The Apollo 11 lunar landing on July 20, 1969, was the culmination of the longtime human dream of reaching the moon. The Apollo missions ended one phase of the U.S. space program, and started another.
Apollo 11 expanded the reach of human beings, and offered a vision of the earth from a new perspective.
Mission commander Neil Armstrong radioed the first words from the moon to Mission Control in Houston.
"Tranquility Base, here. The Eagle has landed," he said.
When Armstrong and Buzz Aldrin planted the American flag on the lunar surface, it was also a proud moment for the United States, which had lagged behind the Soviet Union in the early days of the space race.
In 1957, the Soviet Union launched the first satellite, Sputnik, followed by a succession of other firsts: a probe on the moon, a lunar orbiter, and the first human in space, cosmonaut Yuri Gagarin.
Retired National Aeronautics and Space Administration engineer Jim Burke was part of the early competition.
"We were in a peaceful but very vigorous contest with a determined adversary who was eager to get to each new destination in the solar system ahead of us. So it was a wonderful contest. We had huge enthusiasm on both sides," he said.
The United States kept its focus on human space flight through five more Apollo missions that took astronauts to the moon. In all, 12 people have walked on the moon, the last two, Harrison Schmitt and Eugene Cernan, in 1972.
The United States and Soviet Union also continued their unmanned space programs, sending probes to the far reaches of the solar system, and robotic landers and rovers to the moon and Mars. Other nations would join them.
Since 1990, the Hubble Space Telescope, a joint effort of NASA and the European Space Agency, has sent back dramatic pictures of distant star systems and other cosmic objects.
In the 1980s, the United States restricted its manned missions to earth orbit, first with the space shuttle program and, starting in 1998, with the International Space Station. The project is another cooperative effort of the United States, Russia, Japan, Canada, Brazil and 11 European nations.
Two tragic accidents would lead to a change of course in the U.S. space program. The space shuttle Challenger was destroyed after liftoff in 1986, killing all seven astronauts, and the Columbia was lost on re-entry in 2003, also killing the seven on board.
At the Planetary Society in Pasadena, California, executive director Louis Friedman says the losses prompted a reassessment and renewed commitment to manned exploration beyond earth orbit.
"Humans are explorers," he said. "The idea of making the cost and risk of human space flight requires destinations because you have to explore new worlds. And that only became obvious after the sad loss of life in the tragic shuttle accidents."
In 2004, President George W. Bush announced the new goal of returning to the moon by 2020 and, with the moon as a base, getting humans to Mars sometime after 2030.
"That's one small step for 'a' man, one giant leap for mankind," said Neil Armstrong, when he landed on the moon.
The Apollo missions started it all, and opened the new vistas," he said when he landed on the moon. "Apollo was a demonstration that humanity is not forever chained to this planet," he added.
Planetary scientistsays the mission was a great human accomplishment.
"It still stands as a symbol of what great countries can do and what great people can do when they put their effort behind it. And that is important for the positioning of the United States in the world. It is important for the positioning of China in the world, and India now is going to the moon," he said.
An Indian probe landed on the moon last November. China sent its first astronaut into space in 2003 and launched a second manned mission two years later. Chinese officials have announced plans for a space station and manned lunar landing.
Friedman says the legacy of peaceful competition of Apollo has continued, and also opened the way to cooperation, as humans expand their understanding of Earth and space.