Voyager and Pioneer missions to the boundaries of the heliosphere

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Abstract
The interaction of the solar wind with the interstellar medium is expected to result in a complex, probably dynamic outer heliospheric boundary region. In increasing distance from the Sun the boundary region includes the solar wind termination shock, the heliopause, and perhaps a heliobow‐shock. The continuing missions of Voyager 1 and 2 and Pioneer 10 and 11 provide a unique opportunity to make in situ particle and field observations of the boundaries of the heliosphere and associated phenomena. Observations already made by these spacecraft suggest that the termination shock may be located 60‐100 AU from the Sun. Voyager 1 will reach 60 AU in 1995 and 100 AU in 2006. NASA's Space Physics Division is planning with the Voyager and Pioneer Projects to ensure timely and maximum scientific return from spacecraft encounters with the solar wind termination region. Initial emphasis is on being able to reliably determine when an encounter is imminent, so that spacecraft tracking and spacecraft and instrument configurations can be changed to encounter modes. It is anticipated that multiple encounters with heliospheric boundaries will occur due to motion of the boundaries in response to large variations in the solar wind pressure.