If we talk to a person on mars it would take about 30 minutes before they would see the communication, but lets say we stayed in contact 24/7 by video calling starting from Earth, what would happen?

Would the video start to lag? Where would we first notice the difference?

  • $\begingroup$ The time between a question and the answer to it would increase. It will be noticed when the delay is greater than about a second. $\endgroup$
    – Uwe
    Jun 16, 2017 at 10:20

1 Answer 1


Initially you'll be able to have a fairly normal conversation with the astronauts. The light speed delay in their responses will be noticeable almost immediately though - even the moon is far enough away for a two second delay. Eventually it will increase to somewhere between 6 and 44 minutes round trip, which is clearly not a normal conversation.

You aren't likely to notice the increase as it happens though. For a 6 month trip that works out as a maximum of 8 seconds per day.

So what about the video itself? Ignoring relativistic effects that are on the order of microseconds per year, 24 hours of video transmitted from the spacecraft just arrived on Earth over 24 hours and 4 seconds.

In theory an analog signal might play imperceptibly slower, but digital will just assume your internet connection dropped out and will pause briefly until it gets sufficient data.

The return trip is a little trickier - with the delay decreasing, you receive video faster than it will naturally play, so you'll need to speed up playback unless you want to maintain the 20 minute delay.


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