I'm assuming there has to be a limit set by NASA of how far astronauts can be from each other for their safety. If something happens to an astronaut's EVA suit, the second astronaut can use the "buddy system" to share their oxygen supply, hence the need for a limit.

The OPS [Oxygen Purge System] provided a maximum of about 30 minutes of emergency oxygen for breathing and cooling. This could be extended to 75 to 90 minutes with a "buddy system" hose that used the other astronaut's functional PLSS for cooling (only).

Question: Was there even a limit of the distance the astronauts are allowed to be from each other during a lunar EVA? If so, what was the limit?

  • $\begingroup$ Wait. If I use my buddies system to extend my life support to 90 minutes, what happens to my buddy's life support? $\endgroup$
    – Octopus
    Aug 22, 2019 at 22:02
  • 1
    $\begingroup$ @Octopus It gets "split" 50-50. $\endgroup$
    – Star Man
    Aug 22, 2019 at 23:26
  • $\begingroup$ So If we share my supply rises to a potential 45 mins from 30. That's good karma. :) $\endgroup$
    – Octopus
    Aug 23, 2019 at 17:52
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    $\begingroup$ @octopus you confused me thoroughly with your username. $\endgroup$ Aug 23, 2019 at 19:25

1 Answer 1


Line of sight distance, which was at most 2.5 miles.

According to the Apollo Program Summary Report:

4.9.2 Extravehicular Communications Unit


The small power output of the transmitters in the extravehicular communications units limited lunar exploration travel to line-of-sight distances (less than 2.5 miles from the lunar module).

You would also want to be within the line-of-sight of your partner so you can keep track of where he is, and so you can directly observe (without the need for radio) if he is in trouble. As you noted in the question, there was a Buddy Secondary Life Support System, which required the astronauts to be within walking distance of each other.

The actual distance could be much less than 2.5 miles, depending on the terrain.

  • $\begingroup$ If you weren't line of sight, would you even have radio communication? Maybe by relay through the CM or Houston. $\endgroup$ Aug 22, 2019 at 21:01
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    $\begingroup$ @FredLarson: No. The commander had line-of-sight links to the LMP and to the LCRU (either on the rover or the LM, with the big dish that went back to Houston). He had no link to the CM. The LMP's voice and telemetry were forwarded through the commander's communications unit; he had no direct link to the rover, the LM, the CM, or Houston. If they are out-of-sight of each other, no one would hear the LMP at all! $\endgroup$
    – DrSheldon
    Aug 22, 2019 at 21:53

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