Pioneer anomaly
The Pioneer anomaly refers to deviations in projected courses for several spacecraft sent to the outer solar system. The effect is believed to have influenced the courses of both Pioneer spacecraft, Galileo, and Ulysses. The courses for the Voyager spacecraft and Cassini were not able to be predicted accurately due to known but unpredictable effects on these craft, and if the anomaly affects these spacecraft, it would not be distinguishable from other effects.
Critics of general relativity cite the Pioneer anomaly as evidence that the theory is in error.
Discovery
Every spacecraft sent to the outer solar system is intended to follow a specific course. The theory of general relativity is used to predict these courses. The spacecraft are then tracked during flight. By using Doppler radar, scientists have found that the courses for both the Pioneer spacecraft show a constant acceleration towards the sun of <math>8.74 \times 10^{-10} \frac{m}{s^2} \ </math> which was not part of the predicted course.
Although the Galileo and Ulysses spacecraft showed some unexpected sunward acceleration, other unpredictable factors, such as the Yarkovsky effect and the thrust caused by radioactive material onboard, prohibit any accurate measurement of the effect on these two spacecraft.
Other craft suffer the same difficulties as the Galileo and Ulysses craft, although more significant, preventing even a discussion of whether these craft are affected at all.
The Pioneer anomaly is about 1000 times bigger than the two effects contributing to the difference between the acceleration predicted by general relativity and that predicted by classical (Newtonian) gravity. The effect of the increase in inertia due to the Lorentz transform is less than <math>10^{-12} \frac{m}{s^2} \ </math>, and the difference in acceleration due to the Schwarzschild metric is also less than <math>10^{-12} \frac{m}{s^2} \ </math>.
Explanations
There are many possible explanations for the effect.
- The anomalous acceleration could be due to the spacecraft venting energy in certain directions.[1].
- The theory of general relativity and the law of universal gravitation could be wrong; the gravitational force could be slightly stronger than predicted. In this context, it is important to note that other spacecraft flying by celestial bodies have also experienced anomalous gravitational effects.
- There could be gravitational forces from other celestial bodies that have not been taken into account. Dr. Russell Humphreys has explained the Pioneer Anomaly as part of his new cosmology, as the pull from the center of mass of the universe.