Electric Propulsion
The most effective measure of a rocket’s performance
is the specific impulse (the time it takes to burn a pound of propellant to make a pound
of thrust). The highest level chemical rockets today have a specific impulse of 450
seconds and today’s nuclear thermal rockets get approximately 900 seconds.
However, another method exists to create a higher specific impulse. It’s called an "ion drive." This drive uses the attractive and repulsive forces of an electrostatic grid to accelerate ionized (decreased electron amount) gas particles. Through this method it is possible to make specific impulses of up to thousandths of seconds and barely heat the exhaust gas. Actually, this is a fact because electric propulsion is used on satellites for station maintaining propulsion. However, a large amount of electric power is required to accomplish such a task. For instance, a 120 ton spacecraft would require a power of five megawatts to generate 280 newtons (about 60 lbs.) of thrust with a specific impulse of 5,000 seconds. Such a ship could make it from low earth orbit to Mars and back in approximately one year while under continuous thrust. In fact, ion thrusters already exist in the form of kilowatt sized units, and the changing of them into megawatt sizes poses no challenges.