Further evidence for collisions among asteroids

The 64 asteroids with reliably known rotational properties [rotation period P, magnitude B(1, 0) and maximum change of magnitude Δm] are studied. A plot of B(1,0) vs P illustrates that smaller asteroids tend to rotate faster than larger asteroids. The mean P for all 64 asteroids is 8.8 hr. The class of irregular asteroids (taken to be those with Δm >0.38, i.e., those whose axes differ by more than 40%) called group C are studied separately; they are shown to rotate much faster (mean P = 7.7 hr) than the remaining more regular asteroids (mean P = 9.2 hr). The smaller bodies are more irregular on the average. These results are interpreted in terms of a model in which collisions break asteroids into irregular fragments. Since angular momentum is transmitted in such collisions, significant increases in the mean can occur in the angular velocity of the largest fragment. The effect of interasteroid collisions on the mean orbital parameters is briefly discussed and is shown to be masked by selection effects.