In an emotional goodbye broadcast on Twitter, the European Space Agency (ESA) in July powered down its last remaining link to a pioneering spacecraft named Philae. The robotic lander, about the size of a washing machine, made history in November 2014 after leaving its Rosetta mothership and achieving the first-ever soft landing on a comet nucleus. That it made it to the surface at all was a dramatic stroke of luck. A harpoon system on Philae meant to attach the lander to the surface of Comet 67P/Churyumov-Gerasimenko after touchdown failed to engage, causing the spacecraft to "bounce" four times and finally settle in an undetermined location.
While amazingly intact and operational, Philae's luck came with a time limit. The site it had bounced to was in the shadow of a cliff or crater, giving it a limited amount of sunlight with which to power its batteries. Over the next several days, ESA team members scrambled to extract as much information from the surface as possible. Some 64 hours after leaving Rosetta, Philae went into hibernation mode and, with the exception of a few pings in June 2015, has never been heard from again.
But on Sept. 5, the lander's final resting place was found. Shortly before the end of the Rosetta mission, the spacecraft's high-resolution camera showed Philae to be hiding in a dark crevice on Abydos, the comet’s smaller lobe. “This remarkable discovery comes at the end of a long, painstaking search,” Patrick Martin, ESA’s Rosetta Mission Manager, said in a press release. “We were beginning to think that Philae would remain lost forever. It is incredible we have captured this at the final hour.”
“This wonderful news means that we now have the missing ‘ground-truth’ information needed to put Philae’s three days of science into proper context, now that we know where that ground actually is,” added Matt Taylor, ESA’s Rosetta project scientist.
The European Space Agency's Philae lander touched down on Comet 67P/Churyumov-Gerasimenko in 2014, but its final location wasn't known until recently due to the probe's rough landing. (Photo: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
Despite the robotic lander's rough landing and untimely demise, the mission was still able to complete an incredible 80 percent of its first science sequence goals. Below are just a few of the amazing discoveries Philae, the little lander that could, made from the surface of Comet 67P.
1. A comet with a diverse surface
The one unexpected benefit of having Philae bounce uncontrollably several times on Comet 67P was the surprising surface readings it captured. Scientists had assumed the surface was likely very loose due to its low gravity. There were even fears that Philae might "sink" upon final touchdown into the soft dirt. Instead, the spacecraft encountered a surface both soft and hard, with the latter unyielding even to an onboard hammer that attempted to crack the surface. "In the future, one would have to think about a mechanism that can cope with pretty hard material," project manager Stephan Ulamec told Nature.com.
2. And a porous surface
While the comet is surprisingly hard, it nonetheless is also extremely porous. Even with all of the dirt, rocks and large boulders strewn across its surface, Philae's instruments found Comet 67P has a high porosity of 70 to 80 percent empty space. According to the ESA, the comet's interior structure is likely comprised of "weakly bonded ice-dust clumps with small void spaces between them."
3. A comet rich in organic compounds
Could the building blocks for life on Earth come from early impacts with comets? That theory received a boost after Philae discovered that Comet 67P contains 16 organic compounds, including four never-before detected on comets. Some of these compounds "play a key role in the prebiotic synthesis of amino acids, sugars and nucleobases: the ingredients for life," the ESA says.
4. A chilly reception
When Philae touched down on Comet 67P, it turned on its thermostat and recorded some very cold temperatures. At a distance of 3 Astronomical Units (about 280 million miles) from the sun, the surface of the comet shifts between -292 degrees Fahrenheit (-180 Celsius) and -229 degrees Fahrenheit (-145 Celsius) during the day. As Comet 67P approached the sun, Philae briefly noted a dramatic improvement in its internal temperature, at one point reporting a reading of 32 degrees Fahrenheit.
5. Comet 67P's nucleus is not magnetized
Because comets contain some of the most pristine materials in our solar system, scientists often look to them for clues as to how bodies such as planets and stars came to form. For the longest time, researchers wondered if the nucleus of a comet contained its own magnetic field — a clue that might help explain how the icy rock initially coalesced. Philae, during its bounces across Comet 67P's surface, ended up returning a surprising answer.
"If the surface was magnetized, we would have expected to see a clear increase in the magnetic field readings as we got closer and closer to the surface," Hans-Ulrich Auster, co-principal investigator for the mission, told the ESA. "But this was not the case at any of the locations we visited, so we conclude that Comet 67P/Churyumov-Gerasimenko is a remarkably non-magnetic object."
6. A remarkable alien landscape
As Philae descended towards the surface of Comet 67P, it captured a dramatic series of photos that presented a celestial body with tremendous character. Massive boulders, towering cliffs and plains littered with soil and broken debris can be seen in the 1,776 photos captured by the lander. They offer a view of an alien landscape that conflicts with the traditional images of comets as nothing more than glowing balls of rock and ice.
Editor's note: This story has been updated since it was originally published in July 2016.