On the deceleration of cometary fragments in aerogel

Determining the thermal history of the cometary grains captured by the Stardust mission presents a difficult problem. We consider two simplified models for the deceleration of hypervelocity particles captured in aerogel ; both models assume a velocity squared drag force. The first model assumes that the mass of the particle remains constant during capture and the second that mass is lost due to ablation of the particle through interactions with the aerogel. It is found that the constant mass model adequately reproduces the track lengths, found from experiments by Hörz et al. in 2008, that impacted aluminium oxide spheres into aerogel at hypervelocities y6 km sx1. Deceleration in aerogel heats volatile particles such as organic ices to high temperatures greater than 1,000 K, for durations of y1 ms : more than sufficient to completely ablate the particle. Refractory particles also experience significant heating greater than 2500 K, greater than the particle’s melting point, over similar timescales. This suggests that the fragments recovered to Earth by the Stardust mission were considerably altered by hypersonic capture by aerogel, and so limits the amount of information that can be obtained regarding the formation of mineral and organic particles within Kuiper Belt comets. Received 14 July 2008, accepted (revised)12 September 2008, first published online 23 December 2008