This Week on Galileo: On the Threshold
28 Oct 2002
(Source: Jet Propulsion Laboratory)
October 28 - November 3, 2002
On the Threshold
The week begins with the Galileo spacecraft at a distance of 80 Jupiter radii (5.7 million kilometers or 3.5 million miles) from the giant planet. Though this may seem close based upon recent history, there were five previous orbits during Galileo's seven-year tour during which the spacecraft never reached this far out from Jupiter. By week's end on Sunday, Galileo will have closed that span to 25 Jupiter radii (1.8 million kilometers or 1.1 million miles), and will be rapidly gaining speed.
The majority of the week is spent in collecting real-time fields and particles science data with the Dust Detector, the Energetic Particle Detector, the Heavy Ion Counter, the Magnetometer, the Plasma Subsystem, and the Plasma Wave Subsystem instruments. During periodic gaps in our ground tracking station coverage, the data are stored in a computer memory buffer and copied to the on-board tape recorder for playback after the coming encounter. This occurs six times during the week.
On Thursday, October 31, the last orbit trim maneuver of the mission was scheduled to occur. This propulsive motor burn would be designed to fine-tune the final position of the spacecraft flyby of the small inner satellite Amalthea. The most recent tracking data have shown that the current path of the spacecraft is close enough to the desired location that the maneuver will not be necessary. Of the 109 maneuvers originally scheduled to occur during Galileo's tour of the Jupiter system, 21 of them have been cancelled as being unnecessary. The Navigation team has done a truly remarkable job of steering the spacecraft to its desired location over the years.
On Friday, November 1, a final conditioning of the tape recorder is performed. This 16-hour-long activity prepares the tape for the recording of the closest approach data to come.
On Saturday, November 2, at 2:44 a.m. [see Note 1], the sequence of commands that will govern Galileo's activity during the flyby begins. The sequence starts by configuring the fields and particles instruments for the coming data collection. This configuration involves setting internal voltages and biases, and selecting the appropriate measurement channels to monitor for this portion of the Jupiter magnetosphere. At the moment, this means that the instruments are configured to measure more sensitively the relatively quiet outer portions of the system. As the spacecraft plunges deeper in towards Jupiter, and the expected environment changes, these configurations will be adjusted to provide the most accurate measurements possible.
At 5:14 p.m., the tape recorder is moved to position the record heads to begin storing the encounter data. When this positioning is complete, at 10:50 p.m., the memory buffer that has been collecting fields and particles data is copied to the tape.
On Sunday, November 3, Galileo begins measuring a series of Plasma Sheet crossings. Jupiter's Plasma Sheet is a flat portion of the magnetosphere far from the planet at Jupiter's magnetic equator where plasmas, or charged particles, accumulate. Since Jupiter's magnetic field is tilted 10 degrees with respect to its rotation axis, the sheet waves up and down as the planet rotates every 9.9 hours. This waving causes the sheet to move past the spacecraft, and the instruments can measure the density and energy of the particles. Galileo has been in a unique position in being able to repeatedly measure this phenomenon over the past seven years. This long-term study has shown how the structure has changed over time, and helps scientists understand more about similar processes that are taking place in Earth's magnetosphere, affecting the environment of our home planet. The Plasma Sheet crossings are predicted to occur at 3:22 a.m., 8:10 a.m., 1:21 p.m., 6:12 p.m., and 11:24 p.m.
At midnight Sunday, the spacecraft is less than 24 hours from the close flyby of Amalthea, and the best is yet to come!
Note 1. Pacific Standard Time (PST) is 8 hours behind Greenwich Mean Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occurred is known as Earth Received Time (ERT). Currently, it takes Galileo's radio signals 44 minutes to travel between the spacecraft and Earth. All times quoted above are in Earth Received Time at JPL in Pasadena.
For more information on the Galileo spacecraft and its mission to Jupiter, please visit the Galileo home page.