Wedge-shaped asymmetrical ejecta pattern surrounding a ~1 km diameter mare crater. Image M154799629R width is 2 km and north is up, 0.5 m/pixel scale.
The ejecta pattern of this unnamed crater left a slice of Mare Undarum (Sea of Waves) uncovered, east of the crater Firmicus. When an impact ejecta blanket is not uniform, the ejecta is defined as asymmetric. Craters with asymmetric ejecta are either caused by pre-impact differences in composition, unusual topography, or an oblique angle of impact. Asteroids hit the Moon at fantastically high speeds, greater than 16 km per second (or 35,000 miles per hour), and most of the craters left by these impacts are circular. However, the shape of the crater (or the distribution of the ejecta blanket) changes when the the angle between the asteroid path and the surface becomes small, 15° or less. 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 extent of ejecta extends to the north and south and a lack of ejecta to the east, probably indicating that the impactor probably came from the east. Another example of an oblique impact is Messier, where the shape of the crater is elliptical due to the angle of impact.
NASA's Goddard Space Flight Center built and manages the mission for the Exploration Systems Mission Directorate at NASA Headquarters in Washington. The Lunar Reconnaissance Orbiter Camera was designed to acquire data for landing site certification and to conduct polar illumination studies and global mapping. Operated by Arizona State University, LROC consists of a pair of narrow-angle cameras (NAC) and a single wide-angle camera (WAC). The mission is expected to return over 70 terabytes of image data.