Abstract
For over 20 years NASA has struggled to find an explanation for the Pioneer anomaly, an unmodelled weak acceleration towards the sun (≈ 8.5×10^{-10} m s^{-2}), observed in deep space probes Pioneer 10, 11, Galileo and Ulysses (Anderson et al. 1998, 1999; Katz 1999). No consensus explanation has been given since the anomaly was first announced, suggesting that new physics is involved. The riddle may be solved if we assume that c, the speed of light, is not a universal constant. Newtonian mechanics, together with the hypothesis by Céspedes-Curé (2002) that the index of refraction is a function of the gravitational energy density of space, leads to values of c slightly higher for interstellar space dominated by the primordial energy density ρ_{*}^{} due to galaxies and far away stars, far from the gravitational influence of Earth, Moon, and Sun. The value derived for the index of refraction of space (n' < 1), implies a Doppler shift of the radio signal received from the probes which results in a decrease of the frequency received at Earth and interpreted as a weak acceleration towards the Sun.
