In 1991, Japan’s budding space agency sent its first spacecraft to the moon: Hiten, alongside a smaller orbiter named Hagamoro. Hagamoro was supposed to orbit the moon, but mission control lost contact with it after it detached from Hiten. Meanwhile, Hiten didn’t have enough fuel left to enter orbit around the moon in the usual way. So a pair of mathematicians, then based at California’s Jet Propulsion Laboratory, helped Japanese engineers plot a new trajectory for Hiten, allowing Japan to become the third nation to flyby, orbit, and land on Earth’s natural satellite. The new path used ballistic capture, also called low-energy transfer.
Now one of those mathematicians, Edward Belbruno, says he’s made the calculations to use ballistic capture to bring spacecraft to Mars more cheaply and safely than ever before.
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:
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.
Recently, Belbruno and a colleague, Francesco Topputo, plotted out a ballistic capture trajectory designed to get a craft into orbit around Mars. Since Japan’s Hiten mission, NASA and the European Space Agency have only used ballistic capture to send missions around the moon. Belbruno and Francesco Topputo have posted a paper about their new work to the free database arXiv, but haven’t yet had the paper peer-reviewed. They estimate their method could use 25 percent less fuel, although it would also take more time to reach Mars than a spacecraft using a Hohmann transfer.
At its best, ballistic capture on Mars would save money, be less risky for the spacecraft, and work with larger launch windows, Belbruno tells Scientific American. In the meantime, the idea is still in its theoretical phase. There are many challenges left. For example, researchers still have to figure out how the gravitational pulls of other bodies out there, such as Jupiter, will affect their calculations.