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 Mission Overview by NASA. launch, mission history and planetary encounter chronology
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]
NASA's Voyager 2 Probe Enters Interstellar Space by NASA Jet Propulsion Laboratory. 10 December 2018 announcement; plasma-current and particle measurements dating the crossing to 5 November
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.
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