Generational interstellar travel

In a subset of science fiction novels and stories, the journey to the stars is longer than a human lifetime. The space vehicles are huge, as they must be to house many cosmonauts during thousands of years. The speeds of these spacecraft are not very high, on the order of a few million kilometers per day. At that rate it takes thousands of years to cross the interstellar chasms.

The cosmonauts who reach the end of the journey are not the same ones who started: hundreds or even thousands of generations have passed their entire lives aboard a spaceship and don’t know the Sun, or life in the open air. The vehicle where they travel is a small planet: a closed entity, in perfect ecological balance. Nothing is wasted. Wastes from normal activity are recycled and reused. The cycles of water, oxygen, carbon, nitrogen and other essential elements of life must be perfectly controlled. They only eat hydroponic vegetables.

The main problem is energy expenditure. The maintenance of life on Earth is possible thanks to solar energy, but during an interstellar journey the closest stars are too far away for their light and heat to be used. The spacecraft must carry its own energy sources, in sufficient quantity to ensure the survival of the inhabitants.

Assume the journey is undertaken by one thousand people; that the number of cosmonauts remains constant throughout the generations; and that the maintenance of human life is the only energy expenditure in the space vehicle. Each human being needs about 2,000 kilocalories per day, i.e., 8.36 million joules. The total energy expenditure per year will therefore be equal to 3.05×10¹² joules. If the journey lasts, say, fifty thousand years, the ship must carry an energy source capable of supplying more than 10¹⁷ joules.

But human beings cannot absorb energy in any form, we need other living beings to manufacture the organic matter that we are going to use. Plants absorb energy from visible light and use it to generate glucose and other organic substances. This process cannot be carried out without losses, so it will be necessary to raise the previous figure, at least by an order of magnitude. Therefore, they’ll need at least a quintillion joules. Where can this be obtained from? And remember we haven't taken into account the energy required to accelerate the ship and stop it when it reaches its goal.

Since several decades ago, technology has been announcing that we are about to make practical use of nuclear fusion energy, which is released when several hydrogen nuclei fuse to make a helium nucleus. This is the reaction by which the Sun produces energy. Starting from a mass M of hydrogen, an energy equal to 0.007× M× c² may be obtained, where c is the speed of light.

If the crew of the spacecraft used for the generational journey could produce nuclear fusion reactions with the same efficiency as the Sun, 1,600 kilograms of hydrogen would be needed to generate a quintillion (10¹⁸) joules. This is a minimum limit. In practice, it should be increased considerably, but it is quite reasonable. The generational journey would be energetically feasible.

Chad Oliver

Our current technology is not yet in a position to accomplish this feat. But, even if it were feasible, who would be willing to embark on such a journey, condemning their descendants to never see sunlight? It seems to me that very critical circumstances would have to concur that would make it necessary to leave the solar system, for this solution to the problem of interstellar travel to be considered, and even then, it would only be a last resort, a desperate flight.

On the other hand, a generational journey raises many psychological and social problems that would have to be taken into account if human beings ever attempt to cross interstellar spaces in this way. Let us cite, for example, the following:

a)      Control the population.

b)      Prevent travelers from suffering from claustrophobia.

c)      Keep a high morale and avoid conflicts.

d)      Distribute the work so that no one is idle.

e)      Educate the new generations to live in a restricted society.

f)       Avoid the loss of knowledge, superfluous for the trip, but necessary to colonize the planet where they are going.

g)      Adapt the last generation to a strange environment.

h)      Keep the meaning of the mission alive.

 As an example of the problems that the last generation would have to face when they reached the end of their journey, see the short novel The Wind Blows Free (1957), by the American ethnologist Chad Oliver.

The same post in Spanish

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Manuel Alfonseca