Complementary data collected by the Rosetta Plasma Consortium fluxgate magnetometer (RPC-MAG) on board ESA’s Rosetta orbiter, and by the Rosetta Lander Magnetometer and Plasma Monitor (ROMAP) on board the Philae lander on the comet’s surface, have been used to investigate the magnetic properties of 67P/Churyumov-Gerasimenko.
The pictures on the left indicate the positions of the Rosetta Plasma Consortium fluxgate magnetometer (RPC-MAG) on the orbiter and of the Rosetta Lander Magnetometer and Plasma Monitor (ROMAP) on the lander. The graph on the right shows the data collected by the instruments. The data shown cover the period immediately prior to the second touchdown of Philae on November 12, 2014. Image credit: ESA / Auster et al / ATG medialab.
Changes in the magnetic field surrounding Rosetta allowed RPC-MAG to detect the moment when Philae was deployed on November 12, 2014. Then, by sensing periodic variations in the measured external magnetic field and motions in its boom arm, ROMAP was able to detect the touchdown events and determine the orientation of Philae over the following hours.
Combined with information from Rosetta’s CONSERT instrument, timing information, images from the OSIRIS camera, assumptions about the gravity of the comet, and measurements of its shape, it was possible to determine the trajectory of the lander.
The Rosetta science team soon discovered that the lander not only touched down once at a site called Agilkia, but also came into contact with the comet’s surface four times in fact.
“The unplanned flight across the surface actually meant we could collect precise magnetic field measurements with Philae at the four points we made contact with, and at a range of heights above the surface,” explained Dr Hans-Ulrich Auster of Technische Universität Braunschweig in Germany, lead author of the paper published in the journal Science.
The multiple descents and ascents meant that the scientists could compare measurements made on the inward and outward journeys to and from each contact point, and as it flew across the surface.
ROMAP measured a magnetic field during these sequences, but found that its strength did not depend on the height or location of Philae above the surface. This is not consistent with the nucleus itself being responsible for that field.
“If the surface was magnetized, we would have expected to see a clear increase in the magnetic field readings as we got closer and closer to the surface. But this was not the case at any of the locations we visited, so we conclude that 67P/Churyumov-Gerasimenko is a remarkably non-magnetic object,” Dr Auster explained.
Instead, the magnetic field that was measured was consistent with an external one, namely the influence of the solar wind interplanetary magnetic field near the comet nucleus. This conclusion is confirmed by the fact that variations in the field that were measured by the lander closely agree with those seen at the same time by Rosetta.
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Hans-Ulrich Auster et al. The nonmagnetic nucleus of comet 67P/Churyumov-Gerasimenko. Science, published online April 14, 2015; doi: 10.1126/science.aaa5102
