IMP (Interplanetary Monitoring Platform) was a series of NASA probes, managed by the Goddard Space Flight Center, aimed at investigating plasma (ionized gas) and magnetic fields in interplanetary space. The placement of IMPs in a variety of solar and Earth orbits enabled the study of spatial and temporal relationships of geophysical and interplanetary phenomena simultaneously by several spacecraft. The IMP network also provided a crucial early warning of solar flare activity for Apollo manned missions that ventured beyond the Van Allen belts. IMPs were a subprogram of the Explorer series and also designated Explorers 18, 21, 28, 33, 34, 35, 41, 43, 47, and 50 (see Explorer entry for orbital details, etc).