Will we always use rockets?

Some rockets, like ion drives, are always likely to be useful. But it would be ideal if we could replace rockets, both to get away from Earth and for the kind of long-term acceleration needed to make travel to the outer Solar System, or even other stars, commonplace.

One alternative to rockets is the space elevator. This involves running an extremely long cable from a satellite down to Earth’s surface. A mechanical device would then be used to climb up it, hauling a payload into space. It’s a great concept as it would be much cheaper than rockets, and doesn’t need to carry its fuel. But we don’t have any materials strong enough to build a space elevator from Earth. The cable would have to be nearly 38,000km long. A typical 28mm steel cable of this length, capable of supporting around 50 tonnes, would weigh 115,000 tonnes. In principle, though, we have materials strong enough to build a space elevator on the Moon.

When it comes to deep space, we could replace some of the requirements for rockets with solar sails, which use the pressure of sunlight to gradually accelerate a vessel, or mass drivers, which are like external thrusters that push the whole ship. But the most dramatic alternative, dreamt up in 1958, is to propel a ship by exploding tiny nuclear charges behind it, riding the shockwave. The original Project Orion had the motto “Mars by 1966, Saturn by 1970”. It was never built (in part because the original idea to use these nuclear charges for taking off from Earth was, to say the least,