Press Release

APL Instrument on NASA Cassini Spacecraft Aims at Saturn’s Space Environment

By SpaceRef Editor
July 1, 2004
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APL Instrument on NASA Cassini Spacecraft Aims at Saturn’s Space Environment

Cassini-Huygens Investigation Peers into Ringed Planet’s Dynamic Magnetic Field

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 (EDT). The MIMI experiment, an international team effort
led by the Johns Hopkins University Applied Physics Laboratory (APL),
Laurel, Md., 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," says Dr. 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 Plank
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," says MIMI Instrument Scientist Dr. 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 says. 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 says. "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 –
launched on Oct. 15, 1997 – will also release its piggybacked Huygens
probe, built by the European Space Agency (ESA), this December 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 in Pasadena, manages the $3.3-billion
mission for NASA’s office of Space Science, Washington, D.C. 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 in Toulouse, France.

On the Web:

For information and live coverage of mission events, visit the
Cassini-Huygens mission home page:

For information on the MIMI instrument, visit:

The Applied Physics Laboratory, a division of The Johns Hopkins University,
meets critical national challenges through the innovative application of
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SpaceRef staff editor.