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Apollo 11 – Lunar Descent and Moonwalk

As the descent began, Armstrong and Aldrin found themselves passing landmarks on the
surface two or three seconds early, and reported that they were “long”; they would land
miles west of their target point. Eagle was traveling too fast. The problem could have been
mascons—concentrations of high mass in a region or regions of the Moon’s crust that
contains a gravitational anomaly, potentially altering Eagle’s trajectory.

Five minutes into the descent burn, and 6,000 feet (1,800 m) above the surface of the
Moon, the LM guidance computer (LGC) distracted the crew with the first of several
unexpected 1201 and 1202 program alarms. Inside Mission Control Center, 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 its tasks in real-time and
had to postpone some of them. Margaret Hamilton, the Director of Apollo Flight Computer
Programming at the MIT Charles Stark Draper Laboratory later recalled: “To blame the
computer for the Apollo 11 problems is like blaming the person who spots a fire and calls
the fire department. 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. The computer, rather than almost forcing an abort, prevented an
abort. 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-foot-diameter (91 m) crater, so he took
semi-automatic control. Throughout the descent, Aldrin called out navigation data to
Armstrong, who was busy piloting Eagle. Now 107 feet (33 m) above the surface,
Armstrong knew their propellant supply was dwindling and was determined to land at the
first possible landing site.

Armstrong found a clear patch of ground and maneuvered the spacecraft towards it. They
were now 100 feet (30 m) from the surface, with only 90 seconds of propellant remaining.
Lunar dust kicked up by the LM’s engine began to impair his ability to determine the
spacecraft’s motion.

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 shut the engine down. Aldrin immediately said “Okay, engine
stop.”

Eagle landed at 20:17:40 UTC on Sunday July 20 with 216 pounds (98 kg) of usable fuel
remaining. Information available to the crew and mission controllers during the landing
showed the LM had enough fuel for another 25 seconds of powered flight before an abort
without touchdown would have become unsafe, but post-mission analysis showed that the
real figure was probably closer to 50 seconds.

Armstrong acknowledged Aldrin’s completion of the post-landing checklist with “Engine
arm is off”, before responding to the CAPCOM, Charles Duke, with the words, “Houston,
Tranquility Base here. The Eagle has landed.” Duke 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.”

Preparations for Neil Armstrong and Buzz Aldrin to walk on the Moon began at 23:43 UTC.
These took longer than expected; three and a half hours instead of two. Six hours and
thirty-nine minutes after landing, Armstrong and Aldrin were ready to go outside, and
Eagle was depressurized.

Eagle’s hatch was opened at 02:39:33. Armstrong initially had some difficulties squeezing
through the hatch with his portable life support system (PLSS). At 02:51 Armstrong began
his descent to the lunar surface. Climbing down the nine-rung ladder, Armstrong pulled a
D-ring to deploy the modular equipment stowage assembly (MESA) folded against Eagle’s
side and activate the TV camera.

Despite some technical and weather difficulties, black and white images of the first lunar
EVA were received and broadcast to at least 600 million people on Earth.

After describing the surface dust as “very fine-grained” and “almost like a powder”, at
02:56:15, six and a half hours after landing, Armstrong stepped off Eagle’s landing pad and
declared: “That’s one small step for [a] man, one giant leap for mankind.”

Armstrong intended to say “That’s one small step for a man”, but the word “a” is not
audible in the transmission, and thus was not initially reported by most observers of the live
broadcast. When later asked about his quote, Armstrong said he believed he said “for a
man”, and subsequent printed versions of the quote included the “a” in square brackets.

About seven minutes after stepping onto the Moon’s surface, Armstrong collected a
contingency soil sample using a sample bag on a stick. Twelve minutes after the sample
was collected, he removed the TV camera from the MESA and made a panoramic sweep,
then mounted it on a tripod. Aldrin joined Armstrong on the surface. He described the view
with the simple phrase: “Magnificent desolation.”

Armstrong said moving in the lunar gravity, one-sixth of Earth’s, was “even perhaps easier
than the simulations … It’s absolutely no trouble to walk around.” Aldrin joined him on the
surface and tested methods for moving around, including two-footed kangaroo hops. The
PLSS backpack created a tendency to tip backward, but neither astronaut had serious
problems maintaining balance. The fine soil was quite slippery.

The astronauts planted the Lunar Flag Assembly containing a flag of the United States on
the lunar surface, in clear view of the TV camera. Aldrin remembered, “Of all the jobs I had
to do on the Moon the one I wanted to go the smoothest was the flag raising.” But the
astronauts struggled with the telescoping rod and could only insert the pole about 2 inches
(5 cm) into the hard lunar surface. Before Aldrin could take a photo of Armstrong with the
flag, President Richard Nixon spoke to them through a telephone-radio transmission, which
Nixon called “the most historic phone call ever made from the White House.”

They deployed the EASEP, which included a Passive Seismic Experiment Package used to
measure moonquakes and a retroreflector array used for the lunar laser ranging
experiment. Then Armstrong walked 196 feet (60 m) from the LM to take photographs at
the rim of Little West Crater while Aldrin collected two core samples. He used the
geologist’s hammer to pound in the tubes—the only time the hammer was used on Apollo
11—but was unable to penetrate more than 6 inches (15 cm) deep.

The astronauts then collected rock samples using scoops and tongs on extension handles.
Many of the surface activities took longer than expected, so they had to stop documenting
sample collection halfway through the allotted 34 minutes. Aldrin shoveled 6 kilograms
(13 lb) of soil into the box of rocks to pack them in tightly. Two types of rocks were found in
the geological samples: basalt and breccia.

While on the surface, Armstrong uncovered a plaque mounted on the LM ladder, bearing
two drawings of Earth, an inscription, and signatures of the astronauts and President Nixon.
The inscription read: “Here men from the planet Earth first set foot upon the Moon July
1969, A. D. We came in peace for all mankind.”

Mission Control used a coded phrase to warn Armstrong his metabolic rates were high, and
that he should slow down. As metabolic rates remained generally lower than expected for
both astronauts throughout the walk, Mission Control granted the astronauts a 15-minute
extension.

Aldrin entered Eagle first. With some difficulty the astronauts lifted film and two sample
boxes containing 21.55 kilograms (47.5 lb) of lunar surface material to the LM hatch using a
flat cable pulley device called the Lunar Equipment Conveyor (LEC). Armstrong then
jumped onto the ladder’s third rung, and climbed into the LM. After transferring to LM life
support, the explorers lightened the ascent stage for the return to lunar orbit by tossing out
their PLSS backpacks, lunar overshoes, an empty Hasselblad camera, and other equipment.
The hatch was closed again at 05:11:13. They then pressurized the LM and settled down to
sleep.