The composition and plasma signature of a large dust impact on the giotto spacecraft

At about 14,800 km from the comet Halley nucleus, on the inbound leg, at least six of the sensors onboard the Giotto spacecraft observed an unusual, brief (approximately 30 to 500 ms) event: The ion mass spectrometer data show a brief flow of energetic (up to several hundred electron volts) plasma consisting of protons, water group, and heavier ions. The Johnstone plasma analyzer data show a short burst of plasma, while the dust impact detector system data show an impact event in four of its detectors. The magnetometer signature of the event shows two brief dips in the field. The sudden change in the spacecraft attitude and spin rate observed by the camera at that same time has been interpreted as the result of a large (greater-than-or-equal-to 5 mg) dust particle impact on the front bumper shield of the spacecraft. In addition, at about the same time the spacecraft star-tracker suffered damage. Here we give the first report combining direct measurements of the composition and dynamics of a dust impact plasma cloud, the dust particle mass and the location of the impact on the spacecraft. Analysis of the data indicate that the impacting particle was water or ice-bearing, possibly loosely compacted, and was composed of one or more of: carbon, nitrogen, and silicon.