SECONDARY FRAGMENTATION OF COMET SHOEMAKER-LEVY-9 AND THE RAMIFICATIONS FOR THE PROGENITORS BREAKUP IN JULY 1992

Comprehensive analysis of discrete events of secondary fragmentation l eads to a conceptually new understanding of the process of disintegrat ion of comet Shoemaker-Levy 9. We submit that the jovian tidal forces inflicted extensive cracks throughout the interior of the original nuc leus but did not split it apart. The initial disruption was apparently accomplished by stresses exerted on the cracked object by its fast ro tation during the early post-perijove period of time. We argue that th is disruption was in fact a rapid sequence of episodes during July 199 2 that gave birth to the 12 on-train, or primary, fragments: A, C, D, E, G, H, K, L, Q (later Q(1)), R, S, and W. The discrete events of sec ondary fragmentation, which gave birth to the off-train fragments, are understood in this scenario as stochastic manifestations of the conti nuing process of progressive disintegration. Of the 13 off-train fragm ents considered, nine were secondary-B, F, G(2), M, N, P (later P-2 or P-2a), Q(2), U, and V-and four tertiary (J, P-1, P-2b, and T). The se paration parameters of 11 off-train fragments were determined. The vec torial distribution of separation velocities of these fragments shows a strong concentration toward a great circle, unquestionably an effect of the approximately conserved angular momentum of the progenitor com et since the time of its initial disruption. Also apparent is their cl umping (except for P-1) to a segment along the great circle, implying that the fragments were consistently released from one side of their p arents, thus explaining for the first time why the off-train fragments preferentially appeared on one side of the nuclear train. In order to obtain a consistent solution, our model requires that the points of s eparation be on the antisolar side of the parent fragments, where ther mal stresses are likely to enhance the effect of rotation. The episode s of secondary fragmentation are found to have occurred in a period of time from a few weeks to at least nine months after the close encount er with Jupiter in early July 1992, and the separation velocities rang ed between 0.36 and 1.7 m s(-1). The spin-axis position is determined to have been nearly in the jovicentric orbit plane, which rules out th e Asphaug-Benz-Solem strengthless aggregate model as a plausible break up hypothesis; Since the separation velocities are rotational in natur e, they cannot substantially exceed the critical limit for centrifugal breakup and offer an estimate for the original nuclear dimensions. Th e comet's nucleus is found to have been approximately 10 km in diamete r and spinning rapidly. With the exception of P-1, and apparently also P-2 and F, no nongravitational deceleration was detected in the motio ns of the off-train fragments. Serious doubts are cast on continuing a ppreciable activity of any of these fragments. Indeed, when it was nec essary to introduce a deceleration into the equations of motion, the e ffect appears to have been due to the action of solar radiation pressu re on the centroid of centimeter-sized particulates in the disintegrat ing condensations. (C) 1998 Elsevier Science Ltd. All rights reserved.