Jupiter Orbiter Nears First Visit to Small Moon, Dusty Ring

by Jet Propulsion Lab

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/.