Today on Galileo

July 1, 1999

Spacecraft Flies Through Closest Approach to Galileo's Moons

Today, the spacecraft flies through closest approach to Ganymede, Europa, Jupiter, and Io. The remote sensing instruments take advantage of today's distant flybys to make observations of Io and Jupiter. The Fields and Particles instruments perform a high time resolution recording of the Io torus. The torus is a region of intense plasma and radiation activity, in which there are strong magnetic and electric fields. It is constantly replenished by the volcanic activity on Io and is a vital part of the Jovian magnetosphere.

Closest approach to Ganymede is first on the flyby schedule. It occurs at 5:58 pm PDT (see note 1) at a distance of 157,000 kilometers (97,600 miles). It is followed by closest approach to Europa at 9:58 pm PDT at a distance of 118,000 kilometers (73,500 miles). Closest approach to Jupiter is next on the schedule, occuring at 10:05 pm PDT at a distance of 7.3 Jupiter radii (522,000 kilometers, 324,000 miles) from Jupiter's center. Finally, closest approach to Io occurs at 10:12 pm PDT at a distance of 124,000 kilometers (77,300 miles).

In the first three observations of the day, the Utraviolet Spectrometer and the Photopolarimeter Radiometer take a look at Io, while the Near-Infrared Mapping Spectrometer looks at Jupiter. The Ultraviolet Spectrometer observation captures Io while the volcanic moon is in eclipse, shadowed from the Sun by Jupiter. The observation is designed to look at Io's atmosphere in hopes of detecting Io's auroral glow, unperturbed by sunlight. The Photopolarimeter Radiometer observation is performed later in the day and is designed to characterize regions that are low in temperature, but contain a lot of energy. The observation will also obtain temperature maps of the surface which will provide insight into how well different regions of the surface retain heat. This is the first of three observations of this type performed by the Photopolarimeter Radiometer during this encounter. Like the previous observations of Callisto, the Near-Infrared Mapping Spectrometer observation of Jupiter is designed to obtain spectral data that will allow scientists to study different materials in Jupiter's atmosphere and advance ongoing studies. These studies address questions of Jupiter's atmospheric structure, such as cloud heights, and can obtain information on particle sizes and cloud densities, as well as information on the temperatures and composition of atmospheric constituents.

Late in the afternoon, the Fields and Particles instruments perform the second in a series of recordings designed to measure the plasma, and electric and magnetic fields of the Io torus. These recordings are to be repeated during each of the four Callisto encounters of the Perijove Reduction Campaign. As Galileo's perijove distance is reduced from orbit to orbit, the Fields and Particles instruments will examine the Io torus at different distances from Jupiter.

The Solid-StateImaging camera takes the stage next with a pair of observations of Io. The first series of images forms a regional map at a resolution of 1 to 1.5 kilometers (0.6-0.9 miles) per picture element. The second series of images captures a global view of Io at a resolution of 2.6 kilometers (1.6 miles) per picture element. The images taken today will provide the lowest resolution views of Io obtained by Galileo prior to the close encounters of Io that occur later this year.

Toward the end of the day, the Photopolarimeter Radiometer performs the second in the series of three observations of Io's surface temperatures. The Near-Infrared Mapping Spectrometer performs the final observation of the day. It is designed to obtain high resolution spectral and spatial data of Io's surface. This will be the highest resolution map of Io made to date, including Galileo's primary mission, by the Near-Infrared Mapping Spectrometer. The spectral data will allow scientists to identify different materials on Io's surface, while the spatial data will help them determine where these materials lie.

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 1, 1999

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