Pioneer 10/11 Anomaly

The astrophysics community has been baffled by the apparent anomalous behavior of Pioneer 10/11, as they have been observed to be attracted toward the sun slightly more than predicted by the force given by Newtonian gravity.

The anomalous Pioneer 10/11 acceleration (8.74 +/- 1.33) * 10^-8 cm/s^2 has been reported. For more info, visit http://www.planetary.org/programs/projects/pioneer_anomaly/ .

The acceleration of the Earth in its orbit is 0.593 cm/s^2 on average (1 AU)*(2*pi/yr)^2 making this anomalous acceleration 1.5*10^-7 of that.

However, Pioneer 10 was 67 AU from the Sun in 1997, when its mission was downgraded to a sort of background status, and Pioneer 11 was 40 AU from the Sun when contact with it was lost in 1995. This means that the anomalous acceleration is ~10^-4 of the Sun's acceleration of them at that distance.

Finally, it is noted that calculated nongravitational effects, such as radiation reaction from the spacecraft's radio transmissions and the glow of the RTG's, are not much smaller than the anomalous acceleration itself, suggesting that one may have to model such effects more carefully.

Also, there are no similar effects reported for the Voyager spacecraft, so these may be due to some quirk of the Pioneers.

The following is a quote from the report from NASA at JPL and LANL.
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Dispassionately, the most likely cause of the anomalous acceleration of the Pioneer spacecraft is on-board systematics, but the smoking gun has not yet been found. The only other possibility is the existence of new physics. This dichotomy represents a healthy win-win situation because either one of these two explanations for the Pioneer anomaly would constitute an extremely important discovery. (Author Slava Turyshev and John Anderson are at the NASA Jet Propulsion Laboratory, Pasadena and Michael Martin Nieto at the Los Alamos National Laboratory, US)
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One possible explanation of this anomaly may be found within the theory of dipole gravity. Since the rotating parabolic antenna of Pioneer represents a dipole gravity rotor, the observed additional acceleration toward the sun can be explained if the antenna is focused toward the sun which means the wider (open) side of the parabola faces the sun. And Pioneer is reported to be spinning at 14.1 sec per revolution which makes it spinning at 4.25 rpm. However, since the effect was not observed from Voyager as prominently(if at all) as it was observed from Pioneer, this may still be an open question.

If Voyager has different mechanical parameter (it also has parabolic antenna) compared to that of Pioneer in such a way that the dipole gravity effect is minimal (for example, the spinning rate is lower and it has more longitudinal axially symmetric configuration), then the mystery could be resolved. For this purpose, the detailed mechanical configuration including the rate of the spin rotation for both Pioneer and Voyager may be needed. This data may be found in the detailed original engineering design in terms of the mass distributions inside the craft.

In the mean time, the following information has been available which can be very significant.
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Is the same effect seen with the Voyager spacecraft?
The Pioneers are spin-stabilized spacecraft. The Voyagers are three-axis stabilized craft that fire thrusters to maintain their orientation in space or to slew around and point their instruments. Those thruster firings would introduce uncertainties in the tracking data that would overwhelm any effect as small as that occurring with Pioneer.

This difference in the way the spacecraft are stabilized actually is one of the reasons the Pioneer data are so important and unique. Most current spacecraft are three-axis stabilized, not spin stabilized.
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If the above information is true, in the three-axis stabilized spacecraft case, the anomalous effect would have been automatically canceled or diluted, because of the continuous adjustment of the orientation of the craft. However, in the case of the spin-stabilized craft, the dipole gravity effect will not be corrected which will make the effect prominently visible.

The following information is exactly what can be expected from a dipole gravity rotor placed in the empty space.
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What really puzzles scientists, Turyshev said, is that the anomaly is constant with respect to time and distance from about 1 1/2 billion miles away from the sun to about 6 1/2 billion miles out. The anomalous behavior has been observed in other deep space vehicles such as Voyager and Ullyses.
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The magnitude of anomaly seems larger than one would expect from a spin rotation of 4.25 rpm regardless of the size of the rotor. But the exact mechanical configuration of the craft is not yet available at the moment. And there is an uncertainty on the average distance and the amount of the mass of the universe which are not well defined relative to the location of the dipole gravity rotor. So, in case a terrestrial experiment is planned for the second confirmation, the Pioneer data will make a good starting point.

In a sense, the alternative experiment that has been proposed for the test of dipole gravity in the page http://dipoleantigravity.blogspot.com/2007/04/alternative-method-of-detecting-dipole.html may have already been done in the space without our knowledge.