Terms I’ve picked up over twelve years of following the space program. Explained the way I understand them, which may not be perfectly scientific, but should be pretty close.

Abort

Ending a mission early because something has gone wrong. During a launch, the crew can fire the launch escape system — a rocket on top of the capsule that yanks them away from an exploding Saturn V. Nobody ever had to use it for real, but it was there. During the mission, an abort means firing the engines to come home early. Apollo 13 was, in a sense, one very long abort.

Astronaut

From the Greek for “star sailor.” The Americans use this term; the Soviets say “cosmonaut” (sailor of the cosmos). Both mean the same thing: someone who has been to space. The military definition at the time was above 50 miles altitude. The international definition is above 100 kilometers (62 miles). Either way, you’ve left Earth’s atmosphere.

CapCom

Capsule Communicator. The person in Mission Control who talks directly to the crew. Always an astronaut — the thinking being that a fellow astronaut can communicate most efficiently with the men in space. During Apollo 11, Charlie Duke was CapCom for the landing. His voice is the one saying “60 seconds” when the fuel was running low.

Command Module (CM)

The cone-shaped capsule at the top of the Saturn V where the crew lives during the trip to and from the Moon. It’s about as big as a large walk-in closet for three men. It’s the only part of the entire Saturn V that comes back to Earth — everything else is either jettisoned or left on the Moon.

EVA

Extravehicular Activity. Walking in space, or on the Moon. The suit they wear outside the spacecraft is a self-contained life-support system. On the Moon, they wore a backpack called a PLSS (Portable Life Support System) — it provided oxygen, removed carbon dioxide, and regulated temperature. Without it, the Moon would kill you in seconds.

F-1 Engine

The main engine of the Saturn V’s first stage. Five of them fire at liftoff, producing 1.5 million pounds of thrust each — 7.5 million total. It’s the most powerful liquid-fueled rocket engine ever built. The combustion chamber is the size of a car. It burns RP-1 (a refined kerosene) and liquid oxygen. Each engine consumes 40,000 pounds of propellant per minute.

Free-Return Trajectory

A flight path that uses the Moon’s gravity to slingshot the spacecraft back to Earth without requiring an engine burn. Apollo 8 used one. Apollo 13, after the accident, was maneuvered onto one — which is why they were able to get home even though their main engine was useless. It’s the orbital equivalent of a rescue rope already tied to the spacecraft.

Go/No-Go

The decision call in Mission Control before each critical event in a mission. The Flight Director polls the controllers: “Guidance, go or no-go?” “Go.” “FIDO, go or no-go?” “Go.” All the way around the room. Only when everyone is “go” does the mission proceed. During Apollo 11’s landing, the Flight Director polled the room while the 1202 alarms were sounding. Everyone said go.

LM (Lunar Module)

The Lunar Module — pronounced “Lem.” The spidery spacecraft that actually lands on the Moon. Built by Grumman Aircraft. It has two stages: the descent stage (which lands and stays on the Moon forever) and the ascent stage (which lifts off and rendezvous with the orbiting Command Module). The ascent engine cannot be tested before firing on the Moon — it either works or it doesn’t. It always worked.

LOI (Lunar Orbit Insertion)

The engine burn that slows the spacecraft enough to be captured by the Moon’s gravity and enter lunar orbit. It happens on the far side of the Moon, out of radio contact with Earth. Six agonizing minutes of silence while you wait to hear if they made it into orbit. They always did.

LOR (Lunar Orbit Rendezvous)

The mission architecture NASA chose for Apollo. Instead of going directly to the Moon and back (which would require a huge rocket), the spacecraft goes into lunar orbit, a smaller vehicle descends to the surface and returns to orbit, and the two craft dock there. Cheaper, lighter, and achievable. The alternative — a direct landing — would have required a rocket too large to build in time.

Mission Control

The Flight Control Room at the Manned Spacecraft Center in Houston, Texas. The rows of consoles, each controller monitoring a specific system of the spacecraft. The Flight Director sits near the back center and makes the final calls. When you see the TV footage of the men in white shirts and ties cheering at splashdown, that’s Mission Control.

Rendezvous

Two spacecraft meeting in orbit. Harder than it sounds — orbital mechanics are counterintuitive. If you speed up, you go to a higher orbit and actually fall behind the target. You have to slow down to go lower, which moves you faster. Gemini taught the astronauts and ground controllers how to do this. Apollo required rendezvous in lunar orbit, which has no ground tracking stations.

Saturn V

The launch vehicle that sent men to the Moon. Three stages: the S-IC (five F-1 engines, burns for 2.5 minutes), the S-II (five J-2 engines), the S-IVB (one J-2 engine, which also fires to send the spacecraft toward the Moon). Total height: 363 feet — a 36-story building. Total weight at launch: 6.5 million pounds, mostly propellant. The loudest machine ever built.

Service Module (SM)

The cylindrical section attached below the Command Module. Contains the main engine (the Service Propulsion System) that puts the spacecraft into and out of lunar orbit, plus the fuel cells that generate electricity, and the oxygen tanks. In Apollo 13, an oxygen tank in the Service Module exploded. The SM is jettisoned just before reentry — you can see it tumbling away in the Apollo 13 footage, and you can see the hole in its side.

Trans-Lunar Injection (TLI)

The engine burn — the third stage of the Saturn V firing again — that sends the spacecraft out of Earth orbit toward the Moon. It’s about a 6-minute burn. After TLI, the spacecraft is committed to the Moon. The journey takes about three days each way.