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Philae May Have Collided With A Crater During Its Second Touchdown

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Analysis of the comet lander's data suggests it bounced at least three times, and not twice as earlier reported.

Data collected by Philae during its descent and subsequent bounces is being used by ESA team members to reconstruct the lander's journey to its final resting place on Comet 67P. They're analyzing perturbations of the magnetic field generated by Philae's ROMAP instrument to get a better sense of what happened.


A picture taken by Rosetta just a few days ago. Comet 67P is starting to let off some steam. (ESA)

During the seven-hour descent, all measurements were normal, and ROMAP recorded the first touchdown at 15:34:04 GMT spacecraft time.


After this first touchdown, the spin rate started increasing. As the lander bounced off the surface, the control electronics of the flywheel were turned off and during the following 40 minutes of flight, the flywheel transferred its angular momentum to Philae. After this time, the lander started spinning at a rate of about one rotation per 13 seconds.

Then, at 16:20 GMT spacecraft time, Philae apparently collided with a surface feature, most likely a crater rim.


Dynamic power spectral density plot describing the intermediate 'touch' that Philae experienced between the first and second touchdowns. See text for details. (Image and caption: ESA/Rosetta/ROMAP/IGeP_TU/Braunscheig/Hungarian Academy of Sciences/Centre for Energy Research/ Space Research Institute Graz)

"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." noted team member Hans-Ulrich. "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."


After this, the main rotation period decreased slightly to one rotation per 24 seconds. At 17:25:26 GMT Philae touched the surface again, initially with just one foot but then all three, giving the characteristic touchdown signal. At 17:31:17 GMT, after traveling probably a few more meters, Philae found its final parking position on three feet.

This could mean that Philae experienced (at least) three bounces and four landings.


According to the ESA, the dramatic change in its x, y, and z rotation axis "can only be explained by a collision with the ground to provide a pivoting point for the lander to change its rotation axis."

Despite all this, Philae's instruments remained operational in the following two days. UK researchers in particular received "rich" data from the lander before it died. Using ROMAP data and other instruments onboard Rosetta, the team is still hoping to pinpoint the exact location of the lander.


[Via Rosetta Blog]