Tranquility Base (Latin: Statio Tranquillitatis) is the site on the Moon where, in July 1969, humans landed and walked on a celestial body other than Earth for the first time.

The astronauts spent two hours and 31 minutes examining and photographing the lunar surface, setting up several scientific experiment packages, and collecting 47.5 pounds (21.5 kg) of dirt and rock samples for return to Earth.

[1] The U.S. states of California and New Mexico have registered Tranquility Base as a heritage site associated with them, but Texas, the U.S. National Park Service, and UNESCO have declined to do so, due to the technicality that it is not located within their borders.

[3] On the landing, a combination of thrust from residual pressure in the docking tunnel that connected the Lunar Module with the command module Columbia 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.

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.

According to Aldrin (with apparent confirmation from later Lunar Reconnaissance Orbiter photos[10]), the US flag planted at the site during their moonwalk was blown over by the ascent rocket exhaust, but remains on the surface of the Moon.

[11] As the site of the first human landing on an extraterrestrial body, Tranquility Base has cultural and historic significance.

[13] 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.

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.