Weighing 405 lbs. (184 kg), this 35-inch (89-cm) pressurized stainless steel sphere measured the density, composition, pressure and temperature of Earth’s atmosphere after its launch from Cape Canaveral on April 3, 1963. The mission was one of three that Goddard Space Flight Center specifically conducted to learn more about the atmosphere’s physical properties, knowledge that they ultimately used for scientific and meteorological purposes. Explorer XVII carried two spectrometers, four vacuum pressure gauges and two electrostatic probes. Before it reached its intended orbit that ranged from 158 to 570 miles (254-917 km) above Earth, the satellite was spun up to about 90 rpm.