On a dark stretch of the chilly Comet 67P/Churyumov–Gerasimenko the lander Philae has begun a lonely and silent vigil. After it landed awkwardly and bounced across the comet’s surface on November 12, 2014, Philae operated for just under three days—its planned primary mission length—before running out of energy and falling into hibernation. As the comet approached the sun in 2015 and the solar-powered spacecraft was able to warm up and recharge its batteries, the European Space Agency (ESA) reestablished contact on June 13, but messages were sporadic and the craft went silent on July 9. After tireless attempts to regain contact with the first spacecraft to land on a comet, the ESA will officially end Philae’s mission this week. Rosetta, the probe that carried Philae to the comet, is still orbiting the body and will continue to collect data.

Losing contact with a spacecraft, be it a lander that ran out of energy or an orbiter that was intentionally crashed, is always bittersweet—and also expected—for the humans who build and operate it. “If you plan out your mission, you’re aware you only have certain given time to collect your data,” says Stephan Ulamec, the Philae lander manager for the Rosetta mission.

Philae has had a rocky life on Comet 67P. For a landing like Philae’s there was only so much planning the engineering team could do. Unlike for landers destined for Mars or the moon, Ulamec and his team did not know what Philae’s landing site would look like in advance and knew very little about the comet itself, because no previous spacecraft had ever imaged it up close. Mission planners had to wait until the Rosetta probe reached the comet and could send back surface images before they could begin to rapidly select a good landing site. Because of these constraints, rather than design the lander for the specific terrain, they had to pack it with redundancies and make it flexible enough to operate on a variety of possible topographies.


Space Graveyard 
Defunct spacecraft litter the solar system from decades of expired missions. Some of them are landers and impactors that studied the surface of our astronomical neighbors and eventually lost contact with Earth. Others are orbiters sent to various bodies that were intentionally deorbited and crashed after their missions were complete. Here we show the final resting sites for a selection of these dead or purposely terminated spacecraft. 
Graphic by Amanda Montañez

These redundancies were vital when Philae’s initial bounce across Comet 67P/C–G’s surface landed it in an inhospitable region of the comet, called Abydos. Without sunlight, Philae’s solar array could not collect enough light to keep the lander going. It was able to use its backup battery to operate for a few days, during which it took measurements of the comet’s composition, surface strength and magnetic properties while also snapping pictures and scanning the comet’s internal properties. In fact, it was able to complete roughly 80 percent of its primary science goals in its short operational life.

If Philae had been able to anchor itself to its quickly selected landing spot, it might have stayed in touch with Earth for months, not days. Unfortunately, it was unable to secure itself to the comet’s surface on its initial touchdown. A combination of harpoons and thrusters attached to the lander that were designed to anchor it both failed; ice screws meant to drill into soft material could not penetrate the hard surface. Because of the comet’s extremely low gravity, after touching down it bounced to a less favorable location, and is thought to have come to rest shadowed from the sun, thereby unable to use its solar panels to recharge its batteries.

Serendipitously, the bounce helped reveal some details about the comet’s surface, which was initially expected to be relatively soft. “Many people were warning us that we would sink into the surface, like if you drop a stone into new-fallen snow,” Ulamec says. Instead, the fact that Philae bounced at all proved, surprisingly, that the comet’s surface was hard.

Losing contact with the lander does not disappoint Ulamec, considering how challenging the mission was and how much data Philae managed to collect despite its difficulties. “What was a bit disappointing was reaching contact again last summer and feeling there was a real chance at getting additional data,” Ulamec says. But saying good-bye to Philae had always been the plan. “The fact that we have a limited lifetime, that’s just how these missions have to be.”

Many times that limited lifetime is longer than anticipated. The NASA rover Opportunity, for instance, has explored Mars’s surface for the past 12 years whereas its initial life span was conservatively expected to be just 90 days. “The Energizer Bunny wouldn’t have lasted a day on Mars, yet this rover lasted over 4,000 days and it keeps going,” says John Callas, Mars Exploration Rover Project manager for Spirit and Opportunity. “This is like your 95-year-old grandmother playing a tough game of tennis every single day.”

Opportunity’s twin, Spirit, was also long-lived in comparison with Philae, but its ultimate demise was for similar reasons. After six years Spirit became mired in sand and could not position its solar panels toward the sun to collect enough energy to power its heaters so it survive the Martian winter.

After decades of probes launched to space on behalf of NASA, ESA and other international space agencies defunct spacecraft now dot the solar system, either drifting through space, in orbit or lying in rubble or at rest on the surfaces of planets, asteroids, comets and moons. “These vehicles are our proxies for exploration. We put them in these harsh environments and we send them on these one-way trips—we’re not bringing them home,” Callas says. These robotic vehicles deepen our understanding of the solar system by going where humans cannot. Although it is always sad to say good-bye, what they discovered endures and fuels future missions.

In the meantime the Rosetta probe is still going strong and will ideally enter a low orbit of the comet later this year. At that point, it should be able to get a visual of the lander on the comet. So although we may not be able to communicate with Philae, this likely is not the last we’ll see of the robotic explorer.