Juno II was a launch vehicle developed by America's space program to enable boosting larger scientific payloads into space. The modified Jupiter C (sometimes called Juno I) that was used to launch Explorer I had minimum payload lifting capabilities, as Explorer I only weighed 31 pounds.
Juno II successfully launched a Pioneer IV satellite on March 3, 1959, and an Explorer VII satellite on October 13, 1959. Pioneer IV passed within 37,000 miles of the moon before going into permanent solar orbit. Explorer VII had a total weight of 91.5 pounds and carried a scientific package for detecting micrometeors, measuring the Earth's radiation balance, and conducting other experiments.
On November 3, 1960, a Juno II sent Explorer VIII into a 1,000-mile deep orbit within the ionosphere. In 1961, Juno II launched an 82-pound satellite into orbit to conduct a complex gamma-ray astronomy experiment.
Subsequently, improved launch vehicles replaced the Juno II as the primary launcher for the Explorer satellite series.