When Neil Armstrong and Buzz Aldrin descended to the moon in the Lunar Module, Michael Collins stayed up in lunar orbit in the Command Module.

Why did they need someone to stay behind?

At this point, rendezvous and docking with an unmanned spacecraft in orbit had already been done. Was there something else they needed someone up there for?

  • $\begingroup$ Relay of messages back to mission control, only two guys could fit in the LM, etc. $\endgroup$
    – user12
    Commented Aug 16, 2013 at 17:29
  • $\begingroup$ What do you mean by "relay of messages"? Did Collins have to do something so mission control could communicate with the guys on the surface? $\endgroup$
    – Joe
    Commented Aug 16, 2013 at 17:31
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    $\begingroup$ Well, you would want someone to able to kick the computer if it started not working. $\endgroup$
    – user12
    Commented Aug 16, 2013 at 17:32
  • $\begingroup$ As a reserve? If either Armstrong or Aldrin fell ill etc, Collins could take their place. $\endgroup$
    – Gnubie
    Commented Aug 16, 2013 at 19:41
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    $\begingroup$ @Gnubie no, the CMP didn't have the appropriate training to substitute for CDR or LMP. $\endgroup$ Commented Sep 24, 2014 at 6:00

3 Answers 3


This is an outcome of several requirements and constraints:

  • The crew size was fixed at three astronauts very early (on 28-29 July 1960, in a large planning conference).

    [Space Task Group's] "Richard S. Johnston presented three demands: "shirt-sleeve" environment, three-man crew, and radiation protection. He discussed the need of the crews for a safe environment and for atmospheric control." (source)

  • The mission design switched to Lunar-Orbit Rendezvous (thanks to work by John Houbolt), and the Lunar Excursion Module could have the mass budget only for two astronauts.

    "By the end of 1961, the newly named Manned Spacecraft Center had virtually swung over to the lunar-orbit rendezvous idea. Gilruth, Faget, and the other Apollo planners conceded that this approach had drawbacks: a successful rendezvous with the mother craft after the bug left the lunar surface was an absolute necessity, and only two of the three crew members would be able to land on the moon." (source)

  • The docking argument is not really valid since CSM's Apollo Guidance Computer (AGC) could receive commands directly from Mission Control and hence, could maintain fixed attitude required for docking without any intervention from the lunar module crew. (I read it somewhere in the handbooks - see http://www.ibiblio.org/apollo/links.html, see also letters from Grumman to ASPO cited here)

  • However, the overall doctrine during the Apollo program hinged on using crew members as backup for computers. If computers or communications failed, command module pilot would be instrumental in mission success. (A sextant, a slide rule, pencil and paper were provided for manual navigation).

    "Shea and his staff reviewed these studies and presented the results to the rest of the manned space flight organization early in October [1962]. The contractors agreed that either two-man direct flight or earth-orbit rendezvous was feasible but both were less attractive than lunar rendezvous because the probability for mission success was lower, the first landing would be later, and the developmental complexity would be greater. The vote was still for three-man, lunar-orbit rendezvous." (source)

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    $\begingroup$ So, more or less, the answer is: they decided to send 3 people. Then designed the equipment. The equipment they designed only fit 2 people, but by then the decision had been made. Plus a little bit of "Hey since he won't fit in the LEM, he can at least be a backup for the PC"? Did I understand that right? Makes sense when you think about it. $\endgroup$
    – Shane
    Commented Sep 25, 2015 at 16:18
  • $\begingroup$ @Shane They probably could have designed the LM to fit three people, but it would have needed to be considerably larger (and consequently heavier) to do so. All the human consumables would scale linearly with the number of crewmembers, whereas electronics consumables (cooling water, batteries, etc.) wouldn't necessarily do so. Surface area and thus skin materials weight would scale, to a first order approximation, to second or third order exponentially with pressure vessel volume. They ended up pushing the limits of the Saturn V; any additional craft mass would likely have meant less payload. $\endgroup$
    – user
    Commented May 29, 2016 at 15:17

In addition to the above answers, the third crewman on the CSM performed visual checks of the LM after separation before descent, photography duties, regular purging of fuel cells, reconfiguring and relaying communications from the LM to mission control and platform alignments. Some of these may be possible from the ground.

I guess it was also a big comfort to have a guy on board just in case of some failure which would result in the loss of attitude or communication (and control) of the CSM. It's not just for computer backup. For example, a mechanical failure like some gas leak could push the CSM off attitude and through gimbal lock. That might be easily correctable by a guy on board, yet pose major problems to fix from the ground if communication was lost, and could prevent the LM from docking and kill everyone.

  • $\begingroup$ Just nitpicking, but "above" has no real meaning in an environment that orders answers by votes and not by time... Except, maybe, if you are aiming for giving the least voted-for answer... ;) $\endgroup$ Commented Feb 23, 2018 at 8:49
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    $\begingroup$ Maybe I recognise that the voting results are too correlated to how quickly someone posts an answer. $\endgroup$
    – Innovine
    Commented Feb 24, 2018 at 6:09

If the Lander had a problem, the CSM needed to be able to accommodate unusual docking circumstances. Sure, they had done unmanned docking before, but the computer on the CSM was not exactly great by today's standards.

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    $\begingroup$ So his role was just for handling docking? (Not to sound like I'm minimizing it -- docking with the CSM was critical.) $\endgroup$
    – Joe
    Commented Aug 16, 2013 at 17:32
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    $\begingroup$ @JonEricson The statement "crew members as backup for computers" in Deer Hunter's answer is an understatement. Even with commands sent by ground control, those space crafts were nowhere near of being fully controllable by a computer. Back then, docking was still rather new. Docking in lunar orbit was considered to be insane. Docking was not the exclusive reason. But ensuring successful docking manoeuvres was a key issue. Documents of the 1960s referred to astronauts as redundant components - a number of incidents in space requiring astronauts to solve them told us otherwise. $\endgroup$
    – s-m-e
    Commented Aug 17, 2013 at 23:40
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    $\begingroup$ @ernestopheles: The point of my comment wasn't to dispute the accuracy of the answer, but to point out that this answer needs more support (such as those documents you mentioned). Either answer seems possible, but in light of the other answer this post seems bareboned. $\endgroup$ Commented Aug 18, 2013 at 0:46
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    $\begingroup$ Maybe for human redundancy? OK, so Armstrong and Aldrin are designated as the LEM crew from 'way back in the crew selection process, but what if one of them develops a really bad cold while enroute to the Moon? Collins swaps roles with whoever got sick and lands on the Moon. I wonder if there was similar logic to putting two crew in the LEM - contingency against one becoming incapacitated while on the Moon (for whatever reason) to the point that he could no longer perform the tasks required for successful ascent, rendezvous, and docking. $\endgroup$
    – Anthony X
    Commented Jun 14, 2014 at 3:12
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    $\begingroup$ A backup plan; should the lm ascent fail and leave it in a low orbit, was to fly the cm to the lm for rendezvous. Since communications were blocked by the moon 50% of the time, having someone onboard the cm greatly improved its capability in this regard. $\endgroup$
    – Innovine
    Commented Nov 24, 2018 at 8:55

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