Contact: Jane Platt

August 16, 1999

Galileo Survives Unexpected Whopper Dose of Radiation

NASA's Galileo spacecraft survived an unexpected whopper dose of radiation -- the strongest it has experienced since its closest-ever approach to Jupiter in 1995 -- before completing the third in a series of flybys of Jupiter's moon Callisto.

The radiation bombardment occurred as Galileo headed toward its Callisto encounter, when the spacecraft flew within 452,000 kilometers (281,000 miles) of Jupiter's cloud tops at 3:59 a.m. Pacific Daylight Time on Thursday, August 12.

"We anticipated the spacecraft's star scanner would detect about 300 to 400 pulse counts of radiation, so imagine our surprise when the instruments showed Galileo had flown through 1,400 pulse counts!" said Galileo Project Manager Jim Erickson. "Then again, that's why we're exploring Jupiter and its moons -- to discover these unusual phenomena."

The cause of the outburst is unknown, but several factors may have played a role. First, at the time the spacecraft was near its close approach to Jupiter and its high radiation environment. Second, it was also near a "plasma sheet," crossing, an area brimming with charged particles trapped in a thin disc that rotates with Jupiter's magnetic field. In addition, it took place one week after the largest heat output since 1986 from Jupiter's volcanic moon Io. The Io heat output was measured by ground-based infrared telescopes and may coincide with an increase in volcanic activity.

The radiation level endured by Galileo during this flyby was many times higher than the levels engineers expect the spacecraft will encounter during its flybys of Jupiter's volcanic moon Io in October and November.

"This was a great dress rehearsal for the Io encounters," said Erickson. "We've been wondering how the spacecraft might hold up when it gets close to Io. This latest brush with radiation makes us think that the odds of survival may be fairly good."

The radiation exposure did have its tense moments. It appears the radiation triggered four spacecraft faults, but all of them were handled correctly by onboard software. The radiation apparently triggered a computer reset of the non-spinning portion of the spacecraft three times -- on Thursday, August 12 at 6:07 a.m. PDT, at 10:29 a.m. PDT, and at 5 p.m. PDT. Because the onboard tape recorder was in use during the first reset, the recording was disrupted temporarily. This resulted in the loss of a few observations -- one from the instruments that study magnetic fields and charged particles and two by the Near-Infrared Mapping Spectrometer.

The radiation apparently caused one other anomaly at 8:21 a.m. PDT on August 12, when a spin detector, used as a backup monitor of the spacecraft's spin rate, experienced a hardware glitch. Onboard fault protection software disabled the faulty detector.

After Galileo passed beyond the high radiation zone and corrected its faults, the spacecraft went on to fly by Callisto at an altitude of 2,299 kilometers (1,429 miles) on Saturday, August 14, at 1:31 a.m. PDT. The flyby included experiments involving changes in the spacecraft's signal as it passed behind Callisto (as seen from Earth). During that period, Galileo's radio signal was weakened by Callisto's tenuous atmosphere. By studying these changes, scientists can learn more about the structure of the atmosphere.

Galileo has begun transmitting to Earth pictures and other science information gathered during the flyby and stored on its onboard tape recorder. On Thursday, August 19, the recorder will be paused so the spacecraft can make a minor flight path adjustment to position it for its next scheduled Callisto flyby on September 16. Galileo's instruments are gathering valuable science information during the Callisto flybys, but the primary purpose is to lower Galileo's orbit to prepare for the two close flybys of Io. The Io encounters will provide the closest-ever look at the most volcanic body in our solar system.

Galileo has been orbiting Jupiter and its moons since December 1997. The spacecraft is more than halfway through a two-year extended Galileo Europa Mission, a follow-on to the primary mission that ended in December 1997. JPL, a division of the California Institute of Technology, Pasadena, CA, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.