Galileo's intrepid flight controllers take the spacecraft back to Io in the next few days as part of the second encounter of the Galileo Millennium Mission, the extension to Galileo's mission at Jupiter. Dubbed as I27 ("I" for Io), the encounter is the 27th flyby of Galileo's orbital tour, which started back in June 1996 with Ganymede. This Io flyby will mark the fourth time the spacecraft has flown past Io since arriving at Jupiter in December 1995, and is the closest-ever approach, with a flyby altitude of only 200 kilometers (124 miles) above Io's fiery surface. That is the same distance at which Galileo flew over Europa in December 1997, and is about the same as the distance between Los Angeles and San Diego!

As has been the case for recent encounters, Galileo's flyby of Io places the spacecraft at risk of being affected by Jupiter's intense radiation belts. Galileo's components have already survived more than twice the radiation they were originally designed to withstand, and any passage through the Jupiter system that can reach Io also adds significantly to the total radiation dose experienced by the spacecraft. The risk associated with this endeavor is well worth the promise of new information on Io and its unique status as the most volcanically active body in the solar system.

The Io flyby is scheduled to occur on Tuesday at 1347 GMT (8:47 a.m. EST). Radio signals indicating that the flyby has occurred, however, won't be received on Earth until 45 minutes later, or at 1432 GMT (9:32 a.m. EST), which is denoted as Earth Received Time (ERT). The time difference is due to the fact that the spacecraft is approximately 5.4 astronomical units (811 million kilometers, or 504 million miles, 1 astronomical unit is equal to the average distance between the Earth and the Sun) from Earth and it will take radio signals just over 45 minutes to travel between the spacecraft and Earth.

Encounter commands for the Io flyby begin to execute late Saturday night at 0400 GMT Sunday (11:45 p.m. EST Saturday-ERT). They will continue to execute through mid-Wednesday when data playback will be initiated. Prior to the start of the encounter, the spacecraft performs standard maintenance on its onboard tape recorder. The majority of the observations performed during the encounter will be stored on this device for later processing and transmission to Earth.

A striking burst of white light near Io's eastern equatorial edge is sunlight being scattered by the plume of the volcano Prometheus. Prometheus lies just beyond the visible edge of the moon on Io's far side. Its plume extends about 100 kilometers above the surface, and is being hit by sunlight just a little east of Io's eastern edge. Scattered light from Prometheus' plume and Io's lit crescent also contribute to the diffuse yellowish emission which appears throughout much of the sky. However, much of this emission comes from Io's Sodium Cloud: sodium atoms within Io's extensive material halo are scattering sunlight at the yellow wavelength of about 589 nanometers. Photo: NASA/JPL

The encounter also features distant flybys of the other Galilean moons. Although no observing is done during those flybys, the only one close enough to mention is the flyby of Europa that occurs on Tuesday at 0351 GMT (11:36 p.m. EST-ERT) at a distance of 409,000 kilometers (254,000 miles).

Around midday Sunday, Galileo's radio signal begins to pass through Jupiter's upper atmosphere on its way to Earth. The signal is weakened and refracted by the extremely tenuous atmosphere approximately 25,000 kilometers (15,500 miles) above Jupiter's cloud tops. The small changes in the signal will be measured by the Radio Science Team here on Earth, and will allow scientists to gain more knowledge of the structure and electron density of Jupiter's upper atmosphere. Approximately 12 hours later, the spacecraft passes behind Jupiter as seen from the Sun. The eclipse lasts just short of two hours.

Late Sunday, the Fields and Particles instruments resume their survey of the Jovian magnetosphere. This survey has been performed from orbit to orbit and allows scientists to study the long term variations of the inner portions of Jupiter's magnetosphere. The survey data also provide context for higher resolution recordings performed by the instrument suite. The Fields and Particles instruments are comprised of the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument.

Monday sees the first remote sensing observations of the encounter. The Photopolarimeter Radiometer (PPR) takes two observations of Jupiter's atmosphere. The polarimetric observations will allow scientists to learn more about the vertical cloud structure of Jupiter, including cloud particle shape and size. This particular pair of observations will provide the best PPR resolution from Galileo's mission to date. PPR is also the first to look at Io. Its first observation captures the dark side of the volcanic moon. The map will be used to describe night time thermal emissions on Io and will aid scientists in the development of heat flow models.

Anticipating the flyby of Io early Tuesday morning, the Radio Science team begins to carefully measure changes in the frequency of Galileo's radio signal just after 11:32 p.m. EST-ERT on Monday. The changes are caused by Io's gravitational pull on the spacecraft. If this sounds familiar, it is because similar experiments have been conducted on each of the previous four Io flybys, with differences only in the spacecraft's flight path. Radio scientists will track these changes for 20 hours, centered on the point of closest approach to Io. The data will be added to the existing repository that provides the basis for a model of Io's gravity field and internal structure.