NUCLEAR POWER One of the most feasible forms of future space travel is also the most controversial.

Is it safe? The Earth's environment could be protected by launching these spacecraft with conventional chemical rockets. but... The production of radioactive waste would pose a problem for sending manned missions on nuclear spacecraft rather than robotic probes.

Our current chemical rocket engines produce relatively little power. They have to make use of planetary alignments, or 'launch windows', and use the gravity of other planets to catapult them further into space. Nuclear rockets would be more powerful and wouldn't need to take advantage of these chance planetary patterns.

Types of nuclear rocket

There are two types of possible nuclear rocket which rely on different types of nuclear reactions:

• fission, when atoms are split apart
• and fusion, when they join together.

Nuclear plants on Earth produce energy from nuclear fission and the technology needed to build a rocket powered by fission is not far beyond our current means. They could fly within the next ten years.

Fission-powered rockets

Rockets run by nuclear fission are more fuel efficient, and so much lighter, than chemical rockets. This means that nuclear spaceships could travel twice as fast as our current chemical spacecraft. A nuclear powered craft could reach Saturn in as little a three years, rather than seven. What's more, because the fuel lasts longer, the spacecraft would still have enough energy left to tour the Solar System for up to 15 years.

Nuclear waste

The main problem with fission engines is the controversy over nuclear waste. The Earth's environment could be protected by launching these spacecraft with conventional chemical rockets. Only when the spaceship was well away from the Earth would the nuclear reactors fire up, ensuring that the radioactive waste wouldn't find its way back home. However, the production of radioactive waste would pose a problem for sending manned missions on nuclear spacecraft.

Fusion-powered rockets

Nuclear fusion produces even more energy and is the process that occurs inside stars and thermonuclear weapons. A propulsion system using nuclear fusion would be 10 million times more powerful than chemical rockets and would emit less radiation. There are many obstacles involved in making fusion-powered rockets, however. In experiments here on Earth, no one has been able to create controlled fusion reactions that produce more energy than they consume. In addition, the best fuel for fusion reactions, Helium 3, is scarce on our planet and the nearest plentiful source is the Moon.