Tranquility Base (Latin: Statio Tranquillitatis) is the site where humans first landed on the Moon and walked on another world, on the flight of Apollo 11. American astronauts Neil Armstrong and Buzz Aldrin landed their Apollo Lunar Module Eagle at approximately 20:17:40 UTC on July 20, 1969. Six hours later, they left the spacecraft and spent 2 hours, 31 minutes on the lunar surface, examining and photographing it, setting up some scientific experiment packages, and collecting 47.5 pounds (21.5 kg) of dirt and rock samples for return to Earth. They lifted off the surface on July 21 at 17:54 UTC. Tranquility Base has remained unvisited since then.
Its lunar coordinates are 00°41′15″N, 23°26′00″E, in the south-western corner of the lunar plain called the Sea of Tranquility (Mare Tranquillitatis), near the craters Sabine and Ritter, and a rille unofficially called "U.S. Highway Number 1".
For more than two years, NASA planners considered a collection of 30 potential sites for the first manned landing. Based on high-resolution photographs taken by the Lunar Orbiter spacecraft, and photos and data taken by the unmanned Surveyor landers, this list was narrowed down to five sites located near the lunar equator, ranging between 45 degrees east and west, and 5 degrees north and south of the center of the Moon's facing side; these were numbered 1 to 5 going from east to west. Site number 2, centered at 0°42′50″N 23°42′28″E / 0.71389°N 23.70778°E / 0.71389; 23.70778, was the Sea of Tranquility site ultimately chosen. Since a precision landing was not expected on the first mission, the target area was an ellipse measuring 11.5 miles (18.5 km) east-west by 3.0 miles (4.8 km) north-south.
On the actual landing, a combination of thrust from residual pressure in the docking tunnel that connected the Lunar Module with the Command Module in orbit, and an imperfect understanding of the Moon's uneven gravitational field, resulted in navigation errors which pushed the powered descent initiation point about 3 miles (4.8 km), and thus the computer-targeted landing spot about 4 miles (6.4 km), downrange (west) of the planned target. The automated targeting system was taking Eagle toward what Armstrong described as a "football-field sized crater, with a large number of big boulders and rocks for about one or two crater diameters around it", which he avoided by assuming manual control and flying a bit further downstream. Thus the landing was still within the target ellipse.
Armstrong named the site at 20:17:58 UTC, approximately 18 seconds after his and Aldrin's successful landing, as he announced:
"Houston, Tranquility Base here. The Eagle has landed."
During training, Armstrong and Aldrin had exclusively used the callsign "Eagle" in simulated ground conversations, both before and after landing. Armstrong and Aldrin decided on using Tranquility Base just before the flight, telling only CAPCOM Charles Duke before the mission, so Duke would not be taken by surprise.
The name has become a permanent designation for the site. Unlike most names given to lunar landmarks by Apollo astronauts, the International Astronomical Union officially recognizes the designation "Tranquility Base". It is listed on lunar maps as Statio Tranquillitatis, conforming to the standard use of Latin names for lunar place names.
About 100 man-made objects - and footprints left by Armstrong and Aldrin - remain at Tranquility Base. The descent stage of the Lunar Module remains at the original point of landing. According to Aldrin (with apparent confirmation from later lunar reconnaissance orbiter photos), the US flag planted at the site during their moonwalk was blown over by the ascent rocket exhaust, but ostensibly remains there. A laser reflector was placed at the site to allow precise ongoing measurements of the distance to the Moon from Earth. A solar-powered seismometer was also left to measure moonquakes, but this stopped functioning after 21 days. Various gear that was no longer needed for the return phase of the mission - including Aldrin's boots - was left behind to lighten the craft for return to lunar orbit.
As the site of the first human landing on an extraterrestrial body, Tranquility Base has cultural and historic significance. The U.S. states of California and New Mexico have listed it on their heritage registers, since their laws require only that listed sites have some association with the state. Despite the location of Mission Control in Houston, Texas has not granted similar status to the site, as its historic preservation laws limit such designations to properties located within the state. The U.S. National Park Service has declined to grant it National Historic Landmark status to avoid violating the Outer Space Treaty's prohibition on any nation claiming sovereignty over any extraterrestrial body. It has not been proposed as a World Heritage Site since the United Nations Educational, Scientific and Cultural Organization (UNESCO), which oversees that program, limits nations to submitting sites within their own borders.
Interest in according the site some formal protection grew in the early 21st century with the announcement of the Google Lunar X Prize for private corporations to successfully build spacecraft and reach the Moon; a $1 million bonus was offered for any competitor that visited a historic site on the Moon. One team, led by Astrobotic Technology, announced it would attempt to land a craft at Tranquility Base. Although it canceled those plans, the ensuing controversy led NASA to request that any other missions to the Moon, private or governmental, human or robotic, keep a distance of at least 75 meters (246 ft) from the site.
Lunar Orbiter 5 image cropped to show the vicinity of the landing site of Apollo 11, used in mission planning. The image is centered precisely on a small crater called West Crater (185 m in diameter), and the lunar module Eagle touched down about 550 m west of West Crater. The area shown is approximately 25 km x 25 km across.
High-resolution Lunar Orbiter 5 image cropped to show the landing site of Apollo 11. The landing site is indicated by a red dot. The prominent crater at right is called West Crater and is about 185 m in diameter.