The Moon is the Earth's only natural satellite. Its the second celestial object to be inhabited by humans (the first being the Earth and the third being Mars) and the closest to the Earth. As of 1990, its population stands at 57 people on the Lunar surface (48 Americans, nine Soviets) and 24 people in Lunar Orbit (all American). The exploration of the Moon began in the mid 20th century with Artificial Probes but later moved to humans in the late 1960s and 1970s.
The first Manned Mission to visit the Moon occured on December 24, 1968 when Apollo-8 entered a low orbit around the Moon. The crew of three (Jim Lovell, William Anders, Frank Borman) spent one day taking pictures of the surface and performing experiments. Their Christmas eve television broadcast of the reading of Genesis from lunar orbit reached an estimated 500 million people.
In May 1969 America's Apollo-10 mission, designed to test the Lunar Module and Command/Service Module in lunar orbit, succeeded in becoming the second Manned Lunar orbital mission.
In June, 1969 the third and final Soviet circumlunar spaceflight, L1-3 took off and made its measurements and took its pictures of the Moon. This would be the last Soviet mission to the Moon for several years.
On July 20, 1969 Apollo 11 touched down on the Lunar surface. Neil Armstrong and Buzz Aldrin became the first human beings ever to walk on the surface of the Moon. The crew spent just 23 hours in the Sea of Tranquility before departing.
In November 1969 Apollo 12 arrived in the Ocean of Storms, just meters away from the Surveyor-3 lander that had arrived a mere two years prior.
In April 1970 Apollo-13 suffered a near-fatal accident while halfway to the Moon, its oxygen tank ruptured resulting in an immediate abort via a gravity assist around the backside of the Moon. While this delayed the Apollo Program by 10 months it did increase United States public's enthusiasm and support for the Space Program.
In the year 1971 two Apollo moon-landing missions (Apollo 14 & 15), each with three astronauts, a Command and Service Module (CSM), and a Lunar Module (LM) launched on a three-stage Saturn V rocket. They constituted the continuation of the Apollo lunar landing missions that had begun with Apollo 11. These H-class missions included precision landings and 36 hour surface stays. Apollo-13 visited the original landing site planned for Apollo-12 and met up with the unmanned Surveyor-3 lander (which landed on April 20, 1967).
In the year 1971 the first two Extended Apollo missions launched. An uprated Saturn VB rocket could launch three astronauts, an Extended CSM (XCSM) capable of 16 days of flight, and an Extended LM (XLM) capable supporting two astronauts for three days. The XLM would had a landed payload capacity of 1000 pounds. NASA flew two Extended Apollo missions per year from 1971 through 1975 and one in 1976.
Early in 1976, a Saturn VC (a three stage Saturn V with a reusable Space Tug as a fourth stage) launched a 50,000-pound Space Station Module (SSM) and a fully fueled Space Tug/LM-B to near-polar lunar orbit. During 1976, 1977, and 1978, nine Saturn VCs launched four Space Tug/LM-Bs and five four-man QCSMs to the lunar-orbit SSM, enabling a continuous lunar orbit population of four astronauts. The QCSM, was an interim system like the Saturn VC. Two-man crews landed on the moon in Space Tug/LM-Bs four times in 1976, five times in 1977, and four times in 1978. Each trip to the lunar surface and back expended 50,000 pounds of LN2/LOX propellants and lasted 14 days.
The lunar-orbit Space Station Module kept two fully fueled Space Tug/LM-Bs on hand at all times. One would land on the moon and the other would stand by to rescue the surface astronauts in the event that their Space Tug/LM-B malfunctioned. After a year of operations, Space Tug/LM-Bs based at the lunar-orbit SSM would be stripped down and turned into tankage for a propellant depot in lunar orbit. Occasionally they would be deorbited with high resolution cameras and be used as impactors.
Beginin in 1978 with the introduction of the NERVA shuttle, a permanent base was built up with unmanned, one-way Space Tugs landing Space Station Modules on the lunar surface. Thus, by the end of 1978 there existed a continuous population of six Astronauts on the Moon at all times and also a continuous population of six astronauts in Lunar Polar Orbit at all times. This new phase allowed continued expansion both of the Bases. By 1980 the population in Lunar Orbit had grown to 24 Men while only six remained at the surface. By 1984 there were 24 people in the Lunar Orbit Base and 48 people at the Lunar Surface Base. The ice available at the Clauvius Crater made it the perfect location for a Base because it allowed Breathing Oxygen, Drinking/Irrigating Water, Hydrogen/Oxygen rocket propellant to be manufactured.