In my world there exists a planet called Solara Prime. The planet orbits a large yellow star, healthy and capable of sustaining the system. There is one planet and 4 moons whose orbits are roughly perpendicular to the star.

Begin Handwaving

The smallest moon is roughly 6000 miles in diameter and has a similarly sized counterpart on the far side of the system, both roughly 750,000 miles from the planet. The closest moons are roughly 12,000 miles in diameter and 250,000 miles from the planet. The planet itself is roughly 24,000 miles in diameter and 95 million miles from the sun. The planet and its moons rotation, axis, and orbits allow atmosphere and for human habitation on all 5 planets.
enter image description here All of the moons are habited, organically; that is they didn't colonize from the planet. Each of the moons and the planet have thriving societies of one type or other. Additionally, they have all figured out there are people on the other moons and planet.

End Handwaving

Here's the question: Knowing there are people in relatively close proximity on other worlds, how would space travel have developed relative to what has happended here on earth assuming current level of technology and the fact there are 3× as many people? That's very broad so here are some smaller questions to support it?

  • How long would it take to create a craft that could safely traverse planet to moon, moon to moon, ?
  • How long would it take to create a reusable craft capable of traversing the entire system?
  • Which force would be the greatest driver - trade, war, unification?

I have my own ideas about these questions, but am struggling to adequately describe the priority and time frame involved in the history of the worlds leading up to where they are now — regular, though expensive, space travel for leisure, business, trade, etc.

  • $\begingroup$ How much of a concentrated effort they put into it could be anything. Look at the Mercury/Apollo program. Or it could languish for centuries with no progress due to social issues and problems with agriculture etc. $\endgroup$
    – JDługosz
    Commented Aug 24, 2016 at 22:58
  • $\begingroup$ How long it takes depends not just on the number of people and tech level but their motivations, both extrinsic and intrinsic, and their resources. Are they wealthy, well-fed, curious scientists, people just getting by in low-end jobs, people more interested in developing "home" than exploring (or vice-versa)? I know you asked about driving forces; I think to keep this from being too broad you're going to need to specify those conditions instead. (Feel free to ask about several scenarios in different questions.) $\endgroup$ Commented Aug 25, 2016 at 1:08
  • $\begingroup$ They could start communicating first if two moons develop radio or one has a lighthouse with focusing and the other a telescope? $\endgroup$ Commented Aug 25, 2016 at 16:08
  • $\begingroup$ I would think radio technology would be enough initially. The interesting problem to overcome initially would be language. $\endgroup$ Commented Aug 25, 2016 at 16:12

5 Answers 5



That's what powers us and has powered us since the first primate stood upright and wondered just what was over the horizon. Trade, friendly or unfriendly rivalry, coveting resources available elsewhere, personal ambition, religious push, co-operation for mutual support, scientific exploration, plain tourist sight-seeing - everything else that motivates space travel comes later.

The closest moons are about as close as the moon to us and very large. They would be very visible to the planet and vice-versa, especially through telescopes. The far moons are about thrice the distance and less visible, yet close enough for observation. The changes they can see on one another's planetary/lunar surfaces, even without telescopes or visual aids of any sort, are enough to seriously grab public attention - recollect the Mars canal fever and Mars is much further away. Large scale constructions/deforestations/plantations, events and other signs of sentience would strike up a very lively curiosity to figure out what was going on. Unlike us who are not sure who's out there and where, these people would definitely know for sure there were other people right there across the 'gap', if only they could cross over. (I can close my eyes and see a crowd of village people gaping up at a festival fireworks display going on across that larger moon - they've started out from the same base people, do they have similar deep-rooted folk beliefs? Or do they think there are wish-granting moon fairies living up there?)

A lot would depend on how the 5 civilizations start out and what technology they have at their disposal. Even if they start out at agricultural level, it would not be long before they could figure out each other's existence. I would imagine that, first, people would try to communicate with each other. Smoke signals, giant written messages, something like morse code, not necessarily intelligible at first but steadily improving with time. Then people might send out probes to cross the distance. Even primitive rockets, fireworks-style, again not necessarily successful at first but as a stepping stone to the future.

The greater chances are that the attempts from the closer moons will manage to cross over first to the planet. Once the basics are worked out, namely propulsion, trajectories and escape velocity, people could build very rapidly on it (Interplanetary parcel service?). Given that it took us about 350 years from the first telescope to the moon landings, the timeline could be as short as that, if not shorter, for these people to start travelling to each other's places. If there are rare things that are only found or made on one of the places, as it usually is, it would be added incentive to get there and back (Moon-route merchant spaceships instead of silk-route merchant ships?)

Once they have the initial system in place, it would be a very short leap for them to have sophisticated spacecraft that could range further and do more. Just the research they put into this will be enough to push all 5 civilizations towards discovering and understanding more complex things like radio communication or nuclear physics.

(I'm not going to question the handwaving part but I'm assuming the planetary system has some amount of interference to make it the way it is, especially the two sets of two moons being at the same distance opposite to each other)


Interplanetary spaceflight of the kind you are talking about is more or less possible with today's technologies. Lets put the Apollo space program in perspective. As of 1942 no craft (as opposed to projectile) on earth had broken the sound barrier. That is the year the V-2 rocket did. A human manned craft broke the sound barrier in 1948. In 1946, a V-2 was used to get a camera to the edge of space to take pictures and send back telemetry. In 1957, the first man-made satellite went into orbit, and 1961 the fist human went into space. Luna-2 was the first impact on the moon in 1959, the first soft landing in 1966 as well as the first object in lunar orbit. Then the 1969, Neil Armstrong took that great leap for mankind.

Think about that technological development curve. 27 years from no craft capable of breaking the sound barrier to a man on the moon. Hell, it was only THREE YEARS from the first successful soft landing on the moon to trying it again with people....successfully!!

Want I wanted to show is the incredible rate of technological progress when there are money and resources put into a project. There was no money wasted on contracts in Senator Porky's district, there were no approvals by committee, there were no safety regulations, there were just people who thought that these engineering problems had to be solved.

If we discovered that there was life on the moon, we could get there from 1940s technology in under 30 years. A supersonic flight in 1948 was turned into a working airliner within 20 years (Tu-144). Assuming that level of engineering determination, I would say that routinely visting other moons would take not more than 50 years.

  • $\begingroup$ This is along my thought process. We don't really have a "good" driver anymore, propelling the technology. How much farther could we have gone had we kept up the momentum? How much faster if we new there were people on Mars? $\endgroup$ Commented Aug 26, 2016 at 18:56
  • $\begingroup$ @SteveMangiameli - we certainly do have a good driver propelling the technology. The SpaceX company has produced several breakthroughs in the last couple of years, and their stated goal is to get a colony on Mars in as short a time as possible, and the way they're going I would say that they have a realistic chance of succeeding. $\endgroup$
    – Spudley
    Commented Aug 26, 2016 at 19:05
  • $\begingroup$ How long has SpaceX been working toward space flight in general and what accomplishments have they attained on the level of putting a man on the moon? I'm not downplaying the importance of what they are doing, but they are privately funded and don't have the resources of the planet behind them as it was back in the golden age of space exploration. So when I say a good driver - we used to have a Mack Truck with rocket engines. Now we're rocking a '69 VW bug on biodiesel. $\endgroup$ Commented Aug 26, 2016 at 19:30
  • $\begingroup$ Just to fact check, the first human flight to break the sound barrier (in level flight) was on 1947 Oct 14. People did break the sound barrier before that, by diving in their airplanes which happened a lot during WW2. That's why it's important to say level flight. $\endgroup$
    – DrZ214
    Commented Aug 27, 2016 at 6:41
  • $\begingroup$ Turns out there was a lot of pork in the Apollo program. It was just so over-funded that it didn't end up mattering much. Not a huge deal, just something I thought I'd mention. $\endgroup$ Commented Mar 16, 2019 at 0:24

There are a couple of factors to consider. Like other answers have discussed, imagine how fast we managed to get to the moon once we really poured resources into the endeavor. Once it had been discovered with certainty that other sapients were present on the moons and planet, I imagine reaching those people would have become something like a cultural icon.

Just think how unimaginably significant it would be for us to meet just one other sapient species—if we knew that there were four other races out there within easy race, along with an entire moon or even planet of alien life to study? The drive, the fervor, to get to those worlds would be immense. I cannot imagine that it would take very long for interplanetary travel to become a reality. Certainly it would happen much faster than it has happened on Earth.

But how long until a craft was developed that could traverse the whole solar system? That's an interesting question. On the one hand, the races would be quite close to such a technology, but the impetus to make such a craft would likely be far less than it was to reach the planet and moons—unless you got some interracial competition.

And now we come to the most interesting question: what would be the strongest driving force? If we were talking about humans, the answer would almost definitely be trade, or at least monetary value of exploration. The only way most groups of humans could be convinced to go through the enormous effort of frequent interplanetary travel is if they could get some serious benefits out of it. Scientists would do it just for the science, but they don't have the kind of money needed for a program like this.

Again, if we were talking about humans, coming very close after trade or monetary incentives would come a burning, passionate curiosity. We would be dying to know everything we could: how do those other sapients see the world? What's their culture like? What's happened in their history? How has life evolved over there? What kinds of technologies have the developed? Have they developed tech in answer to problems similar to ours?

But if we're talking about races other than humans, or not very similar to humans? Well, in that case, it would entirely depend on the disposition of the race. An aggressive one, for example would probably seek to dominate the other races for resources.

There is also another point to consider: what if the races of this planet and moons are like us in that they are not unified? What if they have independent nations like we do? That would probably change the answer to every one of these questions. Not only would different races respond differently to these questions, but within those races different nations would respond differently, too.

If the nations were competing with each other, that would make monetary incentives all the stronger, and curiosity would probably now be closely followed with a patriotic drive—look at how great our country is, we got to the other races before that other country!


It would take a while to produce a fully safe spacecraft. Once the planet and moons know about each other they would probably try and co-operate on reaching each other with viable space crafts appearing shortly after planes. Ships are likely to travel moon to planet first of all as this requires the least fuel. I would suggest between 50 and 200 years after heavier than air flight. Probably towards the 200 year end as the planet/moons probably won't have 2 World Wars and a Cold War speeding things up.

Reusable crafts between moons and planet shouldn't take to long given that they can refuel on arrival but cross Solar System crafts would take far longer. Probes and stuff would be created early but crafts carrying life across the Solar System would take a long time to discover. I'm thinking at least 1000 years after intermoon travel.

Main driving force originally exploration, desire to make contact and possibly slight rivalry with the other moons to see who can get there first. Later main factors would be trade followed by exotic holiday market opening up. These driving forces will only have a small impact on Solar System wide exploration though.

System wide exploration will be sped up as ships can launch from low gravity moons rather than having to fight planetary gravity reducing fuel consumption but main driving factor for system wide exploration will be scientific research.

  • $\begingroup$ I assume the two smallest moons will have spaceflight first, since they are already better suited to handle lower air pressures (because of the smaller moon and resulting lower gravity), they have a much shallower gravity well (facilitating launch), but they can share knowledge with at least two more planet)oid)s (the other two probably being hidden from view by the planet). Interestingly, as a side node, the smaller moons have a much shorter route to space elevators (again the gravity well). $\endgroup$
    – Burki
    Commented Aug 26, 2016 at 15:21

If the Solara Prime system has five worlds that are inhabited. This will create an interesting challenge for the beings on any one world arising from the presence of beings on the other four worlds. Although you suggest that the inhabitants of all five worlds are human, this does seem biologically and evolutionary improbable. George Gaylord Simpson has argued this in The Nonprevalence of Humanoids, Science 21 Feb 1964: Vol. 143, Issue 3608, pp. 769-775. This can be found here.

A system with five different species of sapients would be a major cultural driver. This would cover everything from religion, philosophy, exploration, military conflict, science, and politics. The possibility of interacting with multiple sapients and their worlds would be a massive impetus driving the development of all five worlds. This would influence almost every aspect of their lives and cultures to a degree that can be barely imagined by the inhabitants of a planet with a single dead moon.

Technologically speaking, this system of five worlds are all within easy reach of each other using chemical rocket propulsion. Earth's humans long dreamed on travel to their Moon. Solara Prime's sapients will be able to dream to four other worlds on any one of the five worlds. When solar electric ion propulsion systems are developed spaceships will be able to travel more easily and efficiently through and around this five world system.

  • $\begingroup$ Organically indigenous is probably not the right term here. I'm still developing that bit as well, but the inhabitants were placed on the moons/planets long ago by colonists, wanting the humans to "start over". Otherwise I'm with you on the rest - the civilizations would develop independantly of each other and be quite unique. $\endgroup$ Commented Aug 25, 2016 at 13:54
  • $\begingroup$ @SteveMangiameli Five inhabited planets within easy travel of each other, in comparative terms with respect to interplanetary travel, would all have major incentives to visit each other. Afterwards I realised signalling would play a big role in contact. When early radio was developed there were attempts to contact, with a penta-planetary set-up such attempts would be an imperative. Communication before contact & travel. The diversity of five human cultural domains adds to its interest. $\endgroup$
    – a4android
    Commented Aug 26, 2016 at 5:17
  • $\begingroup$ @SteveMangiameli Nice that you have explained why there are humans on the five worlds. That puts at rest any concerns about the independent evolution of sapients on the five worlds. Actually you've devised a really interesting scenario that is rich with possibilities. Go with it and have fun. $\endgroup$
    – a4android
    Commented Aug 26, 2016 at 5:21

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