Harnessing Sunlight and Orbital Momentum
Another technology that scientists expected to fine-tune by 2005 is the solar-thermal rocket. This system would use large inflatable mirrors about 10 meters (33 feet) in diameter to collect sunlight and focus it into an engine to heat a propellant, such as liquid hydrogen. The heated propellant would expand and emerge at high speed through a narrow nozzle in the rear of the vehicle to provide thrust. Solar-thermal rockets would have about twice the exhaust velocity of the best chemical rockets. One possible application of solar-thermal rockets is to boost satellites from low Earth orbit to much higher orbits.
Space tethers are another experimental propulsion system that could be in operation by 2005. Unlike traditional propulsion systems, tethers require little or no propellant. Tethers exploit differences in the momentums of different orbits to change the velocities of orbiting objects, such as satellites. Objects in high Earth orbit move slower than ones in lower Earth orbit. A strong tether connecting objects in different orbits provides a potential to do work, because the object in low Earth orbit can be used to pull the object in higher orbit down, and the object in higher orbit can pull the lower object up. Using this principle, a series of spinning space tethers in orbit around the Earth could be used to fling payloads to the Moon, or even to Mars. Such a system would involve several spinning tethers, each in a higher orbit around the Earth than the last. The payload would be transferred between the tethers, picking up momentum before being released.
Between 2005 and 2010, scientists should be making progress in developing a “solar sail,” another propulsion system that would require little or no propellant. A solar sail would be a large reflective sheet propelled by the pressure of sunlight, similar to the way a sailboat's sails catch the wind. Because photons (particles of light) exert little pressure, the main challenge of building solar sails is developing ultra-lightweight films to enable even the slight pressure of sunlight to achieve an adequate rate of acceleration. Despite this challenge, solar sails may one day be the supertankers of the solar system, slowly transporting cargo between planets while consuming no propellant.
A concept similar to the solar sail, called a “magnetic sail,” could provide another way to use the sun to propel a spacecraft. A magnetic sail would harness the force of the solar wind, a steady stream of charged particles blowing outward from the outermost layer of the sun's atmosphere. A magnetic sail would create strong magnetic fields that the solar wind could push against, creating thrust to move the spacecraft forward.