The plasma probe is designed for use on multiple sounding rocket missions. The instrument is designed to operate either in the daytime D region of the ionosphere( ISIS ) or in the nighttime E region (HEX II). For the purposes of the ISIS mission, only the space environment in the D region of the ionosphere is considered. The plasma probe is intended to operate with all collectors deployed at altitudes greater than 65 km and nominally less than 100 km. In this region, the plasma density normally ranges from 10 cm - 3 to 6 x 10 4 cm -3 . Atmospheric pressure above 65 km is less than 10 -1 mbar with temperatures are in the range of 200 to 250 Kelvins. Under these conditions, the Debye length ranges from 0.4 to 8.0 cm. In order for the tethered collectors to measure conditions in the region undisturbed by the probe payload structure, a tether length of at least 50 cm is required.