Forming Binary Near-Earth Asteroids From Tidal Disruptions

Table 4.1.Lifetimes for binary NEAs.Semi-Major Axis (R pri ) Critical q for 50% ejection Lifetime (Myr)2 1.6 10.4 3.0 2.6 4.0 1.68 5.0 1.010 6.0 0.715 8.0 0.425 12.5 0.17Note. — Predicted lifetimes for NEA binaries for systems with encountersof v ∞ = 16 km/s and a mass ratio of 0.1. The limiting close approachdistance, q, was selected for the distance at which 50% of the randomlyoriented binaries were disrupted as a result of the close approach. Thelifetime is then how often an encounter at that critical distance is expectedto occur.due to the relative quickness of tidal eccentricity damping). Thus each set of parametersassumed a probability for binary disruption, which was then used in the Monte Carlo simulationto determine the fate of binary encounters. This code was tested against the resultsin Bottke & Melosh (1996a,b), for the case of v ∞ = 12 km s −1 , a = 6 km and mass ratioof 0.125, yielding very close matches (see Fig. 4.3). The results also match well with theanalytical calculation of Agnor & Hamilton (2006), relating the Hill sphere of the binaryand its semi-major axis,r td ≈aR priR ⊕ (4.5)where r td is the tidal disruption distance for a binary encounter with Earth.A statistical estimate for binary lifetimes against separation due to planetary encountersis calculated for binaries of various semi-major axes (assuming a relatively averagebinary encounter scenario with v ∞ = 16 km s −1 and a mass ratio of 0.1). A critical en-90