By Doug Adler on May 25, 2022
There’s more than one way to make a lunar voyage.
When China’s Chang’e 5 became the first mission to return samples from the Moon since 1976, it made headlines around the world and captured the attention of spaceflight enthusiasts everywhere. For many, part of the fascination was that it evoked a bygone era of spaceflight, returning images reminiscent of Apollo: lunar landscapes, Moon rocks, and spacecraft rendezvouses. The shape of the Chang’e 5 lander resembled an Apollo lunar module. And the mission landed in Oceanus Procellarum, the Ocean of Storms, as did Apollo 12 astronauts Pete Conrad and Alan Bean.
But Chang’e 5 was similar to the Apollo lunar landings in an even more fundamental way: the method it used to get to and from the Moon, known as lunar orbit rendezvous (LOR). In this approach, an assemblage of modules travels to the Moon, each to carry out a specific purpose.
But it didn’t necessarily have to be this way: There are many ways to get to the Moon, and NASA studied multiple approaches before opting for LOR. And though LOR has ruled the roost until now, it’s possible that future Moon missions may look very different.
NASA chose a lunar orbit rendezvous approach for the Apollo program, but only after careful consideration of alternatives.
NASA
Lunar orbit rendezvous
The Chang’e 5 spacecraft followed a familiar, Apollo-style playbook, consisting of a service module that supplied power and propulsion to a lunar lander and a module for returning samples to Earth. Upon reaching lunar orbit, the lander detached, leaving the service module and Earth return module behind. The lander consisted of two parts: a descent module and an ascent module. After touchdown and acquiring lunar soil samples, the ascent module used the descent module as a launch pad, blasting off from the moon’s surface to rendezvous with the patiently waiting components of Chang’e 5 in lunar orbit.
Once the lunar samples had been transferred to the Earth return module, the ascent stage undocked from the service module. The service module, still linked to the Earth return module, then lit its engine to return to Earth where, in the final minutes of the flight, the return module — carrying the precious lunar samples — separated and reentered the atmosphere, landing in Mongolia.
LOR is a highly efficient means of getting to and from the moon: It only puts hardware onto the surface of the moon that actually needs to go to the surface. The Earth return module, the service module, and all of the fuel needed to return to Earth from lunar orbit do not need to actually land on the moon, so they don’t. This allows the lunar lander to be much lighter and use less fuel and a smaller engine. Similarly, as only the ascent module takes off from the moon, even less fuel and an even smaller engine are needed to get this component back into lunar orbit.
This image from the Chang’e 5 lunar lander shows off its spindly, Apollo-like legs.
CNSA
The downside of missions that utilize LOR is that the actual rendezvous above the Moon is tricky and has to work perfectly — otherwise the entire mission will fail. Chang’e 5 was able to perform this rendezvous autonomously using microwave radar, whereas Apollo missions relied on their human pilots to dock their spacecraft manually. Indeed, the main function of the lone astronaut remaining in lunar orbit in the Command and Service Module during the Apollo missions was to be able to rescue the moonwalkers from a low or erratic orbit should they encounter troubles after lifting off from the moon itself.
Direct ascent
LOR has proven itself to be an effective way to get to and from the lunar surface, but it’s not the only way to go. In fact, during the 1960s, engineers at NASA and within the Soviet Union (which was also making plans to put cosmonauts on the moon) looked at a variety of different mission profiles. Like LOR, these each had advantages and disadvantages.
Direct ascent is, at least conceptually, the simplest way to get to the moon and back. Direct ascent involves building a single rocket, taking off from the Earth, flying to the moon and landing said rocket on the moon. After the lunar phase of the mission is complete, the entire rocket takes off from the moon and returns to the Earth.
To fly a direct ascent mission to the Moon, NASA considered building a rocket dubbed Nova (right) that would be significantly larger than the Saturn V rocket (depicted here in an early version, center). A predecessor rocket, the medium-lift Saturn C-I, is shown at left.
NASA
Direct ascent was notably featured in the 1950 science fiction film Destination Moon. In fact, before the Apollo program, direct ascent was how most lay people conceived of how a flight to the Moon would actually work.
The advantage of direct ascent is its simplicity, as it obviates the need for any rendezvous phases. The main, and largely insurmountable, disadvantage of direct ascent is that it requires an almost unfathomably massive spacecraft to make it work. NASA looked at a few different direct ascent spacecraft designs, but these were considered too large or unworkable.
Earth orbit rendezvous
Earth orbit rendezvous (EOR) was another pre-Apollo competing method conceived for getting to the Moon. The hallmark of EOR approaches was several launches of different spacecraft components into Earth orbit where assembly into a larger spacecraft could be performed. This larger spacecraft would then fly to the moon and land on it, later taking off and returning to Earth.
EOR has the advantage of allowing the tricky rendezvous phase of the mission to occur close to Earth. If trouble developed, astronauts could simply reenter the atmosphere and be rescued; if the rendezvous during an LOR mission failed, the moonwalkers would perish. NASA’s Gemini program convincingly proved that on-orbit rendezvous techniques worked and were safe and could be used for LOR approaches.
EOR’s disadvantages include its requirement that more than one rocket launch from Earth and that it generally sends more mass to the lunar surface — and is therefore less fuel efficient — than LOR.
The way of the future
NASA’s Artemis program, which aims to return humans to the surface of the Moon, centers on using the Space Launch System (SLS) rocket to launch an Orion crew capsule to conduct an LOR. In that sense, it’s similar to the mission profile used by Apollo.
A Starship Human Landing System vehicle descends to a lunar base in this artist’s rendering.
SpaceX
However, there’s a twist. The craft that NASA has chosen as a lunar lander is an under-development variant of SpaceX’s Starship that will launch separately as an uncrewed vehicle. And before the Starship lander (dubbed Starship Human Landing System, or Starship HLS) can travel to lunar orbit to rendezvous with the crew on Orion, it must fill up at a refueling station in Earth orbit. SpaceX’s current plan calls for the refueling station to be a tanker variant of Starship, which itself must be fueled on-orbit by anywhere from 4 to 12 Starship flights carrying fuel as payload.
This series of launches is reminiscent of the early Earth orbit rendezvous concepts. Though not as fuel efficient, the strategy will allow Starship to deliver over 100 tons of payload to the lunar surface and back — orders of magnitude more than Apollo.
For all its promise, Artemis may not even be the next crewed mission to the Moon. The Artemis program is extremely complex, requires enormous amounts of funding, and relies on components that are still in development and, in many cases, behind schedule. NASA’s first crewed Artemis lunar landing has been delayed from a target date of 2024 to 2025, and it seems inevitable that it will slip to 2026 or beyond.
It’s possible that private space ventures, such as SpaceX, Blue Origin, and others could mount independent crewed lunar missions before NASA. In particular, SpaceX’s main Starship vehicle is planned to be capable of a direct ascent flight to the Moon. It will be interesting to see what sort of methods these ventures might use to land on, and return from, the moon.
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