Artist impression of the Pioneer 10 probe (NASA)

The Pioneer Anomaly: A Deviation from Einstein Gravity?

Article Updated: 24 Dec , 2015


Both Pioneer probes are approximately 240,000 miles (386,000 km) closer to the Sun than predicted by calculation. Scientists have been arguing over the cause of this mysterious force for a decade and reasons for the Pioneer anomaly range from the bizarre to the sublime. Is it a simple fuel leak, pushing the probes of course? Is it phantom dark matter dragging them down? Or do the gravity textbooks need to be re-written? Unfortunately there’s still no one answer, but some researchers believe there might be a small deviation in the large-scale space-time Einstein described in his famous theory of general relativity. See, I knew there would be a simple explanation…

The Pioneer 10 and 11 deep space probes were launched in 1972 and 1973, visiting Jupiter and Saturn before pushing on toward interplanetary space, into the unknown. The Pioneer program really lived up to its name, pioneering deep space exploration. But a few years on, as the probes passed the through the 20-70 AU mark, something strange happened… not suddenly, but gradually. Ten years ago Pioneer scientists noticed that something was wrong; the probes were slightly off course. Not by much, but both were experiencing a slight but constant sunward acceleration. The Pioneer probes had been measured some 240,000 miles (386,000 km) closer to the Sun than predicted. This might sound like a long way, but in astronomical terms it’s miniscule. 240,000 miles is a tiny deviation after 6.5 billion miles (10.5 billion km) of travel (it would take light 10 hours to cover this distance), but it’s a deviation all the same and physicists are having a very hard time trying to work out what the problem is.

That is until NASA physicist Slava Turyshev, co-discoverer of the anomaly, rescued a number of Pioneer magnetic data storage disks from being thrown out in 2006. These disks contain telemetric data, temperature and power readings that both Pioneer probes had transmitted back to mission control up to 2003 (when Pioneer 10 lost contact with Earth). From this, Turyshev and his colleagues teamed up with Viktor Toth, a computer programmer in Ottawa, Ontario, to design a new code designed to extract the vast quantity of raw binary code (1s and 0s), revealing the temperature and power readings from the crafts instruments. It sounds as if the search for the culprit of the Pioneer anomaly required a bit of forensic science.

Now the researchers have a valuable tool at their disposal. Turyshev and 50 other scientists are trying to match this raw data with modelled data in an effort to reconstruct the heat and electricity flow around the craft’s instrumentation. Electricity was supplied by the on-board plutonium generator, but this is only a small portion of the energy generated; the rest was converted to heat, lost to space and warmed up the probe’s bodywork. Heat lost to space and warming of the probe’s instruments are both thought to have a part to play in altering spacecraft momentum. So could this be the answer?

Tests are ongoing, and only a select few simulations have been run. However, early results indicate that around 30% of the Pioneer anomaly is down to the on-board heat distribution. The rest, it seems, still cannot be explained by probe dynamics alone. The team are currently processing a total of 50 years of telemetry data (from both Pioneer 10 and 11), so more simulations on the rich supply of transmissions from the probes may still uncover some surprises.

But on the back of everyone’s mind, and it keeps cropping up in every Pioneer anomaly article I find, that the fundamental physics of our universe may need to be brought into question. Sending long-distance deep space probes gives us a huge opportunity to see if what we observe locally is the same for other parts of the Solar System. Could Einstein’s general theory of relativity need to be “tweaked” when considering interplanetary (or interstellar) travel?

The researchers are excited if a mundane solution does not present itself (i.e. probe heat distribution effects), therefore indicating some other cosmic reason is behind this anomaly:

If we actually had a means in the solar system here to measure deviations from Einstein’s gravity, that would be phenomenal.” – Viktor Toth

In the mean time, Pioneer 10 is drifting silently toward the red star of Aldebarran and (barring any more anomalous behaviour) will arrive there in 2 million years time…

Sources: Scientific American, Symmetry Breaking News

42 Responses

  1. Ian O'Neill says:

    To RL:

    I’m thinking that if the craft is losing heat to space, it is emitting electromagnetic (EM) radiation of a certain wavelength (i’m not sure of the wavelength as I don’t know the temperature of the probe). Photons are therefore being emitted into space, carrying momentum (p=h/λ), giving the probe a small, photon-sized kick. Over enough time, there should be a measureable effect from this heat loss.

    The problem with this is there must be the correct distribution of heat around the craft. If the heat is coming from all surfaces, surely there will be no effect on the craft’s motion…

    It’s probably a bit more involved than that, but that’s the gist 🙂

    Hope it helps,


  2. Ian O'Neill says:

    Hehe, no problem Jorge, will convert it now…

    Cheers, Ian

  3. Ian O'Neill says:

    Hi Silver Thread:

    I believe it has been pretty much constant, but it cannot be attributed to the solar wind. It is a subtle sunward drift, which is the opposite direction to the solar wind flow. This is why the heat-loss scenario is quite attractive, but the Pioneer anomaly has been observed on othe probes (inc. Voyager), for them all to have a sunward drift seems like a huge coinsidence… I think there will need to be some lateral thinking to this problem. My brain aches, so I think I’ll leave the lateral thinking to you guys 😉

    Cheers, Ian

  4. Haplo says:

    Deep-space probes… lol.

    A little presumptuous if you ask me.

  5. RL says:

    Could someone explain how heating the spacecraft slows its momentum? Any help understanding this is greatly appreciated.

  6. Jorge says:

    Ian, could you please add to those distances their values in km? Those of us from outside Imperialmeasureland would surely appreciate. Thanks.

  7. Silver Thread says:

    Is the deviation increasing with distance from the sun or has it remained constant? I am sure it’s been considered but I wonder if interstellar wind could cause such an anomaly. It would be akin to shooting a bullet into the wind, the effects are minimal, but present.

  8. Stephen Farrugia says:

    I wonder if the connecting signal itself [Earth to Pioneer] is the unknown culprit to this anomaly. Just like gravity, that pulls masses together, there may be a miniscule miniscule pull effect between a sender-receiver electromagnetic system, as yet never detected on Earth, that is causing this slow down. In the distances on EARTH the effect is negligible but over the long distances and the time span we are talking about the affect becomes noticeable.

    Transmission is a recent phenomenon in Science and I’m sure there’s a lot more we need to learn about it.

    Stephen, your man from Kibish

  9. Fabio Knopf says:


    # Haplo Says:
    April 16th, 2008 at 7:43 pm

    Deep-space probes… lol.

    A little presumptuous if you ask me.

  10. Brad says:

    So objects in the rear view mirror may actually be FURTHER than they appear. Great.

  11. GBendt says:

    I think, the Pioneer anomaly isn´t an anomaly at all: It is caused by the drag of the mass in the outer solar system. The space outside the Pluto orbit is not empty. Since Pioneer has been crossing that space during the lst decades, the mass of the dust and the comets and the kuyper belt objects conzained within that space adds to the mass of the solar system, increasing it by one millionth and creating a slight additional gravity pull.



  12. >So objects in the rear view mirror may actually be FURTHER than they appear.

    AND, they happened a long time ago. 😉

    @ Günther, I was thinking the same thing..

  13. Wolfgang Keil says:

    Certainly all internal and external energetic effects, which may lead to an imbalance of perturbation have to be considered. The probe is not undisturbed by particle radiation (GCR, solar particles) and the according absorption/transmission/reflection factors are not homogeneous for the probe. Sun depending parameters change, Sun has lost mass. etc. etc. There are a lot of fine studies triggered to get the problem solved…

  14. john says:

    A nice little Mystery. Enjoy!

  15. joe says:

    Does this influence occur on asteroids and comets? That would determine whether it is a spacecraft issue or external influence.

  16. dilip says:

    To really test any gravity theory applicable, a ballistic or free fall object must be tracked, not a complex spacecraft like the Pioneers, which is affected by its internal, and initially Earth-controlled, dynamical forces. Of course, even such a ballistic object is subject to nongravitational effects, which include photon drag near Sun (Poynting-Robertson drag), solar wind, cosmic rays, possible effects of an asymmetry in Kuiper belt, Oort Cloud (which is of course rather far away at 10**5 astronomical units or so, but an asymmetry in it can have an effect, if the cloud is massive enough). I’m sure the experts (are expected to) take account of all possible effects of sufficient magnitude. However, one can sometimes lose sight of an effect thought to be minor in another context, but which could be cumulative in this one.

  17. Max says:

    Higher powered photons have more of an acceleration than lowered powered (i.e. heat) so that the net effect of any radiation should be an acceleration away from the sun (seeing as that is where the principle external warming force will be coming from) whereas the observed acceleration is towards the sun. My personal pet theory is all about observation points, in other words it looks to us as though it is being slowed down from our observation point, however if you were aboard this probe I reckon you would observe an acceleration away from the sun, the clocks run at different speeds. Does anyone know if there is an internal clock on this probe and if it can be measured?
    (That would put paid to my theory!)

  18. PHWilson says:

    Why not astral weather? There’s a solar wind -where there is wind, there are patterns formed by forces larger than the wind itself. Jet Streams? Turbulence? Cyclones? All totally imperceptible except by the results of a long probe.

  19. alphonso richardson says:

    That should keep the guys busy for a LOONG time…………………..

  20. Have a look at this paper from 7 years ago that came to the conclusion that non-isotropic heat radiation is the answer and that “by accounting for the bulk of the acceleration, the proposed mechanism makes it much more likely that the entire effect can be explained without the need for new physics.”

    Could it be that the “anomaly” has been kept on life-support by eager physicists when it was dead in the water already in 2001, thanks to a sound engineering study …? Anyone remember the “N-rays” of 100 years earlier? Go google for them and be astounded …

  21. Ed2 says:

    Great news. My conjecture on the alpha constant requires the speed of light to be 3.00000 and not 2.99792458. if you use that correction factor in the distance calculation, the probes could easily be 240,000 km closer to the sun. The correction factor is simply 1.000692286. Ed2…4/17/08

  22. Gavin says:

    “In the mean time, Pioneer 10 is drifting silently toward the red star of Aldebaran and will arrive there in 2 million years time…”

    I wonder how much of the spacecraft will have sublimed by then?

  23. Uclock says:

    The distance of the probe and the rate of accelleration from the gravity of the Sun will cause a real time dilation between us here on Earth and the intruments on the probe. It is the understanding of time that is wrong and that is where the scientists should be looking. The further the probe travels the more its time will dilate compared to us.

  24. Vanamonde says:

    We where discussing this in the forum. Was not the Voyager spacecraft and even maybe Mercury MESSENGER also experiencing this analomy?I don’t think it was just a Pioneer thang.

  25. Fermin says:

    I guess we’ll find out the real answer in about 10 years. Isnt the New Horizon probe supposed to pass Pluto and onward to outerspace?

  26. John says:

    Dust. There is a lot of it out there and Pioneer is going fast, so it will encounter more of it in a short time. Space objects clear a path through it. Pioneer is making its own path. I don’t think NASA has any way of measuring this. The effects have been observed with slingshots past Earth, where there is a speedup as it transits earth’s path. I don’t see a mystery here.

  27. Dave says:

    This is just for Pioneer 10 spacecraft, but are we seeing the same thing happening to other spacecrafts as well, such as Voyager 1 and 2…because if those 2 spacecraft have nothing going on like that, then this might be something else

    Most of all, this is interstellar space travel, so there are many things to be considered which can take lot of time and research.

  28. Silver Thread says:

    Ian O’Neill Said:

    “Hi Silver Thread:

    I believe it has been pretty much constant, but it cannot be attributed to the solar wind. It is a subtle sunward drift, which is the opposite direction to the solar wind flow…”

    I was referring to Wind Effects produced by other stars. I called it “Intersteller Wind” in an effort to make that distinction. Presumably in the areas closer to the Sun where the Solar wind can effectively negate Flow from other stars this anomaly would not be noticed, but beyond the outer orbits of Planet within our Solar System.

    Is it not possible that the Solar Wind’s Effect is decreased and as a result spacecraft traveling into the headwind of insterstellar space caused by the cumulative effects of the billions of other stars? Think of the Sun as a big fan, blowing into the face of many much bigger fans.

    In very close proximity to the Sun, it’s Solar Wind might effectively counter Interstellar Wind, but as you get further from it, the effect decreases and the overwhelming force of Wind from Other Stars would begin to slow you gradually, perhaps even eventually brining you to a halt entirely?

  29. Jason Leary says:


    Lateral thinking —is goofy, equivocal , thinking.. qute wrong thinking, and , hence , is quite contrary to systematic science . Any paydirt afforded by lateral thinking is due merely to epistemic luck .

  30. Ian O'Neill says:

    Actually Jason, lateral thinking isn’t goofy – it is a different way of looking at things, departing from traditional logic. I think a heavy dose of lateral thinking was required by Einstein when breaking traditional physics rules to envision special relativity. Or by Pauli when understanding how subatomic particles interact in small volumes. Or quantum entanglement – I have no idea how physicists came to work out that theory! Mathematics and physics can only predict so much, “thinking out of the box” often leads to a new direction of thinking and philosophy. From there new science evolves.

    I for one wish I could think more laterally 🙂

    Cheers, Ian

  31. zylstra says:

    Wouldn’t the pros have taken into consideration everything you guys have mentioned thus far?

    Also, what about the flyby anomaly?

  32. Aztral says:

    Something I’m trying to spend some time looking into is the correlation between these anomolies, orbital velocities of stars at galaxy (and globular cluster) egdes and Neptune’s/Uranus’ formation (apparently they’re too large according to current solar system formation theory).

    To me it is becoming apparent that G is too low, and that something about the space-time in areas of relatively high density is slowing down orbital velocities near solar system/galaxy/clusters centers.

  33. Wally says:

    It’s none of the above! It’s that darn Planet X. The fact the Pioneer is slowing down (so to speak) is proof the world is going to end in 2012 because Nibiru and Annunkais are returning to earth.

    Why can’t scientists connect the dots like the rest of us? Kidding, kidding….

  34. Michael says:

    I have several ideas about what might cause such an anomaly that I have not seen articulated, but before I put them out there I need to confirm some assumptions. My understanding is that the location of the spacecraft is determined using Doppler. Where might I find detailed information on how the measurement is being made (radio telescopes, arrays, times of year, triangulation, etc.)? Also, would it be possible to get a detailed schematic of the spacecraft?



  35. C. Y. Lo says:

    The cause of the pioneer anomaly has been identified as the due to the charge-mass interaction interaction, a recently discovered neutral force that makes the unification of gravity and electromagnetism necessary.

  36. C. Y. Lo says:

    The cause of the pioneer anomaly has been identified as the due to the charge-mass interaction, a recently discovered neutral force that makes the unification of gravity and electromagnetism necessary.

  37. Jamahl Peavey says:

    There too many variables internal and external to the probe to say what the source of the error in position actually is.

  38. NASA CHRIS says:

    One word, well.. three.

    MASS. Dark Matter.

    Or if you prefer, DUST.

  39. mt0ne says:

    C. Y. Lo
    Site your sources please.

  40. pioneerf90bt says:

    I really enjoyed reading this page…Thanks for this 🙂

  41. timstro59 says:

    it could be a combination of space dust, unknown keiper belt objects, the gravitational pull of the sun and jupiter combined, the unseen dark matter halo surronding the sun and just plain old bad luck

  42. JK says:

    C Y Lo is a good physicist. I’ve read a paper of his on vertical arm interferometers. What is he doing here with these people? We’ve got to get you out of here doc, don’t worry we’ll get you out…

Comments are closed.