Hypervelocity impact into low density material and cometary outburst

As a mechanism of cometary outbursts, the impact and the penetration of denser asteroids into lower density comet are discussed. First, the results of the previous hypervelocity impact experiments with higher density projectiles and lower density targets are summarized. The depth, the maximum cavity diameter, and the volume of the cavities which are produced by the penetrations of projectiles are investigated. Then a model of the penetration including projectile fragmentation is proposed. The calculated penetration depth and the maximum cavity diameter agree with the experimental data. The model is applied to the impact of asteroids into comets. The mass fractured by the vapor, which is produced by the penetration of asteroids, and the velocity of the vapor expanding into vacuum (space) are estimated. The results are consistent with the typical mass and ejection velocity observed in outbursts.