During long voyages, the health and fitness of astronauts is reduced by exposure to radiation and weightlessness. Long missions also require more air, water and food, and increase the chances of something going wrong. But how can missions be shortened?
One way is to make rocket engines more efficient. ESA engineers have already built an electric propulsion system for the SMART-1 mission to the Moon. Electric motors are much smaller and lighter than normal engines. Although they use much less fuel, they are able to produce a higher exhaust velocity - the speed at which gas from the engine shoots into space.
Using electricity made by sunlight falling on 'wings' made of solar cells, they can slowly increase a spacecraft's speed until it is travelling many times faster than a chemical rocket.
For missions far from the Sun, nuclear engines will be needed. Using such engines it may eventually be possible to fly to Mars in just a few days.
Last update: 2 December 2004