From: baalke@zagami.jpl.nasa.gov   
      
   The Final Day on Galileo   
   Sunday, September 21, 2003   
      
   The Wind-Up   
      
   Well, after twelve years of pre-launch development and planning, six years   
   of interplanetary cruise, and nearly eight years in orbit, our exciting,   
   quarter-century odyssey has finally come down to this: the final 19 hours   
   of existence for the Galileo spacecraft. It began life in October 1977 as   
   the Jupiter Orbiter Probe mission, was launched in October 1989, and   
   arrived at Jupiter in December 1995. After circling the solar system's   
   largest planet 35 times, it is about to plunge into the atmosphere of   
   Jupiter, becoming only the second man-made object to do so, following the   
   smaller Galileo atmospheric probe that accompanied the Orbiter to Jupiter.   
    From launch to impact, the stalwart spacecraft has travelled 4,631,778,000   
   kilometers (2,878,053,500 miles) on 925 kilograms of propellant (246   
   gallons), not counting the fuel for the shuttle. In all that time, and   
   across all those miles, Galileo has returned over 30 gigabytes of data,   
   including 14,000 pictures.   
      
   One chapter of the volumes of scientific data produced by Galileo over the   
   years includes the discovery of likely sub-surface water oceans on the icy   
   satellite Europa. This has fueled speculation about the possibility of life   
   existing in that environment, and is prompting plans for future spacecraft   
   to return to Europa to search for life. Since the Galileo spacecraft was   
   never designed to specifically search for life, it was never subjected to   
   the rigorous sterilization procedures such as those mandated for craft   
   going to Mars. To prevent any possible future biological contamination of   
   Europa, the decision was made to provide a final resting place for the   
   Galileo Orbiter that guarantees the spacecraft will never collide with any   
   of the Jovian moons. That resting place is Jupiter itself.   
      
   The Pitch   
      
   The final day for Galileo begins on Saturday evening at 5:52 p.m. PDT [see   
   Note 1], when the spacecraft is just over 18 Jupiter radii (1.32 million   
   kilometers or 822,000 miles) from the center of the planet, and closing   
   fast. At that time, the instruments that measure the magnetic and electric   
   fields and the particle environment stop collecting data for a while. These   
   instruments are the Dust Detector Subsystem (DDS), the Energetic Particle   
   Detector (EPD), the Heavy Ion Counter (HIC), the Magnetometer (MAG), and   
   the Plasma and Plasma Wave Subsystems (PLS and PWS). They have been   
   collecting data continuously for the past six months, storing these data in   
   a computer memory buffer for later transmission to Earth. By stopping this   
   collection for 7 hours, this buffer is allowed to drain, and subsequent   
   data collected can be transmitted to Earth almost immediately.   
      
   At 10:52 p.m. the spacecraft attitude control system is told to base its   
   orientation calculations on the observations of a single star. Normally,   
   three stars are used, but in the intense radiation environment near   
   Jupiter, noise in the star sensor circuits overwhelms the signals from   
   fainter stars, and a single, bright star is selected which can rise above   
   the noise and still be detected. Today that star is Vega (Alpha Lyrae), the   
   fourth brightest star in the sky.   
      
   At 12:52 a.m. Sunday, the 70-meter-diameter (230 feet) Deep Space Network   
   tracking station antenna near Madrid, Spain, is listening to the   
   spacecraft. The science instruments are configured properly, and begin   
   again to send their data in real-time to Earth. Galileo has closed the   
   distance to 13.5 Jupiter radii (965,000 kilometers or 600,000 miles).   
      
   At 5:55 a.m., the tracking antenna at Goldstone in the Southern California   
   desert joins the Madrid station, and for the next three hours both stations   
   are collecting the faint signals sent from a half a billion miles out in   
   the solar system.   
      
   At 5:07 a.m., the distance has closed to 10 Jupiter radii (715,000   
   kilometers or 444,000 miles) and MAG changes the sensitivity of its   
   measurements in anticipation of the stronger magnetic fields to come. At   
   6:24 a.m., all instruments except MAG stop collecting data for just over an   
   hour. During this time the data collection rate on the spacecraft is   
   greater than the rate the ground stations can reliably receive. If the   
   collection continued, the data would accumulate in the storage buffer to   
   such an extent that the buffer would not have a chance to empty before the   
   spacecraft is lost from view. This brief pause allows the most valuable   
   data collected nearest to Jupiter to be sent without buffering.   
      
   At 7:22 a.m., the radio signal sent from the spacecraft is changed to   
   provide more power to the underlying carrier signal. This will help the   
   ground stations keep track of the signal as Jupiter's increasing   
   gravitational pull speeds the spacecraft up and alters the apparent   
   transmission frequency, due to the familiar Doppler effect.   
      
   The Swing   
      
   At 9:05 a.m., Galileo crosses the volcanic satellite Io's orbit at a   
   distance of 6 Jupiter radii (422,000 kilometers or 262,000 miles). The   
   spacecraft has spent most of its 8-year travels around Jupiter outside of   
   this distance, to keep the received radiation dose down. It has ventured   
   significantly inside this distance only twice. Once, in December 1995, as   
   Galileo first entered Jupiter orbit, when we reached 4 Jupiter radii (3   
   radii over the clouds), and again in November 2002, when a flyby of the   
   small inner moon Amalthea took the craft down to 2 Jupiter radii (1 Jupiter   
   radius, 71,500 kilometers or 44,400 miles over the cloud tops). This time,   
   though, it's a one-way trip. The distance will only get shorter.   
      
   By 9:42 a.m., the intensity of the radiation noise has reached a point   
   where even a bright star like Vega can no longer reliably be seen by the   
   attitude control star scanner. The software is now told to expect to see no   
   more stars, ever.   
      
   At 11:31 a.m., Galileo is two Jupiter radii above the clouds (143,000   
   kilometers or 89,000 miles) and the Magnetometer instrument has taken its   
   final data for the mission. At this distance from Jupiter, the magnetic   
   field is so strong that the instrument, even in its most robust   
   configuration, would produce a signal that would be completely saturated,   
   and of no further scientific value.   
      
   Seventeen minutes later, at 11:48 a.m., the spacecraft passes the orbit of   
   the tiny satellite Amalthea, and at about 12:17 p.m., passes the orbits of   
   the innermost moons, Adrastea and Metis. Galileo is now just 57,500   
   kilometers (35,700 miles) above the clouds, closing fast, and picking up   
   speed. As the spacecraft passes Amalthea a special measurement will be   
      
   [continued in next message]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)