Detection of large grains in the coma of Comet C/2001 A2 (LINEAR) from Arecibo radar observations

Arecibo S-band (λ=13cm) radar observations of Comet C/2001 A2 (LINEAR) on 2001 July 7-9 showed a strong echo from large coma grains. This echo was significantly depolarized. This is the first firm detection of depolarization in a grain-coma radar echo and indicates that the largest grains are at least λ/2π or 2 cm in radius. The grains are moving at tens of m s−1 with respect to the nucleus. The nondetection of the nucleus places an upper limit of 3 km on its diameter. The broad, asymmetric echo power spectrum suggests a fan of grains that have a steep (differential number ∼a−4) size distribution at cm-scales, though the observed fragmentation of this comet complicates that picture.