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The recent publication by Siraj & Loeb (2021; Nature Scientific Reports 11,3803) attempts to revive the debate over whether the Chicxulub impactor was acomet or an asteroid. They calculate that ~20% of long-period comets impactingEarth will have first been disrupted by passage inside the Sun's Roche limit,generating thousands of fragments, each the needed size of the Chicxulubimpactor. This would increase the impact rate of comets by a factor ~15, makingthem as likely to hit the Earth as an asteroid. They also argue that a cometwould be a factor of 10 more likely to match the geochemical constraints, whichindicate the Chicxulub impactor was carbonaceous chondrite-like. Theseconclusions are based on misinterpretations of the literature. Siraj & Loeb [1]overestimate the number of fragments produced during tidal disruption of acomet: tens of fragments are produced, not thousands. They also conflate'carbonaceous chondrite' with specific types of carbonaceous chondrite, andignore the evidence of iridium, making comets seem more likely than asteroidsto match the Chicxulub impactor, when in fact they likely can be ruled out.Rather than a comet, an asteroidal impactor similar to CM or CR carbonaceouschondrites is strongly favored.

Matters Arising on "Breakup of a long-period comet as the origin of the dinosaur extinction" by Siraj & Loeb