It is also only the second close flyby of Io by the spacecraft since December 1995. This pair of Io flybys was made possible by the four preceding flybys of Callisto, the outermost Galilean moon, which were used to lower the orbit of Galileo towards Jupiter.

Galileo's flyby of Io is a risky endeavor which also promises a wealth of new information about both Io and Europa. The spacecraft is beginning to show the effects of 10 years in space and dozens of passages through the radiation belts near Jupiter.

One example is the Ultraviolet Spectrometer (UVS), which will be turned off in order to protect the instrument from additional radiation damage and to allow for possible annealing of some damaged electronic components.

Such problems were anticipated and are the reason the Io encounters were placed at the end of the Galileo Europa Mission. Balanced against these risks are unique data to be obtained from Io, including high-resolution images and spectra of Io's volcanos and plumes, and the only opportunity to determine if Io possesses an intrinsic magnetic field.

Encounter activities begin on Wednesday night and continue through Friday night. The bulk of the encounter activities are scheduled for Thursday, with the peak surrounding the close flyby of Io at 8:05 pm PST at the spacecraft's location.

The radio signals indicating that the flyby has occurred won't be received on Earth until 35 minutes later, or 8:40 pm PST. The time difference is due to the fact that the spacecraft is approximately 621 million kilometers (386 million miles) from Earth and it will take radio signals just under 35 minutes to travel between the spacecraft and Earth [see Note 1].

The flyby will take the spacecraft to within 300 kilometers (186 miles) of Io's surface. That is closer than the altitude at which the International Space Station flies over the Earth's surface!

The flyby is also unique in that the spacecraft will be passing over Io's south pole. This will be the first detailed look at what lies at one of Io's poles. During the previous flyby, the spacecraft flew above Io's equatorial regions.

Prior to Wednesday night, the spacecraft activities are focused on preparations for the I25 encounter. Playback of data acquired during Galileo's previous flyby of Io and stored on the spacecraft's onboard tape recorder will complete on Tuesday morning.

On Wednesday, the spacecraft performs standard maintenance on its onboard tape recorder, thus preparing it to store data acquired in the following days.

The spacecraft also has distant flybys of Jupiter and the other Galilean moons during this encounter. The flyby of Callisto occurs Wednesday afternoon at 3:54 pm PST-SCET (4:29 PST-ERT) at a distance of just over 1.5 million kilometers (950,000 miles).

The remaining flybys occur on Thursday, including a relatively close flyby of Europa on Thursday morning at a distance of 8,642 kilometers (5,371 miles) from the moon's surface. Most of this encounter's observations are focused on Io, its interior, its volcanic surface, and its electromagnetic environment.

However, given the relatively close flyby distance, a number of observations focus on Europa. The Europa flyby turns out to be the only opportunity of the combined prime and Europa missions that provides a view of the Jupiter-facing hemisphere of Europa at reasonable observation resolutions. Due to Jupiter's size and brightness, this part of Europa's surface is difficult if not impossible to observe from Earth.

One observation is initiated on Wednesday evening, just after the start of the encounter. As with previous encounters, the start of this encounter marks the resumption of the magnetospheric survey performed by the Fields and Particles instruments.

The Fields and Particles instruments are comprised of the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument.

During the survey, the instruments take measurements of plasma, dust, and electric and magnetic fields. These measurements are not recorded on board, but rather are processed and returned to Earth in near real time.

This survey has been repeated from encounter to encounter allowing scientists to study the long term variations within the inner portions of Jupiter's magnetosphere. The measurements will also provide a broader context for higher resolution measurements made by the Fields and Particles instruments later in the encounter period.

Thursday is a heavy day of activity on Galileo. Come back to learn all about it!