Introduction

The primary goal of the Ulysses mission, a joint venture of ESA and NASA launched in October 1990, is to explore for the first time the region of space above the poles of the Sun. Although the importance of this exploration to our understanding of the Sun's environment, the heliosphere, has long been recognised, the practical implementation of such a mission has, until relatively recently, been impossible. While we are able to place satellites into a polar orbit around the Earth without much difficulty, the energy needed to launch a space probe into a polar orbit around the Sun is far greater. So much energy is required in fact, that even with the powerful launch vehicles available today, direct injection from the Earth cannot be achieved. This is because the Earth itself orbits the Sun at a speed of 30 km/s in a plane perpendicular to the desired solar polar orbit. The energy imparted to a space probe must cancel out this motion in addition to providing the correct polar trajectory.

A polar orbit can be achieved, however, by taking advantage of a gravity assist by another planet. Jupiter is the nearest body capable of meeting the requirements. The need to make use of Jupiter in order to carry out its primary mission has resulted in the recent flight of the ESA-built Ulysses spacecraft through the Jovian magnetosphere. Even with this gravity assist, the combined power of the space shuttle and three upper-stage rockets were needed to send the 370 kg space probe on its way. As it left the confines of the Earth's gravitational field, Ulysses was travelling at 11.3 km/s, making it the fastest interplanetary spacecraft ever launched.

Ulysses arrived at Jupiter 16 months after departing from Earth, having travelled nearly 1 billion kilometers in the ecliptic. (See ESA Bulletin No. 67, August 1991, for a report on the first scientific results from the in-ecliptic phase of the mission). Closest approach to the planet occurred at 12:02 UT on 8 February, 1992. As explained above, the primary aim of the flyby was to place the spacecraft in its final heliocentric out-of-ecliptic orbit with a minimum of risk to the onboard systems and scientific payload. Scientific investigations at Jupiter are a secondary objective of the mission. Nevertheless, the opportunity to study Jupiter's magnetosphere was exploited to the greatest extent possible. The results exceeded all expectations of the scientists involved.

Figure 1: Schematic of Jupiter's magnetosphere. The solar wind, approaching from the left, is deflected around the magnetosphere by the bow shock. The outer boundary of the magnetosphere, called the magnetopause, is indicated. Major structural features inside the magnetosphere are shown.

Jupiter is a strongly magnetised, rapidly rotating planet. Its magnetosphere is the largest object in the solar system, a fact reflected in the long interval of 12 days from 2 to 14 February (days 033 to 045 of 1992) that it took for Ulysses to travel through it. The large Galilean satellites are embedded within the magnetosphere and Io is known to be a prolific source of ions and neutral particles (Fig.1). Ions, predominantly of sulfur and oxygen, are distributed around the orbit of Io to form a large torus. Electrons and ions from Io, Jupiter's ionosphere and the solar wind are all present and are transported throughout the magnetosphere. A substantial fraction of these particles are accelerated to extremely high energies to form intense radiation belts. Upstream of the magnetosphere, in the free-streaming solar wind, a detached bow shock forms which slows the solar wind and allows it to be deflected around the magnetosphere. A wide variety of complex physical phenomena are available for study.

Figure 2: The Ulysses trajectory past Jupiter. The open circle represents the point of closest approach at 6.3 Jovian radii (450,000 km) from the centre of the planet. Vertical lines denote intervals of 3 hours relative to closest approach.

The inbound trajectory (Fig.2) was rather similar to those of the four spacecraft which flew past Jupiter previously: Pioneer 10, 11 (1972, 1973) and Voyager 1, 2 (1979). In contrast to these missions, however, Ulysses reached high latitudes (40 deg. north of Jupiter's equator) near closest approach. A unique aspect of the Ulysses flight path was the outbound passage through the hitherto unexplored dusk sector (18:00 hours local time) of the magnetosphere, this time at high southern latitudes. Another unique aspect of the flyby was the penetration of the Io Plasma Torus (IPT), a few hours after closest approach, in a basically north-south direction which contrasted with the nearly equatorial Voyager 1 traversal. In addition to this direct penetration, the spacecraft radio signal passed through the IPT for a significant length of time making it possible to probe the electron density distribution in the Torus.

Although the instruments that make up the scientific payload are optimised for the conditions encountered in the solar wind, including their orientation on the spinning spacecraft, they have produced a wealth of new information relating to the Jovian magnetosphere. In this report we summarise some of the initial findings. As is the case for the primary mission, many of the observations made in the magnetosphere by the different experiments are complementary in nature, making a correlative approach the most fruitful in terms of data interpretation.