On 20th July 1969, Armstrong and Aldrin had to attempt something no one had done before – landing on the lunar surface. They were in orbit some 50,000 feet above the Moon, traveling at several thousand miles per hour, and had to pilot the lunar module Eagle down to the Moon. The entire process, which was little more than a controlled fall, would take just 12 minutes.

Flight controllers during lunar module descent
[Image credit: NASA]

It wasn’t a smooth ride. They had trouble with communications with Earth. Then the onboard computer, which was controlling the craft’s speed and orientation, began sounding an alarm – it was overloaded with tasks. It produced two error messages: 1201 “Executive Overflow – No Core Sets” and 1202 “Executive Overflow – No VAC Areas”. In Houston, flight controller Stephen Bales had the unenviable task of deciding whether the alarm meant the mission had to be aborted, but ultimately it wasn’t considered to be a show stopper.

NASA used integrated circuits (what we now call microchips) in the computers that flew the Apollo command module and the Apollo lunar module. With the U.S. Air Force they were the first significant customers for a technology that was little more than three years old – IBM wasn’t even using them yet. NASA’s demand for them, and its insistence on near-flawless manufacture, helped to create the world market for microchips and to cut the price by 90% in five years.

Integrated circuits in the Apollo Guidance Computer
[Image credit: NASA]

The Moon has no atmosphere, meaning that neither wings nor parachutes are any use at all; Eagle’s descent rocket was their only means of controlling the descent. The lunar module is the only crewed spacecraft that has been designed solely for off-planet use. It couldn’t be flown on Earth, and training to use it could only be done in simulators. In total, 14 flight-ready lunar modules were produced, at a cost of $1.6 billion ($11 billion in 2019 dollars). Ten flew in space, with six landing on the Moon.

The Eagle Lunar Module, after undocking
[Image credit: NASA]

Whether it was possible to land a spacecraft on the Moon at all was a question that occupied NASA in the mid-1960s during the planning of the Apollo missions. It wasn’t an easy question to answer, and thousands of hours of scientific effort and a long, drawn-out investigation requiring millions of dollars were needed to produce an answer. In the end, the debate was only ended by the successful landings of the Soviet Luna 9 and the American Surveyor in 1966.

By the time Apollo 11 launched, NASA had been considering where to land for over two years, whittling down a list of 30 potential sites to three, all located near the lunar equator. They used high-resolution photographs taken by the Lunar Orbiter spacecraft, and photos and data from by the uncrewed Surveyor landers. The sites were chosen because they had relatively few craters and boulders, were located away from large hills, high cliffs, and deep craters that could cause incorrect altitude signals to the lunar module landing radar, required the least expenditure of spacecraft propellants and had less than a 2° slope in the approach path and landing site.

The chosen landing site was in the Sea of Tranquillity (Mare Tranquilitatis), which actually has a high density of craters. Due to a slight navigational error and a faster-than-intended descent speed, Eagle overshot the intended landing spot by four miles. In the last few seconds of the descent, Armstrong had to take manual control of the lunar module to avoid a sharp-rimmed ray crater around 180 metres across and 30 metres deep. The descent engine was all but out of fuel when they landed safely around 6 km from the intended landing site, in a spot henceforth known as ‘Tranquillity Base’ (and which is listed on lunar maps as Statio Tranquillitatis).

The view out of the window, just after landing
[Image credit: NASA]

Before the mission, there had been some doubt within NASA that they would be able to pinpoint the lunar lander’s location on the moon once it had landed – and in the end, they couldn’t. Armstrong and Aldrin had landed in a place devoid of obvious landmarks. Up in orbit, Michael Collins looked for them using a telescope, but travelling at 3,700 mph, 69 miles up, he had just two minutes in each pass to find a spacecraft that was only 31 feet wide. Since then, NASA has figured it out, and all six Moon landing bases have been photographed by orbiting probes.

Tranquility Base is now considered to be a heritage site, and NASA request that any future Moon missions keep a respectful distance from the Apollo mission sites.

The plan was that Armstrong would cut the engines when the lander was a few feet from the surface; Eagle was built with legs that would crumple upon impact to absorb the shock. But the ace pilot landed extremely softly on the surface, and the legs never crumpled, meaning that the astronauts had to exit the lander several feet higher than intended. When Aldrin exited the lander, his urine collector broke; when he relieved himself, one of his boots filled with pee. Knowing that every word was being recorded for posterity, he kept that bit of information to himself. But more about the moonwalk tomorrow!