Spacecraft carrying imaging equipment have visited a total of six comets. Four of them have binary nuclei (1P/Halley, 19P/Borrelly, 103P/Hartley, and 67P/Churyumov–Gerasimenko) and four appear to have layered structures (67P/Churyumov–Gerasimenko, 19P/Borrelly, 81P/Wild, and 9P/Tempel). How such morphologies came to be has been something of a mystery for astronomers. Erik Asphaug of Arizona State University and Martin Jutzi of the University of Bern now think they’ve solved at least a piece of that puzzle. They ran some 100 collision simulations, each taking one to several weeks to complete, to see what happens when two loosely packed icy spheres—something like giant snowballs—collide at velocities consistent with comet-sized bodies falling gravitationally toward each other.1
The two astronomers’ efforts were motivated by the “talps” model (splat spelled backward) proposed in 2007 by Michael Belton, an emeritus astronomer at Kitt Peak National Observatory in Arizona, and his collaborators, based on images of comet 9P/Tempel...
