"In recent years, pressure to reduce the costs of interplanetary missions has led to an emphasis on designing missions with shorter flight times, smaller launch vehicles and simpler flight systems," said Victoria Coverstone-Carroll, a professor of aeronautical and astronautical engineering at the U. of I. "These requirements have renewed interest in low-thrust propulsion systems because of their high propellant efficiencies, but the need to optimize their flight paths has posed certain challenges."

Low-thrust systems, such as solar-electric propulsion -- which uses huge solar arrays to collect photons and convert them into electricity to drive an ion engine -- possess little impulse power but may be operated continuously. Conventional chemical propulsion systems, on the other hand, provide short, but intense, bursts of thrust. Flight trajectories for the two systems differ markedly.

Coverstone-Carroll and graduate student Bill Hartmann developed a Pareto genetic algorithm capable of optimizing low-thrust trajectories. With Steven Williams at the Jet Propulsion Laboratory in Pasadena, Calif., the researchers used this special algorithm to evaluate different mission scenarios.

"We analyzed a number of proposed missions, including a rendezvous with the asteroid Vesta, a mission to Mars and a Pluto flyby," said Coverstone-Carroll, who presented the team's findings at the American Astronautical Society/American Institute of Aeronautics and Astronautics national meeting in Monterey, Calif., in February. "In each of these missions, the low-thrust propulsion technology delivered more payload capability than the equivalent chemical propulsion mission."

Genetic algorithms work by creating a population of individual solutions that then evolves over a series of generational cycles, with each solution undergoing alterations to its respective parameter set. With the U. of I. algorithm, the automated search procedure provides a mission planner with an array of compromise solutions trading off such system performance characteristics as time of flight and amount of propellant consumed.

"Every pound of propellant that must be launched into space represents one less pound of instrumentation for the mission," Coverstone-Carroll said. "One of the big advantages of solar-electric propulsion is that if you are willing for the flight to take a little longer to reach its destination, you not only can launch larger scientific payloads, you also can avoid the limitations of launch windows."

Chemical propulsion systems, with their ballistic trajectories, are dependent upon launch windows during which the geometry of Earth and the target are favorable, Coverstone-Carroll said. "We can avoid that restriction with low-thrust systems, however. We can launch at any time of year."

While not practical for manned missions, where time of flight must be minimized, low-thrust systems could be used for supply missions, sample-return missions and rendezvous with distant planets.