The plasma instrument, with a mass of 9.9 kilograms, was designed to view in two directions: one toward the Earth and Sun, primarily to study the solar wind, and the other sideways, looking toward the direction plasma would flow if it were caught up in the rotating Jovian magnetic field. If it is desired to look in other directions, the entire spacecraft must be tipped, a maneuver that was carried out several times near Jupiter. The detectors directly sense the flow of electrons, protons, and alpha particles (helium nucleii, made up of two protons and two neutrons each). Analysis of the energy spectra can also yield data on positive ions of higher mass. [Illustration: Frederick L. Scarf, plasma wave Principal Investigator] Plasma Waves The plasma wave investigation on Voyager was a late addition to the scientific payload. It was selected to broaden the capability of the mission to study a wide variety of plasma processes. Because of the electrically charged nature of the plasma, it responds to energy inputs in ways that ordinary gas cannot. One of these modes of response yields plasma waves, which are oscillations in density and electric field that generally cover the audio range of frequencies. Measurement of such waves characterizes the density and temperature of the local plasma surrounding the spacecraft, and it also allows remote sensing detection of distant events from the plasma waves they produce. The plasma wave Principal Investigator is physicist Frederick L. Scarf of the TRW Defense and Space Systems Group of Redondo Beach, California—he is the only Voyager Principal Investigator to come from industry. Scarf has been associated with many particles and fields investigations in the terrestrial magnetosphere, although he is better known as a theorist than as an experimenter. A member of the Space Science Board of the National Academy of Sciences, he is familiar in Washington as an eloquent advocate of space physics—the study of plasma-physical processes in the space environment. The plasma wave instrument shares with the planetary radio astronomy investigation a pair of 10-meter-long antennas. Whereas the PRA uses these as electric antennas to detect radio radiation, the plasma wave system uses them to detect directly the oscillations in the plasma near the spacecraft. Waves are measured over a broad frequency range, from 10 hertz (a bit deeper than the lowest bass note we can hear) to 56 kilohertz (about three times higher than the highest pitch to which the human ear responds). The instrument electronics have a total mass of only 1.4 kilograms. Low Energy Charged Particles