On Jan. 31, 1958, Explorer 1 became the first satellite launched by the United States. Its primary science instrument, a cosmic ray detector, was designed to measure the radiation environment in Earth orbit. Though its final transmission was in May 1958, it continued to revolve around Earth more than 58,000 times. As those looping orbits continued, NASA was busy building other groundbreaking instruments to observe and better understand Earth's systems.
