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Voyager 1 is a spacecraft launched on 5 September 1977 to investigate the outer Solar system. It flew past Jupiter in March 1979 and Saturn in November 1980, then continued outward on an extended mission to study the environment beyond the planets.^[ref]

On 25 August 2012 it crossed the heliopause, the boundary between the solar wind-dominated heliosphere and the surrounding interstellar medium. This boundary concerns the plasma environment; it is not a boundary beyond which the Sun's gravity vanishes. Its scientific instruments include detectors of charged particles, a magnetometer and a Voyager plasma-wave instrument. Together, measurements of particles, magnetic fields and plasma waves allow a moving probe to investigate a remote environment that cannot be sampled in a terrestrial laboratory.^[ref]

The Voyager plasma-wave instrument measures fluctuating electric fields associated with plasma waves. Its original design combines a multichannel spectrum analyser with a wideband waveform receiver, making both frequency content and selected time-domain waveforms available.^[ref]

When an observed feature is identified with electron plasma oscillations, its frequency constrains electron number density through $f_p=(2\pi {)}^{-1}\sqrt{n_e{e}^2/({\epsilon }_0{m}_e)}$. Thus a fourfold density increase doubles the characteristic frequency. Wave identification, instrument calibration and the surrounding plasma conditions enter the inference. On Voyager 1, such measurements have provided local information about plasma beyond the heliopause.^[ref]