Explorer 17 carried four pressure gauges for the measurement of total neutral particle density, two mass spectrometers for the measurement of certain neutral particle concentrations, and two electrostatic probes for ion concentration and electron temperature measurements. April 3, 1963 at 02:00 UTC · Launch Successful Thor Delta B — United States Air Force, to Low Earth Orbit Space Launch Complex 17A, Cape Canaveral SFS, FL, USA The world's 236th orbital launch attempt, and 12th of 1963. Data: Launch Library 2, The Space Devs.