Jupiter Radiation Belts Harsher Than Expected

Jupiter Radiation Belts Harsher Than Expected

March 28, 2001

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Guy Webster/Jet Propulsion Laboratory (818) 354-6278

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.

Additional information about Cassini-Huygens is online at http://saturn.jpl.nasa.gov.

The Cassini spacecraft is scheduled to arrive at Saturn in July 2004 to
begin a four-year exploration of the ringed planet and its moons. The
Cassini mission is managed by NASA's Jet Propulsion Laboratory in Pasadena,
Calif., for NASA's Office of Space Science, Washington, D.C. JPL is a
division of the California Institute of Technology in Pasadena.

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