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[[File:Dn11304-1 600.jpg|right|300px|thumb|Pioneer anomaly]]

[[File:Dn11304-1 600.jpg|right|300px|thumb|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''' 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.

[[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]].

[[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]].

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 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" />

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 [[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 Project|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.

Although the [[Galileo Project|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.