Five Robotic Milestones That Redefined Deep-Space Exploration

Mariner 2's 34,854-kilometre Venus flyby completed the first fully successful interplanetary mission.

Mariner 4's first close-up Mars photographs mattered: 21 complete images and a partial twenty-second image showed Mars close-up.

Venera 7's first transmissions from another planet's surface mattered because recovered data lasted 22 minutes 58 seconds.

Voyager 2's 1989 Neptune encounter culminated a gravity-assist route spanning the outer planets.