# We Found a New and Cheaper Way to Get to Mars

By Sarah Zhang on at

There are myriad reasons why getting from Earth to Mars is hard, but chief among them are two 1) the massive amount of fuel needed and 2) a launch window that is limited to every 26 months, when the two planets are in optimal alignment. A couple of mathematicians have calculated a new path to Mars that solves both—and it's far from a straight line.

Mapping a route to Mars, of course, is more complicated than mapping any Earth-bound route. The distance between the two planets is constantly growing or shrinking, depending on their orbits around the sun. (That's why the optimal launch window only opens every 26 months.) And you have to take into account how gravity from the Earth, Mars, and sun will pull a spacecraft off its course.

Mathematicians Francesco Topputo and Edward Belbruno have calculated a path that actually takes advantage of the Mars' own motion. The strategy is called ballistic capture, as opposed to what we now use, the Hohmann transfer. Scientific American explains both:

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.

A crucial detail is that the spacecraft will no longer need hundreds of litres of extra fuel to brake as it approaches Mars. Twenty-five per cent fuel means less weight, which means a smaller and cheaper rocket to boost the whole thing into space. Or, alternatively, more stuff in the payload.

Ballistic capture also erases the need to wait for when Mars and Earth are in optimal orbital alignment, making it a lot easier to schedule the multiple missions to drop supplies necessary for humans to survive on Mars.

But it also has a downside. The journey will take longer, adding several months to the already six-month haul. For human space travellers, that would take a toll that is both physical (mental exposure) and mental (cabin fever).

Still, ballistic capture adds another promising option for getting to Mars. James Green of NASA told Scientific American this is paper is an "eye-opener." To learn a lot more about the backstory of ballistic capture—including why it took so longer for anyone to think of it for Mars—be sure to read the whole story at Scientific American. [Scientific American, arXiv]

Top image: JPL/NASA