Doomed Philae lander accidentally did a science by denting the comet

The close-ups highlight the bright ice exposed in the boulders when Philae struck them during its second touchdown (green box above).
Enlarge / The close-ups highlight the bright ice exposed in the boulders when Philae struck them during its second touchdown (green box above).

The Rosetta mission’s attempt to drop the Philae lander on a comet in 2014 didn’t go according to plan. The harpoon mechanism meant to stick Philae to terra-not-quite-firma didn’t work, and poor Philae ended up bouncing around and landing under a dark cliff overhang, unable to deploy its solar panels and complete its tasks. But let it not be said that Philae failed to leave its mark. Because it did. Quite literally.

To extract value from Philae’s accidental adventure, researchers have worked hard to identify the spots where the craft impacted the surface of the comet. This required painstaking analysis of Philae’s motion sensors to reconstruct its trajectory, along with a terrifically complex game of “one of these things is not like the others” played with before-and-after images of the comet’s jumbled surface.

The site of the initial bounce was easy enough to find, but the path from there to its resting place was another story. A new study led by the European Space Agency’s Laurence O’Rourke reveals another spot where Philae dented comet 67P. And the size of that dent actually tells us something remarkable about what comets are like.

Dented

Researchers eventually found a spot they dubbed “skull-top ridge” where a pair of boulders separated by a crevice appeared to have met Philae. After the landing, a bright spot appeared in that crevice, as if surface dust had been removed to expose water ice in the boulder. And indeed, spectral data from imagery confirms that the bright spot is largely water ice. While water ice makes up a substantial portion of comets—which are often somewhat rudely referred to as “dirty snowballs”—a comet’s surface is composed of a layer of dust left behind as sunlight drives off the outermost ice, so actually seeing ice there is telling.

A simple animation to show how the team thinks Philae interacted with the pair of boulders.

Philae’s initial touchdown location was in a flat spot likely covered by a thick layer of that dust. Its encounter with this boulder represents an interaction with something more similar to the comet’s interior.

The team estimates the depth of the dent it left behind at about 25 centimeters. Using the recorded velocity of the 100-kilogram craft, this allowed them to calculate the boulder’s sturdiness—or lack thereof, as it turns out. They found that the boulder was actually about as soft as fluffy snow on Earth.

Mixed measurements

This illustrates something that Rosetta successfully measured: the comet is extremely porous. The high water ice and CO2 ice content might make you think the comet is a hard, frozen block, but around 75 percent of its volume is void space in between grains of ice and dust. Without strong gravity to pull things together, comets just aren’t that dense.

This isn’t the first estimate of the comet’s material strength to come out

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NASA, human lunar lander companies complete key Artemis milestone

NASA, human lunar lander companies complete key Artemis milestone
Credit: NASA

NASA’s Human Landing System (HLS) Program recently checked off a key milestone in its progress toward landing the first woman and the next man on the Moon by 2024. The HLS Program conducted Certification Baseline Reviews (CBR) with the three U.S. companies competing to provide landers that will deliver Artemis astronauts to the Moon. These virtual meetings were the culmination of critical work by NASA and the companies since NASA announced the base period selections in April.


Since then, NASA has worked closely with the Blue Origin-led team, Dynetics, and SpaceX to better understand their human landing system proposals and approach for the agency’s Artemis program. The primary purpose of the CBRs was to finalize the functional and performance requirements for the companies’ landing system designs, confirm the standards to be applied to lander development, establish the baseline designs, schedules, and management plans for HLS contract execution and human spaceflight certification. Dr. Lisa Watson-Morgan, the HLS program manager at NASA’s Marshall Space Flight Center in Huntsville, Alabama, chaired the CBR board that approved the certification baseline for each contractor.

Seeking to leverage NASA’s human spaceflight experience and the commercial sector’s speed and innovation, the agency specified a concept of operations and high-level requirements and standards but did not dictate approach or design, allowing the contractors to propose their own designs. This was a departure from NASA’s traditional procurement approach of providing contractors with highly detailed specifications for building spacecraft hardware.

“We wanted to be as open as possible in our procurement approach, to accelerate the process and to encourage innovation,” said Watson-Morgan. “It worked. Within one year, we were able to select three very different design solutions to accomplish the bold and challenging objective of sending astronauts to the lunar South Pole.”

During the CBR meetings, NASA examined how each contractor has been proceeding with the design of their landing system, and NASA and the contractors confirmed the results of an intensive adjudication process that established design, construction, safety, and health and medical standards for each proposed landing system. Companies also provided development and testing schedules, identified top risks, and provided plans for safety and mission assurance, verification, validation, and certification.

The CBR is part of the base period for the three contracts. Running from May 2020 to February 2021, the base period is about mid-way through—the ideal time to conduct the CBR in the fast-paced development process, according to Watson-Morgan. “With firm-fixed price contracts it is important to come to an agreement up front about how each contractor will proceed,” she noted. “While NASA wants to be as flexible as possible to achieve success, late changes can be costly and add to schedule risk.”

Next Steps to Land Artemis Astronauts on the Moon

Concurrent with the base period, NASA is running an active federal procurement for the next phase of HLS development, Option A, which will determine which design(s) will be selected to continue development to flight. The three HLS base period contractors, having passed CBR, are

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