Today on Galileo - November 4, 1996
Galileo's closest approach to Jupiter's moon Callisto will occur today at 5:34 am PST. At that time, the spacecraft will pass only 1118 km over Callisto's surface, the closest distance from which any spacecraft has viewed Callisto! For 7 hours around this "closest approach", the instruments will be making intense measurements of Callisto's surface, atmosphere, and magnetic environment. Galileo will pass near the moon's equator at a speed of 8 kilometers per second. Signals from the spacecraft will take 46 minutes to make their journey to Earth, since Galileo is now over 500 million miles away.
The surface structure and composition of three different regions on Callisto will be studied in depth by the imaging camera, NIMS, and UVS instruments. The Asgard Ring Structure has two overlapping cocentric ring structures and will be imaged with at least 1.1 km/pixel resolution. Higher resolution imaging (up to 28 meters/pixel) of the Valhalla region will be done in the minutes just surrounding closest approach. NIMS will study one of Callisto's central pit craters and dome to determine its origin and composition. UVS will look for an oxygen and hydrogen atmosphere. The Fields and Particles instruments will investigate the presence of a magnetic field around the moon, and studying the interaction with Callisto of dust particles, plasma, and the magnetic field of Jupiter.
After leaving Callisto, Galileo will turn its attention afternoon to Jupiter. The imaging camera will observe the northern auroral region to see how the atmosphere moves there and what the clouds underneath are made of. Later the imager, NIMS, UVS, and PPR instruments will study the atmospheric belts in the equatorial region. The day ends with a unique study of the lines of Jupiter's magnetic field that cause plasma (hot ionized gases) to be driven into Jupiter's atmosphere and cause aurora. Galileo will for the first time actually pass through these lines andwill take measurements that can help us understand why the aurora happen.