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Line 4: + − + − + − 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>. + − + − + − + +
Added description of anomaly from Nieto 2005
==Discovery==
==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<ref name="Nieto">Michael Martin Nieto and John D Anderson. "[http://www.iop.org/EJ/abstract/0264-9381/22/24/008 Using Early Data to Illuminate the Pioneer Anomaly]". ''Classical and Quantum Gravity'', 2005</ref>, including the hypothetical [[Oort cloud]].
Every spacecraft sent to the outer solar system is intended to follow a specific course. The theory of [[General relativity|general relativity]] is used to predict these courses. The spacecraft are then tracked during flight. By using [[Doppler effect|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.
Around the time of Pioneer 11's flyby of Saturn, it was found to be slightly off-course.<ref name="Nieto" /> (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 between 20 to 70 [[Astronomical Unit|AU]] from the sun, the farthest any spacecraft has gone in near-free-fall. By using [[Doppler effect|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.<ref name="Nieto" />
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.
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==
==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.<ref name="Nieto" />
There are many possible explanations for the effect.
There are many possible explanations for the effect.
* The anomalous acceleration could be due to the spacecraft venting energy in certain directions.<ref>http://www.symmetrymagazine.org/breaking/2008/04/13/pioneer-spacecraft-a-step-closer-to-being-boring/</ref>.
* The anomalous acceleration could be due to the spacecraft venting energy in certain directions.<ref>http://www.symmetrymagazine.org/breaking/2008/04/13/pioneer-spacecraft-a-step-closer-to-being-boring/</ref>. However, such effects would be expected to be more significant earlier on, when the power sources were less degraded<ref name="Nieto" />. 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 by celestial bodies have also experienced anomalous gravitational effects.
* 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 [[Starlight_problem#Humphreys.27_model|his new cosmology]], as the pull from the center of mass of the universe.
* 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 [[Starlight_problem#Humphreys.27_model|his new cosmology]], as the pull from the center of mass of the universe.
==References==
{{reflist}}
{{Relativity}}
{{Relativity}}