PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011

NASA’s Galileo spacecraft has begun beaming volcano pictures
and other science data to Earth, now that it has successfully
completed its third and closest-ever flyby of Jupiter’s fiery
moon Io.

Despite intense radiation near Io, the spacecraft completed
all its planned activities during the flyby at 6:32 a.m. Pacific
Standard Time on Tuesday, Feb. 22, at an altitude of 198
kilometers (124 miles).

Data gathered during the flyby include observations designed
to study changes in Io’s volcanoes since Galileo’s previous
flybys of Io in October and November of 1999. There was also a
radio science experiment performed while Jupiter was partly
blocking the radio path from the spacecraft to Earth. By
studying distortions in radio signals in these situations,
scientists learn more about Jupiter’s atmosphere.

While Galileo was approaching Io, radiation did apparently
trigger two computer resets, but previously-installed onboard
software in essence told the spacecraft they were “false alarms,”
and the flyby continued unaffected. The resets occurred on Feb.
22 at approximately 1:38 a.m. PST, and again sometime between
5:30 and 6:30 p.m.

Nearly two days after the close flyby, a third reset
occurred on the spacecraft at 4:45 a.m. PST on Thursday, Feb. 24.
This put the spacecraft in “safing,” or standby mode, which
temporarily stopped all non-essential operations until further
commands were received from Earth. Normally, that reset would
have been handled by the same onboard software that took care of
the first two resets. However, the third reset happened when the
spacecraft had already completed its Io flyby and had begun
playing back data from its onboard tape recorder. During
playback, the software that would prevent safing is disabled.
Once the flight team diagnosed the problem, normal operations
were restored on the spacecraft later in the day, at 9:30 p.m.
Playback of the Io data will resume on Saturday, Feb. 26.

Galileo engineers were somewhat surprised that this third
computer reset happened well after the Io flyby, when the
spacecraft was quite a distance away from Jupiter (29 Jupiter
radii, which is 2.1 million kilometers or 1.3 million miles) and
therefore not as close to the most intense radiation. This
served as another reminder of the powerful effects of natural
radiation in space. Galileo has already survived more than twice
the radiation it was designed to withstand, and its experiences
will help mission planners design future spacecraft headed for
high-radiation environments.

Galileo was launched from the Space Shuttle Atlantis in 1989
and arrived in orbit around Jupiter in December 1995 for a two-
year study of the huge planet, its moons, and its magnetic
environment. That primary mission was completed successfully,
and was followed by a two-year extension, which ended last month.
Galileo is now embarking on another extension, called the Galileo
Millennium Mission.