Energetic ions at comet Grigg—Skjellerup measured from the Giotto spacecraft

Abstract

ON 10 July 1992, the Giotto spacecraft flew within about 200 km of the nucleus of comet Grigg—Skjellerup; this is only the third comet at which in situ measurements have been made, and the encounter constituted the closest approach so far of a spacecraft to a cometary nucleus. Here we report the detection by the EPONA instrument on Giotto of charged, energetic particles deep within the inner coma of Grigg—Skjellerup. In contrast to previous spacecraft encounters with comets Giacobini—Zinner (in 1985) and Halley (in 1986), well defined, periodic intensity variations recorded in the particle fluxes suggest that the ions close to the nucleus were strongly coupled to the ambient magnetic field. The present data indicate that Giotto flew on the nightside of the nucleus.