matter. However, this acceleration does not show up in the orbits of the outer planets, so any generic gravitational answer would need to violate the equivalence principle (see modified inertia below). Likewise, the anomaly does not appear in the orbits of Neptune's moons, challenging the possibility that the Pioneer anomaly may be an unconventional gravitational phenomenon based on range from the Sun. Drag The cause could be drag from the interplanetary medium, including dust, solar wind and cosmic rays. However, the measured densities are too small to cause the effect. Gas leaks Gas leaks, including helium from the spacecraft's radioisotope thermoelectric generators (RTGs) have been thought as possible cause. Observational or recording errors The possibility of observational errors, which include measurement and computational errors, has been advanced as a reason for interpreting the data as an anomaly. Hence, this would result in approximation and statistical errors. However, further analysis has determined that significant errors are not likely because seven independent analyses have shown the existence of the Pioneer anomaly as of March 2010. The effect is so small that it could be a statistical anomaly caused by differences in the way data were collected over the lifetime of the probes. Numerous changes were made over this period, including changes in the receiving instruments, reception sites, data recording systems and recording formats. New physics Because the "Pioneer anomaly" does not show up as an effect on the planets, Anderson et al. speculated that this would be interesting if this was new physics. Later, with the Doppler shifted signal confirmed, the team again speculated that one explanation may lie with new physics, if not some unknown systemic explanation. Clock acceleration Clock acceleration was an alternate explanation to anomalous acceleration of the spacecraft towards the Sun. This theory took notice of an expanding universe, which was thought to create an increasing background 'gravitational potential'. The increased gravitational potential would then accelerate cosmological time. It was proposed that this particular effect causes the observed deviation from predicted trajectories and velocities of Pioneer 10 and Pioneer 11. From their data, Anderson's team deduced a steady frequency drift of 1.5 Hz over eight years. This could be mapped on to a clock acceleration theory, which meant all clocks would be changing in relation to a constant acceleration: in other words, that there would be a non-uniformity of time. Moreover, for such a distortion related to time,