Pioneer 4 was a spin-stabilized unmanned spacecraft launched as part of the Pioneer program on a lunar flyby trajectory and into a heliocentric orbit making it the first U.S. probe to escape from the Earth's gravity. It carried a payload similar to Pioneer 3: a lunar radiation environment experiment using a Geiger–Müller tube detector and a lunar photography experiment. It passed within 60,000 km of the Moon's surface. However, Pioneer 4 did not come close enough to trigger its photoelectric sensor. The spacecraft was still in solar orbit as of 1969. It was the only successful lunar probe launched by the US in seven attempts between 1958–60; only in 1963 would Ranger 6 surpass its success by accomplishing all of its mission objectives.