Recent studies of the low‐energy plasma population in the Earth’s space environment have revealed that this plasma population is much more complex than previously supposed and that a simple model of ionospheric evaporation cannot explain the distributions. There was a need to develop an advanced instrument to study this plasma in detail, and this paper describes the scientific background, design, development, and in‐flight characteristics of such an instrument, the Light Ion Mass Spectrometer (LIMS). This instrument combines a magnetic mass spectrometer, a planar‐grid retarding potential analyzer, and multidirectional sensor heads to measure the mass composition, density, temperature, and flow velocity of low‐energy (E〈100 eV) plasma. The studies which were conducted leading to the final design will be discussed in detail and will illustrate certain effects which arose in the combining of energy and mass analysis into a single sensor. The instrument was flown on a high‐altitude satellite in February 1979, and selected flight data will be presented to demonstrate the instrument performance.