This Week on Galileo

July 3-11, 1999

Galileo's 10th Encounter Ends

The tenth encounter of the Galileo Europa Mission comes to an end on Saturday, July 3, at 4:00 am PDT (see note 1). Shortly after, Galileo begins processing images and science data stored on the spacecraft's onboard tape recorder during the previous few days and returning them to Earth. This playback of data is interrupted four times during the week. On Saturday, July 3, the spacecraft performs a standard gyroscope performance test. On Thursday, July 8, playback is paused to perform a small flight path correction, and a small turn to keep its antenna pointed toward Earth. On Sunday, July 11, playback is interrupted to allow the spacecraft to perform standard maintenance on its propulsion systems.

The Solid-State Imaging camera performs several observations this morning, prior to the end of the encounter command sequence. In the first pair of observations, the camera captures Io just prior to exiting Jupiter's shadow (Io is eclipsed from the Sun by Jupiter). The observation is designed to gather data on a cloud of sodium with neutral electrical charge that has been found to exist in the vicinity of Io's orbit. These data will help scientists identify the source(s) of the cloud and advance the understanding of how sodium and other neutral elements are removed from Io. The last two observations of the encounter complete the camera's campaign to monitor Io's plume activity. The data will be used to prepare for two close flybys of Io later this year, and will also allow scientists to compare Io's volcanic activity to measurements of the Io torus made by the Fields and Particles instruments.

Six observations are scheduled to be returned before the end of the week. Two are returned by the Solid-State Imaging camera, one by the Near-Infrared Mapping Spectrometer, one by the Utraviolet Spectrometer, and two by the Photopolarimeter Radiometer. The camera's observations are high resolution images of dark material found near a ringed structure on Callisto's surface. The images are designed to study variations in the appearance of the surface in hopes of better understanding the processes that lead to resurfacing of the surface and apparent lack of small craters. The Near-Infrared Mapping Spectrometer's observation is designed to obtain high resolution spectral and spatial data describing Callisto's surface. The data will allow scientists to identify the various materials found on the surface. The Ultraviolet Spectrometer returns an observation of Callisto's surface which will provide additional information on surface properties. Finally, the Photopolarimeter Radiometer observations capture data describing the surface temperatures on Callisto's surface. The data will characterize relatively large regions of the surface whose temperatures are slightly higher than their surroundings They will also provide insight into how well different regions of the surface retain heat.

At the end of the week, the Fields and Particles instruments begin the return of a two hour recording made as the spacecraft flew through the Io torus. The recording contains measurements of plasma, magnetic and electric fields. The torus is a region of intense plasma and radiation activity that is constantly replenished by the volcanic activity on Io. It is a vital part of the Jovian magnetosphere.

Note 1. All times listed correspond to the Pacific Time zone (currently daylight time) and spacecraft event time. Radio signals indicating that an event has occurred on the spacecraft reach the Earth 33 to 50 minutes later, depending on the time of year. Currently, Pacific Daylight Time (PDT) is 7 hours behind Greenwich Meridian Time (GMT), and it takes radio signals 44 minutes to travel between the spacecraft and Earth.

For more information on the Galileo spacecraft and its mission to Jupiter, please visit the Galileo home page.

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Last updated: July 3, 1999

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