It's interesting when you look back at the history of space exploration and realize that propulsion technology hasn't really changed very much.
The earliest rocket prototypes were nothing more than elaborate versions of weapons used during World War 2 and fireworks used during civil celebrations. Even the Space Shuttle made use of solid rocket fuel technology in its pair of solid rocket boosters. But, with the liquid rocket fuel propulsion in the external tank, this combination has proved to be highly effective and launched hundreds of astronauts into space.
The approach works -- albeit not very efficiently -- and to get out of the gravitational well of the Earth, it seems for now that the extra punch from exothermic processes is needed.
In deep space, however, there are alternatives receiving very serious consideration -- such as the "eco-friendly" solar sail.
The solar sail concept is simple: any surface exposed to electromagnetic radiation 'feels' a pressure known as radiation pressure and it's this pressure that exerts a tiny pushing force against the surface. If the surface happens to be a spacecraft or part of a spacecraft, it could act against it to provide propulsion through space.
The concept of radiation pressure isn't particularly new. The idea was first alluded to by Johannes Kepler in 1610 when he suggested the reason why the tail of a comet points away from the sun was in some way caused by the sun. Kepler even made reference to using this unknown force for exploration when he wrote in a letter: 'Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.'
By 1864, it was accepted that the light carried momentum and would exert a pressure on anything it meets. A great demonstration of this can be seen in the Nichols radiometer, which is a sealed bulb with tiny silvered glass mirrors attached by a very thin wire inside the glass. On being exposed to light the mirrors start to rotate, driven by radiation pressure excerted by photons from the bulb filament.
The technique is already being used in space exploration to for course corrections and fuel savings. For example, NASA's Mercury MESSENGER probe successfully used solar radiation pressure to make small course corrections during its journey to the innermost planet.
To make the most out of radiation pressure for space exploration every bit of solar energy needs to be eked out. For any useful form of propulsion, giant solar sails need to be used and exposed to as much light as possible.
The first interplanetary test of a solar sail was conducted by the Japanese Aerospace Exploration Agency in May 2010 with the launch of Ikaros. This was the first time a solar sail was deployed and tested in space and used as its primary propulsion. NASA also launched the orbital solar sail prototype Nanosail-D in November 2010, successfully completing its mission after 240 days in Earth orbit. Ikaros, on the other hand, continues its journey around the sun after passing Venus in December 2010.
With Ikaros' 27 square meter sail deployed, the full effect of radiation pressure from the sun on the sail produces about 0.0002 pounds of force, thats equal to about 0.1 grams -- less than the average goose feather! The acceleration offered by this method of propulsion is small but over a long period of time, incredible speeds could be reached.
The downside to this mode of transportation is that heavier craft will take longer to accelerate, so larger sails would need to be manufacturered. Ikaros' sail was impregnated with solar cells to power the electronic equipment and a matrix of liquid crystals around the outside whose reflectivity could be altered to change the attitude of the spacecraft.
Future missions will take these tests further from the sun. The challenge here is that the further away from the sun you go, the weaker the radiation pressure, so acceleration through interstellar space will be limited. Innovations in laser technology may extend the range of solar sails.
The technology is no doubt in its infancy, but new ideas of rotating solar sails in various configurations shows great promise.