JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, A09S01, doi:10.1029/2004JA010742, 2004
Cassini//Huygens flyby of the Jovian system
Scott J. Bolton, Candice J. Hansen, Dennis L. Matson, and Linda J. Spilker
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
Jean-Pierre Lebreton
Space Science Department, European Space Research and Technology Centre/European Space Agency, Noordwijk,
Netherlands
Received 13 August 2004; revised 16 August 2004; accepted 24 August 2004; published 30 September 2004.
[1] This is an introduction to the series of 15 papers that follow. These report scientific
results from the fields-and-particles instruments on Cassini/Huygens. For these works,
their data acquisition started as early as 16 months before the passage by Jupiter and,
for some, continued up to the approach to Saturn. The flyby of Jupiter provided a
gravitational assist, which was necessary for the spacecraft to reach its destination, the
Saturnian system. In addition to the scientific results, the Cassini/Huygens team gained
much because Jupiter provided a dress rehearsal for skills needed at Saturn. The flyby
also fostered international cooperation between the Cassini/Huygens teams, the Galileo
teams, and the scientific teams using instruments in orbit about the Earth and on the
ground. A number of important discoveries and insights came from these cooperative
efforts. The boundary physics and compression of the Jovian magnetosphere was observed
by both fields and particle instruments. The propagation of the solar wind between
Earth and Jupiter was studied. The source of Jovian radio emission was further
characterized. The first images of energetic neutral atoms at Jupiter were
INDEX TERMS: 6220 Planetology: Solar System Objects: Jupiter; 6297 Planetology: Solar
obtained.
System Objects: Instruments and techniques; 2756 Magnetospheric Physics: Planetary magnetospheres
(5443, 5737, 6030); 2794 Magnetospheric Physics: Instruments and techniques; KEYWORDS: Cassini,
Jupiter, Cassini-Huygens, flyby
Citation: Bolton, S. J., C. J. Hansen, D. L. Matson, L. J. Spilker, and J.-P. Lebreton (2004), Cassini/Huygens flyby of the Jovian
system, J. Geophys. Res., 109, A09S01, doi:10.1029/2004JA010742.
1. Introduction
[2] Cassini/Huygens is an international mission that
consists of the Cassini Saturn Orbiter spacecraft and the
Huygens Titan Probe. Huygens is targeted for entry into the
atmosphere of Saturn’s largest moon, Titan. Between its
launch on 15 October 1997 and its arrival at Saturn in July
2004, Cassini/Huygens will travel over 3 billion km. Once
in orbit about Saturn, the required trajectory for properly
inserting the Probe into Titan’s atmosphere is set up. Some
6 months after arrival at Saturn, Huygens is released from
the Orbiter and enters Titan’s atmosphere. After the completion of the Huygens mission, the Orbiter continues its
4-year tour of the Saturnian system. Forty-five flybys of
Titan are used not only to investigate Titan but also for
gravitational deflections needed for maneuvering the
Orbiter on to multiple encounters with the icy moons and
the different regions of the magnetosphere. Thus Titan, the
rings, icy satellites, the magnetosphere, and Saturn itself
are all studied, as well as the many interactions among
them.
Copyright 2004 by the American Geophysical Union.
0148-0227/04/2004JA010742$09.00
[3] On 30 December 2000 the Cassini/Huygens spacecraft passed by Jupiter. The Jupiter flyby provided a unique
opportunity for two spacecraft to simultaneously study
Jupiter’s magnetosphere. During the approach to Jupiter,
Cassini/Huygens monitored the solar wind in front of the
Jovian bow shock, while Galileo provided in situ measurements deep within the magnetosphere. As Cassini/Huygens
flew past Jupiter, the spacecraft skirted along the edge of the
magnetosheath, crossing the boundaries a number of times.
Observations by the two spacecraft were complemented by
coordinated images from Hubble Telescope, Chandra X-ray
Observatory, and radio telescope observations of Jupiter’s
synchrotron emission by the Very Large Array and the Deep
Space Network. This first time occasion of two spacecraft
simultaneously at Jupiter resulted in a number of outstanding investigations of the solar wind, Jupiter’s satellites, its
atmosphere, and the magnetosphere. The results reveal a
number of clues to the dynamical environment that shapes
Jupiter’s magnetosphere and its interaction with the solar
wind and the Jovian system. Initial results were published in
a series of eight papers in Nature (415, 965– 1005, 2002).
More detailed results from the remote-sensing instruments
were published as a series of 11 papers in Icarus (see
Hansen et al. [2004] and sequel). The series of 15 papers
contained in this issue provide the corresponding set of
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Figure 1. Cassini was launched in October 1997 on a trajectory that arrived at Saturn in 2004. En route,
Venus, Earth, and Jupiter provided ‘‘gravity assists.’’ The Jovian flyby let us exercise the spacecraft, the
instruments, and all the Cassini teams in a ‘‘dress rehearsal’’ for the coming work at Saturn. The Jupiter
flyby also yielded new scientific results for the Jovian system. These are presented in this special section.
See color version of this figure in the HTML.
detailed results from the fields-and-particles instruments and
concludes the overall series of papers on results from the
Cassini-Huygens flyby of Jupiter.
2. Spacecraft and Payload Overview
[4] On the launch pad, the mass of the fully fuelled
spacecraft weighed 5636 kg. Cassini consists of several
sections. Starting at the bottom of the ‘‘stack’’ and moving
upward, these are the lower equipment module, the propellant tanks together with the engines, the upper equipment
module, the 12-bay electronics compartment, and the highgain antenna (HGA) which has a diameter of 4 m. These are
all stacked vertically on top of each other. Attached to the
side of the stack is the approximately 3-m diameter, diskshaped Huygens Probe. Cassini/Huygens accommodates
some 27 different scientific investigations that are supported
by 18 specially designed instruments, 12 on the Orbiter and
six on the Probe. Most of the Orbiter’s scientific instruments
are installed on one of two body-fixed platforms. These are
called the remote-sensing pallet and the particles-and-fields
pallet. An 11-m-long boom supports sensors for the dual
technique Magnetometer (MAG) experiment. Three skinny
10-m-long electrical antennae point in three orthogonal
directions. These are sensors for the Radio and Plasma
Wave Science (RPWS) experiment. The Orbiter carries the
Magnetospheric Imaging Instrument (MIMI) to provide
measurements of energetic particles and the Cassini Plasma
Spectrometer (CAPS) to provide measurements of the
plasma. Figure 3 shows the energy range capabilities of
CAPS and MIMI.
[5] Two-way communication with Cassini is via the Deep
Space Network (DSN) with an X-band radio link, which
uses either the 4-m-diameter high-gain antenna (HGA) or
one of the low gain antennae. The high-gain antenna is also
used for radio and radar experiments and for receiving
signals from Huygens. At Saturn, communications will be
via the HGA. Additional information on the spacecraft is
available in the literature. For example, see Burton et al.
[2001], Matson et al. [2002], and Spilker [1997].
3. Mission Overview
[6] Paradoxically, Cassini/Huygens did not immediately
head for the outer solar system but went inward toward
Venus to pick up additional gravitational assistance because
even the great thrust of the Titan-Centaur was insufficient to
propel the massive spacecraft on its way to Saturn. Two
Venus swingbys would be necessary, followed by an Earth
gravity assist plus one at Jupiter before the spacecraft had
sufficient energy to climb far enough out of the Sun’s
gravitational potential well to reach Saturn.
[7] The second swingby of Venus occurred at an altitude
of 598 km on 24 June 1999. This provided a unique
opportunity for the fields and particles instruments to study
the interaction of the solar wind with Venus, a planet with
no intrinsic magnetic field. Cassini/Huygens has a much
more capable fields-and-particles payload than previous
Venus missions. This was followed 3 months later by a
swingby of the Earth. The results of these encounters are
published in a series of 11 papers (Burton et al. [2001] and
sequel).
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[8] The primary purpose of the flyby of Jupiter was to
provide the gravitational deflection necessary to send Cassini-Huygens to Saturn, arriving in July 2004. The Jupiter
flyby also provided an opportunity to obtain important
calibration data for the experiments and essentially carry
out a dress rehearsal for science operations at Saturn. The
spacecraft flew to within a range of 9,800,000 km or
138 Jovian radii of Jupiter on 30 December 2000.
[9] Observations of Jupiter and the solar wind began
earlier, providing an extended monitoring of the solar wind
conditions near 5 AU. Figure 1 depicts the Cassini-Huygens
trajectory from launch in October 1997 though Saturn
arrival in 2004. The opportunity for a two-spacecraft
experiment was afforded by the fact that the Galileo
spacecraft, in orbit around Jupiter since December 1995,
continued to be in operational good health. Figure 2 shows
both the Cassini-Huygens trajectory and the Galileo orbits
for a period from approximately 60 days prior to CassiniHuygens Jupiter closest approach along with model predictions for the location of the bow shock based on Voyager
measurements. The trajectories of Voyager 1 and 2 are also
shown for reference. Unlike the Galileo dual spin spacecraft
design, Cassini/Huygens is a three-axis stabilized spacecraft
without a scan platform; thus the entire spacecraft must be
turned to point the instruments. This is important to the
Figure 3. The energy ranges for the Cassini fields-andparticles instruments. The horizontal axis is energy in eV.
The coverage is comprehensive, as is required for a mission
with a broad range of scientific objectives.
interpretation of fields-and particles-data, as full threedimensional distributions of the plasma and energetic
particles is not usually available and quality of science
observations are occasionally limited by the sharing of
pointing between competing science objectives (see Hansen
et al. [2004] and Matson et al. [2002] for further discussion
of these points).
Figure 2. The Cassini Jovian flyby trajectory is compared
with the paths of the Galileo and Voyager spacecraft. The
trajectories are projected on the ecliptic plane. The sun is in
the +X direction, off the top of the figure. Cassini’s slow
flyby enabled a 6-month study of the dynamics of the
atmosphere of Jupiter and Io’s torus. During the approach
the Cassini spacecraft was out in the solar wind in front of
the bowshock of Jupiter’s magnetosphere, while Galileo
was in Jupiter’s magnetosphere. As the Cassini spacecraft
flew on past Jupiter, the spacecraft skirted along the edge of
the magnetosheath. The boundary crossed over the spacecraft a number of times as it left Jupiter’s environs. See
color version of this figure in the HTML.
4. Science Observations
[10] Early results from the flyby of Jupiter demonstrated
the value of having two spacecraft at Jupiter and are
discussed in a series of eight papers in Nature (Hill
[2002] and sequel). Kurth et al. [2002] report multiple
boundary crossings and evidence for a well-developed
boundary layer just inside of the magnetopause. Gurnett
et al. [2002] suggest that three shocks were responsible for
triggering increases in radio and auroral emissions, and
Krimigis et al. [2002] report on the first observations of a
hot and fast magnetospheric neutral wind extending away
from Jupiter. The flyby also offered an opportunity to map
Jupiter’s synchrotron emission at high frequency, providing
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insights into the ultrarelativistic electron populations
trapped in Jupiter’s inner radiation belts [Bolton et al.,
2002]. Additional results discussing details of the CassiniHuygens remote sensing observations at Jupiter and its
satellites appear in Icarus (Hansen et al. [2004] and 10paper sequel). In this paper we present an introduction and
overview of the 15 papers contained in this issue which
describe the Cassini measurements of the solar wind and
Jupiter’s magnetosphere and comparison to measurements
made by near-Earth satellites, Ulysses, and at Jupiter by
Galileo.
4.1. Solar Wind Observations
[11] Comparisons between Cassini-Huygens solar wind
measurements at 5 –8 AU and near-Earth spacecraft provided an opportunity to study structure evolution between
1 AU and the outer solar system. Lario et al. [2004] and
Hanlon et al. [2004a] present fields and particles measurements from the solar maximum of cycle 23. Effects of
transient structures on the propagation of particles are
evident in the measurements of energetic particles and
plasma as well as the magnetic field.
4.2. Multiple Spacecraft Magnetospheric Observations
[12] Dual spacecraft measurements of Jupiter’s radio
emission revealed important results on the source and
propagation of magnetospheric radio emission. On the basis
of simultaneous measurements by Galileo and CassiniHuygens, Hospodarsky et al. [2004] suggest the nature of
Jovian quasi-periodic (QP) bursts is more strobe-like
(pulsed) as opposed to a rotating searchlight. Using measurements from Ulysses and Cassini-Huygens, Kaiser et al.
[2004] report a similar finding for the Very Low Frequency
(VLF) radio emission at Jupiter. Both studies state the
observations rule out a source that corotates with the planet,
suggesting the emission is modified by interaction with the
magnetosheath. These observations have important implications for the study of Jupiter radio emissions, as well as
potentially affecting our understanding of similar phenomena at Earth. Dual spacecraft observations in and out of the
magnetosphere also provide new insights on the signature of
compression events [Hanlon et al., 2004b].
4.3. Results From Cassini-Huygens Observations of
Jupiter’s Magnetosphere
[13] Results from electron measurements near Jupiter’s
magnetopause are reported by Svenes et al. [2004], providing a characterization of the magnetopause boundary
effectively acting as a hard boundary to low-energy
plasma. Achilleos et al. [2004] discuss details of the
Cassini measured magnetic signatures of bow shock crossings and compare the observations to a Voyager and
Pioneer based model. They conclude that the global shape
of the Jovian bow shock and intrinsic compressibility of
the magnetosphere is stable over long periods of time. The
Cassini-Huygens trajectory fortuitously skirted along the
magnetospheric boundary for an extended period, allowing
for an in-depth study of the physics associated with the
magnetosheath, magnetopause, and bow shock. Krupp et
al. [2004] report on energetic particle pitch angle distribution and flux level variations associated with the cross-
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ings of these boundaries. The data indicate surprising
structure in the dynamics at distances out to 400 RJ,
suggesting Cassini-Huygens may have passed through a
dusk-midnight magnetosheath region where Jupiter feeds
energetic electrons to the heliosphere. Mitchell et al.
[2004] report on the first-ever energetic neutral atom
(ENA) images of Jupiter’s magnetosphere. The measurements show the emission is dominated by hydrogen atoms
associated with the neutral gas in Jupiter’s exosphere and
to a lesser extent by heavier atoms (i.e., sulfur and
oxygen) from the Io torus. Mauk et al. [2004] use in situ
energetic ion measurements from Galileo to aid in the
interpretation of Cassini ENA observations. Among other
significant findings, they suggest that plasma pressure
from energetic sulfur ions is important in supporting the
magnetodisk against the magnetic pressure from the lobe
regions at distances planetward of 46 RJ. Recent Hubble
Space Telescope observations of Jovian aurora revealed
new features poleward of the main oval suggested to be
related to a Jovian cusp. Bunce et al. [2004] report on
analysis investigating slow and fast flow models
corresponding to varying values of the interplanetary
magnetic field. Using high-resolution observations of the
broadband kilometric radiation, Farrell et al. [2004] suggest the emission characteristics are consistent with a
source of plasma density bubbles affected by an interchange instability.
4.4. Instrument Calibrations
[14] The Jupiter flyby also offered unique opportunities to
obtain instrument calibration data useful for interpretation
of observations at Saturn. Zarka et al. [2004] and Vogl et al.
[2004] describe two examples of calibration of the Radio
and Plasma Wave Subsystem experiment.
[15] Acknowledgments. A portion of this work was carried out at the
Jet Propulsion Laboratory, California Institute of Technology, under a
contract with the National Aeronautics and Space Administration.
[16] This paper was reviewed by editor Arthur Richmond.
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S. J. Bolton, C. J. Hansen, D. L. Matson, and L. J. Spilker, Jet Propulsion
Laboratory, California Institute of Technology, Pasadena, CA, USA.
(scott.j.bolton@jpl.nasa.gov)
J.-P. Lebreton, Space Science Department, European Space Research and
Technology Centre/European Space Agency, Noordwijk, Netherlands.
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