1. One Astronaut Circled the Moon Alone While Two Landed
While Neil Armstrong and Buzz Aldrin were on the lunar surface, Collins flew the command module, named Columbia, alone around the Moon. The Smithsonian National Air and Space Museum describes him as the first person to go behind the Moon completely alone, out of radio contact for part of every orbit[1].
It is tempting to read that as a lonely, sad job. Collins himself saw it differently. He said the venture "has been structured for three men, and I consider my third to be as necessary as either of the other two"[1]. So the first puzzle is a good one: why would a plan need a person to stay behind?
2. The problem was getting home
The space race gave the project its deadline. On May 25, 1961, U.S. President John F. Kennedy told Congress that the United States should commit itself to "landing a man on the moon and returning him safely to earth" before the decade was out[2]. Notice the two halves of that goal. Landing is not enough. The astronaut has to come back.
NASA, the U.S. space agency, looked at three main ways to do it. In direct ascent, one giant rocket flies straight to the Moon, the whole spacecraft lands, and the same spacecraft lifts off again. In Earth orbit rendezvous, several launches carry pieces into orbit around Earth, where they are joined together before heading for the Moon. In lunar orbit rendezvous, a small lander (the lunar module) carries two astronauts to the surface while a third stays in orbit aboard the command module[3].
Direct ascent needed a rocket called Nova, with a liftoff thrust of around 12 million pounds, which NASA's history page describes as nearly twice the Saturn V's capacity[3]. The Saturn V's five first-stage engines produced about 7.65 million pounds of thrust at liftoff, according to the Apollo 11 press kit[4].
Why does landing the whole ship make everything so heavy? Here is one way to think about it. Anything you land must carry the fuel to take off again, and a bigger ship needs more fuel just to carry that extra fuel. Picture rowing to a beach from a big ship. You would not drag the whole ship onto the sand. You would anchor it offshore and row in with a small boat. The real command module was not anchored, of course. It kept moving in orbit with one astronaut aboard, so the lander had to catch up with it later. That is Collins's job answered: someone had to fly the ship that would carry everyone home.
NASA's own summary lists the advantages of the small-lander plan: only one Saturn V was needed, no huge spacecraft had to be assembled in Earth orbit, and much less mass had to be set down on the Moon[3].
3. How NASA Came to Choose the Two-Ship Plan
The idea of lunar orbit rendezvous did not win easily. John Houbolt, an engineer at NASA's Langley Research Center, believed in it so strongly that on November 15, 1961, he sent a nine-page letter straight to Robert Seamans, NASA's second-highest official. NASA's own history book on the subject says he feared it might cost him his job, because he was skipping proper channels[2].
The letter opened "Somewhat as a voice in the wilderness" and asked: "Do we want to go to the moon or not?" It also asked why the huge Nova rocket was simply accepted while "a much less grandiose scheme involving rendezvous" was treated as suspect[2].
Houbolt's critics had a real worry, and it was not silly. Two spacecraft would have to find each other and connect in lunar orbit, about 240,000 miles (385,000 km) from Earth, where rescue looked remote. Wernher von Braun's team at the Marshall Space Flight Center said so openly. But on June 7, 1962, after months of study, they told their own colleagues they had concluded that this disadvantage "is far outweighed by" the advantages[2]. The idea was not chosen because it had no weakness. It was chosen by people who knew the weakness and judged that the benefits were bigger.
NASA's Manned Space Flight Management Council came out in favor on June 22, 1962, and Administrator James Webb announced the decision on July 11. President Kennedy's science adviser, Jerome Wiesner, stayed firmly opposed[2]. So this was not one person winning an argument. Many people studied the problem and changed their minds, and Houbolt's letter was one of the things that got them studying. The result was the plan that left Collins in orbit.
4. Why Apollo 11 Aimed Ahead of the Moving Moon
Apollo 11 lifted off from Florida at 9:32 a.m. Eastern Daylight Time (U.S. East Coast summer time) on July 16, 1969[5]. It did not head straight for the Moon. First it settled into orbit around Earth, circling about one and a half times, so the crew could check that everything worked. Then, 2 hours and 44 minutes after launch, the Saturn V's third stage fired a second time for 5 minutes and 48 seconds[5]. The press kit's plan had this burn raising the speed from 25,567 to 35,533 feet per second[4]. That is roughly 7.8 to 10.8 kilometers per second (17,400 to 24,200 miles per hour).
Now comes the part that surprises most people. The Moon is not standing still. NASA's Moon fact sheet gives its average speed along its orbit as about 1.02 kilometers per second, and one lap around Earth takes about 27.3 days[6]. The burn that sent Apollo toward the Moon happened at 2 hours 44 minutes after launch, and the flight plan in the press kit scheduled the burn that slowed the ship into lunar orbit for 75 hours 54 minutes[4] (the real burn came at about 75 hours 50 minutes[5]). That is about 73 hours of travel. In that time the Moon moves along its orbit by roughly 40 degrees, which is about four fists held at arm's length.
So the crew did not aim at the Moon where it was. They aimed at where it would be when they arrived. It is the same idea as throwing a ball to where a running friend is going to be, not where they are now. The difference is that the Moon's path is very predictable, so the destination can be worked out ahead of time.
Along the way, the spacecraft rolled slowly like a chicken on a rotisserie, so the Sun's heat would not bake one side. Engineers called this the "barbecue" mode[4]. And the path had a built-in safety net: it was a "free-return" trajectory, one that would swing around the Moon and bring the ship back to Earth without firing an engine, so the mission could be called off at any time before it entered lunar orbit. NASA notes that Apollo 11 was meant to be the last Apollo mission to fly this way[5].
5. Slowing Into Lunar Orbit, Then Landing the Lunar Module
A ship falling toward the Moon speeds up. If it does nothing, it swings past and leaves. So about 75 hours and 50 minutes into the flight, on the far side of the Moon where radio cannot reach, the main engine fired backward for 357.5 seconds. That slowed the spacecraft enough for the Moon's gravity to capture it into lunar orbit[5].
Then Armstrong and Aldrin climbed into the lander, Eagle, and separated from Columbia, leaving Collins aboard. The descent did not go smoothly. At about 33,000 feet (10,000 m), an alarm the crew had not expected sounded. NASA's account says it simply meant the computer was overloaded with too much data. After a few seconds of analysis, Mission Control gave the crew a "go" to continue[7]. Around 600 feet (180 m) up, Armstrong saw the computer steering them toward a boulder-strewn area near a crater. He took manual control, tilted Eagle more upright to slow the descent, and flew on, looking for smoother ground[7].
At about 100 feet (30 m), a warning light showed only about 5 percent of the fuel left, which NASA's account says gave Armstrong about 90 seconds of hover time[7]. Eagle touched down in the Sea of Tranquility on July 20, 1969, about 102 hours and 45 minutes into the mission[5].
6. The Lunar Module Had One Ascent Engine—and No Backup
Armstrong and Aldrin spent 21 hours and 36 minutes on the Moon, and their moonwalk lasted a little over two and a half hours[5]. Then came the moment the whole design depended on. Only the top half of Eagle could leave. The bottom half, the descent stage, stayed behind and served as a launch pad for the top half[4].
The top half had one ascent engine, with about 3,500 pounds of thrust[4]. It had no spare. A NASA technical paper quotes the people who built it: "You can't have redundancy for ascent from the moon. You've got one engine. It's got to work. There is no second chance," and says weight restrictions ruled out a backup[8]. The engine fired 124 hours and 22 minutes into the flight and ran for 435 seconds. Then Eagle's ascent stage caught up with Columbia, which had been waiting in orbit, and docked with it 128 hours and 3 minutes into the flight[5].
Collins had been circling the whole time. After the two moonwalkers came aboard, Columbia had all three again, and it was time to go home.
7. Only the Command Module Came Home, and the Split Design Paid Off
The command module entered Earth's atmosphere at about 36,194 feet per second in the press kit's plan. That is about 11 kilometers per second, or roughly 24,700 miles per hour[4]. Friction with the air makes the outside extremely hot. The heat shield was built from stainless steel honeycomb covered with an outer layer of epoxy resin designed to burn away and carry heat with it[4].
On July 24, 1969, at 12:50 p.m. Eastern Daylight Time, Columbia came down under parachutes in the Pacific Ocean, 13 miles (21 km) from the recovery ship USS Hornet. The whole flight took 195 hours, 18 minutes, and 35 seconds, a little over 8 days[5].
Only the command module returned. It was a cone about 11 feet 5 inches (3.5 m) tall, weighing 12,250 pounds (5.6 tonnes) at launch, with 210 cubic feet (about 6 cubic meters) of living space for three people[4]. The huge rocket stages, the service module, and both halves of the lander were all left behind.
The two-ship design later proved its value in a way nobody planned. On Apollo 13, in April 1970, an oxygen tank exploded about 56 hours into the flight, and the crew moved into the lunar module. Using the lander's descent engine, they made a burn that put the spacecraft on a free-return path around the Moon. They splashed down in the Pacific on April 17 after 142 hours, 54 minutes, and 41 seconds[9]. The second spacecraft, built just for the Moon, turned out to be useful for something else.
Try It Yourself: Watch the Moon Move, or Read the NASA Records
- Watch the Moon move. The Moon travels around the sky by about 13 degrees a day, from 360 degrees divided by 27.32 days. A fist held at arm's length covers about 10 degrees. Ask an adult to come with you, and watch from a safe spot such as a window, garden, or yard. Some evening when the Moon is out, note where it sits next to a bright star or planet, at a particular time. Look again at the same time the next night, from the same spot. The Moon should have moved a bit more than one fist to the east. If it is cloudy, skip it. This is the same drift that Apollo's navigators had to plan around.
- Read Houbolt's letter. The NASA history book "Enchanted Rendezvous" quotes it at length and is free to download. You can decide for yourself whether it sounds like a voice in the wilderness.
- Open the Apollo 11 press kit. NASA published it before the flight, and its flight plan chapter lists every burn and its planned time. Try lining it up with the real times in NASA's mission overview and see where the plan and the flight differed.