Pioneer anomaly

Trajectories of the Pioneer and Voyager spacecraft. (The Pioneer anomaly is much too small to be noticeable at this scale.)

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.

The Pioneer anomaly may be evidence that our understanding of gravity (Newtonian mechanics and general relativity) needs to be improved.

Discovery

Pioneer 10 and Pioneer 11 were space probes sent to study the planets Jupiter and Saturn. After following a hyperbolic trajectory around these planets, they had reached escape velocity for the solar system and were flying out. While their main mission was now ended, NASA stayed in radio contact with the craft to study the outskirts of the solar system, including the hypothetical Oort cloud.

Around the time of Pioneer 11's flyby of Saturn, it was found to be slightly off-course. (Every spacecraft sent to the outer solar system is intended to follow a specific course, predicted by the theory of general relativity. Radio transmissions and radar are used to track spacecraft to ensure that they stay on course.) While this in itself was within the range of error, astronomers continued tracking the craft to find that the anomalous sunward acceleration increased. Currently, Pioneer 10 and Pioneer 11 are respectively over 30 and 70 AU from the sun, the farthest any spacecraft has gone in near-free-fall. 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> beyond theoretical preditions.

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. The confounding effects are even more significant on the Voyager spacecraft, preventing even a discussion of whether the Pioneer Anomaly affects these craft 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

Originally, scientists supposed that the Pioneer navigation code was in error. However, the code was verified by an independent team. After a rigorous search for all possible effects, the anomaly was determined to be real: the course actually does diverge from models.

There are many possible explanations for the effect.

• The anomalous acceleration could be due to the spacecraft venting energy in certain directions.. However, such effects would be expected to be more significant earlier on, when the power sources were less degraded. The opposite was actually the case.
• 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 in hyperbolic trajectories around 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, which he locates inside the solar system. However, he does not explain why this new force acts only on spacecraft.