Infrared observations of the broken comet 73P/Schwassmann-Wachmann 3 were performed at 24 μm with the Spitzer Space Telescope in 2006 May. The image reveals the fragments and the meteoroid stream associated with the comet. Heavy numerical simulation of the production and evolution of the meteoroids ejected by the comet during the last century, coupled with a model of thermal emission of cometary grains, allows us to determine the dust size distribution. As found by in situ measurements performed during spacecraft encounters with 1P/Halley and 81P/Wild 2, the size distribution for 73P shows a plateau for the mass range 10–10 to 10–7 kg. Except for this mass bin, the cumulative mass index is in the range 0.85-1, so the size distribution has the same slope for dust and meteoroids separately, but with a significantly different normalization such that there are more large particles than would be expected from an extrapolation of the small-particle size distribution. The abundance of meteoroids with masses larger than 10–5 kg is poorly determined by in situ measurements due to the small number of detections; infrared observations provide a unique way to constrain it. The stream is mainly composed of particles ejected during the 1995 and 2001 passages. We show that the dust production rate increased by a factor of 11 during the 1995 outburst.
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