Pioneer 4 - Credit: NASA. 13,289 bytes. 191 x 348 pixels.

Following the unsuccessful USAF/NASA Pioneer 0, 1, and 2 lunar missions, the U. S. Army and NASA launched 2 more lunar missions. Smaller than the previous Pioneers, Pioneer 3 and 4 each carried only a single experiment to detect cosmic radiation. Both vehicles were planned to flyby the moon and return data about the Earth and Moon's radiation environment. The launch of Pioneer 3 failed when the launch vehicle first's stage cut-off prematurely. Although Pioneer 3 did not achieve escape velocity, it reached an altitude of 102,332 km and discovered a second radiation belt around Earth. The launch of Pioneer 4 was successful, and Pioneer 4 was the first American spacecraft to escape Earth's gravitational pull as it passed within 58,983 km of the moon (about twice the planned flyby altitude). The spacecraft did return data on the Moon radiation environment, although the desire to be the first man-made vehicle to fly past the moon was lost when the Soviet Union's Luna 1 passed by the Moon several weeks before Pioneer 4. Spacecraft: Battery powered. Passive thermal control (paint pattern on external surface). Fibreglass external structure. Payload: 2 Geiger counters. Camera trigger mechanism for planned camera system carried as test (no camera).

Design Life: Several days. Total Length: 0.5 m. Maximum Diameter: 0.2 m. Total Mass: 6 kg.

Failed to reach moon; provided radiation data. Pioneer III, the third U.S.-IGY intended lunar probe under the direction of NASA with the Army acting as executive agent, was launched from the Atlantic Missile Range by a Juno II rocket. The primary objective, to place the 12.95 pound scientific payload in the vicinity of the moon, failed. Pioneer III reached an altitude of approximately 70,000 miles and revealed that the earth's radiation belt comprised at least two distinct bands.

The fourth U.S.-IGY lunar probe effort, Pioneer IV, a joint project of the Army Ballistic Missile Agency and Jet Propulsion Laboratory under the direction of NASA, was launched by a Juno II rocket from the Atlantic Missile Range. Intended to impact on the lunar surface, Pioneer IV achieved earth-moon trajectory, passing within 60,200 km of the moon before going into permanent orbit around the sun.

• 2 - McDowell, Jonathan, The United Nations Registry of Space Objects, Harvard University, 1997. HTML when accessed: http://hea-www.harvard.edu/QEDT/jcm/space/un/un.html
• 6 - JPL Mission and Spacecraft Library, Jet Propulsion Laboratory, 1997. HTML when accessed: http://msl.jpl.nasa.gov/home.html
• 279 - McDowell, Jonathan, Launch Log, October 1998. HTML when accessed: http://hea-www.harvard.edu/~jcm/space/log/launch.html