During this quadrennium, while the Voyager and Pioneer spacecraft moved through the outer heliosphere, ever further from the sun, the Ulysses and Galileo spacecraft were traversing the inner heliosphere. Sophisticated navigation [see for example, D'Amario et al., 1992] directed them from one planet to another, directing them into orbits tailored to match the primary scientific requirements of the missions. In both cases the trajectories were indirect; the desired orbits required flybys of one or more planets to obtain gravitational assists. It is those flybys, visits to secondary targets as it were, on which this review focuses. The flybys provided rich mines of data that are being exploited with great benefit to space plasma physics. The intriguing results obtained more than justify the effort made to grasp opportunities that were in a sense accidental.
The Galileo spacecraft consists of a massive Orbiter and a small Probe. The Probe will be released on a trajectory that penetrates the Jovian atmosphere where it will make history by acquiring in situ measurements. The Orbiter will be placed into a capture orbit (another first) where it will spend at least two years acquiring data on Jupiter's magnetosphere, atmosphere, and satellites [ Johnson et al., 1992]. Although the original mission plans anticipated use of a direct trajectory from Earth to Jupiter and an arrival in the early 1980s, numerous problems eliminated this option. Instead Galileo will reach Jupiter in late 1995, following a six year interplanetary odyssey. The spacecraft sped by Venus once and Earth twice. On its two passages through the asteroid belt, precious maneuvering fuel was dedicated to retargeting the spacecraft so that it could fly by two small asteroids. Although not totally accidental, these flybys were also peripheral to the principal purpose of the mission, so we shall also discuss the asteroid flybys. Below we summarize a few of the key findings from these multiple, diverse flybys.
The primary objective of the Ulysses mission is to explore high heliographic latitudes where the solar wind and cosmic ray fluxes are expected to differ greatly from those typical at low latitudes. But high heliographic latitudes are not readily reached by spacecraft launched from the surface of the Earth with a velocity that differs little from Earth's orbital velocity. An effective way to get out of the ecliptic plane is to head toward Jupiter where the strong gravitational field can redirect the velocity vector. Therefore, Ulysses journeyed first to Jupiter and thence onward towards the solar poles. The Ulysses flyby of its secondary target provided an unparalleled opportunity for the exploration of one of the most remarkable elements of the solar system, the Jovian magnetosphere. An outstanding complement of instruments, though not optimized for the planetary environment, made numerous "first ever" measurements. In a section that follows, some of the new results are described.
In the discussions that follow, unless otherwise stated, the coordinate system is the GSE (geocentric solar ecliptic) system for Earth ( x towards the sun, z towards ecliptic north), or the equivalent planetary solar orbital system for other bodies ( x towards the sun, z normal to the planetary orbit and positive north). When symmetry about the magnetotail axis is important, it is useful to correct for orbital motion by rotating the coordinate system about the z-axis so that the x-axis is aligned with the apparent direction of the average solar wind in the planet's rest frame. This system is referred to as aberrated GSE or equivalent.