This image taken on December 28 by APL's ion and neutral mass spectrometer on NASA's Cassini spacecraft makes the huge magnetosphere surrounding Jupiter visible in a way no instrument on any previous spacecraft has been able to do. Photo: NASA/JPL

Cassini's recent pictures of Jupiter are providing scientists with never-before-seen images of the giant planet's magnetosphere and underlying dynamics.

NASA's $3.4 billion Cassini spacecraft is presently in a 6-month flyby of Jupiter during a gravity-assisted swing toward Saturn and a 4-year study of the ringed planet that will begin in July 2004. Researchers using the flyby as an opportunity to try out some of Cassini's advanced instrumentation are reaping scientific rewards.

"Every new spacecraft carries instruments that expand our ability to see things," says Dr. Stamatios Krimigis, Space Department Head at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and principal investigator for the Magnetospheric Imaging Instrument (MIMI) aboard Cassini. "With MIMI, we're able to visualize the invisible."

The MIMI instrument includes an Ion and Neutral Camera developed by APL, a spectrometer built by the University of Maryland under Dr. Douglas Hamilton, and a high energy particle detector developed by Dr. Stefano Livi of APL and a number of co-investigator institutions. "By detecting various energetic particles and discriminating among them according to energy and mass, the camera is able to obtain remote images of the global distribution of these particles," says Dr. Donald Mitchell of APL, who leads the camera science team.

From a distance of 6 million miles (9.7 million kilometers), MIMI's camera has recorded pictures of Jupiter's energetic particle-filled magnetosphere. Sequenced into a movie, these images will eventually provide a large-scale look at the compression and expansion of magnetospheres as they are buffeted by solar winds.

"These images, when combined with the other MIMI measurements, demonstrate the ability of the camera to capture not only the shape and dynamics of the magnetosphere but also elements of its chemical composition," says Krimigis. "They reveal that the particles we're detecting -- primarily hydrogen, but also oxygen, sulfur and sulfur dioxide -- are spewed from volcanoes on the Jovian moon Io and spun out into Jupiter's magnetosphere, where they are trapped, energized and accelerated to high velocities. Then, when collisions with other particles provide them with an electron, they become neutral and are able to escape the magnetosphere. And that's when we can detect them with our camera."

An artist's concept of Cassini during the flyby of Jupiter. Photo: NASA/JPL

In addition to imaging the Jovian magnetosphere, MIMI's instruments have also detected the presence of a huge nebula of particles enveloping Jupiter and extending out to at least 13 million miles (22 million kilometers) from the planet, according to Hamilton, the developer of another MIMI sensor that detected oxygen, sodium, sulfur, potassium and sulfur dioxide. "All of these are constituents of the gas spewed out by Io's volcanoes, thrown out of Jupiter's magnetosphere and eventually picked up by the flowing solar wind," says Hamilton. Other observations include detection of flowing electrons along the planetary magnetic field inside Jupiter's magnetosphere, according to Livi, who developed the magnetic spectrometer sensor.

Closer to home, MIMI technology -- in the form of a High Energy Neutral Analyzer (HENA) instrument -- is orbiting Earth in NASA's Imager for Magnetospheric to Aurora Global Exploration (IMAGE) satellite, launched in March 2000. IMAGE obtained the first global image of the Earth's magnetosphere, the shell of positively charged ions and negatively charged electrons that lies atop our atmosphere and extends far out into space, says Mitchell.

"By improving our ability to visualize a planet's magnetosphere -- whether here or at other planets of the solar system -- we are better equipped to monitor its space weather," says Krimigis. "This will benefit science and, in the case of Earth, may lead to space weather forecasts that will give advance warning of electromagnetic storms, which in the past have disrupted communications and crippled electrical power grids."