The Pioneer 10 and 11 spacecraft have been described informally as the most precisely navigated deep space vehicles to date. Such precise navigation [24, 27, 260, 262, 390, 391, 392, 393] was made possible by many factors, including a conservative design (see Figure 2.3 for a design drawing of the spacecraft) that placed the spacecraft’s RTGs at the end of extended booms, providing added stability and reducing thermal effects. For attitude control, the spacecraft were spin-stabilized, requiring a minimum number of attitude correction maneuvers, further reducing navigation noise. As a result, precision navigation of the Pioneer spacecraft was possible across multi-year stretches spanning a decade or more .
Due in part to these excellent navigational capabilities, NASA supported a proposal to extend the Pioneer 10 and 11 missions beyond the originally planned mission durations, and use the spacecraft in an attempt to perform deep space celestial mechanics experiments, as proposed by J.D. Anderson from the Jet Propulsion Laboratory (JPL). Starting in 1979, the team led by Anderson began a systematic search for unmodeled accelerations in the trajectories of the two spacecraft. The principal aim of this investigation was the search for a hypothetical tenth planet, Planet X. Later, Pioneer 10 and 11 were used to search for trans-Neptunian objects; the superior quality of their Doppler tracking results also yielded the first ever limits on low frequency gravitational radiation .
The acceleration sensitivity of the Pioneer 10 and 11 spacecraft was at the level of 10–10 m/s2. At this level of sensitivity, however, a small, anomalous, apparently constant Doppler frequency drift was detected [24, 27, 390].
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