The meteoroid environment of comet 9P/Tempel 1 and the Deep Impact spacecraft

Abstract

On the 4th of July 2005 the Deep Impact probe reached comet 9P/Tempel 1. The flyby spacecraft observed the collision from a distance of several hundred kilometres. Here we examine the meteoroid environment of the nucleus and present a method for computing the spatial density of particles, as well as the number of collisions any probe is supposed to endure. The model is calibrated from observations of the comet taken at the Pic-du-Midi observatory. In particular we measured [Afρ]=272 ± 25 cm, at Rh=1.579 au and φ=29.4°. The data acquired by the Stardust spacecraft were also used. Several thousands of 100 μm-size meteoroids are expected to have collided with Deep Impact. Centimeter-size particles were a threat to the probe and the upper limit of the number of collision computed here was higher than unity. However the results depend on the differential size population index s, which is taken to be constant. Preliminary results show that a few collisions were recorded but no cm-size meteoroids were encountered. Interpreting this result as a change of s with the meteoroid size, we derive s>3.5 for cm-size and larger particles ejected by comet 9P/Tempel 1. This can be caused by a fragmentation process and shows the relevance of any observation of large meteoroids ejected by cometary nuclei, as it provides constraints on s. This method can be used in preparation of any space mission to a comet.