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The heliopause is the boundary separating the predominantly solar-wind plasma of the heliosphere from the surrounding interstellar plasma. It is set by the interaction of flows, magnetic fields and pressure, rather than by a fixed spherical surface. It differs from the termination shock, where the solar wind slows abruptly inside the heliosphere.

Voyager 1 supplied a direct test. Plasma-wave oscillations observed in 2013 indicated a much greater local electron density, supporting a retrospective crossing in August 2012. Because the characteristic plasma frequency depends on the square root of electron density, the observed oscillation frequency served as a density diagnostic.^[ref]

Retain the source's idealized solar radial trajectory. Use ${\mu }_{\odot }\simeq 1.32712\times 10^{11}\mathrm{k}{\mathrm{m}}^3{\mathrm{s}}^{-2}$, $r_i=1.5\times 10^9\mathrm{km}$ and $v_i=22\mathrm{km}{\mathrm{s}}^{-1}$. At $r=1.8\times 10^{10}\mathrm{km}$,The speed has decreased because the probe gains gravitational potential energy while moving away from the Sun. This is the supplied simplified model, rather than a reconstruction of Voyager 1's actual three-dimensional trajectory or planetary encounters.

The termination shock is the transition where the expanding solar wind slows from supersonic flow into the slower, compressed and heated flow of the outer heliosphere. It forms because the outward flow encounters pressure from its surroundings. It is distinct from the farther-out heliopause, where solar-wind plasma meets the interstellar plasma.

A shock wave converts organized flow kinetic energy into thermal and nonthermal particle energy. A spacecraft can therefore identify a crossing through changes in the magnetic field, plasma and energetic-particle populations, rather than through a visible solid boundary. Voyager 1 crossed the termination shock on 16 December 2004 near 94 astronomical units from the Sun. Its magnetic-field and particle instruments supplied evidence for entering the heliosheath.^[ref] The original particle report found enhanced low-energy protons but no local maximum of anomalous cosmic ray helium, illustrating why detecting a shock and explaining all particle acceleration are separate questions.^[ref]

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]