An unmanned American spacecraft. The first American spacecraft to escape Earth's gravitational pull and pass by the Moon.

Pioneer 4 was the first successful mission in the Pioneer series. Launched by a Juno 2 rocket on March 3, 1959 from Cape Canaveral, the craft achieved Earth-Moon trajectory passing by the Moon at a distance of 60,200 km, about twice the planned altitude. The craft then went into permanent orbit around the Sun.

The mission was undertaken with the hope to be the first man-made vehicle to escape Earth orbit. However, the Soviet Union won the initial Race to the Moon by launching Luna 1 past the Moon on January 2, 1959.

A tiny probe weighing only 6 kg, it carried a Geiger counter to measure the radiation environment in interplanetary space and a camera trigger experiment that failed because it didn't past close enough to the Moon.

A distance record of 654,860 km was set as the spacecraft was tracked for 84 hours until March 6, 1959 when the on-board batteries were exhausted.