Galileo succeeds in its closest flyby of a Jovian moon

NASA’s Galileo spacecraft has successfully completed a flyby
of Jupiter’s moon Callisto, closer than any of the spacecraft’s
30 previous flybys of Jovian moons.

Galileo’s camera appeared to be working well from the time
it was given a command Thursday afternoon to turn off then back
on, right through and after the spacecraft’s closest approach to
Callisto at 4:24 a.m. (PDT) today, said Dr. Eilene Theilig,
Galileo project manager at NASA’s Jet Propulsion Laboratory,
Pasadena, Calif. Earlier, the camera appeared to be
malfunctioning and probably did not capture some intended images
taken of the moon Io from greater distance.

Other instruments appear to have worked well throughout the
encounter, Theilig said. “This incredible spacecraft has come
through for us again.” she said.

Galileo passed about 138 kilometers (86 miles) above the
surface of Callisto. The spacecraft has been orbiting Jupiter
since 1995. Its closest previous encounter came within about 198
kilometers (123 miles) of the volcanic moon Io in February 2000.

Today’s pass was designed to use Callisto’s gravity to
alter the shape of Galileo’s orbit so that the spacecraft will
fly near Io in early August. As a bonus, the flyby gave
scientists an opportunity to point their instruments for a
close look at Callisto, a heavily cratered moon about the size
of the planet Mercury.

“It appears Galileo is on track for a polar pass by Io in
August,” Theilig said. “Because this spacecraft has already
outlived expectations, the flight team prepared for
contingency situations, but is always relieved to get through
without encountering significant problems.”

If all goes well, images and other data will be
transmitted to Earth by Galileo over the next two months, with
an interruption of three weeks in June when Jupiter and
Galileo will be behind the Sun from Earth’s point of view.

Intense radiation near Jupiter poses a risk to the
spacecraft’s electronics. Galileo’s closest approach to
Jupiter on this orbit was at a distance of about 460,000
kilometers (about 285,000 miles) from the giant planet’s cloud
tops on May 23. It will pass about 20 percent closer than
that to Jupiter the same hour it flies by Io in early August.

Galileo, built at JPL, has already received more than
three times the cumulative radiation exposure it was designed
to withstand and has continued making valuable scientific
observations more than three years after its original two-year
mission in orbit around Jupiter. The spacecraft’s nuclear
electrical power source — two radioisotope thermoelectric
generators – continues to provide power to Galileo’s
instruments, computers, radio and other systems.

The radio signals indicating today’s Callisto flyby had
taken place traveled for about 50 minutes at the speed of
light and reached a large dish antenna at the Madrid station
of NASA’s Deep Space Network at about 5:15 a.m. PDT. The
network relayed the signals to mission controllers at >

JPL.

As of 11 a.m. today, the spacecraft had recorded about 90
percent of the scientific data that its instruments had been
programmed to collect during this swing through the inner
portion of the Jovian system. During the weekend, Galileo is
scheduled to make additional observations of Callisto and of
Jupiter’s clouds.

Magnetometer readings by Galileo during earlier flybys of
Callisto indicated that this moon may have a layer of melted,
salty water deep beneath its surface. However, unlike its
sister moon Europa, which is likely to have liquid water much
nearer its surface, Callisto shows a heavily cratered surface
bearing the record of impacts by comets and other objects over
billions of years. High-resolution images can help scientists
understand the bombardment history of the Jovian system.

Additional information about the Galileo mission is
available at http://galileo.jpl.nasa.gov .

Galileo was launched from NASA’s Space Shuttle Atlantis
on Oct. 18, 1989. It began orbiting Jupiter on Dec. 7, 1995.
JPL, a division of the California Institute of Technology in
Pasadena, manages the Galileo mission for NASA’s Office of Space
Science, Washington, D.C.