Popular Science charts a course to Mars that’s easier and cheaper… in a roundabout way:
Spacecraft usually enter orbit around planets via Hohmann transfer, which requires engineers to send the craft to its target just when the target (that’s the moon, Mars, or any other planet) is coming by. Once the craft meets its target, things really get into action. Scientific American explains beautifully:
Spacecraft screaming along at many thousands of kilometers per hour have to hit the brakes hard, firing retrorockets to swing into orbit. The burn can require hundreds of pounds of extra fuel, lugged expensively off Earth . . . .
With ballistic capture, on the other hand, the spacecraft launches ahead of its target. The craft takes its time—and less fuel—slowing down to some speed that’s less than the speed at which the target orbits the sun. Eventually, the target catches up to the craft and sucks it into orbit around itself using its own gravitational pull.
One of the mathematicians who’ve plotted the ballistic capture path to Mars has already done it before, for Japan’s Hiten moon mission. They think their technique could use 25 percent less fuel.