= Voyager 2
{c}
<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.<Voyager 2 Mission Overview by NASA>[launch, mission history and planetary encounter chronology]{ref}
<Voyager 2> later sampled the transition from the <solar wind> to the surrounding interstellar <plasma>. On \b[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.<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]{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.
Back to article page
