ΔΙΑΣΤΗΜΑ

[johnme]

Μόλισ χθες έλαβα κάποιες νέεσ πληροφορίεσ που αφορούν την εξέλιφη του προγράμματος Cassini.Παραθέτω ατόφιες τις ανακοινώσεις της NASA.

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

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.

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