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Rosetta team track Philae's spin and tumble after comet landing

Paul Sutherland, Feature writer
Nov 30, 2014, 21:58 UTC

Sen—Space scientists are employing ever more creative techniques to discover what happened to their Rosetta lander Philae after it hit the surface of Comet 67P/Churyumov-Gerasimenko.

As Sen reported from mission control, Darmstadt, the washing-machine-sized probe bounced twice on landing on 12 November, following the failure of its harpoons, and was carried more than a kilometre from the intended site.

Close study of imagery revealed the exact point of first touchdown, and then shots of the probe itself flying above the surface, but Philae’s final resting place, in the shadow of a wall, has yet to be accurately determined.

However, using techniques that would impress fictional sleuth Sherlock Holmes, the European Space Agency’s experts are still on the case. And their latest findings suggest it may have grazed the rim of a crater.

Data from one experiment aboard the Rosetta mission has already been used to provide clues to Philae’s wherabouts. Called CONSERT—the COmet Nucleus Sounding Experiment by Radiowave Transmission—its original purpose was to use radar to check the internal structure of the comet, by transmitting a signal from the Rosetta orbiter that would be picked up by a detector on Philae. 

Running this experiment once gave ESA scientists a circle along which the lander must lie. Running it a second time provides a second circle. Philae had to lie at one of the two points where the circles cross.

Now data from a second experiment on Philae is being examined to provide fresh evidence of what happened to the probe as it bounced from its first touchdown. The experiment, called ROMAP—the Rosetta Lander Magnetometer and Plasma Monitor—has allowed the team to track Philae’s trajectory by analysing its magnetic field.

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Enhancement of Rosetta's comet images from 26 November dramatically show the levels of gas and dust now being given off by 67P/Churyumov-Gerasimenko. Image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0 

Both the orbiter and lander generate small magnetic fields of their own because of the electronic circuits on instruments within the spacecraft. These show themselves as perturbations in the magnetic fields produced naturally by the comet and the solar wind.

As part of the programme of experiments during landing, the Rosetta team always planned to measure these perturbations to follow what was happening to their lander as it dropped from the mothership to the comet’s surface.

Now the data is revealing the sequence of events after ROMAP was deployed on a boom during Philae’s seven-hour descent. It recorded first touchdown, then the lander began spinning once every 13 seconds as it bounced back into space.

About 46 minutes after the first bounce, Philae collided with a feature on the surface of the comet, possibly a crater wall, the data suggests.

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The ROMAP experiment and where it sits on its boom on the Philae lander. Image credit: Technische Universität Braunschweig/ESA

ROMAP’s co-principal investigator Hans-Ulrich Auster, of the Technische Universität Braunschweig, Germany, told ESA’s Rosetta blog: “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.

“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 . . . any more, it was a much more complex motion with a strong signal in the magnetic field measurement.”

Rosetta itself is due to lower its orbit from a height of 30 km to 20 km next week which will give the team operating its high-resolution OSIRIS cameras a better chance of finding a “needle in a haystack” and imaging Philae on the comet’s surface. 

Meanwhile the mothership continues to send back dramatic images of the comet which show a steadily increasing amount of gas and dust being emitted as 67P is warmed by the Sun.