A New Way to Reach Mars Safely

Ballistic capture, a low-energy method that has coasted spacecraft into lunar orbit, could help humanity visit the Red Planet much more often

A newfound, lower-energy means for spacecraft to attain Martian orbit could help make Red Planet voyages cheaper, safer and therefore more frequent. Credit: NASA

Getting spacecraft to Mars is quite a hassle. Transportation costs can soar into the hundreds of millions of dollars, even when blasting off during "launch windows"the optimal orbital alignments of Earth and Mars that roll around only every 26 months. A huge contributor to that bottom line? The hair-raising arrivals at the Red Planet. 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, and comes with some risk of failure that could send the craft careening past or even right into Mars.

This brute force approach to attaining orbit, called a Hohmann transfer, has served historically deep-pocketed space agencies well enough. But in an era of shrinking science budgets the Hohmann transfer's price tag and inherent riskiness look limiting.

Now new research lays out a smoother, safer way to achieve Martian orbit without being restricted by launch windows or busting the bank. Called ballistic capture, it could help open the Martian frontier for more robotic missions, future manned expeditions and even colonization efforts. "It's an eye-opener," says James Green, director of NASA's Planetary Science Division. "It could be a pretty big step for us and really save us resources and capability, which is always what we're looking for."

The premise of a ballistic capture: Instead of shooting for the location Mars will be in its orbit where the spacecraft will meet it, as is conventionally done with Hohmann transfers, a spacecraft is casually lobbed into a Mars-like orbit so that it flies ahead of the planet. Although launch and cruise costs remain the same, the big burn to slow down and hit the Martian bull's-eyeas in the Hohmann scenariois done away with. For ballistic capture, the spacecraft cruises a bit slower than Mars itself as the planet runs its orbital lap around the sun. Mars eventually creeps up on the spacecraft, gravitationally snagging it into a planetary orbit. "That's the magic of ballistic captureit's like flying in formation," says Edward Belbruno, a visiting associated researcher at Princeton University and co-author, with Francesco Topputo of the Polytechnic University of Milan, of a paper detailing the new path to Mars and the physics behind it. The paper, posted on arXiv, has been submitted to the journal Celestial Mechanics and Dynamical Astronomy.

"A delicate dance" Ballistic capture, also called a low-energy transfer, is not in of itself a new idea. While at NASA's Jet Propulsion Laboratory a quarter century ago, Belbruno laid out the fuel-saving, cost-shaving orbital insertion method for coasting probes to the Moon. A Japanese vessel, called Hiten, first took advantage in 1991, as did NASA's GRAIL mission, launched in 2011.

Belbruno worked out how to let the competing gravities of Earth, the sun and moon gently pull a spacecraft into a desired lunar orbit. All three bodies can be thought of as creating bowl-like depressions in spacetime. By lining up the trajectory of a spacecraft through those bowls, such that momentum slackens along the route, a spacecraft can just "roll" down at the end into the moon's small bowl, easing into orbit fuel-free. "It's a delicate dance," Belbruno says.

Unfortunately, pulling off a similar maneuver at Mars (or anywhere else) seemed impossible because the Red Planet's velocity is much higher than the Moon's. There appeared no way to get a spacecraft to slow down enough to glide into Mars' gravitational spacetime depression because the "bowl," not that deep to begin with, was itself a too-rapidly moving target. "I gave up on it," Belbruno says.

However, while recently consulting for the Boeing Corp., the major contractor for NASA's Space Launch System, which is intended to take humankind to Mars, Belbruno, Topputo and colleagues stumbled on an idea: Why not go with the flow near Mars? Sailing a spacecraft into an orbital path anywhere from a million to even tens of millions of kilometers ahead of the Red Planet would make it possible for Mars (and its spacetime bowl) to ease into the spacecraft's vicinity, thus subsequently letting the spacecraft be ballistically captured. Boeing, intrigued by this novel avenue to Mars, funded the study, in which the authors crunched some numbers and developed models for the capture.

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A New Way to Reach Mars Safely