Apollo 11 astronauts Neil Armstrong and Edwin E. “Buzz” Aldrin, the first men to land on the moon, plant the U.S. flag on the lunar surface, July 20, 1969. Photo was made by a 16mm movie camera inside the lunar module, shooting at one frame per second. (Nasa via AP)

Remembering Tranquility Base.

It’s been 48 years since we heard those famous words announcing that humans had finally reached another world:

Houston, Tranquility Base here. The Eagle has landed

On July 20, 1969, Apollo 11 landed on the moon. It was only a little over eight years after the flights of Gagarin and Shepard, followed quickly by President Kennedy’s challenge to put a man on the moon before the decade is out:

I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the earth.

After a three-day journey to the moon and a day in lunar orbit Neil Armstrong and Buzz Aldrin climbed into the lunar module Eagle and began the descent to the moon’s surface. Michael Collins continued to orbit the moon in the command module Columbia.

Collins later wrote that Eagle is “the weirdest looking contraption I have ever seen in the sky.”

When it was time to set Eagle down in the Sea of Tranquility, Armstrong was forced to improvise, manually piloting the ship past an area littered with boulders. During the final seconds of descent, Eagle’s overloaded computer was sounding alarms.

The Apollo 11 computers had less processing power than a modern cell phone. Both the lunar module and the command module were guided by an Apollo Guidance Computer, which had only 32KB of fixed memory (aka ROM) and 4KB erasable memory (aka RAM)

When the lunar module landed at 4:18 p.m. EDT, only 30 seconds of fuel remained. Armstrong radioed “Houston,

At 10:56 p.m. EDT Armstrong is ready to take his small step. With more than half a billion people watching on television, he climbed down the ladder and proclaimed:

That’s one small step for a man, one giant leap for mankind.

Aldrin joined him shortly, and gave a powerful description of the lunar surface:

Magnificent desolation.

Armstrong and Aldrin explored the surface for two and a half hours, collecting samples and taking photographs.

They left behind an American flag, a patch honoring the fallen Apollo 1 crew, and a plaque on one of Eagle’s legs. It reads:

Here men from the planet Earth first set foot upon the moon. July 1969 A.D. We came in peace for all mankind.

Armstrong and Aldrin blasted off and docked with Collins in Columbia.

Over the next three and a half years, 10 more American astronauts also explored the moon. Gene Cernan, commander of the last Apollo mission left with these words:

We leave as we came and, God willing, as we shall return, with peace, and hope for all mankind.

The following video showshows the launch of Apolo 11, Armstrong’s first steps on the moon, activities at Tranquility Base, and the crews return to Earth.