NASA’s Galileo orbiter will dart past Jupiter’s moon Io
on Thursday in the veteran spacecraft’s last and closest flyby
of any of the giant planet’s four major moons.

Io’s volcanoes have presented many surprises since they were
first seen in 1979 by NASA’s Voyager spacecraft and especially
during the six years that Galileo has been orbiting Jupiter.
Scientists hope this week’s encounter will reveal how several
regions of Io have changed over the years.

“Galileo’s days are numbered now, so it’s especially exciting
to visit Io one last time,” said Dr. Eilene Theilig, Galileo
project manager at NASA’s Jet Propulsion Laboratory (JPL),
Pasadena, Calif. “An orbital mission like Galileo gives you
the advantage of getting to examine interesting places
repeatedly over a period of time. That’s been great for
studying Io, since it keeps changing so much.”

The Galileo flight team at JPL aimed the orbiter to skim just
100 kilometers (62 miles) above Io’s multicolored surface at
9:09 a.m. EST on Jan. 17. “The reason we’re going so close is
to put Galileo on a ballistic trajectory for impact into
Jupiter in September 2003,” Theilig said.

Galileo has operated in orbit more than three times longer
than its originally planned mission. The resilient spacecraft
has survived about three and a half times as much exposure to
radiation from Jupiter’s radiation belts as it was designed to
withstand. In its 33 loops around Jupiter, it has flown near
Io six times previously and near the other three of Jupiter’s
planet-sized moons – Europa, Ganymede and Callisto – a total
of 27 times.

The tour has relied on expert navigators to calculate several
moves in advance, using each moon’s gravity to help adjust the
spacecraft’s trajectory toward its various encounters.

However, the propellant supply needed for steering the
spacecraft and keeping its antenna pointed toward Earth is now
nearly exhausted. To avoid even a slim chance that Galileo
could crash into Europa after its mission ends, NASA has
decided to send it to a controlled demise in the crushing
pressure of Jupiter’s dense atmosphere. Galileo had earlier
found evidence that Europa has a deep ocean of melted
saltwater under its frozen surface, heightening interest in
keeping Europa pristine for later studies of its potential for
harboring extraterrestrial life.

Before its final plunge, Galileo will make the first close
flyby of Amalthea, a small, inner moon of Jupiter, in November

This week, Galileo will make direct measurements of the
charged particles and magnetic environment around Io. Also,
its camera and instruments for infrared and thermal imaging
have been programmed to make observations during the flyby. As
much of the data as possible will be transmitted to Earth from
the spacecraft’s tape recorder in coming months, Theilig said.

Io, like Earth’s Moon, always keeps the same side facing
inward toward its planet. On Thursday, Galileo will be in
position for its best-ever look at the Jupiter-facing side of
Io. “We’re hoping to see areas we haven’t seen well since
Voyager imaged them back in 1979,” said JPL’s Dr. Torrence
Johnson, Galileo project scientist. “We’d like to know more
about rates of change for volcanic features on Io.” New
observations are also planned for a previously inactive
volcano that unexpectedly lofted a tall plume last summer.

On this swing through the inner portion of the Jovian system,
Galileo will also examine storms on Jupiter itself and the Io
torus, a doughnut-shaped band of charged particles encircling
Jupiter at Io’s distance from the planet.

A sporadic malfunction has affected performance of Galileo’s
camera since mid-2000, apparently due to radiation damage to
an electronic component. The camera worked flawlessly during
the most recent Io encounter in October 2001, but each time
Galileo swings as close to Jupiter as Io’s orbit, odds
increase for more serious damage to the spacecraft from
exposure to the planet’s radiation belts.

Io is the innermost of Jupiter’s four large moons. Heat from
tidal flexing powered by Jupiter’s gravitational pull makes it
the most volcanically active world in the solar system, with
an estimated 200 to 300 volcanoes rapidly resurfacing it.

Galileo left Earth aboard the space shuttle Atlantis in 1989.
JPL, a division of the California Institute of Technology in
Pasadena, Calif., manages the Galileo mission for NASA’s
Office of Space Science in Washington.

Additional information about the mission is available online