Orbits to and Around the Moon
General Theory of Orbits
The motion of one body about another body due to gravity, is known as an Orbit. Orbits are described by a body of theory called "Orbital Dynamics". Kepler discovered that in Newtonian Physics, which ignore Einstein's theories of relativity, orbits are elliptical in shape, at least for the simple case of one body orbiting another body without any influence from any third body. Kepler's Laws are three equations which describe elliptical orbits, and still hold true today.
The Moon is Gravitationally very Lumpy
The Moon is both out of round and does not have a symmetric distribution of mass. The result is that low lunar orbits are not stable for long time periods. For example, the Apollo Command modules would have drifted out of reach for the returning Lunar Lander in only a few days.
The mass distribution of the Moon varies from a perfect sphere by:
- An equatorial bulge
- Mass Concentrations (Masscon) in the Mares
- The Near Side crust being ½ the thickness of the Far Side crust
- A mass deficiency in the South Pole Aitken Basin (which is where we are going)
Result on Lunar Orbits
If you need a long term stable lunar orbit, either you have to maintain considerable altitude or you have to choose from a limited number of low orbits that happen to balance out the lumps. There are about five angles to the lunar equator that have long term orbits. One of these is in the high 80 degrees of inclination and is attractive for polar work.
Having to use only selected low lunar orbits means that the lunar landers must carry additional fuel. The new lander design has large fuel tanks for this reason. Also there will be fewer and more limited launch windows for leaving the Moon.
Most other low lunar orbits simply crash into the Moon within a few weeks.
A Trip to the Moon
The factors controlling the choice of a path from the Earth to the Moon are very different for manned and unmanned vehicles. In all cases, a lunar polar site takes a little more delta V, and therefore rocket fuel, than an equatorial site.
Usually for unmanned vehicles, you are simply trying for the most mass delivered to the Moon for a given type of rocket launched from a specific Earth port. Often you trade off time in transit for additional mass delivered. The transit time may be a few extra days or even weeks longer than manned flights. If an ion drive transfer rocket is used, this trip could take months. The launch windows for this type of trip are wide open.
The safest route is to fly first to an orbit near the International Space Station (ISS). You do not plan to stop, but if you have an emergency you could reach the station. You then have a problem. ISS has a high orbit inclination so that it can be reached from Russian launch sites. This is a poor orbit to use for lunar transfer as it takes a lot of fuel to take out the inclination. One workable solution is to next fly to the Earth-Moon Lagrange Point 2 (L2). At this point the gravity of the two bodies is nearly balanced and you can make a burn of reasonable size to move you on your way to a near lunar polar orbit. Your path will look like a large figure 8 that is twisted at the cross-over point. This transfer path takes more time and fuel and has fewer and shorter launch windows than more direct methods.
Go to WayBackMachine, enter http://science.nasa.gov/headlines/y2006/30nov_highorbit.htm in the take me back window, click the take me back button, and choose 1 December 2006 for the article A New Paradigm for Lunar Orbits.