English: In an artist's rendition, a four-quadrant solar sail propulsion system, complete with payload, hovers at a fixed point in space in Earth orbit. Sunlight reflected off the sail provides the necessary thrust to maintain the spacecraft's orbit. The giant mirror-like sails are made of lightweight, reflective material 40-to-100-times thinner than a single sheet of writing paper and measure approximately 100 meters along each side. The stripes imbedded within the sail during fabrication ensure that any rips that may occur during deployment, or as a result of collision with meteoroids, will not spread across the entire sail. Two teams selected by NASA to lead hardware design and development of solar sail system concepts are scheduled to conduct ground demonstrations of their 20-meter subscale designs in April and June 2005. Solar sail propulsion is under development by NASA scientists and their industry partners, led by the In-Space Propulsion Technology Office at NASA's Marshall Space Flight Center in Huntsville, Ala. The In-Space Propulsion Technology Program is implemented by the Marshall Center on behalf of NASA's Science Mission Directorate in Washington.