The Mendel-Ryberg Basin is a Nectarian (formed during the Nectarian geological period, some 3920 million to 2850 million years ago) impact basin on the southwestern limb of the moon. It is named after the crater Mendel on the west margin and the smaller crater Rydberg north of the centre of the basin. The basin is due south of the larger, younger Orientale basin, and ejecta (ejected particles after a cataclysmic event) and other geomorphological effects from the younger basin have overprinted the older one.

At the centre is a mass concentration, which is a region of a planet or moon's crust that contains a large positive gravitational anomaly, (mascon), or gravitational high.

Because of its mascons, the Moon has only four "frozen orbit" inclination zones where a lunar satellite can stay in a low orbit indefinitely. Lunar subsatellites were released on two of the last three Apollo manned lunar landing missions in 1971 and 1972; the subsatellite PFS-2 released from Apollo 16 was expected to stay in orbit for one and a half years, but lasted only 35 days before crashing into the lunar surface. It was only in 2001 that the mascons were mapped and the frozen orbits were discovered.

The mascon in the Mendel-Ryberg Basin was first identified by Doppler tracking of the 'Lunar Prospector' spacecraft.

More Info: en.wikipedia.org