"Today's spacecraft use propellant-powered thrusters to move from one orbit to another," said Bruce Conway, a professor of aeronautical and astronautical engineering. "But each pound of fuel carried aloft means a corresponding reduction in the weight of the mission payload. A next-generation spacecraft may switch orbits at a substantial fuel savings by dipping into the atmosphere, generating aerodynamic lift and drag on airplane-like control surfaces, and then climbing to a new orbit."

The concept is similar to aerodynamic braking, a technique that uses a planet's atmosphere to reduce the speed of a space vehicle. Aerodynamic braking was used successfully for the Mars Global Surveyor mission, currently in orbit around the red planet.

"It would have been far too costly to send that spacecraft to Mars with all the necessary fuel to place it in the desired orbit through a prolonged 'burn,' " Conway said. "Instead, the mission planners placed the spacecraft in a more accessible but highly elliptical orbit, and used aerodynamic braking to slowly circularize the orbit. The procedure took several months to complete, but required little fuel."

In much the same fashion, aero-assisted orbital transfer vehicles could use a planet's atmosphere to change the altitude, shape or plane of their orbits. Applications in Earth orbit include atmospheric sampling, satellite repair missions, surveillance and missile interception.

In a paper published in the September-October issue of the Journal of Guidance, Control, and Dynamics, Conway and graduate student Kazuhiro Horie applied a new numerical method they developed at the university.

"Optimal trajectories were found for the interception of a target in low-Earth orbit by a vehicle initially in a higher orbit, using aero-assist," Conway said. "We obtained solutions for a wide range of target orbit inclinations and constraints, such as maximum allowed heating rates. We also found that our method could solve problems with demanding constraints that conventional methods could not."

Using aero-assist made some interceptions possible that otherwise were infeasible because the spacecraft carried insufficient fuel. But an interesting and non-intuitive result was that in some cases, even if the spacecraft had enough fuel to intercept the target without entering the atmosphere, using a combination of conventional propulsion and aero-assist yielded a quicker interception.

"Unlike other methods, our technique is particularly well-suited for solving problems where many complicated constraints are placed on the trajectory," Conway said. "Our numerical method looks first for any trajectory that starts with the given conditions and satisfies the desired goal, then it optimizes that trajectory for the best possible solution."