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One of the biggest faults with the concept of a one-shot slower-than-light colony mission was the proportion of the time spent accelerating and slowing down. Take Barnard's star, for example. At 5.9 light years away, with a ship capable of 0.3 lights, a plausible speed for a ramscoop . . . you'd be there in 19.7 years, right?

Wrong. It all depends on acceleration. High-speed acceleration is expensive and creates engineering stresses, to say nothing of the stresses on the biological matter. A slow steady push is best. You accelerate slowly for at least a third of your trip. And then you have to slow down again. If you're going to visit a number of systems, this adds hugely to travel time. What's more, the momentum you've lost has to be built again.

Momentum is expensive. It is energy. Energy, whether taken from solar-pumped lasers or a-bombs is a consumable. Even if it is "free" solar power, it still costs to get it into a usable form, and once it has been used, it is gone. A metal space habitat has a finite lifespan—but it is an enormous one. The depreciating cost, amortized over its space-life, divided by its carrying capacity, makes it the cheapest vehicle humanity ever built.

However: Building the momentum needed to travel between the stars is too expensive to waste on one stop journeys, or even on leapfrogging between stars. Once the colony ship accelerates, it must never slow down again. Never. It will drop space habitat modules at each sun. But it must itself just keep cruising along, a slow train to the stars.

From Slowtrain: The Stars Within Our Grasp, Conquist, A., Mordaunt Scientific Press, NY. 2090.


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