On the surface of 67P/Churyumov-Gerasimenko, an icy comet that looks like a rubber duck about 370 million miles from Earth, lie the remains of the Philae lander. In 2014, it was released from the European Space Agency's Rosetta spacecraft to touch down on 67P, but not everything went according to plan. Harpoons that were supposed to pin it to the comet didn't fire, and Philae bounced off the surface, glanced past a cliff edge and disappeared from sight.
It sent signals back to Earth confirming it was operational, but where exactly it ended up was a mystery.
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Twenty-two months later, it was discovered in new images from the Rosetta probe. Philae was in an awkward position. The landing had left it lying on its side, wedged in a shadowed crack on 67P, its spider-like legs in the air. But while its first touchdown and final resting place were known, the lander's data seemed to suggest it made a brief second touchdown, lasting just two minutes, along the way. Scientists weren't sure where.
"Philae had left us with one final mystery waiting to be solved," said Laurence O'Rourke, a planetary scientist with the European Space Agency.
O'Rourke is the lead author on a study, published in the journal Nature on Wednesday, that uncovers Philae's second touchdown site and reveals how brittle the surface of the comet is. It's an excellent example of deep space detective work, showing how Philae skidded across the surface of 67P and left impressions in a region known as "skull-top ridge."
The lander's final resting place after its unfortunate bounce was a shaded grotto. Its batteries died 48 hours after its messy landing. But even though Philae's landing didn't go to plan, it has provided researchers with a golden opportunity to study 67P, revealing the interior of the comet and boulders that are extraordinarily soft — "fluffier than froth on a cappuccino," according to O'Rourke.
Philae fallin' (in the air tonight)
Comets, like 67P, are small cosmic bodies made up predominantly of dust, frozen together with rock and ice. They contain much more ice than asteroids and generally orbit the sun out in the depths of the solar system beyond Neptune, making larger loops than rocky asteroids. When comets fly closer to the sun, their frozen bodies begin to thaw, sometimes leading to dramatic outbursts. And they're old — they likely formed around the time the solar system was born and have wandered unchanged for some 4.5 billion years.
The Rosetta mission, and the Philae lander, were the first spacecraft to orbit and "soft" land on the surface of a comet six years ago. Their cameras provided one of the most incredible short films ever and gave us the best look yet at the surface and dynamic environment of these ancient bodies.
Using those same cameras, the scientists established a crime scene around Philae's sideways body. They looked at thousands of images taken by Rosetta and data gathered by Philae before its untimely demise. Working backward, they constructed a timeline of events that pointed to the exact location of the second touchdown — and revealed some of 67P's most intriguing secrets.
When Philae bashed into skull-top ridge, it scraped away a layer of dust and debris about 10 inches deep on the surface of 67P, revealing the comet's icy interior. On the images taken by Rosetta, the ice is incredibly bright, and the team was able to show that even in images taken almost two years after Philae's crash, the ice remained visible because it lived in a shadowed region of the comet that receives little sunlight.
The timing of the crash was also significant because it allowed for a calculation of how soft 67P's interior was. By analyzing data from a magnetometer within Philae, the team noticed a significant spike right around the time of second touchdown. The spike lasted for about 3 seconds. Knowing Philae "bounced" on the primitive ice of the interior, the team concluded it had incredibly low compressive strength and was about as soft as "freshly fallen snow."
Reexamining the magnetometer data also helped the team push back the time of first touchdown by 90 seconds.
Small planetary bodies like comets and asteroids are a major focus of two ongoing missions: Japan's Hayabusa2 journey to asteroid Ryugu and NASA's Osiris-Rex flight to potentially hazardous asteroid Bennu. The latter completed an almost-too-successful touchdown on Bennu on Oct. 20, scooping up so much rock from the surface that its sample collector is wedged open. Mission scientists for Osiris-Rex were surprised to find Bennu so crowded with boulders — and that fact highlights why the Philae data is critical for future missions.
Now that planetary scientists have a better understanding of a comet's physical characteristics, they can plan for a touchdown or sample return mission with much more certainty. "That the comet has such a fluffy interior is really valuable information in terms of how to design the landing mechanisms, and also for the mechanical processes that might be needed to retrieve samples," said Matt Taylor, a project scientist on the Rosetta mission, in an ESA press release.
One such mission was submitted as a white paper to Voyage 2050, ESA's long-term planning program, in March 2019. The Ambition project would be the first to return a cryogenically stored sample from a comet's interior to Earth. The Voyage 2050 committee is set to hand down recommendations on mission selection in 2021.