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This Week on Galileo?
Today on Galileo
December 29, 2000
DOY 2000/364

Galileo's mission at Jupiter - Day 3 of the 29th encounter

The spacecraft makes its closest approach to Europa today, the third day of the 29th encounter of Galileo's mission at Jupiter. Closest approach occurs at 1:37 a.m. PST [see Note 1] at a distance of 794,000 kilometers (493,000 miles). Throughout the day, Galileo's instruments make observations of Europa, Io, and Jupiter. Galileo also takes a break from its science endeavors to perform two engineering activities. In the early morning, the spacecraft executes the first part of a standard gyroscope test. In the early evening, the spacecraft performs part two of the test, which involves moving the spacecraft's scan platform to examine how the gyroscopes react to the movement. The scan platform houses all of Galileo's remote sensing instruments and is used to point those instruments at targets of interest (like Jupiter and its moons).

The Photopolarimeter Radiometer (PPR) and Near-Infrared Mapping Spectrometer (NIMS) are again major players in today's observation suite, performing 27 of the day's 29 observations. The Solid-State Imaging camera (SSI) performs the remaining two observations. The first SSI observation is a global color mapping observation of Io at low phase angle. SSI's second observation is also global in nature, and is timed so as to capture the Prometheus volcanic plume in profile on the edge of Io's disk. The shape and structure of the plume will be compared with that seen in previous Voyager and Galileo images.

PPR's six observations consist of four observations of Europa and two of Io. All of these observations measure the polarization of light reflected off of the surface of these Jovian moons. Like the polarimetery observations made yesterday, these data will allow scientists to learn about the surface texture and small-scale surface properties. NIMS performs the remaining 21 observations scheduled for today. One is of Europa, while the other 20 are dedicated to studying Jupiter's atmosphere.

NIMS' observations of Jupiter's atmosphere cover the North Temperate Zone (1), the South Tropical Zone (3), Jupiter's aurora (10), Jupiter's limb (5), and a white oval (1). North Temperate and South Tropical Zone observations will provide scientists with information on cloud activity, and thermal and compositional characteristics. When compared to similar measurements from previous orbits, they can give insight into the change in these characteristics over long time scales. The auroral observations are designed to study the auroral emissions at Jupiter's south polar region. Auroral emissions are much like Earth's northern lights. NIMS' limb observations are designed to look right at the edge of Jupiter's disk and will measure how thermal emissions vary with altitude in the upper cloud levels in Jupiter's atmosphere. Finally, the white oval observation is designed to look at the last white oval. The three long-lived white oval features have merged over the course of the last two years into a single storm. White ovals are storms that occur between two adjacent zonal jet streams, and have lasted for decades. The data obtained in this observation will provide scientists with more information on the composition and cloud dynamics of the region.

Galileo still has a few days of exciting observations ahead of it. Come back tomorrow and learn what is in store for the weekend!

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 35 minutes to travel between the spacecraft and Earth.

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