Perturbation Effects
Gravitational anomalies slightly distorting the orbits of some Lunar Orbiters led to the discovery of mass concentrations (dubbed mascons), beneath the lunar surface caused by large impacting bodies at some remote time in the past. These anomalies are significant enough to cause a lunar orbit to change significantly over the course of several days. "Lunar mascons make most low lunar orbits unstable" but there exist four 'frozen orbits' "where a spacecraft can stay in a low lunar orbit indefinitely." The frozen orbits occur at four inclinations, 27º, 50º, 76º, and 86º."
The Apollo 11 first manned landing mission employed the first attempt to correct for this effect. The parking orbit was "circularized" at 66 nautical miles (122 km; 76 mi) by 54 nautical miles (100 km; 62 mi), which was expected to become the nominal circular 60 nautical miles (110 km; 69 mi) when the LM made its return rendezvous with the CSM. But the effect was overestimated by a factor of two; at rendezvous the orbit was calculated to be 63.2 nautical miles (117.0 km; 72.7 mi) by 56.8 nautical miles (105.2 km; 65.4 mi).
The Apollo 15 subsatellite PFS-1 and the Apollo 16 subsatellite PFS-2, both small satellites released into lunar orbit from the Apollo Service Module near the end of their respective missions, provided later scientists data on Lunar gravitational perturbation effects. PFS-1 ended up in a long-lasting orbit, at 28 degrees inclination, and successfully completed its mission after one and a half years. PFS-2, was placed in a particularly unstable orbital inclination of 11 degrees, lasted only 35 days in orbit before crashing into the Lunar surface.
Read more about this topic: Lunar Orbit
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