Comet 67P shows no sign of magnetism

If astronauts ever go on a walkabout around a comet, they can leave their compasses at home. Comet 67P/Churyumov-Gerasimenko, which the Rosetta spacecraft has been orbiting since August (SN: 9/6/14, p. 8), has no detectable magnetic field, researchers report online April 14 in Science. This finding indicates that magnetism did not help assemble comets and other small planetary building blocks in the early days of the solar system.

Rosetta’s Philae lander looked for, and failed to find, hints of a magnetic field as it descended toward — and bounced across — the comet on November 12 (SN Online: 11/13/14). That means that any magnetic field at the comet’s surface must be less than 2 nanotesla, report Hans-Ulrich Auster, a planetary scientist at the Technische Universität Braunschweig in Germany, and colleagues. That’s less than one ten-thousandth as strong as Earth’s magnetic field.

The Giotto spacecraft also failed to find a magnetic field when it flew within 600 kilometers of comet 1P/Halley in 1986. If magnetism helped bring together shards of interplanetary dust, then there should be some trace of it in iron embedded in the comet.

Philae’s magnetometers did, however, reveal details of the lander’s trip across comet 67P. Both Philae and Rosetta generate their own magnetic fields as a side effect from their onboard electronics. The changing interaction between the two fields as Philae drifted away from Rosetta let researchers deduce that the lander bounced, nicked a crater rim and bounced again before finally settling in its current (yet to be determined) parking spot.