Exodus (The Exodus Trilogy) by Andreas Christensen

options, but none was sufficient for reasonable interstellar flight. Chemical propulsion could theoretically get a starship from Earth to Alpha Centauri, just 4.3 light years away, which is closer than any other star system, in a little less than eight millennia.” She paused, grinning wryly.
    “ Which, of course, would be out of the question. With nuclear fission, the time would be reduced drastically. In principle, with nuclear fission we could generate an exhaust velocity of 4 percent of light speed, and using a gravity assist, such as that of Jupiter or the sun, we would be able to achieve 8 percent of light speed. That would enable the journey to Alpha Centauri to take fifty-four years, although one critical factor needs to be accounted for: deceleration. Using nuclear fission for deceleration would increase transit time to 108 years. Of course, with a magnetic sail for deceleration, which is certainly doable, we would be able to utilize the entire exhaust velocity for acceleration, so 8 percent is probably the number to keep in mind here. But remember that these speed estimates are theoretical possibilities. In the real world, we’d probably not be able to achieve much more than 2 or 3 percent of light speed with fission.
    “ Also, let’s not forget that it’s unlikely that we’re going to find the planet we seek at the shortest possible distance. My guess is that we’ll have to be looking at the stars further out, such as Gliese 581, at a little more than twenty light years away, or 55 Cancri, which is about forty-one light years away. Not much further though. I guess there will be a limit somewhere around fifty to eighty light years away though, and we’ll plan accordingly. But even so, at twenty light years, fission is also impractical.” They all nodded; they’d been through this before, so this first session was mainly to bring their White House attendees up to speed, although Ramon silently felt better getting the condensed facts this way, since sometimes these discussions seemed to go way over his head. Now, having patiently presented the pros and cons of the impractical options, Dr. Shearing turned to the more realistic options as to the prospect of being able to bring the starship from A to B.
    “ Now, a deuterium and helium-3 fusion rocket may be able to produce an exhaust velocity of 5 percent of light speed. With gravity assist and efficient engineering, this could be increased to 10 percent. That would give us a transit time to Alpha Centauri of approximately forty-three years, or eighty-six years if we also use this method for deceleration. For distances of twenty light years or more, it’s still a bit on the slow side, but the technology is within reach. It can be done.
    “ Another option is solar sails. This was already in the works when NASA was disbanded, and the Europeans have already used solar sails for unmanned probes for more than a decade. Although the purely solar-powered sails the Europeans are using are too slow for star flight, with a theoretical maximum speed of little more than 1 percent of light speed, we could increase the speed dramatically by giving the sails a decent push. What we need then is a high-energy laser lens. We’re talking about a 250-meter-wide lens, a huge engineering project by itself, but possible, given the resources we’re willing to spend here. But the energy consumption is dramatic. The power needed would be more than 25 percent of the entire world’s energy consumption for about four months, fission probably. It is a practical issue, really. The lens would have to be constructed in space, and in addition to the fission-powered laser, it would consist of a chemical rocket that would push it out of Earth orbit, and then trail the starship. Then it would point the laser at the starship’s sail for four months, until the starship reached a cruise velocity of about 25 percent of light speed. This option would be devastating to our world economy, and