"A small discrepancy between the velocity derived from ephemeris modeling based on distance ranging and the velocity measured from the Doppler shift has been detected and confirmed. It appears that the probe is subjected to a tiny constant acceleration toward the Sun that neither depends on the time nor on the distance. In addition to this constant acceleration discrepancy, periodic annual and diurnal acceleration variations have been observed. The same acceleration anomaly has been seen with Pioneer 11, which is moving outward in the opposite direction and also with other spacecrafts. No physical explanation has yet been found."
NS:Are there two types of gravity?