Jupiter Radiation Belts Harsher Than Expected

The harshest radiation is within about 300,000 kilometers (about 200,000 miles) of the giant planet. NASA's Galileo has been orbiting farther out than that, and Cassini was nearly 10 million kilometers (6 million miles) from Jupiter when it passed by three months ago on its way to Saturn. Both of those craft have especially durable electronics hardened to withstand radiation.

Cassini's Italian-made main antenna, which serves for communicating with Earth and will later be used to radar-map Saturn's moon Titan, was used during the Jupiter flyby in a listen-only mode, pointed toward Jupiter. It caught details of the radiation belts' natural radio emissions not discernible from Earth or any earlier spacecraft, said Dr. Michael Janssen, team leader for the radiometer instrument. The quality of results is encouraging for radar research at Saturn, he said.

"We got some surprises," said Dr. Scott Bolton, a physicist for NASA's Jet Propulsion Laboratory, Pasadena, Calif. "This has implications not only for understanding the physical processes in the radiation belts, but also for designing any spacecraft for future exploration close to Jupiter." Preliminary results from these radio-science investigations were presented today at meetings of the European Geophysical Society in Nice, France.

High-energy electrons, traveling at nearly the speed of light in spirals shaped by the magnetic field enveloping Jupiter, beam out radio emissions called synchrotron radiation. Synchrotron radiation is not the type that could damage spacecraft, but it provides information about the high- energy electrons emitting it, which are the potential hazards.

Earth-based radio telescopes have mapped some wavelengths of synchrotron emissions from Jupiter's radiation belts, and scientists have used that information to model the belts and estimate their potential to damage spacecraft. But the shortest wavelengths, emitted only by the highest-energy electrons in the belts, get lost in hundred-fold stronger, non-synchrotron radio emissions from heat in Jupiter's atmosphere.

As it flew past Jupiter, Cassini had a better angle for distinguishing atmospheric emissions from radiation belt emissions, though the task was still challenging. The craft had to rock back and forth to scan across the target area several times, then roll 90 degrees and scan back and forth again, to recognize the synchrotron radiation by its trait of polarization.

"Using its antenna, Cassini has been able to anchor the high-energy end of the electron spectrum from Jupiter's radiation belts for the first time," Bolton said.

New measurements made from Earth of Jupiter's radio emissions added context for interpreting the Cassini radiometric measurements. Scientists took readings at several wavelengths using the National Science Foundation's Very Large Array of radio telescopes near Socorro, N.M. And students at 25 middle schools and high schools in 13 states used a large dish antenna near Barstow, Calif., by remote control from their classrooms to monitor changes in Jupiter's emissions from week to week. The students' work, coordinated by a partnership of JPL's Deep Space Network and the Lewis Center for Educational Research, Apple Valley, Calif., helped rule out the possibility that Cassini's measurements happened to be made when emission levels were either unusually high or unusually low.

Cassini's measurements indicate that the highest-energy electrons are sparser than anticipated. That's not good news for spacecraft designers, though. Accounting for the known levels of longer-wavelength synchrotron radiation without having as many of the highest-energy electrons as expected means estimates must be increased sharply for the number of electrons with slightly lower energy levels. Those electrons are still plenty energetic enough to fry electronic equipment. The increase in their numbers is many times greater than the decrease in numbers of highest-energy electrons, compared to the earlier estimates, so the net result is a more hazardous environment than previously estimated, Bolton said.

No approved NASA missions now in development would venture as close to Jupiter as the region with the heightened estimates of radiation hazard, said Bolton. The moon Europa, target of NASA's next planned mission to the jovian system, is about twice as far from the planet. Europa is nevertheless in a hazardous-enough radiation environment that the Europa Orbiter mission is being designed with substantial shielding and durable electronics. The new measurements by Cassini carry direct implications for potential closer-in exploration, such as NASA Discovery mission proposals for orbital studies of Jupiter's atmosphere and internal structure.

The only spacecraft that have experienced the full blast of the radiation belts so far have passed through them quickly. NASA's Pioneer 10 and 11 each sped close to Jupiter during flybys in the 1970s. Galileo's atmospheric probe shot through the belts on Dec. 7, 1995, before plunging into Jupiter's atmosphere. The Galileo orbiter briefly passed close to Jupiter that same day to begin its first orbit. The orbiter will end its tour with a dive into the atmosphere in 2003. Galileo has already endured more than three times the radiation exposure it was built to withstand.

The recent radio observations help with understanding how Jupiter's radiation belts work, as well as what hazards they present, Bolton said. "We would like to know more about their potential interactions with the atmosphere and with the rings," he said. Jupiter's radiation belts provide a useful comparison for better understanding of Earth's radiation belts, too.

Cassini is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. Additional information is available at http://www.jpl.nasa.gov/cassini . JPL, a division of the California Institute of Technology in Pasadena, manages Cassini and Galileo for NASA's Office of Space Science, Washington, D.C.