The Pioneers are considered the most precisely tracked and navigated spacecraft to date. This astonishing accuracy with regards to measuring the acceleration of the Pioneers led to an unusual discovery. It was understood that the probes would slow down to some extent due to the Sun’s gravitational attraction. But while the Pioneers had reached escape velocity from the Sun and would ultimately leave the solar system, by 1980 a researcher at NASA’s Jet Propulsion Laboratory noticed the Pioneers were slowing down more than the data predicted they would. This occurred in both Pioneers, manifesting as a sunward acceleration of 8.74 ± 1.33 × 10m/sWhile this may seem like a very small difference in measured acceleration from predicted acceleration, it led to the Pioneers remaining thousands of kilometers closer to Earth per year than they should have been. The cause greatly puzzled the scientists who studied the problem. Even such a small, unexplained sunward acceleration suggested a possible breakdown in the principle of gravitation as described by general relativity.Numerous people became interested in the problem. Why, they asked, were the Pioneers slowing down? Many factors were considered as possible causes for this unexplained deceleration, which became known as the Pioneer anomaly. Could our understanding of the fundaments of physics be flawed? Could gravitational forces from the surrounding interstellar medium that fills the space between stars be different than had been expected? Could dust produce drag on the spacecraft? Was the anomaly possibly secondary to solar radiation forces? Could our calculations regarding the exact position of Earth relative to the spacecraft be incorrect? Was something related to the spacecraft — waste heat, a faulty thruster, or a gas leak — causing the anomaly?Beyond the theoretical problems facing researchers, another more concrete problem stood in their way: The technical information and recordings of Pioneer data needed to solve the problem were stored either on paper or on 7- and 9-track magnetic data tapes. Identifying these data sources (some of which were literally in boxes under stairwells and in dumpsters on their way to being destroyed), recovering the hardware needed to read the tapes, and converting all of this information to modern media formats was a daunting challenge on several levels, most notably the cost and manpower involved. The Planetary Society helped raise funds to help researchers like Slava Turyshev and his collaborators overcome these challenges.