Getting to Mercury Is Harder Than Getting to the Outer Planets. Earth Is Already Moving 67,000 Miles an Hour Sideways and a Spacecraft Has to Shed Most of That to Fall Inward. Mariner 10 Solved It by Flying 3,600 Miles Over Venus and Letting the Planet Slow It Down.

Getting to Mercury Is Harder Than Getting to the Outer Planets. Earth Is Already Moving 67,000 Miles an Hour Sideways and a Spacecraft Has to Shed Most of That to Fall Inward. Mariner 10 Solved It by Flying 3,600 Miles Over Venus and Letting the Planet Slow It Down.

A spacecraft weighing about 1,100 pounds, built for roughly $100 million in 1970s money, made three passes at Mercury in 1974 and 1975 and returned the first close images anyone had ever seen. It also found something nobody expected: a global magnetic field, about one percent as strong as Earth's, on a planet small enough that it should have cooled solid long ago.