Abstract
The Rosetta lander Philae successfully landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. Philae carries the Dust Impact Monitor (DIM) on board, which is part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME). DIM employs piezoelectric PZT sensors to detect impacts by sub-millimeter and millimeter-sized ice and dust particles that are emitted from the nucleus and transported into the cometary coma. The DIM sensor measures dynamical data like flux and the directionality of the impacting particles. Mass and speed of the particles can be constrained assuming density and elastic particle properties. DIM was operated during three mission phases of Philae at the comet: (1) Before Philae's separation from Rosetta at distances of about 9.6 km, 11.8 km, and 25.3 km from the nucleus barycenter. In this mission phase particles released from the nucleus on radial trajectories remained undetectable because of significant obscuration by the structures of Rosetta, and no dust particles were indeed detected. (2) During Philae's descent to its nominal landing site Agilkia, DIM detected one approximately millimeter-sized particle at a distance of 5.0 km from the nucleus' barycenter, corresponding to an altitude of 2.4 km from the surface. This is the closest ever dust detection at a cometary nucleus by a dedicated in-situ dust detector. (3) At Philae's final landing site, Abydos, DIM detected no dust impact which may be due to low cometary activity in the vicinity of Philae, or due to shading by obstacles close to Philae, or both. Laboratory calibration experiments showed that the material properties of the detected particle are compatible with a porous particle having a bulk density of approximately $250\, \mathrm{kg\,m^{-3}}$. The particle could have been lifted off from the comet's surface by sublimating water ice.
Highlights
On 6 August 2014 the European spacecraft Rosetta (Glassmeier et al 2007) encountered its target comet 67P/ChuryumovGerasimenko and became the first artificial satellite of a cometary nucleus
Dust Impact Monitor (DIM) measurements on 16 and 17 October 2014 were taken at a distance of 9.6 km and 11.8 km from the nucleus barycenter, respectively, when Rosetta was orbiting the comet nucleus on an eccentric trajectory
Measurements with the Dust Impact Monitor (DIM) on board Philae were obtained before the separation from Rosetta, during Philae’s descent to the nucleus surface, and at Philae’s final landing site Abydos
Summary
On 6 August 2014 the European spacecraft Rosetta (Glassmeier et al 2007) encountered its target comet 67P/ChuryumovGerasimenko (hereafter 67P) and became the first artificial satellite of a cometary nucleus. Rosetta carried the lander spacecraft Philae on board (Bibring et al 2007), which on 12 November 2014 successfully landed on the surface of 67P at a heliocentric distance of 2.99 AU (Biele et al 2015). Comet 67P is a short-period comet with a present orbital period of about 6.6 years and aphelion and perihelion distances at 5.68 AU and 1.24 AU, respectively. Nucleus images recently taken by the cameras on board Rosetta show an irregularly shaped body with a size of about 4 km that consists of two lobes connected by a short depression, and a wide diversity in surface morphology (Thomas et al 2015)
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