Lava flows and ridged plains at Prometheus on Jupiter's moon Io as seen by Galileo in February. Photo: NASA/JPL

Galileo continues to cruise through space on the longest and largest orbit around Jupiter since its arrival at Jupiter in December 1995. The spacecraft's current orbit is 222 days long, and will reach apojove, the farthest point from Jupiter for a given orbit, a distance of 290 Jupiter radii (20.7 million kilometers, or 12.9 million miles) on September 8, 2000. Galileo's second largest orbit followed immediately after arrival when the spacecraft traveled for 203 days before its next encounter. During that orbit, the spacecraft reached an apojove distance of 270 Jupiter radii (19.3 million kilometers, or 12.0 million miles).

The Fields and Particles instruments continue their survey of Jupiter's magnetosphere, or magnetic environment, and surrounding solar wind this week. This is the third week of a four-week survey, which is an extension of Galileo's typical surveys of the inner magnetosphere. The extension will allow the Fields and Particles instruments to observe the transitions from Jupiter's inner magnetosphere, through the outer magnetosphere, and into the solar wind. In addition, the survey is part of Galileo's preparation for joint activities with the Cassini spacecraft starting in October of this year. Cassini is enroute to an arrival at Saturn in 2004, but will first fly past Jupiter in late December 2000.

The flight team turns on the Extreme Ultraviolet Spectrometer (EUV) toward the end of the week to perform remote observations of the Io torus. The torus is a ring-shaped region of intense plasma and radiation activity with its inner edge bounded by Io's orbit. The activity in the Io torus is maintained by Jupiter's strong electric and magnetic fields and Io's constant supply of volcanic particles. The data set obtained by these observations will be the most complete since early in Galileo's primary mission. It will allow scientists to continue studying the shape and energy output of the torus, and to perform long term comparisons with data obtained over the last 4 years.