Philae May Have Grazed a Crater and Tumbled Over Comet3 min read

You’ve heard of first-world problems? Well this is an off-world problem: accidentally scraping the rim of a crater with one’s landing gear during a landing on a comet, a mishap that can send you into a tumbling slow-motion spin.

Illustration of Philae’s descent onto comet 67P/Churyumov-Gerasimenko.

Illustration of Philae’s descent onto comet 67P/Churyumov-Gerasimenko.

This is what Rosetta mission engineers think may have happened to the Philae lander last month after its touchdown on comet 67P/Churyumov-Gerasimenko — a historic event that, while scientifically successful, still resulted in a mislaid spacecraft that’s now in silent hibernation. But luck may have been with Philae after all.

After riding piggyback aboard Rosetta through the inner solar system for over ten and a half years, the 100 kg Philae craft was deployed from the orbiting Rosetta on the morning of Nov. 12, 2014, taking about seven hours to descend to the surface of comet 67P/C-G. Although the world cheered when a confirmation of touchdown was received on Earth at 16:03 UTC, it was soon discovered that Philae did not simply settle down and secure itself to the comet as anticipated but rather bounced several times, sending it careening across the surface into a shadowed, uneven, and as-yet-unknown location.

Now, based on magnetic field data acquired by the Rosetta Lander Magnetometer and Plasma Monitor (ROMAP) instrument mounted at the end of a small boom extending from Philae’s “head,” it’s suspected that one of the lander’s legs scraped something on the comet — perhaps a crater rim — after its initial bounce. This brief contact in the near-weightless and airless environment on the 4-kilometer-wide comet was enough to both slow the lander’s descent spin rate and send it into a tumble as it soared away from its planned landing site.

“It was not a touchdown like the first one, because there was no signature of a vertical deceleration due to a slight dipping of our magnetometer boom as measured during the first and also the final touchdown,” said ROMAP co-principal investigator Hans-Ulrich Auster. “We think that Philae probably touched a surface with one leg only — perhaps grazing a crater rim — and after that the lander was tumbling. We did not see a simple rotation about the lander’s z-axis anymore, it was a much more complex motion with a strong signal in the magnetic field measurement.”

OSIRIS images showing Philae after its touchdown on comet 67P. See full series here. Credit: Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Given this scenario, the entirety of which is still being worked out by mission scientists (as is the final location of the lander), it’s really a marvel that Philae managed to end up in a right-side-up orientation on the comet at all — not to mention manage to acquire and transmit data for all of its primary scientific objectives before its batteries drained.

“It was really an exciting and almost unbelievable excursion,” said Hans-Ulrich.

Although Philae is currently in a low-power hibernation mode it is very possible that it will end up receiving enough sunlight to recharge its batteries and awaken as comet 67P approaches the sun during the first half of 2015. In fact, its partially-shadowed position could even help it avoid overheating during perihelion, ultimately prolonging Philae’s operating life.

Just more proof that there are such things as happy accidents.

Sebastien Clarke
Sebastien Clarke

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