Friday, July 17, 2015

Hold the Line, Please! Distance and Communication in space

Pluto has been hot news recently especially among sci-fi fans and gamers. As it is one of the most distant objects in our solar system, the New Horizons spacecraft took nine long years to reach it. That's a long way away, and brings me to the point of this post, which is this:

Like it or not, to venture into outer space you have to deal with the staggering distances involved, both to communicate and to get from Here to There.

Most sci-fi films and TV shows have no sense of scale. Between limited run times and the show, not tell format of the screen, they have to cut down on the boring parts of travel and communication.

Literature does a better job, as it is telling instead of showing.  A solid, classic example of the real hard science problem of communication over great distances is found in H. Beam Piper's novel The Cosmic Computer
"It was late evening, Storisende time, but Rodney Maxwell, who must have been camping beside his own screen, came on at once, which is to say five and a half minutes later. "Well, I see you got in somewhere. Where are you, and how is everything?" Then he picked up a cigar out of an ashtray in front of him and lit it, waiting."

H. Beam Piper. The Cosmic Computer (Kindle Locations 1854-1857). 

Maxwell is 'screening' with his son Conn, who is on another planet a mere 80 million kilometers away.
Here's how the math breaks down:

 One light-second is 299,793 km. That's about the distance from the Earth to the Moon. So a radio call between the two will experience about a 1-second delay. Not too difficult to deal with. Further out though . . .

From Earth to Mars, says the average distance is 225 million km. So the time delay for communication is 750.5 seconds. That's 12 1/2 minutes. It just gets worse from there.

With travel distances in space charting upwards to 1 billion kilometers (TTB, page 54) communication lag can stretch to 3,335 seconds. That's nearly an hour from time sent to time received. Pluto is even further, between 4.2 and 7.5 billion km away. So any radio signal we may send to New Horizons will get there at best 238 minutes or 3.9 hours later.

At those distances, there's just no point in even sending out a distress signal. Even if you made radio contact, nobody's going to get there in less than days. The travel times chart shows that at 6G it takes 2.9 days to travel 1 billion km. If Pluto is currently at its nearest to Earth, that's still a travel time of 12.18 days to get there.

In space combat (TTB, pp 72-79) the combat turn is 1000 seconds, or about 16 minutes. Any 3rd party observer more than 300 million km away, the Earth to Mars run, will be one entire combat round behind what's actually happening. Laser fire will have virtually no chance of arriving on target, and missiles fall hopelessly short of getting there from 29,000 range bands away.

I watched the film Interstellar recently, and while I'll save the movie review for another time, the film did a great job with one thing at least. The characters in the film understood and were much daunted by the fact that by going out into space they would be gone for a long time, and they were far, far away from help.


The next time your PCs take an in-system trip, use the communications lag to remind them that they're far away from the safety of a planet. Or go watch the episode of Firefly "Out of Gas". Space is big; don't let your players forget it. 

Pluto data:


  1. I had a space chase scene in a game where the players were going to do a quick scoop run around a gas giant to refuel and then run for the jump limit. When I ran the numbers, it turned out that it would take the players' ship 7 hours to get to 100 diameters of the gas giant - not exactly a speedy get-away.

  2. That's exactly what I was talking about, David. It is necessarily unrealistic for movies/TV to show ships zipping around planets or through space in short amounts of time. In written or gaming sci-fi it is easier to acknowledge the passage of time and the actual distances involved.