Explorer 19 was an American satellite launched on December 19, 1963 as part of NASA's Explorers program. It was the third of six identical Explorer satellites launched to study air density and composition, and the second to reach orbit. It was identical to Explorer 9.
The second of six identical air density research satellites to be launched, Explorer 9 was the first to successfully reach orbit. It was still operational when the next satellite, Explorer 19, was launched, allowing simultaneous readings to be taken and compared. The spacecraft consisted of alternating layers of aluminium foil and Mylarpolyester film. Uniformly distributed over the aluminium surface were 5.1 cm-diameter dots of white paint for thermal control. The sphere was packed in a tube 21.6 cm in diameter and 48.3 cm long and mounted in the nose of the fourth stage of the launch vehicle. Upon separation of the fourth stage, the sphere was inflated by a nitrogen gas bottle, and a separation spring ejected it out into its own orbit. The two hemispheres of aluminium foil were separated with a gap of Mylar at the spacecraft's equator and served as the antenna. A 136 MHz, 15 mW beacon was carried for tracking purposes, but the beacon failed on the first orbit and the SAO Baker-Nunn camera network had to be relied upon for tracking. Power was supplied by solar cells and rechargeable batteries
Explorer 19's launch vehicle, a Scout X-4, placed it into a slightly lower than planned orbit.