As NASA's Cassini-Huygens spacecraft begins its
four-year orbital tour of the Saturn system, mission
scientists will use an innovative imaging device to deliver
the most detailed look yet at the relationship between the
sun, the giant ringed planet and the diverse collection of
moons looping around it.
The
Magnetospheric
Imaging Instrument — or MIMI — is one of 12
science instruments on the main Cassini spacecraft, which
entered orbit around Saturn on July 1. The MIMI experiment,
an international team effort led by Johns Hopkins' Applied
Physics Laboratory, includes three distinct sensors that
will profile the environment of charged particles at Saturn
and obtain the first visible global images of Saturn's
magnetosphere.
"Until now, measuring a planet's magnetic field and
charged particle environment — its magnetosphere
— has been difficult," said Stamatios "Tom" Krimigis
of APL, MIMI principal investigator. "A spacecraft can
measure it at one point when it flies past a planet, but
you don't know what's happening somewhere else, and you
never get the entire picture. With our instrument, and from
orbit around the planet, we'll be able to look at the
changing patterns and dynamics of Saturn's space weather
— actually see how it reacts to the solar wind over
time and how it affects the surfaces and atmospheres of the
moons embedded in the magnetosphere."
MIMI's sensors combine three critical measurements to
create that picture. Its high-energy particle detector
(developed by APL, the Max Planck Institute and a number of
co-investigator institutions) looks at the distribution and
strength of energetic ions and electrons near the
spacecraft. Its charge-energy mass spectrometer (built by
the University of Maryland, College Park) measures the
charge and determines the elemental composition of these
particles.
MIMI's ion and neutral camera takes a wider approach,
using an APL-developed technique known as energetic neutral
atom imaging to provide a global view of the entire
magnetosphere, a deep-space mission first. The camera
detects the "glow" of energized particles trapped in the
planet's magnetic field, which will allow scientists to
make three-dimensional images of the compression and
expansion of Saturn's magnetosphere as it's buffeted by the
solar wind or as it sends streams of particles toward the
surfaces of Titan and other nearby satellites.
"Magnetospheres can change dramatically over a matter
of hours to days, so flybys such as the Voyager
spacecrafts' only yield a snapshot in time and space," said
MIMI instrument scientist Donald Mitchell, of APL. "With
Cassini we're going to get years and years of nearly
continuous data, which will give us a much more complete
understanding of this complex system. We'll be able to
watch the whole dynamic between the sun and Saturn, and the
planet and its moons."
The camera was pointed toward Saturn's magnetosphere
four months ago and has already collected impressive data,
Mitchell said. MIMI had a successful test run during
Cassini's flight past Jupiter in late 2000-early 2001,
providing scientists with never before seen images of the
giant planet's magnetosphere and underlying dynamics; a
huge nebula of particles, spewed from volcanoes on the moon
Io, enveloping Jupiter and extending some 22 million
kilometers (13 million miles) past the planet; and a large
and surprisingly dense gas cloud sharing an orbit with
Jupiter's icy moon Europa.
While the mission at Saturn is just beginning,
Krimigis expects MIMI's data will eventually give
scientists a better understanding of the space environment
closer to home.
"Planetary magnetic fields have a lot in common, even
if the planets themselves are very different," he said. "We
hope to find some of the characteristics that affect both
the Earth and Saturn and feed this knowledge back into the
models we have for space weather at Earth. Then we can
devise accurate space weather forecasts that give advance
warning of the electromagnetic storms that affect our
satellites, communications, power grids and other
technological systems."
Including the APL-built MIMI, half of Cassini's
instruments are designed to investigate the space
environments around Saturn and its moons. Cassini, which
was launched Oct. 15, 1997, in December also will release
its piggybacked Huygens probe, built by the European Space
Agency, for descent through the thick atmosphere of the
moon Titan.
The Cassini-Huygens mission is a cooperative project
of NASA, ESA and the Italian Space Agency. The Jet
Propulsion Laboratory, a division of the California
Institute of Technology, manages the $3.3 billion mission
for NASA's Office of Space Science. The MIMI team includes
investigators and expertise from APL; the University of
Maryland, College Park; University of Kansas, Lawrence;
University of Arizona, Tucson; Bell Laboratories, Murray
Hill, N.J.; the Max Planck Institute for Solar System
Research, Lindau, Germany; and the Centre d'Etude Spatiale
des Rayonnements, Toulouse, France.
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