New measurements from NASA’s Cassini spacecraft indicate
that any future spacecraft venturing very near Jupiter would
be zapped by the radiation belts there even more severely than
had been previously estimated.

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

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

“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 .
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.