Before American astronauts ventured beyond the Earth, scientists and engineers had to assess the nature of the space environment and the hazards of human exploration. In early 1959-just after the start of Project Mercury and during selection of the seven Mercury astronauts-the Pioneer IV space probe was launched into orbit around the sun, and two monkeys, Able and Baker, made a 579-kilometer (360-mile) high sub-orbital flight.

PIONEER IV
On March 3, 1959, the U.S. launched the Pioneer IV space probe, boosted by a four-stage Juno II rocket. For 82 hours on-board instruments transmitted measurements of radiation in space. When the batteries went dead, the probe was 654,900 kilometers (407,000 miles) from Earth.
Mission planners designed Pioneer IV to pass within 32,000 kilometers (20,000 miles) of the Moon. It carried a photoelectric sensor to be triggered by moonlight in a test for future photographic missions. To accomplish this mission the speed and trajectory of the probe had to be within precise targets. The launch speed was 302.7 kilometers (188 miles) per hour too slow and the probe passed the Moon at 60,016 kilometers (37,300 miles), too far away for the sensor to work.
Pioneer IV was the second object sent into solar orbit from Earth. Two months earlier the Soviet Union's Mechta rocket had passed 5953 kilometers (3700 miles) from the Moon and had been tracked to a distance of 542,200 kilometers (337,000 miles) from Earth.

Pioneer IV Instrument Package