NASA’s Galileo Set To Visit Jupiter’s Dust Ring and Moon Amalthea

Before starting its 35th and final orbit around Jupiter
next week, NASA’s Galileo spacecraft will visit three
intriguing features of the giant planet’s neighborhood for the
first time: a small moon named Amalthea, a dusty ring and the
inner region of Jupiter’s high-energy magnetic environment.

“We’re excited about this encounter because the
spacecraft will be flying closer to Jupiter than it’s ever
flown before,” said Dr. Eilene Theilig, Galileo project
manager at NASA’s Jet Propulsion Laboratory, Pasadena, Calif.
The encounter will set up Galileo for its final orbit, as well
as provide valuable research opportunities.

As Galileo approaches Jupiter, it will skim past Amalthea
at 06:19 on Nov. 5, Universal Time (10:19 p.m. Nov. 4, Pacific
Standard Time). Amalthea is one of four small moons closer to
Jupiter than the four large moons — Io, Europa, Ganymede and
Callisto — that Galileo has inspected during more than 30
encounters since late 1995. Navigators have set a course for
the orbiter to pass about 160 kilometers (99 miles) above
Amalthea’s cratered surface. That’s barely more than half the
length of this egg-shaped moon.

By measuring the strength of Amalthea’s gravity tugging
at the spacecraft, researchers intend to determine the moon’s
mass, said JPL’s Dr. Torrence Johnson, Galileo project
scientist. “We know what Amalthea looks like, but we don’t
know what it’s made of,” he said. More distant pictures taken
by Galileo of Amalthea provide a good estimate of its size.
Learning its mass will allow a calculation of its density, an
important clue to its composition. Galileo’s camera will not
be used during this flyby in order to concentrate resources on
higher priority research, such as the gravity measurements.

Knowing Amalthea’s density might help pin down whether
the origin of moons around Jupiter resembled the origin of
planets around the Sun, Johnson said. The Sun’s inner planets
— Mercury, Venus, Earth and Mars — are rocky worlds much
denser than the gassy and icy planets from Jupiter on out.
Likewise for Jupiter’s four large moons: Io, the innermost, is
mainly dense rock and iron, while Ganymede and Callisto, the
two outermost, are mixtures of rock and ice, and Europa
completes a gradient in between.

Amalthea orbits about halfway between Jupiter and Io. If
it is dense, that could fit a theory that primordial Jupiter,
like the Sun, gave off enough heat to prevent volatile,
lightweight ingredients, such as water ice, from condensing
and being incorporated into the orbiting bodies forming
closest to it.

While near Amalthea, Galileo will fly through a faint
“gossamer” ring that encircles Jupiter. This will be the first
opportunity for any spacecraft to use a dust detector to
directly examine the sizes and movements of dust grains within
a planetary ring.

Another special research opportunity will be Galileo’s
sprint through the inner region of Jupiter’s magnetosphere, a
high-radiation environment of charged particles controlled by
the magnetic field around the planet. “As you get closer to
Jupiter, many of the processes resemble what we think happens
close to a star,” said Dr. Claudia Alexander, a Galileo
scientist at JPL. “Jupiter is a massive planet that didn’t
quite make it to the size of a star. It’s exciting that we’ll
be able to take exploratory measurements of this inner region.
We may get results that enable us to understand a little more
about how stars behave.”

The inner-magnetosphere measurements could also provide
information helpful for designing future spacecraft to Europa
or Io, which would need to operate for extended periods in the
harsh conditions of Jupiter’s radiation belts. To supplement
Galileo’s measurements from within the radiation belts,
observations of Jupiter’s natural radio emissions are planned
with radio telescopes on Earth before, during and after the
encounter. Scientists will use the Very Large Array of radio
telescopes near Socorro, N.M. From several high school and
middle schools across the country, students will remotely
control a large radio telescope at the Goldstone, Calif.,
facilities of JPL’s Deep Space Network.

Galileo has already accumulated more than fourfold the
dose of damaging radiation it was designed to withstand. It
has operated for nearly five years past the end of its prime
mission. “Considering what it has been through, the spacecraft
is in remarkably good shape,” Theilig said. Some electronics
on board have degraded over the years, however, and the
intensity of radiation the spacecraft will experience on this
encounter — almost twice as high as ever before — raises
concerns, she said. The flight team has been preparing Galileo
to be as robust as possible, but achieving all the science
goals of the encounter cannot be guaranteed.

After this flyby, Galileo will be on course to hit
Jupiter in September 2003. Its propellant supply, needed for
pointing the antenna toward Earth and controlling the flight
path, is nearly depleted. While still controllable, the
orbiter is being steered into Jupiter to avoid any risk of the
spacecraft hitting Europa in years to come. That precaution
stems from Galileo’s own discoveries of evidence for a hidden
ocean under Europa’s surface, heightening interest in Europa
as a possible habitat for life.

Galileo left Earth aboard NASA’s space shuttle Atlantis
in 1989. 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. Additional
information about the mission is available online at
http://galileo.jpl.nasa.gov .