A lunar lander is a type of spacecraft designed to land on the surface of the moon. It’s also known as a moon lander.

Humans have been building lunar landers since the 1950s, in anticipation of the first moon landing. Some prominent examples include:

1. Apollo Lunar Module (LM). This is the most well known lunar lander. Originally called the Lunar Excursion Module (LEM), it was developed as part of the United States’ Apollo program to take humans to the moon—which it did successfully for the first time in 1969 with Apollo 11. The specific lunar module involved in the historic event was called the Eagle lander.

2. LK Lander. The Soviet counterpart to America’s LM, the LK Lander (it stands for “lunniy korabl,” meaning “lunar craft”) was part of the USSR’s attempts at a lunar landing. It never reached the lunar surface, although several modules flew in Earth orbit.

3. Surveyor spacecraft. These were robotic spacecraft that NASA sent to the moon from 1966 to 1968. Their goal was to test out soft landings ahead of the Apollo program’s manned moon missions.

4. Project Morpheus. This more recent NASA project from 2010 to 2014 was geared at achieving autonomous flying, vertical takeoff, and landing (VTVL), and using a new, non-toxic propellant system based on methane and oxygen—meaning it could be potentially manufactured on the moon and Mars, as well as more safely on Earth.

Every time humans have sent spacecraft to land on a body in the solar system—whether it was the moon, another planet, or an asteroid—it has come with a new set of challenges. In the case of lunar landing, aerospace engineers have to consider the following factors:

1. The moon has high gravity compared to asteroids. This means the only method of liftoff and landing that has so far provided enough thrust has been with the use of rockets. The goal of all landers is to achieve a “soft landing,” meaning without damage to the vehicle.

2. The moon has no atmosphere. Landers that have gone to Mars, Venus and Saturn’s moon Titan could use parachutes to descend, harnessing the atmosphere for aerobraking. For the moon, much more fuel is needed for the descent.

3. The moon’s days are long. The lunar surface is in direct sunlight for a fortnight and then darkness for another two weeks. This results in extreme temperatures ranging from 260 F to -280 F, making it hard to protect the lander’s instruments.

The LM is split into a two-stage design that corresponds to different needs in the landing mission. The upper section is known as the ascent stage and the lower section, the descent stage.

1. The ascent stage contains the crew compartment, aft equipment bay, reaction control subsystem (RCS), and ascent engine for launching from the moon’s surface. The crew compartment also held environmental control and life support systems. The RCS has 16 thrusters arranged in two independent systems with their own propellant tanks for maneuvering in lunar orbit, but it can also draw fuel from the main propulsion system for ascent from the surface.

2. The descent stage contains the landing gear and descent engine for lowering the craft to its landing site on the moon. The landing gear folds out after launch and is partly made of a crushable aluminum-honeycomb material to absorb the force of moon landing. There are four legs, one with a ladder and porch allowing astronauts access to the lunar surface. The last three Apollo missions (Apollo 15 to 17) also featured a lunar rover, the Lunar Roving Vehicle (LRV), folded into a corner of the descent stage.