Pioneer 10 & 11 are robotic space probes launched by the NASA in the early 1970's. After leaving our solar system, an unusual deceleration of both spacecrafts has been measured to be approximately $$\ddot{r}_p = -(8.74±1.33)×10^{−10} \frac{m}{s^2}$$ with respect to our solar system.

Several attempts were made to explain this tiny effect, called the Pioneer anomaly, but none was fully accepted in the scientific community so far.

Two months ago, Frederico Francisco et al have proposed another solution to the problem. They assume, roughly speaking, that the thermal radiation of the spacecraft caused by plutonium on board along with the actual structure of the probes is responsible for this mystery of modern physics.

Here an image of Pioneer 10 taken from Wikipedia along with a sketch of the radiation model employed in the paper: Pioneer 10 and heat model

Hence my question:

Is the Pioneer anomaly finally explained?


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    $\begingroup$ These are good developments - although mundane ones - and a good question. I would bet that the explanation is mostly right. It's not quite new - Toth and Turshyev wrote similar things previously, see e.g. popsci.com/pioneeranomaly - I guess that to say the least, these derivations refute the statement that the Pioneer provides us with strong evidence for new physics. ... I am afraid that one needs to be an experienced "engineer" - like the authors - to offer a truly relevant appraisal of the validity of the paper. It looks OK to me, at a superficial resolution. $\endgroup$ – Luboš Motl May 11 '11 at 14:08
  • $\begingroup$ @Luboš: This is exactly the point - if the explanation is indeed correct, it implies that the effect is no hint to exciting new science like a change of "constants" on large scales. Hopefully we have someone between us who can give us some details if the ansatz is applicable (also with respect to the confidence ranges given in the paper). $\endgroup$ – Robert Filter May 11 '11 at 14:21
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    $\begingroup$ Dear @Robert, I am pretty sure that if we have someone like that, she will confirm that the tantalizing evidence of some new holographic or alternative dark matter or MOND physics is just gone by now. If the paper were wrong to the extent that the newly pointed out effects can't be of the same order as the anomaly, we would probably know about it by now. $\endgroup$ – Luboš Motl May 13 '11 at 19:31
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    $\begingroup$ Btw, how did they measure that such a tiny difference in speed? $\endgroup$ – Calmarius Apr 23 '13 at 17:38
  • $\begingroup$ @Calmarius "We also discuss the methods, theoretical models, and experimental techniques used to detect and study small forces acting on interplanetary spacecraft." arxiv.org/abs/gr-qc/0104064 $\endgroup$ – Alessandro Jacopson Apr 5 '15 at 17:05

I'll stick my neck out and say that the answer to your question is simply "yes."

First off, these detailed thermal models are complex and hard to do, so we want confirmation from independent groups. We have that: Rievers and Lämmerzahl, "High precision thermal modeling of complex systems with application to the flyby and Pioneer anomaly," gr-qc/1104.3985

Second, we could ask whether these results contradict previous work. The answer is basically no. Previous work was simply sloppy. There is a nice talk on this topic here by Toth: http://streamer.perimeterinstitute.ca/Flash/a2cc528b-1d36-4a2e-af73-5f81b8b17477/viewer.html There is a long history where people did back-of-the-envelope estimates of the thermal effects and said, "Look, the order of magnitude is too small to matter!" It just turns out that the back-of-the-envelope were wrong.

Finally, all of this stuff is very tough to be sure of, because there are so many uncertainties about things like the degradation of the white paint on the RTGs. Therefore it would be good to have independent ways of testing the hypothesis of a gravitational anomaly, without having to use the Pioneer data at all. We do have these independent tests. If the effect obeyed the equivalence principle, it would have had effects on the outer solar system that are not in fact observed: Iorio, "Does the Neptunian system of satellites challenge a gravitational origin for the Pioneer anomaly?," gr-qc/0912.2947

It's dead, Jim.

  • $\begingroup$ Indeed it seems as if we have enough independent proof by now. Also enough to round up this question. Greets $\endgroup$ – Robert Filter Jul 20 '11 at 10:29

A NASA team recently (july 2011) doubled the amount of available data related to the pioneer anomaly, and they saw an exponentially decreasing acceleration with a 27 year half life, consistent with the thermal scenario. More precisely they say :

The rationale for an exponential model is based on the possibility that the acceleration may be due to thermal recoil forces generated on-board. Due to degradation of the RTG thermocouples and changes in the thermal louver system, the resulting thermal recoil force could have a half-life significantly shorter than the 87.74 year half-life of the ²³⁸Pu fuel, with 27 years being in the acceptable range.

For them "The most likely cause of the Pioneer anomaly is the anisotropic emission of on-board heat" and promise a more detailed analysis.

The paper is arXiv:1107.2886 (now published in PRL) and there is a short discussion on it on The Physics arXiv Blog.

From my (scientist but non-expert) point of view, this temporal decay looks like an independent confirmation of the thermal origin of the Pioneer anomaly and basically settles the question.

Edit (2012-04-20): The same team just published arXiv:1204.2507 another confirmation, based on a detailed modelling of the thermal radiation of the spacecrafts. The last sentences of their abstract says:

As a novel element of our investigation, we develop a parameterized model for the thermal recoil force and estimate the coefficients of this model independently from navigational Doppler data. We find no statistically significant difference between the two estimates and conclude that once the thermal recoil force is properly accounted for, no anomalous acceleration remains.

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    $\begingroup$ I would be cautious about believing specific claims about the time-dependence of the effect. Although Toth is one of the authors of the paper, his talk at the Perimeter Institute, which I linked to above, also has some sobering discussion of how difficult it is to reliably extract this kind of thing from the data. $\endgroup$ – user4552 Jul 20 '11 at 23:03
  • $\begingroup$ @BenCrowell : Their April 2012 paper (see my edit above) seems to be immune from the data extraction problems, because it's an independent modelling. $\endgroup$ – Frédéric Grosshans Apr 20 '12 at 8:31
  • $\begingroup$ Another one for the list. spectrum.ieee.org/aerospace/astrophysics/… $\endgroup$ – PearsonArtPhoto Jan 3 '13 at 19:36

See Braking of the space crafts caused by the vacuum fluctuations.

Quantum fluctuations are able to cause the braking of the spacecrafts (Pioneer spacecrafts anomaly), as these fluctuations cause bodies to approach closer to one another (the Casimir effect). A new quantitative formula of wave energy loss in vacuum is proved and presented in this article. It gives the wave energy loss per one oscillation of a de Broglie wave: $W = Hhc/v$. This formula permits direct calculations of the cosmological redshift and braking of the spacecrafts.

  • $\begingroup$ Interesting theory. Quote from the article: "Anomalous braking was discovered for all space crafts, for which such calculations were technically possible. Moreover, there are facts confirming the braking of the asteroids as well.". Do you have a source for observational data documenting the braking of the asteroids? $\endgroup$ – Peter Mortensen Sep 11 '11 at 13:41
  • $\begingroup$ Another quote: "Quantum vacuum fluctuations reveal themselves in the Casimir effect, the spaceship braking effect, the photon aging effect (red-shifting) and the atom orbital electron shaking effect. Quantum fluctuations affect all elementary particles, and, respectively, travelling through the fluctuating vacuum spaceships "shake" as if traveling on the uneven road, which results in their braking. " $\endgroup$ – Peter Mortensen Sep 11 '11 at 13:41
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    $\begingroup$ Please do not post not-broadly-accepted theories here. $\endgroup$ – user68 Sep 13 '11 at 20:04
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    $\begingroup$ @mbq: This is not a reasonable request--- you can explain that vaccuum fluctuations don't cause braking because they are Lorentz invariant. One can post theories that are fringe, if one is ready to see that they are incorrect. There are quite a few correct fringe theories. $\endgroup$ – Ron Maimon May 20 '12 at 17:20

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