Apollo 11 was the spaceflight that landed the first humans, Americans Neil Armstrong and Buzz Aldrin, on the Moon on July 20, 1969, at 20:18 UTC. Armstrong became the first to step onto the lunar surface 6 hours later on July 21 at 02:56 UTC. Armstrong spent about two and a half hours outside the spacecraft, Aldrin slightly less; and together they collected 47.5 pounds (21.5 kg) of lunar material for return to Earth. A third member of the mission, Michael Collins, piloted the command spacecraft alone in lunar orbit until Armstrong and Aldrin returned to it just under a day later for the trip back to Earth.
Launched by a Saturn V rocket from Kennedy Space Center in Merritt Island, Florida on July 16, Apollo 11 was the fifth manned mission of NASA's Apollo program. The Apollo spacecraft had three parts: a Command Module with a cabin for the three astronauts which was the only part which landed back on Earth; a Service Module which supported the Command Module with propulsion, electrical power, oxygen and water; and a Lunar Module for landing on the Moon. After being sent to the Moon by the Saturn V's upper stage, the astronauts separated the spacecraft from it and travelled for three days until they entered into lunar orbit. Armstrong and Aldrin then moved into the Lunar Module and landed in the Sea of Tranquility. They stayed a total of about 21 and a half hours on the lunar surface. After lifting off in the upper part of the Lunar Module and rejoining Collins in the Command Module, they returned to Earth and landed in the Pacific Ocean on July 24.
Broadcast on live TV to a world-wide audience, Armstrong stepped onto the lunar surface and described the event as "one small step for man, one giant leap for mankind." Apollo 11 effectively ended the Space Race and fulfilled a national goal proposed in 1961 by the late U.S. President John F. Kennedy in a speech before the United States Congress, "before this decade is out, of landing a man on the Moon and returning him safely to the Earth."
In addition to throngs of people crowding highways and beaches near the launch site, millions watched the event on television, with NASA Chief of Public Information Jack King providing commentary. President Richard Nixon viewed the proceedings from the Oval Office of the White House.
A Saturn V launched Apollo 11 from Launch Pad 39A, part of the Launch Complex 39 site at the Kennedy Space Center on July 16, 1969 at 13:32:00 UTC (9:32:00 a.m. EDT local time). It entered orbit 12 minutes later. After one and a half orbits, the S-IVB third-stage engine pushed the spacecraft onto its trajectory toward the Moon with the Trans Lunar Injection burn at 16:22:13 UTC. About 30 minutes later the command/service module pair separated from this last remaining Saturn V stage and docked with the lunar module still nestled in the Lunar Module Adaptor. After the lunar module was extracted, the combined spacecraft headed for the Moon, while the third stage booster flew on a trajectory past the Moon and into heliocentric orbit.
On July 19 at 17:21:50 UTC, Apollo 11 passed behind the Moon and fired its service propulsion engine to enter lunar orbit. In the thirty orbits that followed, the crew saw passing views of their landing site in the southern Sea of Tranquility (Mare Tranquillitatis) about 20 kilometres (12 mi) southwest of the crater Sabine D (0.67408N, 23.47297E). The landing site was selected in part because it had been characterized as relatively flat and smooth by the automated Ranger 8 and Surveyor 5 landers along with the Lunar Orbiter mapping spacecraft and unlikely to present major landing or extra-vehicular activity (EVA) challenges.
On July 20, 1969 the lunar module (LM) Eagle separated from the command module Columbia. Collins, alone aboard Columbia, inspected Eagle as it pirouetted before him to ensure the craft was not damaged.
As the descent began, Armstrong and Aldrin found that they were passing landmarks on the surface 4 seconds early and reported that they were "long"; they would land miles west of their target point.
Five minutes into the descent burn, and 6,000 feet (1,800 m) above the surface of the Moon, the LM navigation and guidance computer distracted the crew with the first of several unexpected "1202" and "1201" program alarms. Inside Mission Control Center in Houston, Texas, computer engineer Jack Garman told guidance officer Steve Bales it was safe to continue the descent and this was relayed to the crew. The program alarms indicated "executive overflows", meaning the guidance computer could not complete all of its tasks in real time and had to postpone some of them.
In a letter to Datamation, March 1, 1971, Margaret H. Hamilton, Director of Apollo Flight Computer Programming MIT Draper Laboratory, Cambridge, Massachusetts, stated: Due to an error in the checklist manual, the rendezvous radar switch was placed in the wrong position. This caused it to send erroneous signals to the computer. The result was that the computer was being asked to perform all of its normal functions for landing while receiving an extra load of spurious data which used up 15% of its time. The computer (or rather the software in it) was smart enough to recognize that it was being asked to perform more tasks than it should be performing. It then sent out an alarm, which meant to the astronaut, I'm overloaded with more tasks than I should be doing at this time and I'm going to keep only the more important tasks; i.e., the ones needed for landing...Actually, the computer was programmed to do more than recognize error conditions. A complete set of recovery programs was incorporated into the software. The software's action, in this case, was to eliminate lower priority tasks and re-establish the more important ones...If the computer hadn't recognized this problem and taken recovery action, I doubt if Apollo 11 would have been the successful moon landing it was.
When Armstrong again looked outside, he saw that the computer's landing target was in a boulder-strewn area just north and east of a 300 metres (980 ft) diameter crater (later determined to be "West crater", named for its location in the western part of the originally planned landing ellipse). Armstrong took semi-automatic control and, with Aldrin calling out altitude and velocity data, landed at 20:17 UTC on July 20 with about 25 seconds of fuel left.
Apollo 11 landed with less fuel than other missions, and the astronauts also encountered a premature low fuel warning. This was later found to have been due to greater propellant 'slosh' than expected, uncovering a fuel sensor. On subsequent missions, extra baffles were added to the tanks to prevent this.
Throughout the descent Aldrin had called out navigation data to Armstrong, who was busy piloting the LM. A few moments before the landing, a light informed Aldrin that at least one of the 67-inch (170 cm) probes hanging from Eagle's footpads had touched the surface, and he said "Contact light!". Three seconds later, Eagle landed and Armstrong said "Shutdown". Aldrin immediately said "Okay, engine stop. ACA - out of detent." Armstrong acknowledged "Out of detent. Auto" and Aldrin continued "Mode control - both auto. Descent engine command override off. Engine arm - off. 413 is in."
Charles Duke, acting as CAPCOM during the landing phase, acknowledged their landing by saying "We copy you down, Eagle".
Armstrong acknowledged Aldrin's completion of the post landing checklist with "Engine arm is off." before responding to Duke with the words, "Houston, Tranquility Base here. The Eagle has landed." Armstrong's change of call sign from "Eagle" to "Tranquility Base" confirmed that landing was complete and successful, and Duke mispronounced his reply as he expressed the relief at Mission Control: "Roger, Twan-- Tranquility, we copy you on the ground. You got a bunch of guys about to turn blue. We're breathing again. Thanks a lot."