National Geographic : 1963 May
ter, picking up sunlight reflected from earth. Since the device had to be sensitive enough to pick up earth even at the end of the voyage -when our planet appears no larger than Venus itself appears from earth-the instru ment could not be turned on until seven days after launch. Earlier use could have burned out its mechanism, as the human eye would burn out if it stared at the sun through a lens. When Mariner gave its sensor the "find earth" command, it was already 1,200,000 miles from its home planet. At the order, trou ble immediately developed. The craft's radio signal lost strength instead of gaining. Technicians in the control room at Pasa dena frantically sought to trace the problem. Finally, 36 minutes after the mishap, Mariner II suddenly locked on earth and sent signals of the proper strength. Later, scientists decided that a freak co incidence had caused the trouble. When the earth sensor was turned on, they believe, it happened to be pointing at earth already. Having seen its target, it automatically be gan sending signals through the narrowly focused antenna. But that first glimpse of earth was not enough to stop Mariner's roll ing motion, and therefore the signal was cast around wildly in space instead of being aimed at earth. It took Mariner 36 minutes to make a complete roll back to its proper position. All was well now-except that the space craft was not on course. Mariner's designers knew from the start that the Atlas-Agena combination was not accurate enough to set the craft on a perfect trajectory, so they built in a corrective rocket motor. After a week of tracking, they could determine Mariner's actual path. Without correction, they con cluded, it would miss Venus by 233,000 miles. Mariner's instruments could work only in a narrow target area-between 8,000 and 40,000 miles from the planet. Success de pended on a "midcourse maneuver." Mariner Attempts a Critical Turn On September 4, Mariner, then 1,490,000 miles out in space, was commanded from the tracking station at Goldstone, California, to release its locks on earth and sun and re orient itself so the rocket in its base would aim in the desired direction. When the craft reported itself ready, the command was given, "Fire!" The rocket burned for 27.8 seconds, delicately boosting Mariner's 60,117-mile-an hour speed by a mere 45 miles an hour. It was enough. Almost immediately after the rocket turned off, it was apparent that the maneuver was a success. In its new course, Mariner once more locked on sun and earth. In the Pasadena con trol room, Jack James, like any proud father, passed out cigars. Four days later something moving at the speed of a bullet apparently hit the craft. Mariner shuddered, lost lock on the sun, then, three minutes later, regained it. All the symp toms point to an object weighing between a twentieth and fiftieth of a pound striking the spacecraft at 1,600 feet per second. Mariner weathered this blow, and another, 21 days later, without ill effect.