March 30, 2009 Vol. 2, Issue 3
Fifty years ago, Pioneer 4 became the first U.S. satellite to escape the earth’s gravity as it rocketed toward a close encounter with the moon.
Launched on March 3, 1959 into a lunar fly-by trajectory, the cone-shaped spacecraft passed within 60,000 km of the moon to conduct a lunar radiation experiment using a Geiger-Mueller tube detector and a lunar photography experiment. It followed closely in the footsteps of Pioneer 3, which had been launched in December 1958. Pioneer 4 did not come close enough to the moon to trigger the sensor for the experiment; no lunar radiation was detected. The spacecraft continued on in a heliocentric (solar) orbit, another first for a U.S. spacecraft.
