Back when transistor radios were hip, Mariner 4 sent these first close-up views of Mars across 134 million miles using a puny 10-watt radio transmitter. The pictures reached home at an agonizingly slow rate of 8.3 bits per second. Each grainy image took eight hours to receive, but the payoff was huge: Surprising evidence of deep impact craters banished the popular idea of Mars as a chillier version of Earth.