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

This week, Galileo winds down on its 14-week-long successful collaboration with the Cassini spacecraft to study the influence of the solar wind on the Jovian magnetosphere. The survey data collected by the spacecraft's Fields and Particles instruments (Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instruments) are sent to the onboard tape recorder six times this week, in order to maintain the continuity of data collection. The tape recorder is used because the radio antennas of the Deep Space Network (DSN) are scheduled to listen to Galileo for only about 106 hours (out of a total 168 hours possible in a given week).

On Monday, the spacecraft performs standard maintenance on its propulsion systems. Other than that, it's another quiet week in deep space, as Galileo increases its distance from Jupiter from 12 million kilometers (7.5 million miles) out to 13 million kilometers (8.2 million miles). Scientists frequently measure distances from a planet in terms of the radius of that planet. Since Jupiter, the largest planet in the solar system, has a radius of 71,492 kilometers (44,423 miles), that means that this week Galileo is travelling between 168 and 182 Jupiter radii out from its December 29 flyby. And it still has another five weeks to go before it reaches its most distant point in this orbit! At these distances, Galileo is expected to be outside of the magnetosphere, measuring the solar wind. Cassini's Fields and Particles instruments are making measurements of the magnetotail boundary, about 30 million kilometers (18.7 million miles) from Jupiter.

On Saturday, commands will be sent to the spacecraft which will govern its activities for the next seven weeks. Each of the command sequences is a small computer program that tells the spacecraft what to do at any particular time: "turn here", "point there", "record this", "play back that". All of the activities reported in these bulletins are accomplished by such programs, which, in the long periods between flybys, are sent to the spacecraft every month or so.