2909: Moon Landing Mission Profiles

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Moon Landing Mission Profiles
If you pick a low enough orbit, it gives you a lot of freedom to use a lightweight launch vehicle such as a stepladder.
Title text: If you pick a low enough orbit, it gives you a lot of freedom to use a lightweight launch vehicle such as a stepladder.


Getting astronauts to the Moon (and back) is hard. There are several different strategies to do it. This comic reviews three mission profiles considered for the Apollo Moon landings and one which is absurd. (While the profiles only depict the outbound leg of the trip, in each case the return journey would likely make compatible rocket and trajectory choices.)

Lunar Orbit Rendezvous (LOR)

Description: Using a single large rocket to get the required lunar orbiter and lander systems into trans-lunar orbit, which can then fulfil their eponymous roles.

Status: Chosen by the Apollo Program in the 1960s and 1970s.

Explanation: This was the actual method used in the Apollo missions. It was efficient in terms of fuel and cost. The main spacecraft ('command module') orbits the Moon, as the lander separates and uses its descent-stage to safely reach the surface.

After the Moon mission, the lander (ascent-stage only) ascends to dock once more with the command module in lunar orbit, the crew then return to Earth in the command module (leaving the abandoned ascent stage behind, in most cases purposefully directed to impact the Moon).

Earth Orbit Rendezvous (EOR)

Description: A large lunar-landing system is assembled in Earth orbit through several launches. Once complete, it travels to the Moon as a whole. It is depicted here as not required to orbit the Moon in full, in any way, but is shown needing to orbit Earth, as an unavoidable part of its profile.

Status: Rejected for requiring multiple Saturn V rockets per landing and potentially taking longer.

Explanation: This concept involved launching different parts of the spacecraft into Earth orbit using multiple rockets and then assembling them before heading to the Moon. It would have allowed almost arbitrarily large sizes of equipment to have reached the surface, perhaps to simplify the return journey, but with the complication of adding multiple orbital docking procedures to the project rather than most assembling and spacecraft mating being carried out prior to launch. It should be noted that Randall made a mistake on this point of the comic; the Earth Orbit Rendezvous would have required multiple launches of the Saturn IB, not multiple launches of the Saturn V.

In theory, a returning craft (the final stage that breaks free of the Moon and heads back to Earth) would have made a direct crossing from the Moon's surface back to Earth's atmosphere, unless a Lunar Orbit aspect (perhaps a habitation module left as a waypoint for use by subsequent missions) was included in the plans. (In this particular regard, the Artemis program profile resembles this particular profile.)

Direct Ascent

Description: The lander is launched from Earth directly to the Moon without entering orbit.

Status: Rejected for requiring an unreasonably large rocket.

Explanation: This was a simpler but less feasible approach, where a single huge rocket (or a particularly large rocket stack) would send the lander straight to the Moon. The inefficiency comes in taking a comparatively huge rocket down to the Moon and back up, requiring a lot more fuel than a separate lander. It avoids having to 'park' items in orbit that it must later dock once more with, but then increases the mass that must land on/take off from the lunar surface, without being useful during this phase of the mission.

The return journey would be as direct as the original leg. This option does not preclude discarding various stages of the rocket as various transit phases are completed, but would not involve any complicated rendezvousing to enable the crew module to reach its waypoints.

In reality, this was the approach imagined for the Nova C-8 rocket as an Apollo alternative. This was also the approach used in Destination Moon from The Adventures of Tintin, with the fuel problem addressed by using a nuclear reactor for much of the trip—which would be a really bad idea in reality since "rockets have a tendency to explode"[1]. Science fiction movies have frequently depicted this method of landing, either before the dawn of the actual Apollo program or (to save plot-time or by using a fictional increase in rocket capability) in more futuristic settings.

Lunar Earth Rendezvous (LER)

Description: The Moon transits to rendezvous with a spacecraft in low Earth orbit.

Status: Rejected because, humorously, "I guess no one thought of it?!"

Explanation: This is a fictional and impractical scenario. The Moon cannot propel itself and cannot alter its orbit to rendezvous with a spacecraft.[citation needed] The Moon would also break up because low Earth orbit is within the Roche limit. Astronauts would theoretically land on the Moon, but the hypothetical fragments of the Moon would make the landing impractical. This would be also bad for the Earth's climate, tides, stock markets and ecosystems.[citation needed]

The 'return leg' could involve having the Moon move back to where it should be, which does not add too many extra mysteries to the mission profile, unless those who implement it would prefer not to have to bring it back again for the next expedition.

The title text imagines the Moon coming to very low Earth orbit, low enough to reach with a step ladder. There are many reasons this is wholly impractical, as well as civilization-ending. Dropping the Moon down to within six feet or so of the Earth would likely cover the Earth in moonrock. The only difference from the prior comic is that debris would ultimately be deposited at a significantly high sideways velocity (perhaps briefly preceded by a hypersonic atmospheric shockwave and everything you might expect from catastrophic worldwide ground-tremors), no matter where you were. If this mission profile had been used in the 1960s, the Soviet Union, the United States, and all other nations (whether they like it or not) would have 'won' the space race almost simultanously as the rapid redistribution of rock tried to settle over everywhere to create (on average) a 43 km-deep grave. Arguably the true 'winner' could be determined by which country gets hit by the initial Moon rocks first.

Factual Mission Profiles[edit]

The Apollo Program considered a number of possible mission profiles. Of the four shortlist plans, the Lunar Surface Rendezvous plan was not shown or mentioned in this comic, but clearly inspired the title of the method created in its place.

With the ongoing work to achieve the Artemis program, the successor to Apollo, this comic is probably also making sideways references to the plans and equipment being developed to achieve it, which currently features several aspects of the above examples. A single crewed-launch is intending to rendezvous with, and make use of, additional equipment separately launched (including one that is very nearly one big rocket in its own right). Instead of assembling in Earth orbit, it will probably make use of a lunar near-rectilinear halo orbit, or NRHO, to and from which the landing system will operate. There is also planning, still in an early stage, to establish lunar-surface infrastructure that would simplify the refuelling of the transfer craft and support surface operations.

The only depicted plan that should not play a part in upcoming missions is the one which may be more connected with some other quite different scenario which has previously been referenced in xkcd.


Ambox notice.png This transcript is incomplete. Please help editing it! Thanks.
[Top left panel]
Lunar orbit rendezvous
Spacecraft orbits Moon, drops lander
[A big circle and a small circle, depicting the Earth and the Moon, with a line starting from the left side of the Earth curving, then straight towards the Moon, then forming an orbit around it and then descending on the Moon. A spacecraft is located at about the middle of the line.]
Chosen by the Apollo program
[Top right panel]
Earth orbit rendezvous
Large lander assembled in Earth orbit via several launches, travels to Moon
[The same circles depicting the Earth and the Moon. Two lines start from the left side of the Earth into an orbit around it, with a couple of different stages of spacecraft located in different parts of the orbit. The merged line heads straight towards the Moon to land on its left side, without orbit. A spacecraft is located around the middle of the line.]
Rejected for requiring multiple Saturn Vs per landing and potentially taking longer
[Bottom left panel]
Direct ascent
Lander launched from Earth directly to Moon
[Another big circle and small circle, depicting the Earth and the Moon again. This time, the line curves the same way as in the lunar orbit rendezvous, heads straight towards the Moon, and ends on the Moon's left side. A spacecraft is located slightly on the left, to Earth's side.]
Rejected for requiring an unreasonably large rocket
[Bottom right panel]
Lunar Earth rendezvous
Moon transits to rendezvous with spacecraft in low Earth orbit
[A big circle representing Earth and three small circles representing the Moon at different times. Starting from the right side, the small circle follows a path that curves slightly upwards. The path then straightens. Another small circle is located on the straight part. The path forms an orbit around the Earth, with another small circle on Earth's left side. Another small path leads from the left side of the Earth to the leftmost small circle.]
Rejected because I guess no one thought of it?!

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Direct assent was a common method in many sci-fi movies. Including the classic From the Earth to the Moon Barmar (talk) 19:18, 20 March 2024 (UTC)

It feels like it needs an addition of more detail from the planned Artemis mission. In several ways it's going to be a "multiple rockets, assembled in orbit" plan (if not Earth orbit, then Lunar), with SLS and Orion scheduled to be cooperating with the Lunar-Starship launch (later, maybe, other independently craft)... which is itself almost a "one big rocket" solution, sticky-taped onto the plan. That's with or without the addition of the dedicated and semi-manned Lunar Gateway moon-orbiting element. I mean, most of the prototypical Apollo-era plans (DA, EOR, LSR and LOR) had their own crazy bits to them, and the full Artemis premise definitely seems crazy as well, if only because the LOR version got chosen for Apollo and pretty much got proven to work. (Or worked enough to even get 13 back home safely!) 21:04, 20 March 2024 (UTC)

Which mission profile will the artemis missions use? -- 06:58, 21 March 2024 (UTC)
From wikipedia:
Artemis 3 (2026) is planned to be the first crewed lunar landing. The mission depends on a support mission to place a Starship Human Landing System (HLS) in a near-rectilinear halo orbit (NRHO) of the Moon prior to the launch of SLS/Orion. After Starship HLS reaches NRHO, SLS/Orion will send the Orion spacecraft with a crew of four to rendezvous and dock with HLS. [...]
...so it's a kind of Lunar Orbit Rendezvous With Multiple Launches (not sure what its official name is, probably something as pithy as "Skycrane" was). Certainly not unambiguously any one of the main types considered for Apollo/in this comic. 09:17, 21 March 2024 (UTC)
So it's both an earth orbit rendezvous and a lunar orbit rendezvous? -- 07:15, 22 March 2024 (UTC)
Depends on how you define the SpaceX contribution (in the current plan.
  • In Earth Orbit, there is due to be unmanned refuelling of the Lunar Starship from 'normal' Starships, which is a sort of EOR, I suppose.
    • But the Apollo EOR would have been basicically building up small bits, assembling 'a Moon rocket' in space that could avoid the LOR rendezvousing but was too big to launch in one go. The Saturn V helped there, in becoming good enough at lifting to not need the EOR stage, along with other mission refinements.
  • Lunar Starship and Orion capsule (and, later, other items, including the Lunar Gateway) meet up in Lunar Orbit prior to any landing, fulfiling a "multiple rocket LOR" hybrid, one could say.
    • The equivalent phase for Apollo was the 'pointy bit' command module temporarily disconnecting from the lunar lander (stowed behind it in the same rocket) and reconnecting nose-to-nose (or nose to head?), but it wasn't really a full rendezvous, just a reconfiguration of bits already floating around together. And this happened on the way between Earth and Moon, anyway, so isn't in either Lunar or Earth orbit. Could have been either, but it just happens in a 'quiet' bit of the mission profile, and probably they also needed the TLI burn to happen before they started messing around 'unpacking' the LM from its shroud, etc.
  • Lunar Starship then goes down to land on the Moon (hopefully) mirroring Apollo's LM separating from the CM
    • But none of the plans ever refer to any undocking stages, by name. Even the Direct Launch approach would probably require disconecting from bits of the rocket stack that they'd discard, at various points, rather than the Single-Stage-To-Moon-And-Back of the Tintin rocket and other popular SF versions of the era.
  • LS then LORs with Orion (again, hopefully!), as that Starship model isn't going to be useful in getting back down to Earth. It'll perhaps be left for use by the next Orion to arrive (and maybe they'll get more fuel to it), if everything goes to plan.
    • This is the LOR bit that the used Apollo profile really refers to, because ascent-stage of the lander had to meet up again with the command module in order to return home. Again, this lander was not set up to get the crew back to Earth (no use carrying heat-shields/etc down to the Moon and back if you didn't have to...), though most bits of landers were either left on the Moon (descent stages) or crashed back into it (ascent stages, once the crew had transfered back), without any plans for re-use by later missions.
  • Pretty much every plan (past and current) has whatever kind (and state) of vehicle that departs the Moon then coming straight back into the re-entry (though discarding the last bit of rocketry below the capsule might still be required).
    • It's possible that future missions will add a post-landing EOR (or similar), perhaps so that Orion's successor can be a reusable ferry back and forth between Earth orbit (ISS/whatever) and Moon orbit (Lunar Gateway/etc) and not need the capsule heat-shield/parachutes, instead crews can hitch rides up/down on Crew Dragons or similar HLSs.
Perhaps let's call the Artemis plan something like Multiple Rocket Earth Orbit Refueling Lunar Orbit Rendezvous/Reusable Rockets (MREORLOR/RR, but only cats can pronounce that properly). 11:43, 22 March 2024 (UTC)

I see Lunar Earth Rendezvous will be bad for Earth's climate, tides, stock markets and ecosystems. Has anyone considered the impact this will have on the trout population? 00:02, 21 March 2024 (UTC)

What is the deal with trout? I’ve seen trout population mentioned in 2 places online. 17:42, 22 March 2024 (UTC)
from a quick google search on "Trout and the moon":
> Anglers often report that trout may be caught all night long on a bright moonlight night. It seems reasonable that when fish feed at night they may have a tendency to feed less than usual dur- ing the day, which in turn may affect the catch.
> The idea behind fishing by moon phases is that the bigger the tide, the more active the fish. The strongest tides happen twice a month: during a new moon, when the sun and moon are both pulling in the same direction, and during the full moon, they're pulling on either side of the planet. (talk) 21:07, 15 April 2024 (please sign your comments with ~~~~)

The choice of Lunar Orbit Rendezvous wasn't easy for the Apollo planners. Thanks to John Houbolt, the "voice in the wilderness" as he called himself, NASA finally adopted LOR rather than Direct Ascent or Earth Orbit Rendezvous they were planning in the early 60's. Remembering John Houbolt, NASA’s Voice in the Wilderness Orion205 (talk) 01:09, 21 March 2024 (UTC)

When you think about it, the complex (re)docking procedures for happen entirely beyond Earth orbit (for all Moon-bound missions). The uncovering of the LM and crew-connection made from the CM early in TLI, and the return of the ascent stage to rendezvous with the CM in lunar orbit.
How tempting must it have been to just have a single, 'uncomplicated' lander that doesn't rely on potentially awkward coordination well beyond any conceivably timely 'rescue shot' from Earth (LEO construction variations might realistically be still somewhat inaccesable, at the time, but there might yet still have been a chance to do something with a handy 1B sitting on a pad just in case.)
But it turned out Ok. Maybe better than Ok, as Aquarius (13's LM) was an important temporary 'lifeboat', whereas any single-craft-to-Moon solution with the same engineering failure might have left the crew with nowhere near as survivable a situation.
As Eleven's initial landing maybe showed (had to choose another landing spot from the one initially aimed for), the Lunar Surface Rendezvous seems to me the trickiest 'rendezvous' variation. At best, the preparatory 'reception lander' might have needed to have been sent again, when the first one encountered the bad landing zone (either landed badly or landed okish but revealed awful conditions to send the manned expedition). At worst, the manned craft lands but not realistically close enough to perform the intended fuel transfer, and very little that they can do about it by then.
So, looked at one way, it was a crazy decision. In another, it was just workable enough to avoid all the problems that the various other schemes had? 09:01, 21 March 2024 (UTC)

Randall's been thinking a lot about space recently. No complaints from me. MrCandela (talk) 02:08, 21 March 2024 (UTC)

He's probably playing KSP2 :D Elektrizikekswerk (talk) 08:35, 21 March 2024 (UTC)

Getting astronauts to the moon isn't hard. Getting them there (and back) in one piece is. 10:18, 21 March 2024 (UTC)

Yes, Carl... ;) 13:04, 21 March 2024 (UTC)