The space probe Pioneer 10 was launched in 1972, and Pioneer 11 the following year. Aside from their missions to fly past Jupiter and Saturn respectively, they are on their voyage towards outer space in different directions from the Sun.

The Pioneers have brought to light an extremely interesting phenomenon. By early 1980s, the accumulated data persistently showed that the probes were off course. The discrepancy between radio telemetry and calculations was small - yet significantly larger than what experimental error would allow.

This meant that probes were exposed to an influence of unknown force, which was directed towards the Sun, just like the gravity itself, only much smaller in intensity.

This "Pioneer anomaly", was observed between 20-70 AU and found to be equivalent to constant acceleration of 8.74 ± 1.33 × 10−10 m/s2 directed towards the Sun. The distance of 20 AU is significantly away from the Saturn orbit, and the only force beyond 20 AU that measurably affects the probes is gravity of the Sun. The Pioneers themselves have no propulsion, or any mechanism that could affect motion of the spacecrafts themselves.

Aside from many guesses going in all directions, a question is raised often, is there something new about gravity that we don't know?

A new theory from masstheory.org website deals with synchronicity of interactions, which is applicable to gravity. Underlying idea is that there is a time-lag in all interactions, directly caused by finite speed at which information travels. For example, when one body changes position, it takes some time until this information reaches other bodies it interacts with. This means that position of bodies in space is not known to them in real time, causing them to affect each other with forces which are not time-synchronized. When expressed mathematically, an expression can be derived that has two addends - one is well known gravity force, and the second has much smaller intensity, and depends on the speed of the observed body.

This second force, unknown to theoretical physics until now, makes the dynamic component of gravity, which perfectly matches effect observed with Pioneer space probes.