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Incredible Balance. One compartment of the spaceship housed a piece of equipment that did nothing but take up space and use electric current. It was a dummy ultraviolet light photometer. In ground tests before launch, the real one developed a disturbing habit of high-voltage arcing that not only blew out the photometer but also disrupted the TV system. Unable to replace the instrument and unwilling to risk ruining Mariner's picture mission, the engineers decided to leave the instrument behind. Since the ship was designed to carry the photometer, a replica was made of exactly the same weight; it was polished to give off the same reflection and engineered to absorb the same electric current, lest an incredibly delicate balance be upset.
Although detailed instructions for nearly all of Mariner's maneuvers were programmed in advance and stored in the on-board computer, the journey still had its moments of suspense and anxiety. The first trouble came only 16 hours after launch, when two solar pressure vanes—flaps hinged to the end of the solar panels—stuck in an up-tilted position. Unless corrected or compensated for, this fault would have been enough to head the ship on a course that would have taken it 400 miles farther away from Mars than was anticipated.
Other malfunctions plagued the early days of flight. The solar plasma probe equipment, designed to detect the low-energy protons of the solar wind, was thrown off kilter because of a defective metal clamp. A tube in the ionization chamber conked out, causing a power failure that eventually ruined the whole experiment. Mariner's roving navigation eye also got it into trouble. The bright, bluish-white star Canopus was supposed to serve as Mariner's polestar, but other bright objects began to confuse Mariner's sensor. Once it tracked the wrong star for ten days until a command from JPL directed it back to Canopus. With another command, the engineers solved the Canopus problem by shutting the brightness gate, a mechanism that caused the sensor to begin searching for its assigned star whenever it was fixed on a light considerably brighter or dimmer than Canopus. Sometimes a speck of dust reflecting the sunlight would accidentally trigger the maneuver. "The dust's effect on the sensor," says John Casani, Mariner systems manager, "really threw us for a loop."
Ultimate Accuracy. Despite such "glitches" (a spaceman's word for irritating disturbances), Mariner handled its difficult assignment without a hitch. On Dec. 5, when it was 1,267,613 miles out, Mariner received a command from JPL to fire its rocket motor for the first and only correction of its trajectory. The ultimate accuracy of the encounter with Mars depended on this operation; hopefully it would correct for the drag of the pressure vanes and any other factors that were taking Mariner from its planned course.