When Apollo 11 touched down on the Moon on July 20, 1969, it was one of the biggest milestones in human history. Today, we take it for granted that the lunar landing was possible, but at the time, no one had ever done anything remotely like it – let alone in a craft that had never been fully tested. In view of NASA's promise to return to the lunar surface in the next decade, the approaching 50th anniversary of this monumental feat brings into focus what is still a pertinent question: How does one land on the Moon?

The Apollo Moon landings were made possible by one of the most improbable vehicles ever to leave the drawing board. The Lunar Module (LM) was conceived at the dawn of the Space Race in the early 1960s when the United States and the Soviet Union were locked in a Cold War battle to prove which was the dominant power beyond the Earth's atmosphere.

Though this rivalry has many facets, the main focus was the race to place a man on the Moon by the end of the decade, as set by President Kennedy in a speech in 1962. Sparking the birth of NASA's Apollo program, it was not only an incredibly ambitious endeavor that would cost the equivalent of a small war to realize, it was also something that was absolutely unknown territory that no one had more than a very general idea of how to carry out.

An early concept of the Lunar Module NASA

At the start, the American effort had several different ideas of how to get to the Moon. Each had its advantages and its disadvantages. After much argument and not a little engineering and horse-trading, it was decided to launch a single rocket carrying two spacecraft – one to act as the mothership and the other as the lander.

Introducing the Lunar Module

Cutaway view of the Lunar Module NASA

The mothership wasn't that hard to design. It was, put simply, a larger, more powerful and robust version of manned spacecraft that were already being built and planned for. But the lander was a completely different matter. In terms of engineering and even basic conception, there was no precedent to go on. This would be a spacecraft that would operate only in the vacuum of space, in conditions of low gravity, and land on another world.

Built by Grumman Aircraft, the Lunar Module had a remarkably swift childhood. Starting in 1963, the design took only two years to complete and it went into production in 1965. And even then, it had to deal with a constant stream of problems, redesigns, and flight delays before the first manned version flew in 1969.

To get some idea of how big a jump the Lunar Module is, let's take a look at it. The LM (or "LEM", as it's pronounced) has the appearance of an aeronautical joke, with not a trace of streamlining. Instead, it's an insect-like asymmetrical collection of legs, angles, bulges, and surfaces that's very hard to visualize. Frankly, it looks like it was thrown together on a Friday afternoon by someone in a hurry to go fishing.

Buzz Aldrin leaving the Lunar Module during the Apollo 11 mission NASA

In fact, it's probably the most form-follows-function machine ever built. It was made specifically to do a job and everything in its design reflects this. Its final shape was born out of the need to trade off one requirement against another, to solve one problem by posing another, and, in the end, creating what was the most reliable part of the Apollo program – a machine that never suffered a failure that could not be corrected to complete the mission.

A step-by-step guide

In lunar orbit

Lunar Module descent sequence NASA

Perhaps the best way to understand the Lunar Module is to fly one. Since there aren't any operational ones left, we'll have to make do with our imaginations.

Our flight begins on one of the launch pads at what was then called Cape Kennedy that were built especially to handle the giant Saturn V rockets. The largest boosters ever to enter service, these were the only rockets powerful enough to catapult the Apollo missions to the Moon.

Atop the skyscraper-like rockets is the Command Service Module (CSM) mothership that makes up the nose of the third-stage booster. Below this inside a special protective fairing is the Lunar Module, its legs tucked up as it rests on the S-IVB third stage. Once in translunar orbit, the CSM deploys from the S-IVB and the four doors of the fairing detach and float away. The CSM with its crew of three astronauts then turn about, dock with the Lunar Module, and tow it free of the S-IVB.

Lunar Module infographic NASA

About 64 hours after lifting off from Cape Kennedy, the engine of the Command Service Module roars into life, slowing the trajectory of the CSM and the Lunar Module docked to its nose. Hidden from the Earth by the bulk of the Moon, the two spacecraft go into lunar orbit. After a series of navigational and systems checks, the orbit is tidied up with a couple of small engine burns until it becomes almost circular at an altitude of about 60 nm (69 mi, 111 km).

Until now, the Lunar Module as ridden in powered-down mode – just so much cargo. During the voyage from Earth, the astronauts remained in the Command Module and only entered the LM to temporarily power it up for inspections and systems checks by the Lunar Module Pilot, who also doubled duty as flight engineer.

Now in orbit, the Lunar Module is brought fully to life as the Commander and Lunar Module Pilot enter the LM, leaving the Command Module Pilot behind in the CSM. The lights are on, the life support system is functioning, the onboard computer is awaiting instructions, and the engine waits its turn. The two hatches that connect the CSM and the LM are closed and sealed, the trunk in between is depressurized, and the Commander presses the switch that fires the explosive bolts that free the springs that force the four landing legs to swing out and lock into position.

The Lunar Module inside the S-IVB during Apollo 17 NASA

All the lights reporting green, the Commander undocks from the CSM and uses the Lunar Module's thrusters to move it a safe distance from the Command Module. He then lets the spidery craft slowly spin in the gravity-less void, allowing the Command Module Pilot to confirm that the legs have deployed to their full width of 31 ft (9.4 m), and that there are no signs of damage to the spacecraft.

Tour of the LM

Crew compartment looking forward NASA

The Lunar Module is made up of two stages: the Ascent Stage and the Descent Stage. The Descent Stage makes up the lower half of the spacecraft. It's unmanned and contains the descent engine, which the astronauts will use to leave lunar orbit and land on the surface. The Ascent Stage is on top and is home to the Commander and the Lunar Module Pilot for one to three days, depending on the mission and which version of the LM is used. It provides air, warmth, water, and all the other necessities of life, as well as the machines and instruments needed to land on the Moon, explore it, and then return safely to lunar orbit.

We come into the Ascent Stage through the docking hatch that connects with the Command Module. There were originally two of these with the second in the front of the Lunar Module, but this was later made redundant and turned into a square 32 x 32-in (81 x 81-cm) hatch to allow the astronauts in their full spacesuits to reach the lunar surface.

The Ascent Stage crew compartment is both impressive and disappointing to look at. It may be intended as home to two men for up to three days, but it's about as comfortable as a circuit breaker locker. Unlike previous spacecraft like Mercury, Gemini, or even the Apollo Command Module, it lacks the compact, aircraft cockpit feel. Instead, it's a bewildering collection of panels, hoses, cables, stowage, and general clutter. There's no place to sit and when the astronauts want to sleep they have to put up hammocks.

Lunar Module propulsion elements NASA

On the other hand, the Ascent Stage is the largest and most spacious American manned spacecraft up to its time. The crew compartment is cylindrical with a volume of 235 cu ft (6.7 cu m), which works out to a habitable volume of 160 cu ft (4.5 cu m). The entire stage stands (9.2 ft) 2.8 m tall, is 14.1 ft (4.3) m wide, and 13.1 ft (4 m) deep. Fully loaded, it weighs 10,300 lb (4,700 kg).

Like the rest of the Lunar Module, the crew compartment is protected by a multi-layered hull that includes the aluminum pressure hull, insulating layers of mylar and aluminized Kapton foil blankets, and an outer layer of micrometeoroid shielding.

Stuck in the middle of the crew compartment is what looks like a squat drum that makes getting around a chore. This is the cover of the ascent engine or Ascent Propulsion System (APS) engine, if you want to use the official name. Built by Bell Aerospace, the details of this liquid-fueled engine that generates 3,500 lb of thrust isn't of interest to us at the moment. Its job is taking off from the Moon, and we won't need it for landing – unless things go pear shaped on the way down.

Interior of the Apollo 15 Lunar Module NASA

What is of interest is that the engine is the reason the Ascent Stage has such an irregular appearance, with a great bulge like a stuffed chipmunk cheek on one side. This is because the oxidizer is heavier than the fuel, so one tank has to sit close in on the stage while the other sticks out to maintain the center of gravity.

Also part of the Ascent Stage is the Reaction Control System (RCS), which is made up of 16 hypergolic thrusters similar to those used on the Service Module. Their job is to orient the spacecraft and provide thrust for docking maneuvers.

The Cockpit

Buzz Aldrin inside the Lunar Module NASA

The cockpit is located at the front of the crew compartment, where the Commander operates the flight controls and engine throttle. Meanwhile, the Lunar Module Pilot keeps an eye on the other systems and navigation status. The layout is as similar as possible to that of the Command Module, but since one is a lander they are not identical.

What is odd is that there are no seats for the crew. These were deemed too heavy and unnecessary. Because of the low gravity and low thrust of the engine, it was decided that the astronauts would fly standing up, with a harness to keep them from being jolted about.

Standing up also gives the astronauts a better view. Originally, the Lunar Module was spherical with four huge windows like those on a helicopter. These were removed at an early stage and replaced with a pair of two-ft2 triangular windows made out of layers of chemically tempered glass, which were installed at an angle to provide the best perspective to land. By standing up, the astronauts could put their faces close to the glass and get a surprisingly wide field of view.

Leaving orbit

A model of an early version of the Lunar Module NASA

The first thing to do in landing on the Moon is to leave lunar orbit. And the first step to achieve that is to get the Lunar Module to a low enough altitude. In the first landings, this was done by the module itself, but in later missions, this was carried out by the CSM to conserve fuel. In the latter case, the docked spacecraft would descend to about 50,000 ft (15,000 m) in altitude and then the Lunar Module and the Command Service Module would undock and separate.

Braking

Lunar Module in Earth orbit during the Apollo 9 mission NASA

Now the Descent Stage comes to life. So far, its main purpose has been to supply electricity to the spacecraft from its four or five silver-zinc batteries. But now, it has its single most important job to perform.

Housed inside the octagonal aluminum hull of the Descent Stage is the engine and its four propellant tanks containing the same Aerozine 50 fuel and nitrogen tetroxide oxidizer as used by the ascent engine. However, the ascent engine is a smaller, simpler engine that you just fire and let burn. The descent engine, on the other hand, is a modern marvel that can be throttled and gimbaled as well as restart as needed. Full on, it generates 18,000 lb of thrust. That may not be much on Earth, but it's pretty impressive in one-sixth gravity.

At 50,000 ft above and 260 nm (299 mi, 480 km) uprange of the landing site, the Lunar Module is flying parallel with the surface with its engine pointed in the direction of flight. The trajectory isn't the most fuel efficient, but it does allow the CSM to remain in line of sight during descent.

The Ascent Stage in lunar orbit during Apollo 17 NASA

The crew are "lying" on their backs, which doesn't matter that much in free fall, and they don't have much to do with what is happening, aside from keeping an eye on the instruments. The descent is pre-programmed and carried out automatically by the onboard computer once the Commander has entered the GO order on the keypad.

The engine fires and burns for 30 seconds, dropping the forward and vertical velocity to almost zero as the module descends to 10,000 ft (3,000 m).

Approach

Inside the Apollo 11 Lunar Module NASA

This is when the Lunar Module makes its approach. The Lunar Module is at a 45-degree angle, slowly shifting to vertical as it drops to 700 ft (215 m). At this point, the Commander can see the landing site and assesses the situation. To land or not to land? He only has about two minutes of fuel to spare, so there's little time to make a decision. As he thinks, his finger is near the Abort button, which will fire the ascent engine and jettison the Descent Stage, sending the Ascent Stage back into orbit. It's an extremely dangerous maneuver called "fire in the hole."

Landing

The Lunar Module on the Moon during Apollo 11 NASA

When the spacecraft is about 2,000 ft (610 m) from target, it switches to the landing phase. This is when the computer hands over manual control to the Commander, who guides it in for final touchdown. Slowed to a hover, the module can be steered by tilting it like a helicopter to make any necessary corrections.

This isn't a formality. When Neil Armstrong brought his Lunar Module, Eagle, in to land, he found that the site was strewn with boulders and had to very quickly find somewhere else to put down. Small wonder mission control held its collective breath.

Then, when it's just above the surface, one or more of the 67-in (170-cm) wire-like probes will touch ground and a contact indicator light will go on in the cockpit. The Commander cuts the engine and the module drops the last 3 feet. As it hits, a crushable aluminum-honeycomb cartridge in each primary strut compresses to absorb the shock. The legs are now useless for another landing, but they aren't going anywhere.

Landing pad of the Lunar Module on the Moon showing the landing probe NASA

The Lunar Module is now on the Moon. It's a landing that each astronaut who performed it only ever made once and would never repeat again, so it was definitely a case of getting it right the first time. Now the astronauts have 45 to 78 hours of battery life to explore the surface before returning to the Command Module and home to Earth.

And, strangely, that's the most dangerous bit.