Wittke et al. (1) state
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The impactor mostlikely broke apart in solar orbit before en-countering Earth, as do most comets, includ-ing Comet Shoemaker
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Levy 9.
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However,Shoemaker
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Levy 9 broke up while in orbitabout Jupiter, and the tidal fragmentationprocess leading to impact on Jupiter doesnot apply to comets in solar orbit or for ap-proaches to terrestrial planets (4). A sponta-neous breakup in heliocentric space, suchas one recent example (5), would have tobe exquisitely timed for an expanding cloudof fragments to strike the Earth. Near-Earthcomets have average lifetimes of at least acentury before breaking up. Within monthsafter disintegration the comet fragmentswould be dispersed over an area much greaterthan that spanned by the Earth, preclud-ing many nearly simultaneous impacts.Thus, dispersed impacts of multiple frag-ments would be at least 1,000 times lessfrequent (probable) than the impact of asingle nucleus.
Thanks bogie.
If the breakup of the comet were caused by an encounter with the Earth, the comparison with SL-9 would be apples-apples; Comet ISON had its encounter with the Sun, so at least some of the fragments are likely to collide with the Sun, or be ejected. More fragments means more to track, because without tracking, there’s no way to know whether things are on trajectories to collide with Earth.