Lesson 2 of 6 · Spacecraft & engineering
One journey, several spacecraft
9 minute lesson · 3 checks along the way
Your goal for this lessonExplain the different jobs of the Saturn V, command and service module, and lunar module.
A rocket begins the journey
A lunar mission began with the Saturn V. Its stages fired in sequence, discarding hardware as its job ended. The rocket first placed the crew and spacecraft into Earth orbit. Its upper stage then fired again to send them toward the Moon, a maneuver called translunar injection. The Saturn V was the launch vehicle, not the craft that landed.
An orbit is continuous motion around a world, rather than a place where a spacecraft stops. Engine firings change that motion. Apollo used particular burns to leave Earth orbit, enter lunar orbit, descend, and depart for home. Between burns, the spacecraft could coast. Following those changes of motion is more useful than imagining one rocket thrusting all the way there and back.
Give each spacecraft a job
The command module was the crew’s cabin and the only part built to carry them through Earth’s atmosphere. The attached service module supplied the main propulsion and important support systems. Together they formed the command and service module, or CSM. The lunar module, or LM, carried two astronauts from lunar orbit to the surface and back.
One astronaut stayed with the CSM while the other two landed. That division kept the larger return spacecraft in orbit and reduced the mass that had to descend and lift off. The plan depended on lunar orbit rendezvous: the returning LM had to meet the CSM again. Use the journey diagram to follow where the crew separates and where it reunites.
To the Moon. Then all the way home.
Follow a landing mission from launch to splashdown. Notice how the spacecraft gets smaller as each vehicle finishes its job.
- Saturn V
Launch
The rocket lifts the crew and spacecraft. Used stages separate, reducing the mass still being accelerated.
- Upper rocket stage
Check in Earth orbit
After checks, the Saturn V’s third stage fires again to send the spacecraft toward the Moon.
- Command + service + lunar modules
Travel together
The command and service modules turn around, dock with the lunar module, and pull it free of the rocket stage.
- Service module engine
Enter lunar orbit
An engine burn slows the joined spacecraft into orbit. Two astronauts later transfer to the lunar module and undock.
- Lunar module
Land and explore
The descent stage lands two astronauts. The third astronaut stays in lunar orbit in the command and service modules.
- Lunar module ascent stage
Meet again in orbit
The upper stage lifts off; the descent stage stays on the Moon. Rendezvous and docking reunite the crew and their samples.
- Command module
Bring the crew home
The lunar module is discarded. The service module powers departure, then separates near Earth. Only the command module carries the crew through reentry and splashdown.
Start on the lunar surface. Which vehicles must a Moon walker use to reach Earth?
Follow the answer
The lunar module’s ascent stage reaches lunar orbit. After docking, the astronaut transfers into the command module. The service module provides the engine for the trip home; the command module alone protects the crew during reentry.
Simplified landing-mission sequence; shapes, distances, and orbits are not to scale. Based on NASA’s Apollo 11 mission overview ↗ and lunar module history ↗.
Apollo 9 tested the lunar module in Earth orbit. Its separate descent and ascent stages divided the work of landing and returning to orbit.
Look closely
Find the broad lower stage and its landing legs, then the crew cabin above it. Which part would need to leave the Moon again?
See what to notice
The landing legs attach to the lower descent stage. The cabin sits in the upper ascent stage, which would carry the moonwalkers back to lunar orbit. On a landing mission, leaving the lower stage behind reduces what must be lifted off the surface. Here, both stages are being tested above Earth.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Prepare the spacecraft for the outward trip
After departure from Earth orbit, the CSM separated from the rocket’s upper stage, turned around, and docked with the LM. Docking means physically joining two spacecraft; rendezvous means bringing them together with compatible positions and motion. The CSM then pulled the lander away from the stage so the two spacecraft could travel toward the Moon together.
Near the Moon, the service module’s main engine slowed the joined spacecraft enough to enter lunar orbit. Without that planned change in motion, reaching the Moon’s neighborhood would not automatically make the spacecraft circle it. Once in orbit, the crew checked systems and prepared the LM for its separate flight to the surface.
Land, lift off, and meet again
The LM had two stages. Its lower descent stage carried the landing engine and legs. The upper ascent stage held the crew cabin and its own engine. After surface exploration, the upper stage lifted off while the lower stage remained on the Moon. Equipment needed only for landing did not have to be carried back into orbit.
The ascent stage rendezvoused and docked with the CSM. The moonwalkers transferred themselves and their samples, then the LM was discarded. The service module’s engine sent the reunited crew toward Earth. Before atmospheric entry, the service module was also discarded, leaving the command module with its heat shield and parachutes to complete the return.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Work the decision: what should land?
Apollo planners considered different ways to reach the surface. One approach would land a much larger spacecraft capable of beginning the direct return to Earth. The chosen lunar orbit rendezvous approach left the Earth-return craft above the Moon. Compare the designs by asking what each needs to lower onto the surface and launch upward again.
Carrying more equipment also requires more propellant to move it. A smaller lander eased that burden, but introduced a dependency: its crew needed a successful meeting with another spacecraft far from Earth. The choice traded one kind of difficulty for another. It made rendezvous a mission requirement that training and flight tests would have to demonstrate.
Learn the skills before flying the full route
Mercury helped establish that astronauts could function in space and be recovered safely. Gemini then developed skills Apollo would need, including longer flights, maneuvering, rendezvous, docking, and working outside a spacecraft. Its two-person crews and ground teams could practice these operations in Earth orbit before Apollo depended on them at lunar distance.
These earlier programs were steps toward Apollo’s particular problem, rather than miniature Moon landings. Gemini could teach how spacecraft meet; Apollo still had to prove that its own CSM and LM could separate and reconnect. You now have the route. Next, follow the people who built, tested, and operated each part of it.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.
- Read: NASA: Why Apollo used a lunar moduleCompare direct ascent with lunar orbit rendezvous. Identify what the smaller lander saved and what new dependency it introduced.
- Read: NASA: Follow Apollo 11’s flightTrace departure from Earth orbit, docking with Eagle, lunar orbit, and the final splashdown. Match each event to the journey diagram.
- Read: NASA: Gemini prepares the skillsRead the opening program objectives. Choose two skills Apollo needed and explain where each fits in the Moon journey.