Since the Galileo probe 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 of how to dispose of Galileo safely offers fascinating insights into the prospects for life on Io, Europa, Callisto and Ganymede.
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, Sept. 14, and 200 Jupiter radii (14.3 million kilometers, 8.9 million miles) on Wednesday, Oct. 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.
Routine maintenance activities for the spacecraft in the coming weeks include exercise of the propulsion system on Thursday, Oct. 3, and a standard test of the on-board gyroscopes on Friday, Oct. 4.
On Saturday, Sept. 21, Galileo executed a propulsive maneuver to alter its trajectory for a planned Amalthea flyby on Nov. 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 Galileo's 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 percent, a mass accurate to even 20 percnet 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.
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