The flyby anomaly is a discrepancy between current scientific models and the actual increase in speed (i.e. increase in kinetic energy) observed during a planetary flyby (usually of Earth) by a spacecraft. In multiple cases, spacecraft have been observed to gain greater speed than scientists had predicted, but thus far no convincing explanation has been found.

Source Wikipedia

The speed increase is relatively small, at about 3 to 13 mm/s, nevertheless, investigations by the US Jet Propulsion Laboratory, The Goddard Space Center and the University of Texas have not been able to explain the anomalies, which have now been observed and verified several times, with at least 10 different spacecraft. The first observation was in 1990 with NASA's Galileo mission.

Theories about the anomaly include : the possible presence of Dark Matterplugin-autotooltip__plain plugin-autotooltip_bigDark Matter

"The nature of the dominant component of galaxies and clusters remains unknown."

Source : Measuring the dark matter equation of state (Mon. Not. R. Astron. Soc. 415, L74–L77)"

In the 1930s, astronomical observations of galaxy rotations showed that the outer regions were rotating (about the galaxy's 'centre') at the same speed, or faster, than the central regions. Subsequent calculations referring to the galaxy's mass, and thus its internal gravitational attractions, showed that i…, possible light-speed variations and / or gravity variationsplugin-autotooltip__plain plugin-autotooltip_bigThe Gravitational constant

The gravitational attraction between two objects is dependent on the mass of the objects, the distance between them, and the gravitational constant (G).

The masses and distance can vary, but the constant, as the name implies, is always a fixed number..