The Moon’s gravity is lumpy. Areas where mass is concentrated have stronger gravity such as where impacts have compressed the ground. These mass concentrations, called mascons, pull spacecraft forward, back, left, right, and down—making most low lunar orbits unstable.
 
NASA launched the Gravity Recovery and Interior Laboratory (GRAIL) mission in 2011 to map the location and strength of mascons. GRAIL consisted of twin satellites named Ebb and Flow that were placed in a low lunar orbit of only 34 miles. The distance between the two satellites varied slightly as they flew over areas of greater or lesser gravity caused by masses hidden under mountains and craters.
 
Large mascons were found underneath all the biggest impact craters on both the near and far sides. Unlike the Seas of Rains and Serenity (the “eyes” of the Moon), the Sea of Tranquility where Apollo 11 landed didn’t form by impact, has no ring around it, and GRAIL showed no mascon under it.
 

This map based on GRAIL data shows that the Moon’s crust is up to 37 miles (60 km) thick (white) on the far side. Impacts compressed and thinned the crust under large impact craters (blue and purple). The Sea of Rains, Serenity, and Crises form a row of low spots across the northern near side (left image). The crust is so thin at the Sea of Crises and the Moscow Sea (upper far side-right image) that olivine (purple stars), a mineral from the Moon’s mantle mapped by Kaguya is exposed on the surface. Credit: NASA/JPL-Caltech/IPGP

Because of mascons, the Lunar Prospector spacecraft (1998-99) had to do a maneuver every two months to stay in a 60-mile (100 km) polar orbit, and once a month to stay at 20 miles (30 km) above the surface. Luckily future missions have some better long-term “parking” options.
 
 

 
Apollo 16 released two subsatellites into lunar orbits. PFS-1 stayed in orbit for one and a half years. PFS-2 crashed after 35 days. Scientists soon discovered that mascons lured PFS-2 to its early demise. PFS-1 avoided the same fate because its orbit was inclined (at 28 versus PFS-1 at 11 degrees) so that it spent less time passing over mascons. (Inclination is the angle that the orbit plane makes with the equator.) Their different fates led to the identification of four stable or “frozen” orbits with inclinations of 27, 50, 76, and 86 degrees. These orbits require less fuel to maintain.
 
A first base on the Moon will likely be placed near the south lunar pole because of deep ice deposits (for fuel and life support) as well as tall mountains that offer almost continuous sunlight (for power). Therefore, the 86-degree frozen orbit offers a reasonable parking spot for return vehicles. The disadvantage of parking there is that launch windows from those orbits to Earth occur only about every two weeks.
 

 
Instead of parking near the Moon, Earth return vehicles can remain in Earth orbits. This Moon Direct  plan is explained by Robert Zubrin in his book, The Case for Space. Astronauts would ride commercial rockets to Earth orbit and catch their shuttle (called a Lunar Excursion Vehicle, LEV) from there to the Moon. LEV then fly directly to the surface of the Moon with no rendezvous in lunar orbit on the way out or back. The LEV can be resupplied with fuel in Earth orbit (using lunar water) and support multiple roundtrips.
 
NASA’s Lunar Gateway space station is not planned to be in lunar orbit at all. Instead, it is slated for an ellipse around a place in space called Lagrange Point 2 (L2, above the far side) and never come any closer to the surface than about a lunar diameter. Zubrin and NASA’s former Administrator and others consider this an unfortunate choice.  From the surface of the Moon, it actually takes more fuel to reach the Gateway’s location than it does to reach Earth orbit. It also requires fuel to “park” anything there and can only be accessed once a week. Some have suggested that the Gateway move to an Earth-Moon cycling obit (to the Moon and back on a regular schedule) and serve as a fuel depot.
 
Lumpy lunar gravity requires some creative solutions for parking Earth return vehicles while working on the surface. The main purpose for a first lunar base, in my opinion, is to process lunar water into fuel. This lunar "gas station" will enable easier access to all other destinations in space, including Earth, any Gateway stations, and missions to Mars. A lunar gas station will also hopefully lead to affordable flights for those of us who want to experience lumpy gravity for ourselves!
 

 
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