In Blind faith in Science is just as bad as blind faith in Religion
brings up the question as to why the Pioneer space probes are not traveling at the speed that they are expected to.
The Pioneer 10 and 11 probes, launched in 1972 and 1973
to visit Jupiter and Saturn ventured beyond the nine planets in the 1980s. Researchers monitored their orbits to detect evidence of the hypothesized 10th planet. While they did not find any evidence for Planet X, they did notice that the probes were slowing down faster than was predicted by Einstein's general theory of relativity. There was some force, approximately equal to a ten-billionth of the force of gravity at the Earth's surface was acting on the probes.
In August of 1998, an analysis of the possibilities was looked into. These included instrumental error, interplanetary gas, the gentle push of sunlight and the gravity of planets, comets, and distant stars. There were also hints of the anomaly in two other deep-space missions: Galileo and Ulysses. Voyager 1 and 2 were not sensitive to the effect because they frequently fired their rockets and minute imprecisions in these maneuvers would conceal any anomaly.
By October, several scientists had explanations. Two of these explanations invoked waste heat. As the probes radiate excess infrared radiation into space, they should recoil ever so slightly. An infrared power of 85 watts, if beamed away from the sun would cause a small force to slow it down. Another possibility was asymmetry in how the on board nuclear generator disposes heat, reflecting off the back of the radio dish and away from the sun. The 85 watt figure is roughly equal to the power consumption of the Pioneer instruments. The excess heat would be dumped into space by a radiator on the side opposite the sun. The third possibility was that the spacecraft was venting fuel into space. A leak of just two grams/year (less than 0.01% of the fuel supply) would act as a breaking force.
Unfortunately, each of these explanations has its own problems. Waste heat reflected off the back of the antenna would be unfocused. Dissipated electrical power is also not the answer, because the acceleration did not diminish as power consumption dropped over the years. Past gas leaks have already been observed in the tracking data and accounted for.
While these explanations may be the case, the value of the anomaly matches the critical acceleration in a new law of motion that modifies Newton's second law (F = m * a) for accelerations less than a critical value. At low accelerations, quantum and cosmological effects may reduce the inertia of objects. When this happens, a given gravitational force would have a stronger affect on bodies.
MOND has two predictive effects. First off, bodies within the solar system receive a constant boost. This was observed in the Pioneer data. The second effect is that objects in elongated orbits (such as the probes) would be affected differently from those in nearly circular orbits.
Source: Scientific American, December 1998.