to break on the thruster’s nitrogen tank, but the mission proceeded anyway, pinning its hopes on the harpoons and screws.
In a press conference shortly after the landing, Ulamec noted that the spacecraft seems to have landed, bounced and then settled back down again, even though its harpoons apparently failed to secure the craft to the surface.
Despite today’s historic soft landing, technically Philae is not the first spacecraft to land on a comet: That honor goes to the “impactor” of NASA’s Deep Impact mission, an instrument-laced slug of copper that slammed into the comet Tempel 1 in July 2005. Besides these two cometary landings, humans have only managed to land spacecraft on six other celestial bodies: Venus, the moon, Mars, the asteroids Itokawa and 433 Eros, and Saturn’s moon Titan.
Now that it has successfully landed, Philae’s schedule is packed with checkouts of its science instruments and initial surveys of its landing site. The lander’s main batteries are sufficient to power it for three days of science operations, after which Philae and its suite of instruments will have to rely on solar power. Mission scientists hope to keep the lander operating for perhaps half a year, however, so that it can have a front-row seat for the fireworks that will erupt as the comet swings closer to the sun in its orbit. Water ice and other volatiles will begin to sublimate off the comet in greater volumes, pouring plumes of material into space and building up a thicker coma of gas and dust.
By March of next year, the comet should be too hot for Philae to operate, setting an end-date for even the most optimistic extended mission, but Rosetta will continue on alone for months longer, watching the comet come alive beneath the sun’s broiling heat until its mission ends, too, in December 2015, more than 4,000 days after it began.