1. Characteristics

The Moon is the Earth's only natural satellite and, at just under 3500 km in diameter, it is about a quarter the size of the Earth[1]. About 4.5 billion years ago, when the Solar System was still forming, the Earth and Moon were part of the same gravitationally bound mass. This was hit by an object the mass of Mars, which scraped off the outside. This layer broke away and fell together again to make the Moon[2].

Photograph of the near side of the Moon with the Apollo sites marked.
Photograph of the far side of the Moon.

Near and far sides of the Moon, with approximate locations of the Apollo landing sites. Image credit: NASA's Goddard Space Flight Center Scientific Visualization Studio and NASA/Public domain.

The Moon crashed into hundreds of tiny rocks as it was flung from the Earth, covering its surface with craters. Just as this bombardment was coming to an end, several large objects hit the newly formed surface, blowing away the white soil (the highlands) on the near side, and releasing dark lava, which filled the craters and created the Moon's 'seas' (the maria)[3].

As the Moon solidified, the small amount of iron that it contains sunk to the centre. This is now surrounded by mantle[4], and encased in a crust composed of uranium, thorium, potassium, oxygen, silicon, magnesium, iron, titanium, calcium, aluminium, and hydrogen[5a].

The crust varies in thickness between about 60 km and 100 km, and is generally thicker on the far side. It's covered in a layer of fractured bedrock, known as the megaregolith. An outer layer of dust and rocky debris, known as regolith, lies on top of this. The regolith is about 15 metres thick in the highlands, and about 4 metres thick in the maria. The Moon is not massive enough to retain an atmosphere[5b].

The Moon is now almost 400,000 km from the Earth. Its rotation and orbital periods are both about a month long, and so the same side always faces the Earth. This is known as synchronous rotation[6].

Credit: astrogirlwest.

2. Pre-Apollo missions

There have been over 100 attempts to send spacecraft to the Moon, starting in 1958, and about 80 of these have been successful.

Both NASA and the Soviet Union made multiple attempts to send a spacecraft to the Moon in 1958, NASA with Pioneer 0 - Pioneer 3, and the Soviet Union with Luna 1958A - Luna 1958C. All were failures, with all of the Soviet missions failing to launch at all.

The Soviet Union had the first partial success with Luna 1, which came within 6000 km of the Moon's surface in 1959. The Soviet Union launched three more probes to the Moon that year. The first, Luna 1959A, was a failure, but the second, Luna 2 became the first spacecraft to reach the Moon's surface, crashing into the surface east of Mare Serenitatis, the Sea of Serenity. The third, Luna 3, became the first spacecraft to photograph the dark side of the Moon. The surface was found to be mountainous, covered with large boulders and lots of small craters.

NASA launched three spacecraft to the Moon in 1959, Pioneer 4, Pioneer P-1, and Pioneer P-3, and two more in 1960, Pioneer P-30, and Pioneer P-31. These were all failures, although Pioneer 4 did observe the Moon from a distance. NASA launched three Ranger probes to the Moon in 1962. These were also all failures, however Rangers 6 - 9 were successful, with Ranger 6 and Ranger 7 crashing into the Moon's surface in 1964, followed by Ranger 8 and Ranger 9 in 1965.

The Soviet Union were the first to successfully land a spacecraft on the Moon in 1966, when Luna 9 touched down in the Oceanus Procellarum, the Ocean of Storms. NASA's Surveyor 1 made a successful landing four months later. The Soviet Union launched 24 Luna missions between 1959 and 1976, almost all of which were successful, with eight landing on the Moon. Luna 17 and Luna 21, which launched in 1971 and 1973, contained their own Rovers, Lunokhod 1, and Lunokhod 2.

NASA launched seven Surveyor missions between 1966 and 1968. They all landed on the Moon expect for Surveyor 2, which crashed, and Surveyor 4, which exploded. NASA launched Lunar Orbiter 1 and Lunar Orbiter 2 in 1966, and Lunar Orbiter 3, Lunar Orbiter 4, and Lunar Orbiter 5 in 1967. These all successfully photographed the Moon's surface. NASA also launched Explorer 33 in 1966. This orbited the Earth for 5 years, at a distance that allowed it to pass the Moon's orbit. NASA launched Explorer 35 in 1967, and this orbited the Moon for 6 years.

The Soviet Union had successes with their Zond missions, as well as their Luna missions, in the late 1960s. The Soviet Union launched 11 Zond missions between 1965 and 1970. Zond 3, Zond 1967A, Zond 1967B, Zond 1968A, Zond 1969A, Zond L1S-1, and Zond L1S-2 were all failures, but Zond 4 - Zond 8, which launched between 1968 and 1970, were all at least partially successful.The Soviet Union also had a number of other failures, with Sputnik 25 in 1963, Cosmos 60 in 1965, Cosmos 111 in 1966, Cosmos 300 and Cosmos 305 in 1969, and Soyuz L3 in 1972.

Map of the Moon, with Luna, Apollo, and Surveyor landing sites marked.

Approximate landing sites of missions to the Moon. Image credit: NASA/X-romix/Public domain.

3. Apollo missions

NASA began serious preparations for a crewed mission to the Moon in the late 1960s. They intended to launch their first manned-mission into low Earth orbit in February 1967 with Apollo 1, crewed by Virgil Grissom, Edward White, and Roger Chaffee. However, a fire broke out in the cabin during a rehearsal in January and 34 people were killed, including all three crewmembers.

All launches were postponed while the tragedy was investigated, and the next Apollo mission was officially designated Apollo 4. This would not be crewed, and referred to the first launch of the Saturn V rocket in November 1967. Two more uncrewed missions, Apollo 5 and Apollo 6 launched in January and April 1968. Apollo 5 was the first test flight of the Lunar Module, and Apollo 6 confirmed that the Apollo spacecraft was ready for crewed missions.

The first successful crewed mission in the Apollo program, Apollo 7, launched in October 1968. The crew, comprised of Walter Schirra, R. Walter Cunningham, and Donn Eisele, orbited the Earth for 10 days.

Apollo 8 launched in December 1968. Apollo 8 became the first crewed spacecraft to orbit the Moon. It was also the first crewed flight of the Saturn V rocket, which launched all of the subsequent Apollo missions.

The crew was comprised of Frank Borman, William Anders, and James Lovell, who would go on to command Apollo 13. These became the first people to directly see the far side of the Moon, and the first people to see the Earth rise over the Moon. Images of this were broadcast live on Earth.

Credit: NASA/Hank Schneider.

Apollo 9 launched in March 1969, with a crew comprised of James McDivitt, Rusty Schweickart, and David Scott, who would go on to command Apollo 15. They spent ten days in low Earth orbit testing equipment and docking manoeuvres.

NASA launched Apollo 10 in May 1969. This was considered a 'dress rehearsal' for the Moon landing, which would be attempted in July, and became the second crewed mission to orbit the Moon. Apollo 10 was crewed by Thomas Stafford, John Young, and Eugene Cernan. Young would become the ninth person to walk on the Moon, as the Commander of Apollo 16, and Cernan would become the eleventh, with Apollo 17.

NASA launched Apollo 11 - Apollo 17 between July 1969 and December 1972. These were all crewed missions comprised of three people, two of which would walk on the Moon. The Apollo 13 mission failed, but all the others were successful, and so, in total, twelve people have walked on the Moon, and 24 people have orbited the Moon.

Neil Armstrong and Buzz Aldrin became the first people to walk on the Moon in July 1969 with NASA's Apollo 11, which was piloted by Michael Collins. It took them just over three days to reach the Moon, where they landed in the Mare Tranquillitatis, the Sea of Tranquillity, which had previously been photographed by Ranger 8 and Surveyor 5. They spent two and a half hours on the surface of the Moon.

Apollo 12 launched in November 1969, crewed by Richard Gordon, Pete Conrad, and Alan Bean. Conrad and Bean landed in the Oceanus Procellarum, next to the landing site of Surveyor 3. They were able to take parts of Surveyor 3 back to Earth, and this allowed them to study the long-term effects of being on the Moon.

Photograph of the Apollo 11 rocket launch.

Apollo 11 launch. Image credit: NASA/Public domain.

Photograph of the Apollo 11 lunar module on the Moon.

The Apollo 11 Lunar Module. Image credit: NASA/Public domain.

Photograph of the Lunar Landing Research Vehicle on Earth.

The Lunar Landing Research Vehicle in flight on Earth. Image credit: NASA/Public domain.

Apollo 13 launched in April 1970, crewed by Jack Swigert, Fred Haise, and James Lovell, who had previously orbited the Moon in Apollo 8. Its mission was aborted after one of the oxygen tanks in the Command Module exploded, two days into the journey. The crew used the Lunar Module as a 'life boat' and safely returned home.

Apollo 13 had planned to go to an area of white dust that Ranger 7 had photographed and so, in January 1971, Apollo 14 went there instead. Edgar Mitchell and Alan Shepard landed close to the Apollo 12 landing site, on the edge of a crater in an area known as Fra Mauro, while Stuart Roosa remained in orbit.

In July that year, David Scott and James Irwin landed, with Apollo 15, in Palus Putredinis, the Marsh of Decay, while Alfred Worden remained in orbit. They landed inside the third ring of the Imbrium basin, a crater that covers an area the size of a continent on Earth. This was the first mission to include the Lunar Rover, which allowed them to travel further than before, and to collect more rocks, taking back 77 kg in total. They explored the highlands above the crater and Hadley Rille, a huge ridge that had was carved into the side of the Moon by rivers of lava.

Apollo 16 launched in April 1972. While Ken Mattingly remained in orbit, Charles Duke and John Young, who had previously orbited the Moon with Apollo 10, landed in an area known as Descartes, in the southern Highlands. This site was chosen by looking at photographs taken by Apollo 12 and Apollo 14. This mission also used a Lunar Rover, and brought back over 94 kg of rocks.

Photograph of astronaut Harrison 'Jack' Schmitt on the Moon.

Harrison "Jack" Schmitt at the Taurus-Littrow landing site during the Apollo 17 mission. Image credit: NASA/Eugene Cernan/Public domain.

The final Apollo mission, Apollo 17, launched in December 1972. While Ronald Evans remained in orbit, geologist Harrison "Jack" Schmitt and Eugene Cernan, who had previously orbited the Moon with Apollo 10, landed in the Taurus-Littrow region of the Mare Serenitatis, the Sea of Serenity. This was the first mission to include a geologist.

Cernan and Schmitt spent 75 hours on the lunar surface, the longest of any Apollo mission, and travelled over 30 km in a Lunar Rover. They brought back over 110 kg of rocks. The highlands were found to mostly be composed of anorthosite, a kind of igneous rock that forms when lava cools slowly. These are thought to have formed when the Moon first settled into orbit as a molten ball.

4. Post-Apollo missions

There have been over 20 successful robotic missions to the Moon since Apollo 17.

NASA's Explorer 49 launched in 1973, and orbited the Moon for two years. NASA's Mariner 10 successfully flew past the Moon in 1973, on its way to Mercury, and NASA's International Cometary Explorer flew past the Moon on its way to comet Giacobini-Zinner in 1983.

NASA launched Clementine in 1994, and the Lunar Prospector in 1998. The HGS-1 satellite, a communications satellite owned by a private company in Hong Kong, was salvaged as a lunar probe after it became unusable in 1997, and flew past the Moon later that year.

Japan's Institute of Space and Astronautical Science (ISAS), now part of the Japan Aerospace Exploration Agency (JAXA), launched three missions to the Moon in the 1990s - Hiten, Hagoromo, GEOTAIL, which was a joint mission with NASA, and Nozomi, which was on its way to Mars. JAXA launched KAGUYA in 2009. These missions were all successful apart from Hagoromo, which failed to transmit data back to Earth.

The ESA launched SMART-1 in 2004, which orbited the Moon until 2006.

The China National Space Administration (CNSA) launched Chang'e 1 in 2007, and this orbited the Moon until 2009. They launched Chang'e 2 in 2010, and it orbited the Moon until 2011. Finally, the CNSA launched Chang'e 3 in 2013, which had its own Rover, and Chang'e 5-T1 in 2014.

The Indian Space Research Organisation (ISRO) launched Chandrayaan-1 in 2009, and it orbited the Moon for almost a year. Chandrayaan 1 also contained a lunar impactor, which found frozen water in the lunar soil. This was confirmed by NASA's Deep Impact spacecraft, which passed the Moon on its way to the comet Tempel, and the Cassini spacecraft, which passed the Moon on the way to Saturn.

Shortly after Chandrayaan-1's discovery, NASA crashed a 2200 kg rocket into a 100 km wide crater on the Moon, and detected more than 100 kg of water. This was part of their LCROSS (Lunar Crater Observation and Sensing Satellite) project, which was launched with their Lunar Reconnaissance Orbiter in 2009.

The latest probes to be launched by NASA are ARTEMIS P1 and ARTEMIS P2, which launched in 2011. NASA also launched GRAIL A and GRAIL B in 2012, these successfully mapped the Moons gravitational field. Finally, they launched LADEE in 2013, which studied the Moon's upper atmosphere.

There are a number of planned missions to the Moon, including at least two crewed orbital missions. Commercial space tourism company Space Adventures Ltd is planning to send tourists on a trip around the Moon by 2020, and NASA is planning to send two astronauts to an asteroid that is in orbit around the Moon in about 2024.

5. References

  1. NASA, 'Planetary Fact Sheet', last accessed 15-02-16.

  2. Canup, R. M. and Asphaug, E., 2001, 'Origin of the Moon in a giant impact near the end of the Earth's formation', Nature, 412, pp.708-712.

  3. NASA, 'About the Moon', last accessed 15-02-16.

  4. Dwyer, C. A., Stevenson, D. J. and Nimmo, F., 2011, 'A long-lived lunar dynamo driven by continuous mechanical stirring', Nature, 479, pp.212-214.

  5. (a, b) NASA, 'Lunar Geology', last accessed 15-02-16.

  6. NASA, 'Earth's Moon: By the Numbers', last accessed 15-02-16.

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