The response of cm-scale laboratory impact experiment targets to catastrophic collisions (those removing ~50% of the target's mass) is governed by material strength, while the impact behavior of ~10 km-scale planetary bodies depends on gravity. The boundary between strength and gravity dominance in catastrophic impacts lies at some intermediate size; estimates of that size for silicate bodies range from ~6 km to~100 km diameter. We extrapolate our new Smoothed Particle Hydrodynamics (SPH) catastrophic impact simulation results (Love and Ahrens 1996, "Catastrophic impacts on gravity dominated asteroids," Icarus 124, 141-155) for 10 to 1000 km diameter bodies to smaller sizes, yielding a new estimate of the boundary diameter: 250±150 m. The uncertainty reflects incomplete understanding of how strength decreases with increasing target size. The catastrophic impact specific energies (Q*) at these sizes are ~40 to ~200 J/kg. Our results imply that most numbered asteroids are gravity dominated, that bodies <1 km across may be gravity bound rubble piles rather than solid monoliths, and that km-sized Earth-approaching asteroids may have disruption energies higher than previously estimated (Fig 16).
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- The plot shows the catastrophic threshold (Q*) as a function of size in the strength (left portion) and gravity (right portion) regimes according to this work and previous studies. In the strength regime we plot Q* for laboratory impact experiments in silicate targets, along with the scaling rules of Farinella et al. (1982), Housen and Holsapple (1990), and Holsapple (1994), with power law slopes of -0.5, -0.24, and -0.33, respectively. We refer to the last value in this work. In the gravity regime we plot the results of this work as well as those of Davis et al. (1995) and Holsapple (1994). We also show the gravitational binding energy per unit mass, a firm lower limit on Q*. The power law slopes of these relations are respectively 1.13±0.01, 1.5, 1.67, and 2.0. The intersection of our present results with the suite of strength scaling curves indicates that the gravity regime for silicate bodies may begin at diameters as small as 250±150 m.
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