Is Inter-Stellar Trade Even Possible?
Many science-fiction books and movies have as a central plot convention the existence of very large, multi-star-system confederations that are linked both politically and economically. Unfortunately, unless some method of by-passing what is currently believed to be the limit to the speed at which interstellar travel can occur can be found, such systems are impossible. And that limit is the speed of light, about 186,000 miles per second.
The closest solar system that apparently might have habitable planets is the Alpha Centauri system. Which is approximately 4.25 light-years away. What about this makes, in particular, economic links between stellar systems impossible?
The terrestrial calendar year has 31,536,000 seconds. The speed of light is about 186,000 miles per second. So the distance light travels in one year (one light year, or 1 LY) is 5.8657x1012 miles. And suppose we want to ship some physical object from Earth to a (hypothetical) inhabited planet located 1 LY away. How much will that cost, and what are the implications of such a cost?
I own a car (1997 Honda Accord) which weighs approximately 3,000 pounds, and can carry about 1,000 pounds of load (passengers plus baggage), for a total of 2 tons. My cost per mile of operating the car, excluding any capital costs or depreciation, and ignoring the value of the time of the driver, is approximately $0.10 per mile, or approximately $0.05 per ton-mile. But since we’re talking about moving cargo, let’s suppose we manage to develop extremely cost efficient interstellar transports that travel at very nearly the speed of light. What do I mean by “extremely efficient,” and what would that entail? By “extremely efficient,” I mean a cost of moving 1 ton 1 mile of one one millionth of a cent [($0.01)/(1,000,000)].
What could we move? The space shuttle had a gross vehicle weight of about 2,000 tons, and could carry a payload of about 24 tons. So to move the shuttle 1 mile, fully loaded, at a cost of $0.00000001, would cost $0.000002024. Now we have to move that 1 LY…which would cost approximately $119 million. And that’s a cost of $5 million per ton of cargo, to travel 1 LY. Alpha Centauri is 4.23 times as far, so to get something to or from there would be nearly $21 million per ton of cargo…around $500 million for the flight.
That does not include any costs attributable to staffing the transports. But that is almost irrelevant. Consider a cargo that is valuable enough that we are willing to pay $21 million (or more, for longer distances—the next closest is Tau Ceti, 11.9 LY away—and wait at least 4+ years to get it.
I would argue, incidentally, that the cost per ton mile is unlikely to be as low as I have suggested here. The cost of propelling a large spacecraft at a high rate of speed is likely to be considerably greater than $0.00000001 per ton-mile.
If all this is even anything close to accurate, then the possibility of interstellar trade is remote. Overcoming that sort of cost issue requires that the cost per ton mile be reduced to an almost unimaginably low level. And free energy is not yet on the horizon.
 “List of nearest terrestrial exoplanet candidates,” Wikipedia, August 2015, https://en.wikipedia.org/wiki/List_of_nearest_terrestrial_exoplanet_candidates
 Charles Stross makes the case for the great expense and difficulty of interstellar exploration/colonization at http://www.antipope.org/charlie/blog-static/2007/06/the_high_frontier_redux.html
 One source projects a cost per pound from launch to earth orbit of $10,000 for the ship’s payload…about $20 million per ton just to get it into orbit, let alone to Alpha Centauri. https://launiusr.wordpress.com/2015/03/06/the-space-shuttle-and-the-costly-nature-of-space-access/