He bases these calculations on 27 years of historic energy trends, societal priorities, required mission energy, and the implications of the Incessant Obsolescence Postulate (where newer probes pass prior probes).
Millis considers two possible missions: launching a minimal colony ship where destination is irrelevant, and sending a minimal probe to Alpha Centauri with a 75 year mission duration. In the experiment, the colony ship is assumed to have a mass of 10^7 kg, and the probe 10^4 kg.
The first mission is a human generation ship of 500 people on a one-way journey into space. He assumes that such a mission would require 50 tones per human occupant and that each person would use about 1000W, equal to the average amount used by people in the US in 2007. From this, he estimates that the ship would need some 10^18 Joules for rocket propulsion. That compares to a shuttle launch energy of about 10^13 Joules.
The second mission would be a 71 year journey by an unmanned probe headed for Alpha Centauri which is just over 4 light years away. Such a ship would be some three orders of magnitude less massive than a colony ship so it would require considerably less energy.
"It is found that the earliest interstellar missions could not begin for roughly another two centuries, or one century at best," writes Millis, "Even when considering only the kinetic energy of the vehicles without any regard for propellant, the colony ship cannot launch until around the year 2200, and the probe cannot launch until around 2500."
Problems I have with this paper:
- Millis's extrapolations assume a linear progression of available energy density; technological development is showing a strong tendency to progress non-linearly
- He assumes that there won't be a "wild card" type breakthrough in propulsion technology and energy extraction; it's not unreasonable to assume that there will be a sudden breakthrough that could serve as a significant game changer
- His 500 passenger colony ship is ludicrous; biological humans won't be making such a journey, and most certainly not 200-500 years from now