19
$\begingroup$

The Apollo missions were in communications blackout when the spacecraft's orbit took it to the far side of the Moon. During the missions, the craft had to perform maneuvers such as the lunar orbit insertion burn on the far side. Without ground support, and without any way of even telling the ground what went wrong, if something did. That seems like an unnecessary risk to me!

I see no reason why NASA couldn't have used relay satellites to allow communications on the far side. Multiple lunar orbiters predated the manned missions, and relay satellites had been operational in Earth orbit since 1962. The cost of such relays would surely have been quite small compared to the whole project.

Was this considered at some point? If so, why wasn't it done?

$\endgroup$
  • 3
    $\begingroup$ Apollos 15,16, and 17 actually released subsatellites just before leaving lunar orbit. However, these were scientific particles and fields instruments, not communications relays. [en.wikipedia.org/wiki/… $\endgroup$ – DMPalmer Jul 22 at 4:51
  • 3
    $\begingroup$ Your question make me think of what I've done in KSP: building a ring of communication satellite around any object before landing anything on it. $\endgroup$ – Manu H Jul 22 at 14:11
  • 1
    $\begingroup$ @ManuH - even with the RT mod, the only reason you'd need to do that is for drone control. All Apollo flights were maned missions.... It's not that they couldn't; why would they? The scary part is the ignition of a quarter million gallons of fuel, not the overly dramatized Loss Of Signal. - "Multiple lunar orbiters predated the manned missions" not just orbiters; USSR: first to return a lunar sample. CCCP-1, USA-0 $\endgroup$ – Mazura Jul 22 at 22:50
32
$\begingroup$

It's not mentioned in Apollo documentation. However, these would be the arguments against it:

  1. The idea of two spacecraft was already controversial. Now you want three? The original Apollo concept was a "direct" mission. The entire mission would launch as one vehicle, shedding pieces as you go, but never putting pieces back together. All of the astronauts and their Earth return vehicle would land together on the moon and take off again.

    Apollo ended up using "lunar orbit rendezvous" instead. One vehicle would launch toward the moon. However, once in lunar orbit, a minimal lander would separate from the main vehicle, land on the moon, and return back to lunar orbit to dock with the main vehicle.

    All of the major players at NASA (e.g. Von Braun, Faget, Gilruth) were initially opposed to the idea of LOR. Rendezvous had never been done before. It needed new mathematical models, automated flight computers, miniaturized radar, docking mechanisms, more stations in mission control, crew and mission control training, and so on. It was only when the size and price of a direct mission was determined to be too costly that the LOR approach was taken seriously. And it had to be proven reliable in the Gemini program.

    Now, you want three (or even more) spacecraft? The battle for just two was bad enough, no one wanted to pick that fight again.

  2. Satellites were unreliable. The technology was new, and their lifespan was short. When a satellite died, there was no way to fix it, so now you must deal without it. The long-term effects of radiation, thermal degradation, and micrometeoroids were not known until the Pegasus satellites were launched on the Saturn I test missions AS-103 through AS-105.

  3. The technology was unproven. Communication had only been done between ground and spacecraft. Now you want both ground-to-spacecraft and spacecraft-to-spacecraft? The latter hadn't even been done yet!

  4. We were in a race. It's not that the above issues were impossible, but they would take time to get them to work. (For example, we can do it today.) We were in a race with the Soviets, and in a race with the end of the decade. There wasn't time to develop a solution.

  5. You ought to plan for that contingency anyway. One of the most serious problems with spaceflight is losing communication with mission control. But it is also something you can plan for. You train your astronauts how to diagnose the problem, how to solve the problem, and how to stay safe in the meantime. You are going to plan for that contingency anyway, so why not embrace it?

    Also, going around the far side of the moon is a predictable, regular event. You can make your plans around this schedule. And the LM lands on the near side of the moon, so you don't have to worry about that.

$\endgroup$
  • 13
    $\begingroup$ I disagree with #1 because the main problem was the rendezvous. For a communications relay, no rendezvous is required, you just dump it and never retrieve it (or send it with a different mission beforehand) $\endgroup$ – vsz Jul 22 at 11:28
  • 5
    $\begingroup$ The controversy you mention in point 1 has nothing to do with the possibility of sending a satellite to the Moon, either on the same mission or on a separate mission. Point 5 seems silly, since they also needed a contingency for everything else that might go wrong. They need a contingency for the LEM not separating, so why have it separate at all? They need a contingency for the service module oxygen tank exploding and not being available, so why not just leave it on the ground? $\endgroup$ – David Richerby Jul 22 at 12:00
  • 2
    $\begingroup$ Point #1 is not intended to be a technical issue, it is a political issue. In an ideal world, space programs would be insulated from politics, but in practice they are affected by it. $\endgroup$ – DrSheldon Jul 22 at 14:41
  • 1
    $\begingroup$ @DrSheldon. To run with your point, there are politics in any interaction involving more than one person. We generally assume that "politics" means management making one decision while engineers would prefer something else. This is of course a constant conflict, but with good management and engineers that are aware of the real financial and time constraints, most politics that I've seen comes from equally smart and qualified people knowing (and fighting for) what's best for the project. $\endgroup$ – Mad Physicist Jul 22 at 16:26
  • 2
    $\begingroup$ I think it's also important that there were astronauts on board. While ground control certainly had a huge amount of problem-solving brainpower at it's disposal, the Apollo capsule still had three highly trained specialists onboard which could preform problem-solving without constant communication. Having a "human-in-the-loop" was a much simpler and probably preferred solution when the alternatives are complex and untested communication equipment and computers. $\endgroup$ – Dragongeek Jul 22 at 20:14
14
$\begingroup$

A communications satellite would have to be in a halo orbit around the Earth-Moon L2 point to have continuous, simultaneous visibility of both the Moon's far side and Earth. The first paper describing this kind of orbit was written in 1968, too late to be used for Apollo. It also described the options for communications satellites in medium lunar orbit.

This was followed up by

SUMMARY of the 1968 paper:

This report investigates the feasibility of communicating with Apollo spacecraft behind the moon and of communications between two such spacecraft. Two satellite geometries are considered for signal relay: a libration or "Hummingbird" satellite anchored 65,000 km behind the moon and a lunar-orbiting relay satellite at 1000-km altitude. The signal transmission frequencies of the Apollo spacecraft are VHF and unified S-band (USB) operating at 300 MHz and 2 GHz, respectively. The three propagation links examined in this report include: lunar far side, i.e., Lunar Module (LM) on lunar surface and/or lunar-orbiting command and service module (CSM), to earth; lunar far side LM to CSM; lunar far side surface to surface.

It is shown that as a result of antenna pointing constraints, only the libration or "Hummingbird" satellite is suitable for relaying USB tracking and communication data to earth. It is also shown that acquisition of a 1000-km altitude lunar satellite by an Apollo spacecraft dictates the use of VHF. Various modes-feasible and unfeasible-of lunar far- side radio relays are listed below, with their respective methods of communication.

(emphasis mine)

Lunar Far-Side Communicatio Satellites; NASA TN D-4509, Figure 3

Lunar Far-Side Communicatio Satellites; NASA TN D-4509, Figure 5

So constellations in low orbit were looked at and discarded. Likewise an L2 satellite was not proceeded with.

This tells me that a risk/benefit analysis was made, and the benefit did not justify the effort in designing and building a communications satellite and launching it into an orbit that had never been used before.

A quick search of the NTRS doesn't turn up more information on this though.

$\endgroup$
  • 8
    $\begingroup$ It wouldn't have to be a single relay at L2; you could have a constellation of 3 or 4 relays in lunar equatorial orbit. $\endgroup$ – Russell Borogove Jul 21 at 17:33
  • $\begingroup$ I've added some information from the link for you temporarily, I hope you don't mind; once you can access the PDF feel free to improve. $\endgroup$ – uhoh Jul 21 at 22:44
  • $\begingroup$ cool (from 2017): Preliminary Mission Design and Analysis of a Lunar Far-side Positioning CubeSat Mission $\endgroup$ – uhoh Jul 21 at 23:23
  • 1
    $\begingroup$ @uhoh thanks for adding the links. $\endgroup$ – Hobbes Jul 22 at 10:12
  • 1
    $\begingroup$ @RussellBorogove the paper also looked at those. $\endgroup$ – Hobbes Jul 22 at 10:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.