Pioneer 5 (a.k.a. Pioneer P-2, and Thor Able 4) was launched on March 11, 1960 on a Thor-Able rocket, to explore the space between the orbits of Earth and Venus. It wasn’t a particularly large spacecraft by modern standards, weighing only 43 kg (94 lb) — but it provided the first maps of the interplanetary magnetic field (which had only been suspected to exist), and gave scientists new measurements of cosmic radiation.
Pioneer 5 set a number of technological milestones as well. It was the first spacecraft to use a digital telemetry system in interplanetary space, sporting data rates of 1, 8, or 64 bits per second. In spite of these (now-)low data rates, Pioneer 5 returned some 3 megabits of data to Earth. Controllers were in contact with the spacecraft until June 26, 1960 — at a then-record distance of 36.2 million km / 22.5 million miles from Earth.