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The heliosheath, usually meaning the inner heliosheath, is the region of shocked solar wind between the termination shock and the heliopause. Solar wind plasma entering it has been slowed and compressed, and its flow turns as it approaches the boundary with the interstellar medium. The termination shock and heliopause are different boundaries: crossing the shock does not yet mean leaving solar-wind material.

The Voyager 2 plasma experiment provided direct measurements around several shock crossings between 30 August and 1 September 2007. The observed thermal plasma was cooler than a simple picture of complete conversion of the incoming flow energy into thermal-ion heat would suggest. The researchers inferred that much of the energy instead went into pickup ions or other energetic particles.^[ref]

The instrument used four Faraday cups, three facing approximately into the flow. Currents recorded at different particle energies and viewing directions were fitted with moving Maxwell-Boltzmann velocity distributions to estimate proton temperatures. This measures a selected part of the particle population; it does not directly measure all the energetic ions that may carry the remaining energy.^[ref]

John D. Richardson studies space plasmas, including the solar wind and its interaction with the interstellar medium. MIT identifies him as a principal investigator of the Voyager plasma experiment and describes earlier work on plasma transport in planetary magnetospheres.^[ref]

His 2008 Voyager 2 study analyzed plasma measurements across the termination shock. The thermal ions were cooler than a simple picture of bulk-flow energy becoming thermal-ion heat would suggest. The result made the partition of energy among particle populations central to understanding the shock.^[ref]

The experiment illustrates how a remote boundary becomes a local measurement. Four Faraday cups sampled particle currents over energy-per-charge ranges; combining differently oriented detectors allowed the team to infer plasma density, flow and temperature. These inferred properties depend on the measured spectra and distribution model, rather than being direct readings of a thermometer.^[ref]

Voyager 2 is a robotic spacecraft launched on 20 August 1977 to investigate the outer Solar system. Its encounters with Jupiter in 1979, Saturn in 1981, Uranus in 1986 and Neptune in 1989 made one mission a comparative study of four giant planets. A gravity assist changes the spacecraft's trajectory and energy relative to the Sun by exchanging momentum with a moving planet, allowing a sequence of encounters that would require much more onboard propulsion if accomplished by engine firings alone.^[ref]

Voyager 2 later sampled the transition from the solar wind to the surrounding interstellar plasma. On 5 November 2018, its plasma instrument recorded a steep decline in the outward solar wind flow. The instrument inferred plasma properties from electrical currents collected from particles arriving in different directions. Changes in energetic-particle measurements supplied independent evidence: particles associated with the heliosphere declined while galactic cosmic rays became more prominent. The combined observations identify the heliopause crossing, rather than defining the boundary by a predetermined distance.^[ref]

This is a moving-laboratory experiment: the apparatus travels through regions that cannot be sampled from Earth, while several instruments test the interpretation of a sharp change. Crossing the heliopause means leaving the region dominated by the Sun's outward-flowing plasma; solar gravity does not suddenly end there. The crossing date and the December announcement date are different events.