As space travel becomes more and more expensive, we will need more frugal methods of transport. One proposed method would be the use of somewhat new technology, solar sails. Solar sails are extremely thin sheets of metal that work similar to regular naval sails. Naval sails use the pressure built up behind them as a means of propulsion of their vessel, solar sails analogously use pressure built up behind them as well to propel their payloads. However the pressure used by these sails come from soar wind and radiation pressure. As we know even though the photon is a massless particle it still has a momentum attached to it, this momentum can be harnessed by these sails as a means of propulsion.
One character very important characteristic about these sails is that they must be very large in area. This is to obtain a sizable amount of pressure built up to gain some propulsion. A draw back of of these sails is that like many of the physical forces, the force that propels these sails falls of as one over the distance squared. Also the forces themselves are quite weak to begin with. For example force given by the radiation pressure goes as 10^-6N/m^2, and solar wind goes as 10^-9N/m^2(these are as seen from the distance of the earth and given by our sun). This drawback may not be as bad as it seems, because devoid of other forces in space the sail may be able to maintain a long voyage with no cost of rocket fuel. Also when these are in use the sails are limited in their pointing because the maximal force is given when the sail is perpendicular to the sun's rays. This could greatly hinder navigation other than straight away from the source.
Attack of the Clones solar sail use
IKAROS solar sail, artist rendering
Although this is a somewhat new technology, it has been seen in the media for a while and has been used in sic-fi movies quite realistically. Above is a picture of a solar sail being used in Star Wars: The Attack of the Clones in 2002. This is because until recently people did not believe that these would work. However, in 2010 the Japan Aerospace Exploration Agency (JAXA), actually launched their sail and measured solar propulsion acting on it. Now the next step is making them more efficient. This means the engineering of thinner sails, more reflective sails (because propulsion is better transferred through reflection than absorption of light) and a more sturdy design. This has actually already begun with the creation of sails that spin, which use angular momentum to make the sails retain their shape.
Actual IKAROS sail
Above is a small clip from the movie where you can see the sail being used.
Radiation pressure, given as the time averaged intensity of light divided by the speed of light squared. .
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