An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL

Galileo emerges from a period of unreliable communications this week, only a few days prior to a close flyby of Ganymede. Of the Galilean moons, Ganymede is third closest in distance from Jupiter, preceded by Io and Europa, and followed by Callisto. This flyby is the third of the Galileo Millenium Mission, an extension of Galileo's tour of the Jovian system. It is the second such extension, following both the primary mission, which ended in December 1997, and the Galileo Europa Mission, which concluded in December 1999.

Galileo resumes normal operations late Tuesday night. For a few weeks prior to Tuesday, the orbital motion of the Earth and Jupiter brought the Sun between the two, creating radio interference and making reliable communications between the spacecraft and Earth impossible. This geometric situation is known as superior solar conjunction. As the spacecraft emerges from behind the Sun, the Sun's effect on its radio signal will gradually decrease to the point where communications can again be trusted. If this situation sounds familiar, it is because it has occurred approximately every 13 months while the spacecraft has been in orbit around Jupiter.

The Ganymede flyby is scheduled to occur Saturday morning at 3:10 a.m. PDT. Radio signals indicating that the flyby has occurred, however, won't reach Earth until 50 minutes later, or at 4:00 a.m. PDT, which is denoted as Earth Received Time (ERT). The time difference is caused by the fact that Earth is approximately 6 astronomical units (898 million kilometers, or 558 million miles; 1 astronomical unit is equal to the average distance between the Earth and the Sun) from the spacecraft and it will take radio signals just under 50 minutes to travel between the two. Radio signals travel at a speed of 300,000 kilometers per second (186,000 miles per second), just under 670 million miles per hour.

This is Galileo's fifth return to Ganymede, and its second closest. The spacecraft's previous close flyby of Ganymede occurred in May 1997, but its closest was in September 1996 with an altitude of 262 kilometers (163 miles). During the upcoming flyby, the spacecraft will pass within 808 kilometers (502 miles) of Ganymede's surface. That is about the same as the distance between San Diego and San Francisco.

File Galileo image: A global view of Ganymede's trailing side in which the colors are enhanced to emphasize color differences. The enhancement reveals frosty polar caps in addition to the two predominant terrains on Ganymede, bright, grooved terrain and older, dark furrowed areas. Many craters with diameters up to several dozen kilometers are visible. The violet hues at the poles may be the result of small particles of frost which would scatter more light at shorter wavelengths. Photo: NASA/JPL

Galileo's flyby of Ganymede again places the spacecraft at risk of being affected by Jupiter's intense radiation belts. This risk is only slightly less than during recent Io encounters as the spacecraft won't be passing as deep into the belts as during those encounters, Galileo's instruments have already survived three times the radiation they were originally designed to withstand. Any passage through the Jupiter system adds to the total radiation dose experienced by the spacecraft. Mission planners believe this risk is well worth the promise of new scientific information.

Events leading up to this weekend's high activity period are geared primarily toward spacecraft preparation. Late Tuesday night, the spacecraft will perform standard maintenance on its propulsion systems. On Wednesday, the spacecraft will perform standard maintenance on its onboard tape recorder. The tape recorder is a key component that allows Galileo to store its valuable science data for later transmission to Earth. Finally, on Friday, the spacecraft will perform a small flight path adjustment, if deemed necessary by flight controllers.

The first science activities also start Thursday night when the Fields and Particles instruments begin a month-long survey of Jupiter's magnetosphere. In most orbits, this survey is performed only in the inner portions of the Jovian magnetosphere, in order to study its variation and to provide context for any recorded high-resolution observations. The current survey, however, will span the entire range from the inner to outer regions of the magnetosphere, and the transition from inside Jupiter's vast magnetic bubble out into the solar wind. The Fields and Particles instruments are comprised of the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument.

Late Friday night, Galileo's radio signal will begin to pass through Jupiter's atmosphere on its way to Earth. Within minutes, the spacecraft will pass behind Jupiter as seen from the Earth, completely blocking its radio signal from reaching Earth. About 40 minutes later, the spacecraft will emerge from behind Jupiter and communications are restored. During this passage, Galileo's radio signal is weakened and refracted by Jupiter's atmosphere. The changes in the signal will be measured by radio scientists here on Earth, which will allow them to gain more knowledge of the structure and electron density of Jupiter's upper atmosphere.