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
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
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
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
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.
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.
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.