The steady streams of engineering and scientific data received on Earth are transmitted from the receiving stations of the Deep Space Network (DSN) to JPL, where the Voyager control functions are centered. There, dozens of technicians check and recheck every subsystem to search for the slightest hint of malfunction. In case of problems, there are thick notebooks of instructions and racks of precoded computer tapes ready to be used to correct any apparent malfunction. Normally Voyager runs itself. Detailed instructions are programmed into its onboard computers and command systems for dealing with such potential emergencies as a stuck valve in the fuel system, loss of orientation in the star trackers, erratic gyroscope functions, failure of radio communications, or a thousand and one other nightmares. The instructions for operating the scientific instruments are also stored on board, with new blocks of commands sent up once every few days to replace those for tasks already completed. Whether in the calm of cruise mode or the intense excitement of a planetary encounter, the Voyager craft is alone in space, continuously sensing and reacting to its environment, tied by only a tenuous thread of radio communication to the anxious watchers back on Earth. [Illustration: Project Manager Robert Parks] NASA PLANETARY MISSIONS Spacecraft Launch Destination Encounter Type of Date Date Encounter Mariner 2 8/26/62 Venus 12/14/62 flyby Mariner 4 11/28/64 Mars 7/14/65 flyby Mariner 5 6/14/67 Venus 10/19/67 flyby Mariner 6 2/25/69 Mars 7/31/69 flyby Mariner 7 3/27/69 Mars 8/05/69 flyby Mariner 9 5/30/71 Mars 11/13/71 orbiter Pioneer 10 3/03/72 Jupiter 12/04/73 flyby Pioneer 11 4/06/73 Jupiter 12/03/74 flyby Saturn 9/01/79 flyby Mariner 10 11/03/73 Venus 2/05/74 flyby Mercury 3/29/74 flyby Viking 1 8/20/75 Mars 6/19/76 orbiter 7/20/76 lander Viking 2 9/09/75 Mars 7/07/76 orbiter 9/03/76 lander Voyager 1 8/20/77 Jupiter 3/05/79 flyby Voyager 2 9/05/77 Jupiter 7/09/79 flyby Pioneer Venus 5/20/78 Venus 12/04/78 orbiter 8/8/78 Venus 12/09/78 probe