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.
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