The Last of Io 32 Observations

The pace is much slower now, but a handful of observations still remain on Galileo's plate for the remainder of the week.

Wednesday morning at 2:40 a.m. PDT [See Note 1] the Energetic Particle Detector instrument cycles its power off and on and reloads its computer memory. The high radiation environment near Jupiter has caused upsets in the instrument's computer memory in past orbits, and this pre-emptive reload is to guarantee that the instrument is in the correct configuration for the long haul.

At 10 a.m. the Photopolarimeter Radiometer performs a second calibration sequence, the same as was done early Monday morning when the instrument was first powered on for the encounter. By measuring the same calibration target before and after immersion in the harsh environment near Jupiter, any changes that appear in the response of the instrument can be factored into the understanding of the science observations acquired.

At 3 p.m. the Near Infrared Mapping Spectrometer shuts down operations for this flyby, resting peacefully until it awakens briefly in early December for a calibration activity.

On Thursday, starting at 10 a.m. the Solid State Imaging camera (SSI) spends 2.5 hours viewing Jupiter. This set of observations looks at a hot spot in the atmosphere of the gas giant. Such regions are similar to one into which the Galileo atmospheric probe was dropped in December 1995, when Galileo first arrived at the Jupiter system. These images will help scientists measure wind speeds in great detail near the hot spot, and help determine whether the low water abundance seen by the atmospheric probe is the result of simply local weather conditions, or whether it represents a planet-wide dehydration. Such a dehydration would send the atmospheric theoreticians back to the drawing boards to change their models of why Jupiter is the way it is.

At 8:40 p.m. on Thursday, SSI snaps a picture of a portion of the Jupiter ring system known as the Gossamer Ring. This ring extends out beyond the orbit of the small inner satellite Thebe, which circles Jupiter at a distance of 150,400 kilometers (93,500 miles) above the cloud tops. This span puzzles scientists, since we believe that Thebe is the source of the material in the ring, and that the particles should pass inwards towards the planet as their orbits evolve.

Starting at 12:50 p.m. on Friday a spacecraft maneuver is planned. This pulsing of the rocket thrusters alter the spacecraft trajectory, correcting the flight path for small differences between the planned and actual position of the spacecraft as it swung by Io and targeting for our next Io flyby in January. Prior to this maneuver, the spacecraft performs an on-board test of the gyroscopes that are used to maintain the attitude of the spacecraft during the burn. This test will automatically update software parameters that describe the current performance of the gyroscopes after their recent bath in the radiation at Jupiter.

During this entire time period, the suite of instruments that measure the energetic particles and the electromagnetic fields and plasmas surrounding Jupiter have been quietly and continuously collecting real-time data about the local environment of the spacecraft. This data collection is independent of the recorded data collection talked about over the past few days. The continuous survey of the Jupiter system began on October 15 and will continue for about another week, until our attention turns to playing back the recorded data.

Note 1. Pacific Daylight Time (PDT) is 7 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 41 minutes to travel between the spacecraft and Earth. All times quoted above are in Earth Received Time.