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Photos of recent comet flybys show their nuclei to be barbell-shaped rather than spherical. What are the forces or processes that produce this shape?

Nathan Fawcett, Ithaca, New York
Comets
Barbell-shaped comets 103P/Hartley and 19P/Borrelly differ from most other studied comets (like 9P/Tempel and 81P/Wild, which appear more spherical). Scientists aren’t sure what causes the different shapes. Hartley: NASA/JPL-Caltech/UMD; Borrelly: NASA/JPL; Tempel: NASA/JPL/UMD; Wild: NASA/JPL-Caltech
The research community doesn’t agree on what controls the shape. Some scientists have suggested that Hartley’s smooth waist is due to the nucleus stretching along its long axis. However, it seems to me more likely that the two ends came together as separate cometesimals, and ejecta from other parts of the nucleus filled in the smooth waist. We’ll need detailed simulations to test whether either of these scenarios, or another, is plausible.

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