Crater ellipticity
Most craters are round even when the impactor arrived at a slant; only very low-angle impacts make elongated craters.
You might expect a rock hitting the ground at a slant to make an oval crater, but most craters are almost perfectly round. That is because a crater is not dug by the impactor pushing sideways; it is made by an explosion-like release of energy at the point of contact, which spreads out evenly in all directions.
Only when the impact angle is very low, less than about 10 to 15 degrees from the horizontal, does the crater become noticeably elongated. Such elliptical craters are a small fraction of all craters. At low angles the ejecta pattern also becomes lopsided, sometimes forming a butterfly shape, with no ejecta in the direction the impactor came from.
The ejecta pattern is therefore a better guide to the impact direction than the crater's shape. Laboratory experiments that fire projectiles into sand at different angles, carried out in the 1970s and later, established these rules.
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Level: Expert Advanced