Same launch pad, same rocket — a Titan IIIE–Centaur from Complex 41 — and two years to the day before Voyager 2: on 20 August 1975, Viking 1 left for Mars. It arrived the following June, spent a month circling and looking for somewhere safe to set down, and on 20 July 1976 its lander became the first spacecraft to land on Mars and stay alive long enough to tell us about it. It was built to work for ninety days. It worked for six years.

The lander’s first photograph showed its own footpad resting in a field of rocks. I think about that picture more than I should: here is our foot, and here is another world.