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 tha n 58,000 times. As those looping orbits continued, NASA was busy building other groundbreaking instruments to observe and better understand Earth's systems. ...read more read less