A satellite put into orbit around the moon will stay there, right? Nope! Most lunar orbits aren’t stable enough for a satellite to stay in orbit.
Why is that?
The answer is something called Lunar Mascons. Mascons are “mass concentrations”. It’s a region of the moon’s crust that contains a different concentration of mass than other parts of the moon. The result? The gravity of the moon varies as an object flies overhead. Since a stable orbit requires a stable gravitational pull, the result of these lunar mascons is that almost any object put into orbit around the moon will have their orbit decay rapidly, and eventually crash into the lunar surface.
Apollo had this problem. For Apollo, this problem caused two issues. The first issue is that the Apollo CSM in orbit around the moon was in an unstable orbit. This meant it could not stay in the current orbit for a long period of time. For the short stays of Apollo, this wasn’t a huge issue, but it did result in the need for some corrections to navigational calculations during the course of the stay.
But the bigger problem was on the lunar landing. These mass concentrations made it almost impossible to accurately predict the landing location of the lunar lander. This caused predicted locations for lunar landings to be in some cases 100 times as large as those planned for safety reasons. This was one of the problems that caused Apollo 11 to deviate from its planned landing site, and caused Neil Armstrong to have to take manual control to maneuver the lander to a safer location.
The solution to the orbital decay problem was to use what was called “frozen orbits”. These are known orbital inclination zones where a satellite in lunar orbit can stay in a low orbit for a long period of time (theoretically indefinitely). There are only four known frozen orbits of the moon, and they were discovered during the Apollo days. For a satellite in orbit at around the 60 mile altitude (100 km), which is a standard orbital altitude used during Apollo, the four frozen orbits exist at orbits of 27°, 50°, 76°, and 86° inclinations. The 86° orbit is nearly a polar orbit, while the 27° orbit is the closest to a equatorial orbit.
For the lunar landings, a Tiger Team was used after Apollo 11 to find a solution to the navigation software used on the Apollo lunar landers in order for them to reduce the size of the possible landing areas. By the time Apollo 12 flew, they were able to reduce the size of the landing area small enough that Apollo 12 was able to land within 535 feet (163 m) of its target.
Impact on Belitopia
In the world of Belitopia, this had an impact beyond that of the Apollo lunar landings. The Lunar Skylab space station, which existed in the world of Belitopia, could only exist and stay in orbit if it used one of the lunar frozen orbits. In the case of the Lunar Skylab, the 27° inclination orbit was used, since the near equatorial orbit was convenient for rendezvous with the Lunar Transit missions that were destined to land at Tycho Base and BLA Base. By using an orbit as near equatorial as possible, this made docking with the Lunar Skylab from a ship destined ultimately for or from one of the lunar bases easier. These transit missions used the equatorial orbit as it made lunar landings more convenient. Having the Lunar Skylab in a closely inclined orbit meant they could dock with it easier, faster, and using less fuel.
The existence of mascons was also important for the navigation software aboard the new prototype lunar lander created in the world of Belitopia. This lander was capable of a near pinpoint landing. This meant it had to take the mascons into account as it determined the path it needed to take to land at a particular location.
More Information
If you would like to learn more about mascons and frozen orbits you might want to take a look at the following references: