| The fastest that anything can travel is the speed of light. This is a fundamental limitation described by Einstein's Theory of Relativity. Light travels at approximately 300,000 km per second (186,000 miles per second). It could travel around the world in the time it takes you to blink your eye. Even at this speed, light takes more than four years to reach the earth from Proxima Centauri, the nearest star other than our sun. The fastest current (unmanned) space probes would take sixty thousand years to travel that distance.
Let's suppose that we could somehow build a craft that could travel at one-tenth the speed of light. That's more than a hundred million kilometers per hour. It would need an enormous amount of energy to accelerate to that speed. Kinetic energy is proportional to velocity squared, so to accelerate to high speeds requires insane amounts of energy. For a ten-ton (10,000 kg) spacecraft the energy required is between 1018 and 1019 Joules, which is roughly the same as the amount of electricity consumed in a year by the whole of the United States. And somehow the craft is going to need to expend the same amount of energy again, to slow down at the other end. Oh, and are the astronauts coming back? Double that energy requirement! And a spacecraft for a 40-year mission won't be anywhere near as light as 10 tons.
Suppose we can overcome the energy problem. We then have the problem that at such high speeds we have no way to dodge any interstellar debris that may lie in our path. At a hundred million kilometers per hour, the tiniest pebble would destroy the spaceship. Even if we could detect the pebble in time, at that speed it would require unrealistically-large amounts of energy to dodge it or deflect it.
We often talk of the "vacuum of space", but it's not enough of a vacuum for high-speed inter-stellar travel. The Kuiper Belt, beyond Pluto, is teeming with rocks the size of asteroids and smaller, and it's thought that the Oort cloud, which is even further out, holds the ice and rock from which comets are formed. |
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