TECH SPACEJupiter Radiation Belts Harsher Than Expected
Radiation belts very close to Jupiter would zap any future spacecraft there even more severely than previously estimated, new measurements by NASA's Cassini spacecraft indicate.
The harshest radiation is within about 300,000 kilometers (about 200,000 miles) of the giant planet, closer in than NASA's Galileo orbiter has yet ventured and safely 300 times closer than Cassini's nearest approach when it passed Jupiter three months ago on its way to Saturn.
Cassini's Italian-made main antenna, through which the craft communicates with Earth and will 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.
"Cassini, with its fabulous antenna, has been able to anchor the high-energy end of the electron spectrum from Jupiter's radiation belts for the first time," Bolton said.
Concurrent Earth-based measurements of radio emissions from Jupiter 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. Explaining 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 are currently planned for venturing as close to Jupiter as the region with the heightened estimates of radiation hazard, he said.
The moon Europa, target of NASA's next planned mission to the jovian system, is about twice as far from 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 Discovery mission proposals for orbital studies of Jupiter's atmosphere and internal structure.
The only spacecraft that has experienced the full blast of the radiation belts so far is the Galileo atmospheric probe, which passed through them quickly before plunging into Jupiter's atmosphere in 1995.
The Galileo orbiter, which released that probe, will end its seven-year tour around Jupiter with a dive into the atmosphere in 2003. It 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.
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