Editing 2909: Moon Landing Mission Profiles

{{comic
| number    = 2909
| date      = March 20, 2024
| title     = Moon Landing Mission Profiles
| image     = moon_landing_mission_profiles_2x.png
| imagesize = 709x504px
| noexpand  = true
| titletext = If you pick a low enough orbit, it gives you a lot of freedom to use a lightweight launch vehicle such as a stepladder.
}}

==Explanation==
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.)

'''{{w|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 {{w|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).

'''{{w|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 {{w|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 {{w|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 {{w|Artemis program}} profile resembles this particular profile.)

'''{{w|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 {{w|Destination Moon (comics)|Destination Moon}} from {{w|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"[https://www.youtube.com/watch?v=LHvR1fRTW8g]. 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 {{w|low Earth orbit}} is within the {{w|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 [[2908: Moon Armor Index|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&nbsp;km-deep grave. Arguably the true 'winner' could be determined by which country gets hit by the initial Moon rocks first.

===Factual Mission Profiles===
The Apollo Program considered a number of possible mission profiles. Of the {{w|Apollo program#Choosing a mission mode|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 {{w|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 [https://www.nasa.gov/missions/artemis/lunar-near-rectilinear-halo-orbit-gateway 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 [[2561: Moonfall|has previously been referenced]] in xkcd.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

:[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 space shuttle 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. The merged line heads straight towards the Moon to land on its left side, without orbit. A space shuttle 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 space shuttle 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?!

{{comic discussion}}

[[Category:Space]]