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Pioneer P-31
Pioneer P-31 (also known as Atlas-Able 5B or Pioneer Z) was intended to be a lunar orbiter probe, but the mission failed shortly after launch. The objectives were to place a highly instrumented probe in lunar orbit, to investigate the environment between the Earth and Moon, and to develop technology for controlling and maneuvering spacecraft from Earth. It was equipped to take images of the lunar surface with a television-like system, estimate the Moon's mass and topography of the poles, record the distribution and velocity of micrometeorites, and study radiation, magnetic fields, and low frequency electromagnetic waves in space. A midcourse propulsion system and injection rocket would have been the first United States self-contained propulsion system capable of operation many months after launch at great distances from Earth and the first U.S. tests of maneuvering a satellite in space.
==Mission==
The spacecraft was launched on Atlas vehicle 91D coupled to Thor-Able upper stages including an Able solid propellant third stage on December 15, 1960. The launch was uneventful until T+66 seconds when a severe axial disturbance was recorded, followed by rapid loss of LOX tank pressure and changes in the Atlas's engine exhaust indicative of oxidizer starvation. At T+73 seconds, the Atlas experienced total structural breakup and loss of telemetry. The upper stages continued transmitting data until impact with the ocean. The payload fell into the Atlantic Ocean 12 to 20 km from Cape Canaveral in about 20 meter deep water. The immediate cause of the failure was unclear, but thought to be related to either the adapter mating the Able stages to the Atlas coming loose and being rammed into the LOX tank or else aerodynamic buffeting on the launch vehicle. However, analysis of telemetry data found that the Able second stage had ignited prematurely and detonated the Atlas's LOX tank. This was proven by data indicating that the Able began accelerating at the moment of disturbance while the Atlas decelerated. The crippled booster continued to fly for a few seconds afterwards, but the structural collapse of the Atlas's forward section combined with the loss of LOX pressure to the propellant feed system resulted in engine shutdown and vehicle self-destruction.
The failure was described as "especially disappointing" since it was the final launch in the Able probe series as its successor, the Ranger program, was in the works. In the end, the US space program would not see a completely successful lunar probe until Ranger 7 four years later. It also marked the final launch in the first generation of lunar probes, which used direct ascent trajectories and would give way to the second generation probes which had parking orbits.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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