SOLAR SAILS Robotic space probes may set sail for the stars, riding on the solar wind.

Dreamed up at the beginning of the last century, solar sails could be one of the most feasible ways of travelling into deep space. A solar sail is a lightweight panel made from reflective material that acts like the sails of a boat. Rather than using wind, however, the sails are actually propelled by light. Unbelievable as it may seem, the stream of light particles (called photons) emitted from the Sun is strong enough to push a mini-spacecraft right out of the Solar System and beyond into interstellar space.

Ten times faster than the space shuttle They could accelerate to speeds of 90km per second - fast enough to travel from London to New York in just over a minute.

Travelling light

One of the major limitations on current missions is the weight of the fuel needed to blast rockets out of the Solar System. To travel further, a rocket needs more fuel. But more fuel means more weight, which slows the spacecraft down and makes it less efficient.

As solar sailed craft are powered by sunlight, they don't need to carry onboard fuel like conventional rockets and so are lighter and easier to propel. For long missions, an on-board laser or microwave transmitter would be fitted to provide power when the Sun is just a distant memory.

Steering vanes at each corner of the solar sail panel could pilot the spacecraft through the Solar System.

Speedy sails

The drawback with solar sails is that initial acceleration is low. This means that craft powered by solar sails could only carry robotic probes. However, because sails are so lightweight and have a continuous source of energy, they could accelerate to speeds of 90km per second - fast enough to travel from London to New York in just over a minute. That's over 10 times faster than the space shuttle. And solar sails are also reusable and do not require costly refuelling for new missions.

In 2004, the Japanese Institute of Space and Astronautical Science opened solar sails in space aboard its S-310-34 rocket. Two types of sail were opened sucessfully during the test flight before the rocket splashed into the sea as planned.

NASA is planning an in-flight demonstration in 2007 with a view to launching a deep space mission in the next decade. The Interstellar Probe would unfurl its delicate solar sail in space and would speed to the edge of the Solar System in just eight years, a feat that takes modern rockets such as Voyager 1 over 40 years.

Another feasible option for alternative space fuel in the near future is nuclear power.