The spectacular Galileo flybys of Europa and Io are largely credited with the discovery of frozen water ice and some of the earliest examples of non-solar (tidal) heating anywhere in our solar system.

Jovian moon gives off more heat than it receives from the Sun

Over the coming weeks and months, Galileo scientists are preparing for their next target: probing one of Jupiter’s moons, Almathea, at close-up ranges of <100 miles. Almathea is one of the most unusual moons in the solar system, because it gives off more heat than it receives from the Sun. Since Galileo found what appears to be water-ice among the moons, scientists have speculated about some of the basic environmental ingredients for life (energy, liquid water, non-vacuum) on both the Jovian moons and to a lesser extent in Jupiter's atmosphere. A summary (Scientific Assessment for Galileo Disposal) of how to dispose of Galileo safely offers fascinating insights into the prospects for life on Io, Europa, Callisto and Ganymede.

Curtain call mission

Galileo has functioned in orbit more than three times longer than its originally planned mission. The Galileo spacecraft is still healthy and active as it continues its long trek back in towards Jupiter for its final planned science pass in November. Galileo is now back within ranges that it has traversed before, reaching 250 Jupiter radii from the planet (17.9 million kilometers, 11.1 million miles) on Saturday, September 14, and 200 Jupiter radii (14.3 million kilometers, 8.9 million miles) on Wednesday, October 2. The spacecraft is still well outside the magnetosphere of Jupiter on the sunward side of the planet, and data collection by the Magnetometer, the Dust Detector, and the Extreme Ultraviolet Spectrometer instruments continues to provide scientists with information about the interplanetary medium.

Galileo entered orbit about Jupiter in December 1995 on a 2-year mission to conduct intensive observations of Jupiter’s atmosphere, rings, satellites, and radiation environment. In 1997, the mission was extended for an additional 2-year period to allow for additional studies of Europa and the first close-up observations of Io. In 1999, the mission was extended for another year to enable more studies of Io and Europa, and, in addition, concerted observations of Jupiter’s magnetosphere with the Saturn-bound Cassini spacecraft in December 2000. The spacecraft is currently on a ballistic trajectory designed to intercept Jupiter and burn up in the atmosphere in September 2003.

Amalthea flyby: Close-up

Routine maintenance activities for the spacecraft in the coming weeks include exercise of the propulsion system on Tuesday, September 10, and Thursday, October 3, and a standard test of the on-board gyroscopes on Friday, October 4.

On Saturday, September 21, Galileo executes a propulsive maneuver to alter its trajectory for the Amalthea flyby on November 5. This maneuver will establish the flyby altitude of 134 kilometers (83 miles) over the surface of the irregularly-shaped moon, whose longest dimension is about 135 kilometers.

A relatively close passage by Amalthea, one of Jupiter’s innermost known satellites, is of scientific interest because so little is known accurately about the radiationally-intense area nearest Jupiter. It is the reddest object so far seen within in the solar system and appears to give out more heat than it receives from the Sun, perhaps from either radiation bands around Jupiter or tidal heating.

Although the radiation environment around Jupiter is always difficult for its on-board electronics, this flyby should yield an estimate of the mass and, correspondingly, the bulk density for the satellite. The trajectory itself will allow tracking and thus yield the mass of Amalthea under challenging operating conditions (3-5 times more radiation than even the previously intense flybys of Io).

This density estimate is important because Amalthea may be a fragment of an object that formed closer to Jupiter than the Galilean satellites, where temperatures in the circumjovian nebula would have been higher. Given that Amalthea’s volume is currently known to an accuracy of about 10%, a mass accurate to even 20% may allow conclusions to be drawn about conditions in the jovian nebula and the satellite-formation processes, in general. Mass determination requires no functional instruments, only tracking of the spacecraft’s trajectory through monitoring of its downlinked radio signals. If remote-sensing observations are possible during the flyby, then monochromatic (clear filter) imaging would allow several secondary goals to be pursued. These include high-resolution images of streaks and crater interiors, searches for evidence of layering, and accurate crater counts.

What’s Next

A series of weekly conditioning exercises for the on-board tape recorder continues, with the latest activity starting on Monday, September 9. With this test, the research team will drive the recorder at high speed across the full length of the tape ten times. At the end of the high-speed motion, the team perform a short series of small, slow-speed cool-down motions that will lessen the possibility of the tape sticking to the heads. Following this, the tape is put into a series of low-speed, full-track motions that will occupy the remainder of the week.

Next on the recorder’s agenda is to play back some data acquired during two previous Io flybys, one in October 2001, and the most recent in January 2002. These data will fill in gaps in a Near Infrared Mapping Spectrometer (NIMS) October observation and provide enhanced visibility into spacecraft attitude during a January NIMS observation.

With scarcely two months to go before the next encounter, the flight team is busy refining strategies, identifying contingency actions, and polishing the detailed sequence of activities to be followed by the spacecraft.