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This Week on Galileo?
Today on Galileo
Wednesday, May 23, 2001
DOY 2001/143

Galileo's Mission at Jupiter - Day 2 of the Callisto 30 Encounter

Today begins with an ending. The Radio Science Jupiter-Earth Occultation experiment, which began yesterday at 6:20 p.m. PDT, concludes at 12:50 a.m. PDT (yawn). [See Note 1] At this time the spacecraft resumes sending telemetry bits. Although Galileo itself is about 10.5 Jupiter radii from the planet (750,000 kilometers or 466,000 miles), the radio signal sent from the craft clears the top of the visible atmosphere by only 25,000 kilometers (15,500 miles) on its way to Earth, and the last of the effects that the atmosphere has on the signal should have dissipated.

Just before 6 a.m. PDT, the Fields and Particles instruments [the Energetic Particle Detector (EPD), Magnetometer (MAG), Dust Detector (DDS), Heavy Ion Counter (HIC), Plasma Wave instrument (PWS), and Plasma instrument (PLS)] complete their configuration for the upcoming close pass by Jupiter. Now these instruments begin an 18-hour period of continuous data collection surrounding perijove, the closest point to Jupiter. These data will give information about the current state and variability of the Io Torus. The ionized gases which make up the Torus originate with the volcanic activity on Io, and form a doughnut-shaped ring that encircles Jupiter and extends from Io's orbit outward.

At 6 a.m. PDT the Photopolarimeter Radiometer (PPR) instrument is turned on to be ready to observe the planet and its satellites. The two other optical instruments, the Solid State Imaging camera (SSI) and the Near Infrared Mapping Spectrometer (NIMS) are already on and ready. Their first opportunity is at 7 a.m. PDT, when Io passes into Jupiter's shadow. All three instruments observe the satellite as it passes into darkness, studying how the surface cools, and looking for hot spots.

Three and a half hours later, after Io has left the shadow, PPR, NIMS and SSI take another look. PPR checks to see how the surface is warming up in the sunlight, NIMS does a global observation to monitor volcanic activity, and SSI takes some color pictures of the leading hemisphere of the satellite to help in the planning of observations in upcoming orbits when Io is the prime target of interest. All four of the large Galilean satellites (Io, Europa, Ganymede, and Callisto) are tidally locked to Jupiter, as the Earth's Moon is to Earth, and always show the same face to their parent planet. This means that one side of the globe is always in the lead as the body orbits, hence the name 'leading hemisphere'.

Closest approach to Io comes at 11:08 a.m. PDT, at a distance of 341,771 kilometers (212,412 miles), a far cry from the 198 kilometer (123 mile) distance of the 27th orbit in February 2000. Shortly afterwards, at 11:24 a.m. PDT, Galileo reaches perijove, the closest point to Jupiter, at a distance of 6.3 Jupiter radii (450,400 kilometers, or 279,865 miles) above the cloud tops. Here the spacecraft is also relatively close to the small inner satellite Amalthea, and we take this opportunity to take several pictures of this body. These pictures will help the Navigation Team refine our knowledge of the orbit of the satellite. The long-term plan for the Galileo spacecraft includes a much closer flyby of that tiny body in November 2002.

In the afternoon, a lighter schedule of activities includes PPR looking at a white oval storm in the atmosphere of Jupiter. This white oval is what remains from the merger of three smaller ovals during the past two years. This measurement will provide temperatures of the feature at unprecedented spatial resolution, and will help define models for convection and energy transfer in the atmosphere.

Closest approach to the icy satellite Europa is at 5:41 p.m. PDT, at a distance of 781,513 kilometers (485,707 miles). NIMS takes this opportunity to perform an observation of the body to complete the global mapping which began in Galileo's Prime Mission.

The day's activities wrap up in the evening with more NIMS observations of the Jupiter atmosphere. At 7:00 p.m. PDT, NIMS looks at cloud structures called "brown barges" in the northern hemisphere of the planet. These measurements provide information about the composition of the clouds, and how they change over time. At 8:00 p.m. and 9:00 p.m. the instrument also looks at a region of hot spots just north of the equator, again studying the composition and dynamics of the clouds.

The day concludes at 10 p.m. PDT with SSI once again taking color pictures of Io to look for changes in the surface of the satellite. Io is the most volcanically active body in the solar system, and significant changes in its colorful surface have been noticed even in the geological blink of an eye represented by Galileo's Tour of the Jovian system.

A busy day, with much more yet to come!

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 50 minutes to travel between the spacecraft and Earth.

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