From the December 2011 issue

Why are most impact craters nearly perfectly round? I’d expect the impacts to result from an obtuse or glancing blow that would leave an oblong or ellliptical shape.

Thomas Whelchel, Huntsville, Alabama
By | Published: December 27, 2011
Double lunar craters Messier and Messier A are rare examples of elliptical craters instead of round. Scientists think the impactor that created the pair must have struck the Moon at a very shallow angle — less than 5° from horizontal. NASA/GSFC/Arizona State University
Impact craters are different from holes you might make in a muddy patch of your backyard by throwing rocks or marbles into the ground at various angles. In such familiar cases, an object entering the mud at a substantial angle from vertical would, indeed, make an elliptical or oblong hole.

Craters made by impacting asteroids or comets, however, are not holes or tunnels “dug” into the ground, but instead result from massive explosions. This is because these bodies strike at tens of kilometers per second, and thus when “stopped” by the ground, they liberate enormous energy. The resulting explosion often creates a crater 10 to 20 times the diameter of the projectile. An explosion generally makes a symmetric (circular) crater, no matter what the shape of the projectile or its angle of impact.

The only exception is if the impact angle is extremely shallow — less than 5° from horizontal. Such a glancing blow also results in an explosion, but while the bottom of the projectile starts to explode when it makes contact with the ground, upper portions may shear off and strike farther downrange, resulting in an elongated crater — or, in exceptional circumstances, a double crater, like Messier and Messier A on the Moon. — Clark Chapman, Southwest Research Institute, Boulder, Colorado