The Apollo program was the third human spaceflight program undertaken by NASA, on behalf of the United States of America. Its goal — "...before this decade is out, of landing a man on the Moon and returning him safely to the Earth," (as U.S. President John F. Kennedy announced in 1961) — was accomplished on July 20, 1969, with the Apollo 11 landing of astronauts Neil Armstrong and Buzz Aldrin, and with Michael Collins orbiting above. To date, it is the only series of spaceflights to land humans on another world, and thus is widely respected and an inspiration for future spaceflights. The program used the Apollo spacecraft and Saturn launch vehicle, and was conducted during the years 1961 – 1975, with a total of six successful landings.
The later Skylab program and the joint American-Soviet Apollo-Soyuz Test Project used equipment originally produced for Apollo, and are often considered to be part of the overall Apollo program. As of 2007, there has not been any further human spaceflight beyond low earth orbit since the last mission in the Apollo program, Apollo 17.
Despite the many successes, there were two major failures, the first of which resulted in the deaths of three astronauts, Virgil Grissom, Ed White and Roger Chaffee, in the Apollo 1 launchpad fire (the mission designation was AS-204, which was renamed Apollo 1 to honor the astronauts and their widows). The second was an in-space explosion on Apollo 13, which badly damaged the spacecraft on the moonward leg of its journey. The three astronauts aboard narrowly escaped with their lives, thanks to the efforts of flight controllers, project engineers, backup crew members and the skills of the astronauts themselves.
The Apollo program, specifically the lunar landings, are often cited as one of the greatest achievements in human history.

Background
See also: Moon landing
Once Kennedy had defined a goal, the Apollo mission planners were faced with the challenge of designing a set of flights that could meet this stated goal while minimizing risk to human life, cost, and demands on technology and astronaut skill. Four possible mission modes were considered:
In early 1961, direct ascent was generally the mission mode in favor at NASA. Many engineers feared that a rendezvous, which had never been attempted in space, would be impossible in lunar orbit. However, dissenters including John Houbolt at Langley Research Center emphasized the important weight reductions that were offered by the LOR approach. Throughout 1960 and 1961, Houbolt campaigned for the recognition of LOR as a valid and practical option. Bypassing the NASA hierarchy, he sent a series of memos and reports on the issue to Associate Administrator Robert Seamans; while acknowledging that he spoke "somewhat as a voice in the wilderness," Houbolt pleaded that LOR should not be discounted in studies of the question. The engineers at Marshall Space Flight Center took longer to become convinced of its merits, but their conversion was announced by Wernher von Braun at a briefing in June 1962. NASA's formal decision in favor of LOR was announced on July 11, 1962. Space historian James Hansen concludes that:

Direct Ascent: A spacecraft would travel directly to the Moon, landing and returning as a unit. This plan would have required a very powerful booster, the planned Nova rocket.
Earth Orbit Rendezvous: Two Saturn V rockets would be launched, one carrying the spacecraft and one carrying a propulsion unit that would have enabled the spacecraft to escape earth orbit. After a docking in earth orbit, the spacecraft would have landed on the Moon as a unit.
Lunar Surface Rendezvous: Two spacecraft would be launched in succession. The first, an automated vehicle carrying propellants, would land on the Moon and would be followed some time later by the manned vehicle. Propellant would be transferred from the automated vehicle to the manned vehicle before the manned vehicle could return to Earth.
Lunar Orbit Rendezvous (LOR): One Saturn V would launch a spacecraft that was composed of modular parts. A command module would remain in orbit around the moon, while a lunar module would descend to the moon and then return to dock with the command module while still in lunar orbit. In contrast with the other plans, LOR required only a small part of the spacecraft to land on the Moon, thereby minimizing the mass to be launched from the Moon's surface for the return trip. Choosing a mission mode

Main article: Apollo spacecraft Spacecraft
The command module (CM) was conical in shape, and was designed to carry three astronauts from launch into lunar orbit and back from the moon to splashdown. Equipment carried by the command module included reaction control engines, a docking tunnel, guidance and navigation systems and the Apollo Guidance Computer. Attached to the command module was the service module (SM), which housed the service propulsion system and its propellants, the fuel cell power system, four maneuvering thruster quads, the S-band antenna for communication with Mission Control, and storage tanks for water and air. On Apollo 15, 16 and 17 it also carried a scientific instrument package. The two sections of the spacecraft would remain attached until just prior to re-entry, at which point the service module would be discarded. Only the command module was provided with a heat shield that would allow it and its passengers to survive the intense heat of re-entry. After re-entry it would deploy parachutes that would slow its descent through the atmosphere, allowing a smooth splashdown in the ocean.
Under the leadership of Harrison Storms, North American Aviation won the contract to build the CSM for NASA. Relations between North American and NASA were strained during the Apollo program, particularly after the Apollo 1 fire during which three astronauts died. The cause of the accident was determined to be an electrical short in the wiring of the command module; while determination of responsibility for the accident was complex, the review board concluded that "deficiencies existed in Command Module design, workmanship and quality control."

Command/service module
The Lunar Module (LM) (also known as Lunar Excursion Module, or LEM), was designed solely in order to land on the moon. It had no heat shield and was of a construction so lightweight that it would not have been able to fly in Earth gravity. It carried two crewmembers and consisted of two stages, a descent and an ascent stage. The descent stage incorporated compartments in which cargo such as the Apollo Lunar Surface Experiment Package and Lunar Rover could be carried.
The contract for design and construction of the lunar module was awarded to Grumman, and the project was overseen by Tom Kelly. There were also problems with the lunar module; due to delays in the test program, the LM became what was known as a "pacing item," meaning that it was in danger of delaying the schedule of the whole Apollo program. Due to these issues, the Apollo missions were rescheduled so that the first manned mission with the lunar module would be Apollo 9, rather than Apollo 8 as was originally planned.

Lunar module
When the team of engineers led by Wernher von Braun began planning for the Apollo program, it was not yet clear what sort of mission their rocket boosters would have to support. Direct ascent would require a booster, the planned Nova rocket, which could lift a very large payload. NASA's decision in favor of lunar orbit rendezvous re-oriented the work of Marshall Spaceflight Center towards the development of the Saturn 1B and Saturn V. While these were less powerful than the Nova would have been, the Saturn V was still much more powerful than any booster developed before—or since.

Saturn V
The Saturn IB was an upgraded version of the earlier Saturn I. It consisted of a first stage made up of eight H-1 engines and a second S-IVB stage which was identical to the Saturn V's third stage. The Saturn IB had only 1.6 million pounds of thrust in its first stage—compared to 7.5 million pounds for the Saturn V—but was capable of putting a command and lunar module into earth orbit. It was used in Apollo test missions and in both the Skylab program and the Apollo-Soyuz Test Program. In 1973 a refitted S-IVB stage, launched by a Saturn V, became the Skylab space station.

Saturn IB
See also: List of Apollo missions

Missions
In September 1967, the Manned Spacecraft Center in Houston, Texas, proposed a series of missions that would lead up to a manned lunar landing. Seven mission types were outlined, each testing a specific set of components and tasks; each previous step needed to be completed successfully before the next mission type could be undertaken. These were:
Later added to this were H missions, which were short duration stays on the Moon with two LEVAs ("moonwalks"). These were followed by the J missions, which were longer three day stays, with three LEVAs and the use of the lunar rover. Apollo 18 to 20 would have been J missions. In addition, a further group of flights — the I missions — were planned, which would have been long duration orbital missions using a Service Module bay loaded with scientific equipment. When it became obvious that later flights were being cancelled, such mission plans were brought into the J missions that were actually flown.

A - Unmanned Command/Service Module (CSM) test
B - Unmanned Lunar Module (LM) test
C - Manned CSM in low Earth orbit
D - Manned CSM and LM in low Earth orbit
E - Manned CSM and LM in an elliptical Earth orbit with an apogee of 4600 mi (7400 km)
F - Manned CSM and LM in lunar orbit
G - Manned lunar landing Mission types
Preparations for the Apollo program began long before the manned Apollo missions were flown. Test flights of the Saturn I booster began in October of 1961 and lasted until September 1964. Three further Saturn I launches carried boilerplate models of the Apollo command/service module. Tests of the launch escape system began in 1963 at the White Sands Missile Range and lasted until 1966.
The only unmanned missions to officially include Apollo as part of their name rather than serial number were Apollo 4, Apollo 5 and Apollo 6.
Apollo 6 was the last in the series of unmanned Apollo missions. It launched on 4 April 1968, and landed back on Earth almost ten hours later at 21:57:21 UTC.
Further information: List of Apollo missions#Unmanned missions

Unmanned missions
By the summer of 1968 it became clear to program managers that a fully functional LM would not be available for the Apollo 8 mission. Rather than perform a simple earth orbiting mission, they chose to send Apollo 8 around the moon during Christmas. The original idea for this switch was the brainchild of George Low. Although it has often been claimed that this change was made as a direct response to Soviet attempts to fly a piloted Zond spacecraft around the moon, there is no evidence that this was actually the case. NASA officials were aware of the Soviet Zond flights, but the timing of the Zond missions does not correspond well with the extensive written record from NASA about the Apollo 8 decision. It is relatively certain that the Apollo 8 decision was primarily based upon the LM schedule, rather than fear of the Soviets beating the Americans to the moon.
Between December 21, 1968 and May 18, 1969, NASA launched three Apollo missions (8, 9, and 10) using the Saturn V launch vehicle. Each mission had a crew of three astronauts, and the last two included Lunar Modules, but none of these was intended as a Moon landing mission. The next two flights (11 and 12) included successful Moon landings. The Apollo 13 mission was aborted before the landing attempt, but the crew returned safely to Earth. The four subsequent Apollo missions (14 through 17) included successful Moon landings. The last three of these were J-class missions that included the use of Lunar Rovers. The last Apollo mission returned safely to Earth on December 19, 1972.
Further information: List of Apollo missions#Manned missions

Manned missions

Main article: Moon rock Samples returned

Legacy

Main article: Canceled Apollo missions Canceled missions

Main article: Apollo Applications Program Apollo applications program
The Apollo program stimulated many areas of technology. The flight computer design used in both the lunar and command modules was, along with the Minuteman Missile System, the driving force behind early research into integrated circuits. The fuel cell developed for this program was the first practical fuel cell. Computer-controlled machining (CNC) was pioneered in fabricating Apollo structural components.
Many astronauts and cosmonauts have commented on the profound effects that seeing Earth from space has had on them. One of the most important legacies of the Apollo program was the now-common, but not universal, view of Earth as a fragile, small planet, captured in the photographs taken by the astronauts during the lunar missions. The most famous of these photographs, taken by the Apollo 17 astronauts, is "The Blue Marble". These photographs have also motivated many people toward environmentalism
The cost of the program is estimated at $135 billion (2006 dollars) ($25.4 billion in 1969 dollars). The Apollo spacecraft cost $28 billion (2006 dollars) to develop: $17 billion for the command and service modules, and $11 billion for the Lunar Module. The Saturn I, IB and V launch vehicle development cost about $46 billion.
It appears that much of the original film and telemetry data for the Apollo 11 mission is missing. For more information see Apollo program missing tapes.

Cultural legacy
Further information: List of future lunar missions

Influence on future human space exploration
Several nations have planned to land probes on the Moon as early as July 2007. Countries which have planned unmanned missions include: Japan, China, India, the United States, Russia, and Germany.

Unmanned
As of 2007, it has been thirty-five years since any human has walked on the Moon, with the last mission being Apollo 17 in 1972, and Eugene Cernan being the last person to walk on the Moon.
The words of Apollo 17 astronaut Eugene Cernan as he became the last person to set foot on the Moon (as of 2007).
Currently, several nations have planned future manned Moon landings, the earliest being the United States (NASA) who have announced plans to land on the Moon no later than 2020, in a speech by President Bush titled "The Vision for Space Exploration".
The next generation of NASA spacecraft, the Orion (Formerly the Crew Exploration Vehicle or CEV), which is to replace the Space Shuttle following its retirement in 2010, is influenced largely by the Apollo Program. The most notable difference is that the CEV will return to Earth on land, much like the Russian Soyuz spacecraft, rather than at sea as the Apollos did. Like Apollo, the CEV will fly a lunar orbit rendezvous mission profile, but unlike Apollo, the lander, known as the Lunar Surface Access Module, will be launched separately on the Ares V rocket, a rocket based on both Space Shuttle and Apollo technologies. Orion will be launched separately and will link up with the LSAM in low earth orbit like that of the Skylab program. Also, Orion, unlike Apollo, will remain unmanned in lunar orbit while the entire crew lands on the lunar surface, with the lunar polar regions in mind instead of the equatorial regions explored by Apollo. The ESA has planned to land on the Moon before 2020, and China, Russia, India, and Japan have planned to land by 2030.
Neil Armstrong, the commander of the first successful landing Apollo 11, is often asked by the press for his views on the future of spaceflight. In 2005, he said that a human voyage to Mars will be easier than the lunar challenge of the 1960s: "I suspect that even though the various questions are difficult and many, they are not as difficult and many as those we faced when we started the Apollo (space program) in 1961." Armstrong also recalled his initial concerns about the Apollo 11 mission. He had believed there was only a 50 percent chance of landing on the moon. "I was elated, ecstatic and extremely surprised that we were successful," he said.

Manned

Apollo Missions Tracked by Independent Parties
Apollo Moon Landing hoax accusations
Apollo TV camera
Extra-vehicular activity
List of artificial objects on the Moon
Pad Abort Tests
Soviet Moonshot
Independent evidence for Apollo Moon landings Notes

"Discussion of Soviet Man-in-Space Shot," Hearing before the Committee on Science and Astronautics, U.S. House of Representatives, 87th Congress, First Session, April 13, 1961.
Hansen, James R. (1995). Enchanted Rendezvous: John C. Houbolt and the Genesis of the Lunar-Orbit Rendezvous Concept. NASA. 
Launius, Roger; Howard McCurdy (1997). Spaceflight and the Myth of Presidential Leadership. Urbana: University of Illinois Press. 
Murray, Charles; Catherine Bly Cox (1989). Apollo: The Race to the Moon. New York: Simon and Schuster. ISBN 0-671-61101-1. 
Papike, James; Graham Ryder and Charles Shearer (1998). "Lunar Samples". Reviews in Mineralogy and Geochemistry 36: 5.1-5.234. 
Sidey, Hugh (1963). John F. Kennedy, President. New York: Atheneum. 
Swenson, Jr., Loyd S.; Courtney G Brooks and James M. Grimwood (1979). Chariots for Apollo: A History of Manned Lunar Spacecraft. NASA.