Researchers have finally discovered where unfortunate Philae landed the second time while inadvertently bouncing over comet 67P.
After years of research, researchers have mapped the second landing site of the unfortunate lander Philae on Comet 67P / CG. Philae landed on very porous boulders and then accidentally left its marks in the underlying billions of years old ice. And that leads to an interesting discovery: apparently the comet’s icy interior is softer than cappuccino foam!
The mission
Comet lander Philae landed on Comet 67P / CG in November 2014. That landing was a bit bumpy. Philae failed to cling to the comet at the planned landing site and continued to bounce. Philae flew through the air for two more hours, then collided with a cliff causing him to tumble to a second landing site. Eventually, the landertje came to a stop in a somewhat nasty place: in the shadow of a cliff. The comet lander – which ran on solar energy – was unable to generate enough energy there and fell asleep after several days of research. And that meant end of story.
The lander seems a bit weedy, but still weighed about 100 kilograms. Fun fact: because the force of gravity on Comet 67P / CG is limited, the lander weighed only one gram once on the surface of the comet! The lander most resembled a solar-paneled box on three legs. On board were ten scientific instruments, including a drill that allowed the lander to drill up to 23 centimeters deep and reveal the comet’s chemical composition. The other instruments were intended to determine, among other things, the comet’s density, texture and porosity and to find out how the comet changes as it gets closer to the sun. The collected data should have sent Philae back to his great friend Rosetta, the space probe that brought the lander to Comet 67P / CG and orbited the comet. The purpose of both the probe and the lander was to provide more insight into comets: icy celestial objects seen as sources of ingredients for life.–
However, researchers were determined to locate the previously undiscovered site where Philae came into contact with the comet for the second time. “Philae had left us one final mystery waiting to be solved,” said Laurence O’Rourke. “It was important to find that landing site because sensors on Philae indicated that the lander had dug into the surface, most likely exposing the underlying pristine ice. And that would provide us with invaluable information about this billions of years old ice. ”
Skull
Partly thanks to the ROMAP instrument (that stands for Rosetta Lander Magnetometer and Plasma Monitor) on board Philae, researchers managed to locate the landertje’s precise second landing site. A reanalysis of all landing data revealed that Philae had spent nearly two full minutes at this second landing site, where he then bounced off at four different points of contact with the local rock. As the researchers studied the location a bit more closely, they came to a somewhat spooky conclusion. “The shape of the boulders reminded me a bit of a skull,” says O’Rourke. “The right eye of the skull was created when Philae compressed fabric, creating the gap between the boulders while Philae slid through them.”
This image shows how lander Philae left its tracks on Comet 67P / CG. Image: ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA; Data: ESA / Rosetta / Philae / ROMAP; Analysis: O’Rourke et al (2020)
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Water ice
These contact points of the landertje revealed clear material that the researchers have now labeled as water ice. Philae even made a 25 centimeter deep dent in this ice; a scar that reveals a very soft and porous body. The data shows that Philae only took three seconds to create this particular hole. “Thanks to Philae, we found that this billion-year-old ice cream is extremely soft,” says O’Rourke. “It’s lighter than foam on a cappuccino.”
Porous
The research team further calculated that the affected rock has a porosity of no less than 75 percent. This means that three quarters of the rock is empty space. That figure is consistent with the calculated porosity of comet 67P / CG as a whole, which was measured in a separate study. That study showed that the comet has a homogeneous interior. And that means that the affected, skull-shaped boulders represent the overall state of the comet’s interior when it was formed some 4.5 billion years ago.
The researchers are happy that, after years, they have still managed to unravel the last mystery of the little country Philae. “Not only are we filling the gaps in the story of Philae’s turbulent journey, but we’ve also learned more about the nature of the comet,” said researcher Matt Taylor. “And that is crucial for future landing missions. That the comet has such a porous interior is really very valuable information. Not only to be able to land safely, but also to come up with the best tactics for collecting samples. ”
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