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-eye—as in the Hohmann scenario—is 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 capture—it'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.
Expanding our Martian horizons
Ballistic capture is not the only fuel-saving technique for entering orbit. In another approach, called aerocapture, an arriving spacecraft dives into the Martian atmosphere and lets friction eat away at some of its excess velocity, rather than relying solely on a big fuel burn to do the trick. That method, however, requires a heavy heat shield, which adds extra mass and thus costs to liftoff, offsetting the penny-pinching on fuel for a Hohmann transfer burn on arrival. Ballistic capture, Topputo says, is "slower and gentler."
Ballistic capture therefore offers many advantages over current approaches for heading to Mars. Beyond avoiding the fuel-guzzling of a Hohmann transfer, for instance, it reduces danger to the craft because the vessel no longer must decelerate on a dime in a tight window near Mars, risking over- or undershooting its mark. The approach also drops fuel needs for the overall journey by 25 percent, Belbruno says, in a rough estimate. That reduction could be used to save money but it could also, instead, allow for bigger payloads at comparable prices. Delivering more mass to Martian orbit can then mean getting more robotic rovers, supplies or what have you to the surface. "What we want to do is leverage [ballistic capture] to put more mass on the ground," Green says. "That's the dream."
Avoiding the need to send the rocket up during rare launch windows would also be a big deal because launch delays are notoriously frequent. Missing a window can mean grounding a Mars mission for two years, plus wasted launch prep costs.
For 'bots, as well as bodies?
Ballistic capture does come with plenty of caveats, of course. A straight shot with abrupt braking at Mars takes about six months whereas a trip relying on ballistic capture would take an additional several months. The burn-free, capture altitude is also quite high—some 20,000 kilometers above Mars, far beyond where science satellites set up shop to scrutinize the planet up close. But taking along just a little extra fuel can then gently lower a ballistically captured spacecraft into scientifically valuable, standard orbits of around 100 to 200 kilometers like those achieved with Hohmann transfers—or even onward to the Martian surface for a landing.
For manned missions, ballistic transfer would be a mixed blessing. On one hand, its longer journeys would add to the challenges of ferrying people to Mars. We're already worried about Mars-bound explorers driving each other crazy stuck in a tin can for six months, not to mention soaking up unacceptably high space radiation doses. For that reason, robotic missions look to be the first potential beneficiaries of Belbruno and Topputo's new low-energy transfers.
On the other hand, because the need for launch windows would go away, ballistic capture could maintain a steady stream of supplies to the planet. Any extended Mars habitation effort would probably depend on Earth for materiel, at least until the establishment of self-sufficient farming and manufacturing. "Ballistic capture would be a good way to send supplies to Mars in advance of a manned mission," Belbruno says, "or as part of one."
NASA's Green agrees. "This [ballistic capture technique] could not only apply here to the robotic end of it but also the human exploration end," he says. Accordingly, Green arranged for Belbruno to speak with the agency’s Johnson Space Center staff back in October about how manned missions might exploit the concept.
Even further down the road, ballistic capture would be perfect, Belbruno says, for placing satellites into "areostationary" orbits—the same as geostationary, except at Mars (aka Ares). The upshot: Martian Internet and cell phone networks, anyone? If the new low-energy transfer works at Mars, it could, in theory, also be extended to deliver matter in bulk to any world in the solar system.
This potential breakthrough research is admittedly still in an early, theoretical phase. Ongoing work includes reworking the calculations of the physics by factoring in smaller influences on a Mars-bound spacecraft than the pull of gravity from Mars itself, such as Jupiter’s gravitational pull. NASA's Green said he envisions the agency wanting to test ballistic capture transfers at Mars in the 2020s.
Belbruno has his fingers crossed. "The route to the moon I found in 1991 was thought to be perhaps the only application of my theory," he says. "I am very excited about this Mars result."