Considerations for a Space Opera Setting: Scale

19:36 Sun 22 May 2011
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By “scale” here I mean: how far does space that humans have explored extend? How far is it feasible for humans to travel within the civilization? How many systems make up that civilization?

“Space opera” implies a large setting, but in real terms “large” is fairly meaningless, and allows for rather a lot of variance.

“Space opera” implies many inhabited star systems, so the first major question is what number or proportion of galaxies this setting will involve.

Multiple galaxies is just too big for what I want. Galaxies are mind-bogglingly huge. The Milky Way isn’t a particularly large one, but it’s estimated to have 100–400 billion stars, with 50 billion planets and a potential 500 million planets in their systems’ habitable zones. Cut that number down two orders of magnitude, and discount terraforming technology and the like, and you’re still left with 5 million planets in habitable zones. Start adding in other galaxies—of which there are apparently at least 100 billion—and the numbers start getting even more ludicrous. While I want the setting to be “large”, I also want it to be such that it’s comprehensible as a whole, at least in theory. So, just one galaxy, and, indeed, just a small part of it.

The feel of the setting I want requires there to be more inhabited systems than most people could easily memorize. On the other hand, the figure shouldn’t be completely outrageous; the number in my head for the number of “civilized space” systems is 2000. I don’t have a good reason for this number and am going off of my still relatively vague sense of what the setting is like, but it seems like a number that’s large enough to make cohesion difficult but not too large for it to be possible at all, as well as providing plenty of room for tension and a for big differences between areas, while not getting to the point where events on a given individual planet are utterly insignificant. The scale will determine the numbers of FTL travel and communication also.

A “civilized sphere” of 2000 systems, plus a few hundred systems further out that are inhabited by at least some humans (in varying stages of viability). Using what we understand the real numbers to be makes the number of planetless systems too high; I’m going to arbitrarily decide that on average every fifth system has one or more planets, and thus that there are 10,000 in-sphere systems, 2000 of which have planets (and all the ones with planets are inhabited).

10,000 systems is a ten-millionth of the total systems in the Milky Way (using the low estimate of 100 billion systems); a setting spanning 10,000 of those systems is to the rest of the Milky Way what a setting of about 15 square kilometers or 5.7 square miles is to Earth’s land mass.

On the other hand, assuming that inhabited systems have, say, on average 5% of Earth’s approximate population of 7 billion, that’s 700 billion people across the 2000 systems, a number I’ll probably push higher on the assumption that technological advances increase the amount of habitable areas.

Using the Wikipedia numbers for volume and systems, a set of 10,000 star systems, if distributed according to the averages, would be a cube of 158 light years per side, and spacing within that area would be seven light years between planets, although there’d be a lot of variance.

I stated in my previous post that messages can cross the setting “in days or less”; distance from corner to corner is about 274 light years. I want the communication to be fast enough to allow for reasonable organizational logistics while also requiring a fair amount of independence. For those purposes, I’ll set the fastest FLT communication speed at about 60 light years per hour. Dialogue between neighboring systems would have a round-trip time of about 90 seconds. Orders from the center to the corners would have a round-trip time of about 4.5 hours, and that seems about right.

As for FTL travel, it’s far slower indeed, and I’m putting it at 2 light years per hour for the fastest ships, or 12 light years/day.

A fast fleet would take over 11 days to get from the center to any corner; a fast starship would take about 2.25 hours to get from our system to Alpha Centauri. (To illustrate how ridiculous this is, consider this excerpt from Alpha Centauri’s Wikipedia page: “Crossing the huge distance between the Sun and Alpha Centauri using current spacecraft technologies would take several millennia, though the possibility of space sail, or Nuclear Pulse Fusion technology may cut this down to a matter of decades.”)


  • 10,000 systems, 2000 of which are inhabited.
  • Between 500 billion and 1.5 trillion humans.
  • A cubic volume of space 158 light years per side.
  • FTL communication at 60 light years per hour.
  • FTL travel at 2 light years per hour.

Those seem like good parameters to work with.

2 Responses to “Considerations for a Space Opera Setting: Scale”

  1. Unkie Dave Says:

    I know that FTL travel is a staple of Space Opera, either in defiance of traditional physics or through sidestepping the issue by using a macguffin like travel via an alternate dimension (David Brin’s Uplift series or China Mielville’s latest Embassytown), but I’ve always been interested in settings that attempt to deal with the relativistic issues of those that travel at or near FTl speeds vs those that stay behind (like Joe Halderman’s Forever War or Ken MacLeod’s Engines of Light series), or try to depict Space Opera that operates with slower than FTL travel.

    If you are interested in finding out a bit more about slower than FTL travel and the number crunching behind intersteller travel times I can recommend “Interstellar Travel and Multigenerational Space Ships” edited by Yoji Kondo ( http://amzn.to/k2PTla ), its a good companion to O’Neill’s ‘The High Frontier’.

  2. Tadhg Says:

    I’m a fan of the Forever War series, but never got into Ken MacLeod somehow. The sublight setting is of course more realistic, given that we have no idea how “warp space” would work, but it’s for whatever reason not the setting I have in mind for this particular project, partly because it’s very difficult to handle far-flung events with connected characters if those characters are going out of synch with each other by decades every time they travel between systems.

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