Maximum speed of spaceships

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• 
  shavera
  Jul 20, 2011

  In principle, yes, you can create a ship that approaches the speed of light. In practice, that's a very very difficult challenge. The energy requirements are just absolutely enormous. And that boils down to a matter of fuel. You see, you can't just ask "why not build a nuclear-powered spacecraft?" or "matter/anti-matter" or whatever, you've got to think about how you can push off of the power you're creating. Think about matter/anti-matter. It annihilates into light. And you can only get a very light push from light. And it's gamma rays so you need to build massive absorbers to even catch them. And only half of the rays are even pointed in the right way to ride them. Similar scenario for nuclear (if you're thinking about say, exploding nuclear bombs behind your spacecraft for propulsion). It's just very inefficient, most of the explosion's energy can't be useful for propulsion.

  So far, the best present-day technology seems to be ion engines. We use electric and magnetic fields to accelerate ionized particles out the back. The acceleration is slow, but you can do it for much longer and end up with a higher speed than classic chemical rockets. But because the acceleration is so slow, you can't use them in atmosphere, only in space. So you use chemical rockets to get them to space, and then ion engines to accelerate within space.

• 
  Ólavur Mortensen
  Jul 22, 2011

  I think what shavera is trying to say is, it would take such an enormous amount of energy to actually get up to light-speed.

  The energy it would take to propel something to the speed of light is m×c^2, where m is the mass of the object and c is the speed of light, which is roughly 3×10^8 or 300 000 000. That adds up to a lot of energy.

  So it isn't impossible to build a craft that travels near the speed of light, but it isn't easy.