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Such a maneuver is based on the solution of complex mathematical equations involving all the intricacies of space mechanics. Computers at JPL took into account Mariner's speed and trajectory, its location in relation to the point in Mars's orbit where the encounter should take place, and the influence of the sun, the Earth and Mars itself. Then they calculated the thrust needed to get the ship where it had to be at the proper time. Without correction, Mariner would have strayed 150,000 miles away from target. After the mid-course maneuver, it was aimed well within its programmed 10,000 miles. A second mid-course correction, though possible, was never needed.
On Feb. 11, JPL signaled for a checkout of Mariner's photographic apparatus. The commands turned on and then turned off power to the tape recorder, and pointed the TV camera as it would have to be when it got close to Mars. Everything functioned well. Recalling the dust problem with Canopus sensors, JPL engineers decided to remove the TV lens cover then, instead of waiting until the final encounter. If there was any dust on the cover, they did not want it shaken loose to endanger the sensors at a critical moment.
Planning for Trouble. Well past mid-point in its journey, the spaceship was sailing along smoothly. No problems, only precedents. On April 29, when Mariner reached a straight-line distance from earth of 66 million miles, it surpassed the record for long-distance space communications set two years ago by the unsuccessful Russian Mars 1 probe.
So uneventful was the flight that it began to worry Mariner Project Manager Dan Schneiderman. He was afraid that his 200-man control team might begin to take the mission too much for granted. Determined to guard against the danger, he busied his men with practice Mars encounter exercises all through the final few weeks of the flight. Working with a duplicate of the ship that was far out in space, Schneiderman's team manned their posts and computed answers to a nerve-racking sequence of simulated problems. They dealt with every imaginable glitch, from premature starts of the camera to unprogrammed movements of the scan platform that was designed to pick up the planet and tell the TV camera when to start functioning. Every decision the team made was fed into a duplicate Mariner in the laboratory, just as radioed commands might later be sent into space. The difference was that on these practice runs results could be checked, tactics could be changed.
As the real test approached last week, the months of calm gave way to hours of apprehension. So much could go haywire in such a complex operation, so much could happen to delicate instruments during such a long journey through the hazards of space. JPL statisticians had already calculated that there was only a 17% chance of getting photographs. To be sure, the JPL crew knew that the mission had already produced significant scientific results, but they also realized that only a set of pictures would mean real success. "It's a failure without the pictures," said John Casani. "You're judged on the success of the most difficult part of the mission. That's the pictures, and if we don't get them, then we've failed."