Distinctive asymmetrical ejecta surrounds a 140 meter (459 feet) diameter crater in the lunar highlands. The crater is located on the northeastern rim of the eroded crater Hommel. Image width is 600 meters (1,969 feet). The top of the image faces north. Credit: NASA/Goddard Space Flight Center/Arizona State University
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Many lunar impact craters display an asymmetric ejecta blanket. During the Apollo era, NASA scientists used high speed projectiles to replicate the conditions of impact at different angles. Real asteroids hit the moon at fantastically high speeds, greater than 16 km per second (or 35,000 mph). As the angle between the asteroid path and the surface becomes smaller, no change in the crater shape is seen until an oblique angle of 15 degrees or less. At these small angles, non-circular craters or non-uniform distribution of ejecta occurs. Craters that display these characteristics are known as oblique impact craters, such as the example above.
In such a low angle impact, the ejecta has more momentum in the direction of travel of the impactor, which causes the asymmetric ray patterns. In this case the largest extent of ejecta extends to the north and south and a small amount to the east, probably indicating that the impactor came from the west. The blocks in the bottom of the crater may have been formed by the tremendous pressures unleashed during the impact.