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We present results from a suite of 169 hydrocode simulations of collisions between planetary bodies with radii from 100 to 1000 km. The simulation data are used to derive a simple scaling law for the threshold for catastrophic disruption, defined as a collision that leads to half the total colliding mass escaping the system post impact. For a target radius 100≤ R T≤ 1000km and a mass M T and a projectile radius r p≤ R T and mass m p we find that a head-on impact with velocity magnitude v is catastrophic if the kinetic energy of the system in the center of mass frame, K= 0.5 M T m p v 2/(M T+ m p), exceeds a threshold value K* that is a few times U=(3/5) G M T 2/R T+(3/5) G m p 2/r p+ G M T m p/(R T+ r p), the gravitational binding energy of the system at the moment of impact; G is the gravitational constant. In all head-on collision runs we find K*=(5.5±2.9) U. Oblique impacts are catastrophic when the fraction of kinetic …