Astronomers calculate 4.37 light years as the distance to Alpha Centauri, the nearest star to our sun. Sirius, the Dog Star, lies at 8.6 light years. The single digit numbers disguise the magnitude of the distances: our moon is only about 1.5 light seconds away, the sun about 8 light minutes, and even the farthest planets only a few light hours.
Alpha Centauri: the ancient Greeks named it Rigel Kentaurus, the “Knee of the Centaur.” The closest bright star to the Southern Cross, it remains below the horizon in the higher Northern latitudes.
For comparison, travel to Rigel Kentaurus would take: at the speed of a bicycle, 315 million years; of an automobile, 47 million years; of a commercial jet airplane, 6 million years. Even at the speed of a spaceship (using the arbitrary figure of 50,000 kilometres/hour), 94,000 years.
Doesn’t sound very promising.
We are used to reckoning travel time by a constant velocity. Even our fastest modes of transport spend a small part of their travel time accelerating and decelerating. So most people have little conception of the power of continuous acceleration.
If a spaceship could continuously accelerate at one gravity – which means that a person inside would feel like standing on the Earth – it would (setting aside the laws of relativity) reach the speed of light in about one year.
So if a spaceship accelerated at one G for half a year, “coasted,” and decelerated for half a year, it could reach Rigel Kentaurus in nine to ten years, a timescale similar to that of the sailing ship voyages exploring the oceans several hundred years ago. Even much less acceleration, if constant, could bring interstellar voyages to human life time scales.
Now constant acceleration requires tremendous amounts of energy. For a 1000 tonne spaceship to accelerate to half the speed of light would take about 30 times the amount of energy used by all humans in a year (2017). But the Earth intercepts more solar energy than that in 2 days.
Energy is effectively unlimited. The Sun powers all life on Earth, with minor exceptions. The Earth intercepts an insignificant portion of the Sun’s energy. To intercept all the Sun’s energy at Earth’s distance from the Sun would require more than 2 billion Earth-sized planets. Also, matter is super-compressed energy (E=mc2). The challenge is how to gather, store, and use energy.
With our present knowledge, to accelerate a spaceship also requires, according to Newton’s third law, expelling matter (reaction mass). So the theoretical starship would have to start out with huge amounts of matter to be expelled. There are several possible approaches which would not require a spaceship to carry reaction mass but none so far proved feasible.
Humanity progressed from horses and sailing ships to nuclear energy and spaceships in 200 years. What will it be capable of in one or two thousand years?
Our galaxy is about 100,000 x 10,000 light years. If Humanity can refrain from wiping itself out before establishing beachheads on other planets, within a million years Homo Galacticus, and partner species that it takes along, will make a first approximation of populating this galaxy, and even send out a few intergalactic expeditions. Why not? As William S. Burroughs said, “We are here to go!”