Apollo 11

At 9:32 in the morning on July 16, 1969, the Saturn V rocket lifted off from Kennedy Space Center in Florida. It was as tall as a 36-story building. Three astronauts sat in a small capsule at the very top: Neil Armstrong, Michael Collins, and Buzz Aldrin.

A rocket this big is really three rockets stacked on top of each other. The bottom one did the heaviest lifting and burned out in under three minutes. Then it dropped away, and the next one took over. Less than twelve minutes after liftoff, the crew was circling the Earth.

Range zero, the moment the countdown reached zero, was 13:32:00 UTC on July 16, 1969. The Saturn V was a three-stage launch vehicle. The first stage (S-IC) burned for about 2 minutes 42 seconds; the second stage (S-II) burned until 9 minutes 8 seconds; the third stage (S-IVB) burned until 11 minutes 39 seconds and placed the spacecraft in Earth orbit.

Staging is the key idea: dropping empty tanks and engines means the remaining engines accelerate a lighter vehicle. The Mission Report describes the launch as “essentially nominal,” with the stages cutting off within about a second of plan and the orbit within expected tolerances.

Before heading to the Moon, Apollo 11 circled the Earth one and a half times. This gave the crew and Mission Control about two and a half hours to check that everything worked. Going around the whole planet took about an hour and a half.

Being in orbit does not mean gravity has switched off. Gravity is still pulling the spacecraft toward Earth. The spacecraft is simply moving sideways so fast that it keeps missing the ground, around and around.

After insertion at 11 min 49 s, Apollo 11 spent about 2½ hours in a near-circular parking orbit, roughly one and a half revolutions, while the crew and ground checked the spacecraft and the third stage before committing to the Moon. The orbit was slightly perturbed by hydrogen venting from the S-IVB stage until restart preparations began at 2 h 34 min.

An orbit is free fall with enough sideways velocity that the curve of your fall matches the curve of the planet. Near Earth that is about 7.8 km/s. Nothing holds the spacecraft up; it is falling the whole time.

To leave Earth orbit and head for the Moon, the third-stage engine lit a second time and burned for almost six minutes. This push is called translunar injection, or TLI. After it, the spacecraft was moving fast enough to climb away from Earth for three days.

Half an hour later the crew did a careful bit of flying. The command ship, Columbia, turned around, docked nose-to-nose with the lunar lander, Eagle, and pulled it free of the used-up rocket stage.

The S-IVB reignited at 2 h 44 min 16 s and cut off at 2 h 50 min 03 s: a burn of 5 min 47 s. The added velocity raised the far side of the orbit far beyond the Moon's distance. The Mission Report notes the injection was accurate to about 1.6 ft/s, and the resulting path would have brought the spacecraft back to Earth even if the engine never fired again (a free-return trajectory).

At 3 h 17 min the command and service module (CSM) separated from the S-IVB, turned 180 degrees, docked with the lunar module (LM) at 3 h 24 min, and pulled it out of the stage at 4 h 17 min. Translunar coast had begun.

For about three days the spacecraft coasted toward the Moon with its engines off. The Earth slowly shrank behind them, and the Moon grew ahead. The crew ate, slept, checked the ship, and sent television pictures home.

Only one small engine nudge was needed the whole way, about 27 hours after launch, to aim precisely at the Moon. Use the speed control on the timeline to watch the three days go by.

From ejection at 4 h 17 min to lunar orbit insertion at 75 h 50 min, the spacecraft coasted for about 71½ hours under gravity alone. It slowed continuously as it climbed out of Earth's gravity well, then sped up again as the Moon's gravity became the stronger pull. The single midcourse correction, at 26 h 45 min, changed velocity by only about 21 ft/s.

To keep the sunlit side from overheating, the spacecraft rotated slowly in “passive thermal control,” sometimes called barbecue mode. In our simplified model the coast is a single Earth-centered ellipse tuned to match the mission's times; the real path was computed by Mission Control from tracking data.

Arriving at the Moon, the spacecraft was going too fast to be captured by the Moon's weak gravity. So the big engine fired for about six minutes to slow it down. This burn happened behind the Moon, where no radio signal could reach Earth. Mission Control could only wait.

When the spacecraft came around the edge of the Moon right on time, everyone knew the burn had worked. Apollo 11 was in orbit around the Moon.

Lunar orbit insertion (LOI) was a retrograde burn of the service propulsion system beginning at 75 h 49 min 50 s, on the far side of the Moon and out of radio contact. The resulting orbit was 169.7 by 60.0 nautical miles. The burn ran about 4.5 seconds shorter than predicted because the engine's thrust was slightly higher than expected.

Two revolutions later, at 80 h 11 min 37 s, a circularization burn trimmed the orbit to 65.7 by 53.8 nautical miles. The orbit was deliberately not made circular: the Moon's lumpy gravity field (“mascons”) distorts orbits, and the planners targeted an orbit that would evolve toward the right shape for the landing.

On July 20, Armstrong and Aldrin floated into Eagle and undocked from Columbia, where Collins stayed in orbit. Eagle's descent engine fired for about twelve and a half minutes to bring it down from orbit. In the last minutes, Armstrong saw that the computer was taking them toward a crater full of boulders, so he flew the lander by hand to a smoother spot.

You can fly those last minutes yourself in The Landing below.

Undocking came at 100 h 12 min. The descent orbit insertion burn at 101 h 36 min lowered the orbit's low point to about 50,000 feet, and powered descent initiation (PDI) at 102 h 33 min 05 s started the 12½-minute braking burn. The onboard computer flew the braking phase; the Commander took manual attitude control at about 600 feet and used the rate-of-descent mode from about 400 feet, landing roughly 1,100 feet beyond the original aim point to avoid a crater.

Five computer alarms (four 1202s and one 1201) occurred during descent. They meant the computer was overloaded and dropping low-priority work; it never lost the guidance task. Mission Control's call was “GO.” The propellant low-level light came on at about 102:44:28, indicating roughly 116 seconds of firing time remaining. Fly it yourself in The Landing below.

At 3:17 in the afternoon, Houston time, on July 20, 1969, Eagle touched down on the Sea of Tranquility. It landed gently, sliding sideways about as fast as a slow walk. Armstrong radioed the words that gave the landing site its name: “Houston, Tranquility Base here. The Eagle has landed.”

The two astronauts spent the next few hours checking the lander, eating a meal, and getting their suits ready. They decided to go outside earlier than the plan said.

Landing came at 102 h 45 min 40 s (20:17:40 UTC) at 0° 41′ 15″ N, 23° 26′ E on the Sea of Tranquility, about 4 miles from the targeted point. Touchdown velocity was negligible forward, about 2.1 ft/s to the crew's left, and 1.7 ft/s vertical: well inside the landing gear's limits. The right and forward footpads touched almost together.

The crew ran a full postlanding checkout for about two hours, ate, and elected to begin surface operations early rather than take the planned rest period.

At 9:56 at night, Houston time, Neil Armstrong stepped off Eagle's footpad onto the Moon. A television camera on the lander showed it to hundreds of millions of people on Earth. Buzz Aldrin followed about twenty minutes later.

They walked, hopped, collected rocks and soil, set up science experiments, planted a flag, and took photographs for about two and a half hours. Then they climbed back in. The next day Eagle's top half blasted off to rejoin Collins in orbit.

The hatch opened at 109 h 07 min, and Armstrong's boot touched the surface at 109 h 24 min 15 s (02:56:15 UTC, July 21). The crew deployed a passive seismometer, a laser retroreflector (still used today to measure the Earth–Moon distance), and a solar-wind collector, and gathered 47 pounds of rock and soil. The moonwalk lasted the planned 2½ hours; the hatch closed at 111 h 39 min.

After 21 hours 36 minutes on the surface, the ascent stage lifted off at 124 h 22 min 01 s, using the descent stage as a launch pad. Docking with Columbia came at 128 h 03 min. The ascent stage was jettisoned in lunar orbit two hours later.

To leave the Moon, Columbia's big engine fired for two and a half minutes while behind the Moon. That sped the spacecraft up enough to escape the Moon's gravity and fall back toward Earth. The trip home took about two and a half days.

Near the end, the crew cast off the service module (the part with the engine and the big tanks). Only the cone-shaped command module, with its heat shield, came home.

Transearth injection (TEI) was a 150-second service propulsion burn starting at 135 h 23 min 42 s, during the 31st lunar revolution. It achieved its velocity change to within 1.2 ft/s of plan. One midcourse correction of 4.8 ft/s, at 150 h 30 min, set the entry flight-path angle to −6.51°.

The command module separated from the service module at 194 h 49 min and reached entry interface (400,000 feet) at 195 h 03 min 06 s. Peak deceleration was 6.56 g. Weather in the recovery area moved the splashdown point 215 miles downrange.

Eight days after launch, the command module came through the atmosphere glowing hot, opened its three big parachutes, and splashed into the Pacific Ocean on July 24. Helicopters from the aircraft carrier USS Hornet picked up the crew.

Because nobody knew for sure whether the Moon had germs, the astronauts put on special suits and lived in a sealed trailer for weeks afterward. They were fine. The goal set by President Kennedy in 1961 had been met with five months to spare.

Splashdown came at 195 h 18 min 35 s (16:50:35 UTC, July 24) at 13° 19′ N, 169° 09′ W, about 8 days 3 hours after liftoff. The crew donned biological isolation garments in the raft, were flown to the USS Hornet, and entered the Mobile Quarantine Facility along with the lunar samples for the trip to the Lunar Receiving Laboratory in Houston. The command module was recovered about three hours after landing.

The Mission Report's last sentence: “With the completion of Apollo 11, the national objective of landing men on the moon and returning them safely to earth before the end of the decade had been accomplished.”