Orbiting Vehicle 1–15, launched 11 July 1968, was the first satellite to return long-term information on the density of the Earth's upper atmosphere. Part of the OV1 series of USAF satellites, using standardized designs and sent to orbit on decommissioned Atlas ICBMs to reduce development and launching costs, OV1-15 was launched via Atlas F side-by-side with OV1-16. Before its orbit decayed, causing the satellite to reenter on 8 November 1968, OV1-15 profoundly improved our knowledge of the upper atmosphere, proving that air density increased with solar activity rather than decreasing, as had been the prevailing theory to that time.