For a mission that lasted two days instead of its planned one to six weeks, the scientists behind the European Space Agency (ESA)’s Philae comet lander are surprisingly cheerful. “We’re extremely happy with how the mission went,” lander control team member Valentina Lommatsch said during an ESA press conference November 14. “Beyond words,” gushed Matt Taylor, project scientist for the Rosetta mission, which launched in 2004 and released the Philae lander on November 12.
These reactions might be surprising, given that Philae failed to land on Comet 67P/Churyumov–Gerasimenko as intended. Instead of touching down on a safe spot on the comet and latching on with harpoons, it bounced off the surface twice before finally settling down next to a cliff that is blocking sunlight from reaching its solar panels. As a result, Philae won't be able to take long-term readings as planned; it has also had trouble drilling below the surface to collect samples for analysis, which is a disappointment. Still, the mission has been overwhelmingly successful, accomplishing between 80 to 90 percent of its main science goals, said Stephan Ulamec, lander manager at the German Aerospace Center. And even the elements of the mission that strayed from the plan, such as the bounced landing, have been useful. Because Philae technically touched down in three places before settling, rather than making a single landing, it was able to measure the magnetic field surrounding the comet at three different points, Taylor said.
Furthermore, many of Philae’s “failures” provided valuable data in themselves. For example, the thermometer probe in its Multi-Purpose Sensors for Surface and Subsurface Science instrument was unable to hammer more than a few millimeters into the comet’s surface to take temperature measurements as planned. That told the scientists the comet’s ground was much harder than predicted, with a strength comparable to solid ice—useful knowledge for a mission whose main job was to characterize a comet’s surface. This strength may have caused Philae to bounce as high as it did after its first landing attempt, when its harpoons, intended to attach it to the comet, failed to deploy. “Was it perfect? No,” says planetary scientist Bruce Betts, director of science and technology at the Planetary Society. “Was it a huge success? Yes, definitely.”
Although Philae’s landing was not quite graceful, it was the first ever “soft” landing on a comet. (NASA’s Deep Impact spacecraft dropped an impactor probe into a comet in 2005, but that was meant to crash into and disrupt the surface to create ejecta for measurement.) “We finally have ground truth from a comet,” Betts says. “This was a very hard thing to try to do, and to have achieved getting science data back at all is a success at some level.”
The lack of sunlight to recharge Philae’s batteries cut its mission short, sending the lander into hibernation on November 15, after just two days and seven hours. Given that the spacecraft’s primary science goals were mostly planned to take place during the first 72 hours or so after landing, however, this was not as big a loss as it may seem. In its short active time Philae managed to operate all of its science instruments—it analyzed the chemical makeup of gas around the comet, captured x-ray spectrometry, imaged the surface and measured temperature and magnetic properties there, among other accomplishments.
All of Philae’s events, good and bad, will inform future missions to small solar system bodies. Many of the glitches reinforce just how difficult it is to land on a comet, which does not have enough gravity to pull objects into orbit around it or hold them on its surface. Comet 67P, for example, has just one one-hundred-thousandth of Earth’s gravity. Scientists planning NASA’s Origins Spectral Interpretation Resource Identification and Security Regolith Explorer (OSIRIS-REx) mission, which will launch in 2016 to return a sample from an asteroid, is taking particularly close notes. “We are eagerly watching what you learn from actually operating in this environment and will apply that,” Gordon Johnston, OSIRIS-REx program executive at NASA Headquarters in Washington, D.C., said during the ESA broadcast.
Philae may not have had its last word quite yet. Although the lander is currently “sleeping” after exhausting its onboard battery, there is a chance it will wake again if enough sunlight hits its solar panels as the comet moves closer to the sun. Comet 67P is due to make its closest approach, called perihelion, in August 2015. Even if it doesn’t reawaken, Philae has done its scientists proud. “Let’s stop looking at things we could have done if everything had worked properly,” said Andrea Accomazzo, the Rosetta flight operations director. “Let’s look at what we have done, what we have achieved. This is unique and will be unique forever.”