IT HAS been a busy week for JAXA, the Japanese space agency. While most eyes were on the spectacular return of the Hayabusa asteroid sample probe, which parachuted into the Woomera rocket range on Sunday after the second fastest re-entry in history, another craft was ever-so-slowly picking up speed in the void between Earth and Venus.The experimental spacecraft Ikaros, after many weeks of delicate maneuvering, has successfully deployed the first solar sail. For researchers interested in sending missions into the far reaches of the solar system, Ikaros is a dream come true – a spaceship that doesn’t need fuel to generate thrust.If all goes according to plan, all the heavy work will be done millions of kilometres away, on the sun. All previous spacecraft have had to carry enormous amounts of fuel – the overwhelming bulk on NASA missions to the outer solar system was simply the gas tank and the booster engines, all of which were thrown away once the probes were up to speed, an expensive and inherently wasteful way of getting about.Ikaros needed chemical rockets to escape the Earth’s gravity (it actually piggybacked a ride on the Akatsuki mission, now on its way to study the atmosphere of Venus), but now the sail has been unfurled it will be pushed along by the light of the sun.This is a fragile machine. ”Ikaros has a square sail approximately 14 metres long on each side,” explains Osamu Mori, the project’s leader. ”The sail, and the solar collectors that will generate electricity, are only 7.5 micrometres thick. Human hair is 100 micrometres thick, so you can imaging how thin this is.”Consequently, the unfurling of a sail packed into the confines of a small spacecraft is undertaken with great care. Two previous American attempts, in 2001 and 2005, have failed at this crucial hurdle.The Japanese had the idea of a square sail with a small weight on each corner which was set spinning – very slowly – on a turntable which forms the centre of the vehicle. Eventually centrifugal force stretches the sail into its desired configuration, and the latest word is so far, so good – the sail reached full extension last weekend.Now to the business end – learning how to sail a yacht, millions of kilometres away, using sunlight. How, for instance, do you steer such a vessel? Ikaros incorporates some very clever technology to control direction and speed.”The solar-powered attitude control system uses technology that controls the reflectivity of the sail,” said Mr Mori. ”It works just like frosted glass: normally the entire area of the sail will reflect sunlight, but by ‘frosting’ part of the film, we can reduce the reflectivity of that area. When the reflectivity is reduced, that part of the sail generates less power. So by changing the reflectivity of the sail, we can control its attitude.”So, assuming these subtle changes in reflectivity work as intended, where is Ikaros headed?At first it will travel side by side with the Venus mission, and then head off on its own.”Akatsuki will decelerate to enter Venus’s orbit,” said Mr Mori, ”but Ikaros will pass by Venus and navigate around the sun. Where it heads will really depend on how the solar sail’s orbit control function performs.”Ikaros does not have a destination – as an experimental craft its major objective is is to establish whether solar sailing is feasible at all. But the implications of a successful flight are profound. While sunlight transfers very little propulsion to Ikaros, it never stops doing so, and over time solar sails can, in principle, build up enormous speeds.So if you’re not in a hurry, and want to go a long way without spending a fortune at the bowser, they might be just the ticket. Osamu Mori and his team at JAXA certainly hope so.