On a third and final tour of duty in the Jovian system,
NASA’s dauntless Galileo spacecraft makes its closest pass yet
to Jupiter’s outermost large moon.

Friday, May 25, the orbiter should skim over Callisto, at an
altitude of about 123 kilometers, or 76 miles, at 7:24 a.m.
EDT. If Callisto were the size of a baseball, that would be
just a nickel’s thickness away.

Mission managers expect the pull of the moon’s gravity to
alter Galileo’s orbit around Jupiter. “The main reason we’re
flying so close to Callisto is to set up flybys of Io,” said
Dr. Eilene Theilig, Galileo project manager at NASA’s Jet
Propulsion Laboratory (JPL), Pasadena, CA. Io is an intensely
volcanic moon closer to Jupiter that continually resurfaces
itself with fiery eruptions.

Galileo will pass over polar regions of Io in August and
October to help scientists determine if the seething and
violent moon generates its own magnetic field. “Since we have
to go close to Callisto anyway to get to Io, we’ll take
advantage of the opportunity for studying Callisto,” said
JPL’s Dr. Torrence Johnson, Galileo project scientist.

Unlike the planet’s other large moons, Callisto, which is
about as big as the planet Mercury, appears to be inactive and
still bears craters billions of years old.

Although earlier magnetic studies by Galileo indicated that
Callisto may have a liquid saltwater layer deep beneath its
surface, Callisto hasn’t drawn the excitement generated by Io
or its sister moon Europa, which appears to have liquid water
closer to its surface, or two-toned Ganymede.

“Callisto is sort of the ugly duckling of the moons, but it’s
the one we need to look at to get the bombardment history of
the Jovian system,” Johnson added. “The craters on Callisto
are the visible record of what sizes of comets and other
objects have pelted Jupiter and its moons with what frequency
over the past four billion years.”

Data from the flyby will be transmitted to Earth over the next
two months. Scheduled observations include high-resolution
imaging to study the density of small craters and the details
of how some features appear to be degraded or eroded, said Dr.
Duane Bindschadler, leader of Galileo’s science planning team.
“Some earlier imaging of Callisto has shown fewer small
craters than expected.”

Scientists also plan to snap new pictures of Io, though from a
much greater distance than Callisto, and hope to see if a
volcanic plume detected near Io’s North Pole five months ago
is still active. On Aug. 5, Galileo will pass directly over
the plume’s source area at an altitude of less than 350
kilometers, or about 220 miles.

Another set of planned observations this week will point at
Jupiter. Galileo will make a map of Jupiter’s clouds in
infrared wavelengths. “One goal is to see if fresh clouds are
still being made at the same types of locations they were
during similar mapping more than five years ago,” said Dr.
Kevin Baines, JPL atmospheric scientist. Another is to check
for “brown barges,” a type of dark cloud that was prominent on
Jupiter when NASA’s two Voyager spacecraft flew by in 1979,
but has not been seen during the years since Galileo began
orbiting Jupiter in 1995. Baines believes recent observations
from Earth-based telescopes hint at a return of brown barges.

Galileo’s mission was originally scheduled to end in 1997, but
has been extended repeatedly as the spacecraft continues to
return scientific discoveries. The orbiter has survived more
than three times the cumulative radiation exposure it was
designed to withstand. Some electronic components have been
affected by the radiation, and each swing near Jupiter
increases the odds of more serious damage from exposure to the
radiation belts around the planet.

Galileo has made 30 previous flybys of Jupiter’s large moons,
including seven of Callisto. Before reaching Jupiter, it made
close passes of Venus, Earth and two asteroids. After three
more encounters with Io and one with the small inner moon
Amalthea, Galileo’s mission will end in 2003 with a final
plunge into the crushing pressure of Jupiter’s atmosphere.

JPL, a division of the California Institute of Technology in
Pasadena, manages Galileo for NASA’s Office of Space Science,
Washington, DC. Additional information about the Galileo,
Jupiter and Jupiter’s moons is available online at: