Mission Type: Flyby
Launch Vehicle: Juno II (no. AM-14)
Launch Site: Cape Canaveral, United States, launch complex 5
Spacecraft Mass: 6.1 kg
Spacecraft Instruments: 1) photoelectric sensor trigger and 2) two Geiger-Mueller counters
References:
Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, Monographs in Aerospace History No. 24, by Asif A. Siddiqi

National Space Science Data Center, http://nssdc.gsfc.nasa.gov/

Although it did not achieve its primary objective to photograph the Moon during a flyby, Pioneer 4 was the first U.S. spacecraft to reach escape velocity.

During the launch, the Sergeants of the second stage did not cut off on time and caused the azimuths and elevation angles of the trajectory to change. The spacecraft thus passed by the Moon at a range of 59,545 kilometers (instead of the planned 32,000 kilometers) not close enough for the imaging scanner to function. The closest approach was at 10:25 UT on 4 March 1959.

The spacecraft's tiny radio transmitted information for 82 hours before contact was lost at a distance of 655,000 kilometers from Earth, the greatest tracking distance for a human-made object at the time. The probe eventually entered
heliocentric orbit and became the first American spacecraft to do so. Scientists
received excellent data on radiation in space.