Making space travel simpler with interplanetary highway
Posted: July 21, 2002

Artist's concept of interplanetary superhighway. Credit: NASA/JPL
A "freeway" through the solar system resembling a vast array of virtual winding tunnels and conduits around the Sun and planets, discovered by an engineer at NASA's Jet Propulsion Laboratory, Pasadena, Calif., can slash the amount of fuel needed for future space missions.

Called the Interplanetary Superhighway, the system was calculated by Martin Lo, who used his theory to design the flight path for NASA's Genesis mission, which is currently using this "freeway in space" on its mission to collect solar wind particles for return to Earth.

Most missions are designed to take advantage of the way gravity pulls on a spacecraft when it swings by a body such as a planet or moon. Lo's theory mixes in another factor, the Sun's pull on the planets or a planet's pull on its nearby moons. Forces from many directions nearly cancel each other out, leaving paths through the gravity fields in which spacecraft can travel.

Each planet and moon has five locations in space called Lagrange points, where one body's gravity balances another's. Spacecraft can orbit there while burning very little fuel. To find the Interplanetary Superhighway, Lo mapped all the possible flight paths among the Lagrange points, varying the distance the spacecraft would go and how fast or slow it would travel. Like threads twisted together to form a rope, the possible flight paths formed tubes in space. Lo plans to map out these tubes for the whole solar system.

Lo has turned the theory of the Interplanetary Superhighway into a tool for mission design called "LTool," using models developed at Purdue University, West Lafayette, Ind. The new LTool designed the flight path for the Genesis mission, the first space mission to use the theory of the Interplanetary Superhighway. Genesis launched in August 2001.

The flight path was designed for the spacecraft to leave Earth and travel to orbit the Lagrange point. After five loops around this Lagrange point, the spacecraft will fall out of orbit without any maneuvers and then loop around Earth to a Lagrange point on the opposite side of the planet. Finally, it will return to Earth's upper atmosphere to drop off its samples of solar wind in the Utah desert, at the Air Force's Utah Testing and Training Range.

"Genesis wouldn't need to use any fuel at all in a perfect world," Lo said. "But since we can't control the many variables that occur throughout the mission, we have to make some corrections as Genesis completes its loops around a Lagrange point of Earth. The savings on the fuel translates into a better and cheaper mission."

"It has been exciting and challenging to develop this field. Our work on the Genesis mission is definitely a high point," said Kathleen Howell, co-creator of LTool, and a professor of aeronautics and astronautics at Purdue. "The theory has been known for some time, but this is the first time it has been applied to a space mission."

"For all missions going to a Lagrange point, LTool will speed up computations," Lo said. "Designing the Genesis spacecraft's flight path with traditional methods used to take eight weeks, but now we can design a new flight path in less than a day -- we have redesigned a whole mission in a week."

Illustration of the Genesis mission track. Credit: NASA/JPL
Lo envisions a place to construct and service science platforms around one of the Moon's Lagrange points. Since the Lagrange points are landmarks for the Interplanetary Superhighway, spacecraft could easily be shunted to and from the station for repair. A team at NASA's Johnson Space Center, Houston, working with the NASA Exploration Team (NEXT), proposes to someday use the Interplanetary Superhighway for future human space missions.

"Lo's work has led to breakthroughs in simplifying mission concepts for human and robotic exploration beyond low-Earth orbit," said Doug Cooke, manager of the Advanced Development office at Johnson. "These simplifications result in fewer space vehicles needed for a broad range of mission options."

Lo's and Howell's work on the Interplanetary Superhighway for space mission design was nominated for an annual Discover Innovation Award by Discover magazine editors and an outside panel of experts.

Spacecraft are not the only users of the Interplanetary Superhighway: asteroids and comets are known to travel on it. Comet Shoemaker-Levy 9 collided with Jupiter when it took an off-ramp toward the giant gas planet. Scientists think the asteroid that killed the dinosaurs could have followed Genesis' flight path -- an iridium deposit at the crash site shows the asteroid traveled fairly slowly. Just what we might expect from an asteroid on the Interplanetary Superhighway, Lo said.

JPL is managed for NASA by the California Institute of Technology, Pasadena.

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