Flying toward the sun is like running down a steep hill. Near the bottom of the hill, it is hard to slow down, which is essentially what BepiColombo needs to do before it can swing into orbit around Mercury, instead of just whizzing by.

The first spacecraft to go to Mercury, NASA’s Mariner 10 in 1974, made the trip in less than five months. But that was only a short flyby, passing within 450 miles of the surface. It made two additional flybys but never entered orbit.

A rocket engine could act as a brake, but that would require far more fuel than BepiColombo would be able to carry.