  Radiometric data from the Pioneer 10 and 11 spacecrafts have revealed an
unexplained constant acceleration of a_A = (8.74 +/- 1.33) x 10^(-10) m s^(-2)
towards the Sun, also known as the Pioneer anomaly. Different groups have
analyzed the Pioneer data and have got the same results, which rules out
computer programming and handling errors. Attempts to explain this phenomenon
arguing intrinsic causes on-board the spacecrafts failed or have lead to
inconclusive results. Therefore, the Pioneer anomalous acceleration has
motivated the interest of researchers to find out explanations that could bring
insight upon the forces acting in the outer Solar Systems or a hint to discover
new natural laws.
