In situ measurements of the interplanetary medium have been made in the
region between the orbit of Mercury and the orbit of Pluto. The Mariner II
spacecraft made the first comprehensive measurements within 1 AU. Helios
1 and 2 moved from the Earth at 1 AU to 0.3 AU. The IMPs, ISEE-3, the
Vela spacecraft, and the Soviet spacecraft such as Prognoz made detailed
studies at 1 AU. Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 are
moving far beyond the orbit of Neptune. These remarkable voyages are
historic achievements, representing the accomplishments of thousands of
people and costing hundreds of millions of dollars. The analysis of data from
these spacecraft provides a detailed picture of our Sun's exotic and beautiful
environment. The time is ripe for a book that summarizes some of the
essential results of these measurements and their interpretations