The new solar-electric propulsion system, in conjunction with a series of nine planetary flybys, will allow the unmanned spacecraft to reach its destination against the pull of the Sun's gravity.

After a successful launch from the European Spaceport in Kourou, French Guiana, the BepiColombo mission to Mercury is currently undergoing system checks before deploying its instruments and going to hibernation for the long five-year journey. However, it won't be a quiet sleep.

Unlike traveling to the outer planets, getting to Mercury poses its own peculiar problems. To reach Mars, for example, a spacecraft needs rockets to boost its velocity, so it can go into an orbit farther from the Sun. It's a bit like pushing a wagon uphill. As you do so, you're actually pumping energy into the wagon as it is moved away from the center of the Earth. But in going to one of the inner planets, you're hurtling downhill and hoping there's something soft to crash into.

This suggests that sending a probe to one of the inner planets should be as easy as dropping a stone down a well, but it turns out to be anything but. True, a relatively small rocket could push a spacecraft toward Mercury, but as it traveled, it would accelerate, shoot past its target and slingshot around the Sun going much faster than before.