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Bouncing Philae reveals comet is not magnetised

Jenny Winder, News Writer
Apr 15, 2015, 18:58 UTC

Sen—When the European Space Agency's (ESA) Philae lander bounced across the surface of comet 67P/Churyumov-Gerasimenko, it may have provided scientists with the first detailed investigation of the magnetic properties of a comet's nucleus.

ESA's Rosetta orbiter arrived at comet 67P in Aug. 2014 and released the Philae lander on Nov. 12, 2014. Philae's first touchdown on the comet was close to the target landing site named Agilkia, but when the harpoons, designed to anchor the lander to the surface failed, Philae bounced three times before finally tumbling down at a point named Abydos.

Philae’s magnetic field measuring instrument, the Rosetta Lander Magnetometer and Plasma Monitor (ROMAP), was able to detect each of the touchdown events and the orientation of the lander, while instruments aboard Rosetta were able to determine Philae’s trajectory. 

“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,” said Hans-Ulrich Auster, co-principal investigator of ROMAP, in a statement. 


Reconstructing Philae’s trajectory. Image credit: ESA/Data: Auster et al. (2015)/Comet image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Scientists believe that magnetic fields played an important role in the formation of the Solar System by empowering dust grains containing iron to initially clump together. Whether magnetism continued to play a part as these building blocks grew until gravity took over is unknown. 

As Philae tumbled across the comet the team compared measurements on the inward and outward journeys to and from each of the contact points, but found the magnetic field's strength did not depend on Philae's height or location above the surface. It was more consistent with the external influence of the solar wind's magnetic field. Variations in the field experienced by Philae also closely agreed with those measured by Rosetta at the same time.

“If the surface was magnetised, we would have expected to see a clear increase in the magnetic field readings as we got closer and closer to the surface,” explains Auster. “But this was not the case at any of the locations we visited, so we conclude that Comet 67P/Churyumov-Gerasimenko is a remarkably non-magnetic object.”


The non-magnetic comet. Image credit: ESA/Data: Auster et al. (2015)/Background comet image: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

“If any material is magnetised, it must be on a scale of less than one metre, below the spatial resolution of our measurements. And if Comet 67P/Churyumov-Gerasimenko is representative of all cometary nuclei, then we suggest that magnetic forces are unlikely to have played a role in the accumulation of planetary building blocks greater than one metre in size,” stated Auster.

Work continues to identify Philae's final landing site with the lander still in hibernation, with ony limited illumination from the sunlight that provides it with power. It may be a few more weeks before mission controllers discover if the lander is capable of rebooting as 67P moves closer to the Sun, and it would take until June for Philae to harvest sufficient energy to communicate with the team back on Earth.

Meanwhile the comet's activity is increasing as it approaches the Sun. Dust from the comet confused Rosetta's navigation system at the end of March, when it flew within 14 km of the surface, sending the orbiter into safe mode. Communications and systems were restored and Rosetta was taken to a safe distance.

The increasing levels of activity are dramatically demostrated in a series of images taken by Rosetta from Jan. 31 to March 25. The material will flow under pressure from the solar wind to form two tails, one of gas and one of dust. The comet’s atmospheric coma will eventually span tens of thousands of kilometres, while the tails may extend hundreds of thousands of kilometres.

The team are now carefully monitoring the spacecraft as they try to discover how close the orbiter can get to 67P to continue science observations as it grows more active.

Rosetta project scientist Matt Taylor, told Sen "We will get as close as possible, but we will only get a good feeling of what that value is in the next weeks. We expect this will on average increase in the next months as we approach perihelion due to activity increase. This obviously has an impact on "near" comet science. We are discussing in the next weeks what we can do to mitigate that."


A series of images from Rosetta's NAVCAM show the comet's active nucleus emitting jets of gas and dust as it is warmed by the Sun. Image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0