Mariner 10 recorded an impressive list of space “firsts.” It was the first spacecraft craft to visit two planets, and the first to use the gravity of one planet to reach another. It also was the first to use the solar wind to help keep itself in the right position.
But its most important first took place 40 years ago today: It became the first spacecraft to visit Mercury, the solar system’s innermost planet.
Mercury is tough to study from Earth because it’s small — about half again the diameter of the Moon — and it never moves far from the Sun. So until Mariner 10, Mercury was pretty much a mystery.
Mariner 10 changed that when it flew just 400 miles above the planet’s surface. Its pictures revealed a landscape that closely resembles the Moon’s — a realm of impact craters. But they also showed features that are quite unlike those on the Moon, including tall ridges that run for miles.
Other instruments detected a weak magnetic field around the planet — an indication that it has a large metallic core.
After that first encounter, Mariner 10 looped around the Sun before recording another first — the first craft to visit the same planet twice. It looped past Mercury again in September 1974, and once more the following March. It saw the same side of Mercury on each passage. So we didn’t see the other side of the planet until the arrival of the second Mercury explorer — more than three decades after Mariner 10’s final visit.
Script by Damond Benningfield, Copyright 2014