The Open University ’ s repository of research publications and other research outputs Stardust Wild 2 Dust Measurements

During its passage through the coma of comet 81P/Wild 2 in January 2004 to collect dust particles for return to the Earth, instruments on Stardust made extensive measurements of the dust coma environment. The inner coma was characterized by many narrow jets, imaged by the Navigation Camera. Dust fluxes measured by the Dust Flux Monitor Instrument (DFMI) revealed a highly non-uniform spatial distribution, with short duration bursts of impacts implying localized spatial density changes of orders of magnitude on scales of less than a km as well as a second period of high activity ~4000 km from the nucleus where almost 80% of the detected impacts occurred. The Cometary and Interstellar Dust Analyzer (CIDA) obtained 29 dust impact mass spectra near closest approach. The spacecraft Attitude Control System (ACS) detected one event attributable to a large particle impact. The overall mass distribution in the inner coma was dominated by the largest grains, with an average cumulative mass distribution index of α=0.75±0.05 (where the number of particles of mass m or larger, N(m)∝m). The mass distribution was consistent with the ACS detection but not with the CIDA results which implied an impact rate a factor of 10 lower. The mass distribution was also highly variable during the flyby, and during the second period of high activity, small grains dominated, with α=1.13±0.2. The enormous variations in dust spatial density over distances of a few hundred metres, have been interpreted as the result of jets and distributed particle fragmentation, with the second period of high activity resulting from outgassing and/or fragmentation of a large (10s of metres diameter) boulder. This interpretation has been received with some skepticism. We review the data, the evidence for fragmentation and refute the criticisms leveled at this interpretation. Data from other comets indicate that the same processes may have occurred and therefore may be common in comets in general.