It's been fifty years since we've heard astronauts and ground folks talk over each other ("step on each other" in 1970's CB radio parlance). Sitting on the floor a few feet away from our 1970's X-ray producing family CRT (television) I always worried that they might not hear each other because they both talked at once, and something terrible might happen.

It's possible that since the audio we heard was in the Earth time frame, maybe it sounded different at the Moon. Those NASA folks probably had it under control somehow.

@CamilleGoudeseune's comment under Time-scale for investigation of the "hut" object observed from the Yutu-2 lunar rover has got me wondering if it will be any different in 5-10 years hence:

Today another British news outlet offered: If it is a hut … it could become hot property – if only because the 2.6-second round trip that light takes to travel between Earth and Moon probably makes it impossible to participate in Zoom meetings.

So I'd like to ask:

Question: When cis-lunar Artemis astronauts and folks on Earth communicate via audio (e.g. EVAs) will they still step on each other's words like we heard during Apollo?

I'm interested in both actual stepping-on of words; one person doesn't hear the other, and apparent stepping-on, what we hear at home. Maybe they will do real-time realignment of ground vs space audio channels?

I guess for video it's also possible (I've managed to live zoom-free so far so I don't know what it's like) but without visual cues (body language, mouths moving) I think the problem is worse for audio-only.

  • 10
    $\begingroup$ Nothing can be done about the delay. But a full duplex system should make conversation manageable by eliminating being stepped on. If undisciplined operators of semi-duplex systems can make themselves understood (I'm thinking half sloshed boaters), I think highly trained astronauts will manage fine with a full duplex system. But that's just an opinion. $\endgroup$
    – Woody
    Commented Dec 9, 2021 at 0:18
  • 1
    $\begingroup$ Adapt an elegant protocol from eras of high SNR rather than high latency? Wait for "over" and "over and out." $\endgroup$ Commented Dec 9, 2021 at 2:49
  • 5
    $\begingroup$ Keep in mind that nearly all the astronauts were also test pilots, with hundreds or thousands of flight hours logged. They were all very, very familiar with using radio and dealing with the occasional stepping on each other. $\endgroup$
    – Innovine
    Commented Dec 9, 2021 at 7:10
  • 1
    $\begingroup$ Absolutely anyone that uses Whatsapp phonecalls are used to audio delays that range from 200ms to 1500ms. This is virtually a complete non-event for professionals trained and expecting the delays. $\endgroup$ Commented Dec 9, 2021 at 7:58
  • $\begingroup$ @Innovine - I loved Željko Ivanek in Black Hawk Down; the guy on the radio in the little chopper doing recon, telling them that they're screwed, in an absolute deadpan monotone. $\endgroup$
    – Mazura
    Commented Dec 9, 2021 at 8:09

2 Answers 2


Likely. Lightspeed lag will always be with us. Indeed it did sound different on the Moon--to the astronauts, they were being "stepped on", rather than the other way around.

Ever been on a VOIP or satellite phone call with weirdly high latency? Same thing. The solution: strict radio etiquette (calling "over") or just get used to saying "sorry, you go". Oddly enough for scifi shows, The Expanse depicts this quite accurately whenever folks on the Moon are talking to Earth.

It'll always come with the territory of distant crewed missions--mission control & the astronauts will just have to get used to it or develop protocol to avoid it.

  • 4
    $\begingroup$ Ansibles still don't exist, +1. And "it is now well understood that quantum entanglement does not allow any influence or information to propagate superluminally." $\endgroup$
    – Mazura
    Commented Dec 9, 2021 at 8:01
  • 3
    $\begingroup$ A consequence of the time-lag that embarrassed Mike Collins came just after the lunar landing, when Houston told them, "Be advised there're lots of smiling faces in this room and all over the world." Armstrong responded, "Well, there are two of them up here." and Collins added, "And don't forget one in the command module." However, by the time his words reached Earth, Houston had already said, "That was a beautiful job, you guys." which made it sound as if Collins was asking to be complimented on a beautiful job too, rather than adding his smiling face to the list! $\endgroup$
    – GordonD
    Commented Dec 9, 2021 at 17:04

The engineering solution is ancient: carrier sense multiple access with collision detection.

Edit: as commenters have explained, CSMA/CD is much better suited to the timings of Ethernet packets than to those of human speech, where it would impose even greater latency. Full duplex is better, and is in fact the plot twist at the end of a short SF story about the radio link to a distant spaceship, given to the rocket scientists by a kid and her mom (IIRC) who habitually talk over each other without any problems.

  • 7
    $\begingroup$ This is pretty much a "link only" answer. Could you summarize what the antient solution is? $\endgroup$
    – James K
    Commented Dec 9, 2021 at 6:45
  • $\begingroup$ @JamesK Which part of the link name "carrier sense multiple access with collision detection" is it that you don't understand? ;-) I know it has fallen a bit out of fashion now that every device sports their own patch cable so that the "Ether" in "Ethernet" has become a misnomer ... but still the idea holds for true ethers. $\endgroup$ Commented Dec 9, 2021 at 10:17
  • 2
    $\begingroup$ Added Wikipedia page title to the answer. $\endgroup$
    – MSalters
    Commented Dec 9, 2021 at 13:42
  • 1
    $\begingroup$ That's an engineering solution for a half-duplex medium and costs extra latency. The other option is actual full duplex, perhaps as simple as using different carrier frequencies for the two directions. $\endgroup$ Commented Dec 9, 2021 at 14:59
  • 2
    $\begingroup$ Doesn't CSMA/CD impose limits on link lengths due to the latency in detecting collisions? Can it work reasonably with a 2.6 second RTT? $\endgroup$
    – Barmar
    Commented Dec 9, 2021 at 15:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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