Dust Flux Monitor Instrument measurements during Stardust-NExT Flyby of Comet 9P/Tempel 1

The Dust Flux Monitor Instrument (DFMI) on the Stardust and Stardust-NExT missions measured impacts from coma dust particles with masses from ∼10−15 to >10−6 kg using two kinds of sensors – one based on polyvinylidene fluoride (PVDF) thin films and the other on acoustic detectors (ACs) mounted on the front and second layers of the Whipple Bumper Shield. At the higher encounter speed of 10.9 km s−1 at Comet 9P/Tempel 1 compared with 6.12 km s−1 at 81P/Wild 2 encounter, the mass sensitivity of DFMI sensors increased by between a factor of 2 and 12 (depending on the sensor subsystem), but the spatial resolution (⩾1.09 km) decreased to approximately a half. The coma of Comet Tempel 1 exhibits highly non-uniform spatial distribution of dust, as found at comet Wild 2, with bursts of impacts of up to 1000 particles over km scales near closest approach surrounded by void regions of many kilometers with no impacts. These data are consistent with passage through clouds of particles resulting from fragmentation of larger aggregates emitted from the nucleus. These fragmentation products dominate the total dust production of small particles, with only a small contribution likely from direct emission from the nucleus. The derived overall mass distribution is similar to that found at comet Wild 2 with the total mass dominated by large particles. The average cumulative mass index α = 0.65 ± 0.08 (where the particle flux is defined by ϕ (>m) = km−α) but a better fit is obtained with α = 0.85 ± 0.08 for particle masses below 10−10 kg and a significantly lower value for higher masses.