The United States entered the space age by launching the first successful orbiting satellite, Explorer-I, four months after the Soviet launch of Sputnik on 4 Oct 1957. Explorer-I measured cosmic radiation, and led to the discovery of the Van Allen radiation belt. Its instrumentation included temperature sensors, a micrometeorite impact microphone, and a ring of micrometeorite erosion gauges. Data from these instruments were returned by a 60-mW transmitter operating on 108.03 MHz and a 10-milliwatt transmitter operating on 108.00 MHz. Explorer-I was 80-inch long, 6-in diam, weighed 31-lb with 18-lb of payload and was delivered into orbit using a Jupiter-C rocket. The orbit had a period of 114.9 minutes.

The U.S. launched a 4-year-old male chimpanzee named Ham on a Mercury-Redstone 2 rocket into suborbital flight to test the capabilities of the Mercury capsule. During his 16.5 minute suborbital flight, Ham experienced about 7 minutes of weightlessness, reached an altitude of 108 miles and a speed of 13,000 mph. He was wired to medical sensors to monitor his vital signs. During flight, Ham performed some simple tasks such as pulling levers when a light came on for a reward of banana pellets. Ham was recovered safely 1,425 miles downrange. This was a test flight before risking the lives of human beings. Now, after Ham's successful flight, NASA was ready to launch the first Mercury astronaut, Alan Shepard, into sub-orbital flight three months later.