A Possible Answer to Flyby Anomalies

Article written: 20 Jan , 2008
Updated: 26 Dec , 2015

Strange things are happening to our robotic space explorers. Also known as the “Pioneer effect“ (the unexpected and sudden alterations to Pioneer 10 and Pioneer 11 trajectories measured as they continue their journey into the outer solar system), similar anomalies are being seen in flybys by modern space probes. Earth flybys by Galileo, Rosetta, NEAR and Cassini have all experienced a sudden boost in speed. After cancelling out all possible explanations, including leakage of fuel and velocity measurement error, a new study suggests the answer may lie in a bizarre characteristic of universal physics…

Planetary flybys are an essential aid to interplanetary missions to gain energy as they accelerate on their merry way to their destination. Gravity assists are accurately calculated by mission scientists so the time of arrival can be calculated down to the minute. Considering most missions take years to complete, this degree of accuracy is amazing, but essential.

So, when Galileo completed gravity assist past Earth on December 8, 1990, to speed it toward Jupiter, you can imagine NASA’s surprise to find that Galileo had accelerated suddenly, and for no apparent reason. This small boost was tiny, but through the use of the Deep Space Network, extremely accurate measurements of the speeding craft could be made. Galileo had accelerated 3.9 mm/s.

This isn’t an isolated case. During Earth flybys by the space probes NEAR, Cassini-Huygens and Rosetta, all experienced a unexplained boosts of 13 mm/s, 0.11 mm/s and 2 mm/s respectively. Once technical faults, observational errors, radiation pressure, magnetic instabilities and electrical charge build-up could be ruled out, focus is beginning to turn to more exotic explanations.

A recent study by Magic McCulloch suggests that “Unruh radiation” may be the culprit. The Unruh effect, put simply, suggests that accelerating bodies experience a type of electromagnetic radiation. At very low acceleration, the wavelength emitted will be so large that a whole wavelength will be longer than the dimensions of the Universe (otherwise known as the Hubble Distance). Low acceleration would therefore generate waves that have no effect on the body. However, should the accelerating body (i.e. Galileo getting accelerated by Earth’s gravity during the 1990 flyby) slowly exceed an acceleration threshold, the Unruh radiation will decrease in wavelength (smaller than the Hubble Distance), causing a tiny, but measurable “boost” to its increasing velocity.

Although complex, this theory is very interesting and proves that although we can calculate the arrival time of space probes down to the nearest minute, the Universe will continue to throw up some perplexing issues for a long time yet.

Sources: arXiv Blog, arXiv abstract and paper download

61 Responses

  1. Brian says

    Hmm…I was thinking it could have been some sort of stellar wind or current.

    Did all of these happen near Earth? Maybe it could have been “pushed” by particles (dust, gas, or whatever) orbiting the planet? Might account for the small boost in acceleration.

  2. Clint says

    Wow! Do we have any theory about how this effect behaves with increasing speed? Does it grow exponentially (that would be great for space flight, wouldn’t?) or just linear?

  3. Sam Wise says

    Just to pick a nit — the “Pioneer effect” doesn’t show up “suddenly.” It shows up as a REALLY subtle bias to the otherwise expected acceleration of the spacecraft during a flyby. I’m pretty sure it shows up in all flybys — but the ones at Earth can be more accurately measured. Build some DSN stations at Jupiter, and it could be more accurately measured there, too 😉

  4. Janus says

    Maybe it’s dark energy. 😉

  5. bryant says

    it must be cold fusion

  6. Johnny Blues says

    Asa power boat on a breezy lake, the faster you go, the more skimming effect, further increasing speed capacity. In this case, would gravity waves work as the lake?

  7. Tony says

    This seemingly very subtle observation and corresponding theory could have dramatic implications for the future of space travel…

  8. This is the kind of thing that NASA could figure out if they had more money. They could send probes specifically to measure this effect with sensors to corroborate the theory. is where you can tell the candidates to give NASA more money to study things like this.

    go to to send a fax to Romney, Clinton, and Obama all at the same time!

  9. Chuck Lam says

    Perhaps the speed-up discussed here is related to a passing gravity wave. Compression then exxpansion might appear as a sudden increase in speed. As Eivind Lillevik suggests, “why not?”

  10. Rod says

    Perhaps we only think we’re measuring that accurately . . .

  11. TerryTrudgill says

    The first indication of the effects of MOND pehaps?

  12. ed hammer says

    The MOND special acceleration effect minimum value is related to the fundamental resonance of the universe. It is related to c*2/R. The precise value for the minimum acceleration value is 1.2345679 x 10*6 meters per second per second. It confirms the universe only has 3 spacial dimensions and one time dimension..

  13. Member
    Eivind Lillevik says

    Could gravitational waves influence the speed of objects in space?
    I know it should’t but then why not 😉

  14. ed hammer says

    My value for the special acceleration is based on the alpha constant being precisely equal to 0.00729 and not the NIST accepted value of 0.007297352568

  15. ed hammer says

    My corrected value for the MOND special acceleration is 1.2345679 x 10*-10 meters per second per second.

  16. Rmin says

    We have calculated the expected acceleration using gravity law, we have calculated the mass of planets using gravity law too, but we have measured the space craft mass by accurate equipments in the earth so may be the error is in gravity law, when you are calculating acceleration of planets and stars you will not see the error because you have calculated the mass by the same law but in the case of space crafts you could see the error because we have measured the mass indipendently.

  17. RoyWebb says

    Could this be an effect of time dilation? The faster an object goes the slower time is for that object. Could we be seeing an example of this in action?

  18. W. Scott says

    The Unru radiation hypothesis sounds flimsy.

    Remember, at the speeds we’re talking about, a couple of millimeters per second is NOT a lot of discrepency.

    It’s just as likely that we are failing to account for accelleration-affected time distortion. Einsteins equations describe universe-sized displacement effects at relativistic speeds, but we could be missing a scalar component of accelleration which affects the flow of time at far lower speeds. After all, we are talking about passing by a large, spinning gravity well.

    Consider that if the navigational thrusters were supposed to fire for a very small amount of time, but time is moving slightly more slowly for ground control than it is for the spacecraft (we have measured this effect!). Then the duration of the thrust would be WRONG from ground control’s perspective in time. Not enough thrust for a corridor-control approach burn could easily result in more accelleration than expected after the slingshot around the planet.

    Oh, and for those who are curious… yes, we see this effect when probes pass other planets too. No, it doesn’t look like solar wind or dark matter are playing a role.

    Good old fashioned Relativity might just be all we need to explain this. It only means that there’s a factor that science doesn’t account for which SHOULD be accounted for.

    Considering the numer of X-factors in the universe, this is not so big a mystery.

  19. Essel says

    Perhaps transfer of angular momentum is only calculated w.r.t. earth and actually the whole of solar system contributes at ecliptic.

  20. JamesB says

    This is worse than that magical “dark energy” they’ve conjured up (like “ether” was 150 years go). We simply do not understand gravity well enough. Newtonian gravity was a good approximation and General Relativity even better. The “ether” was a reasonable explanation back when physics was mechanical, but things like the Pioneer Anomaly tell us that we’re still lacking the understanding needed to “grok” gravity. I think Einstein was on the right track, that gravity a consequence of space, but that it and time are two ways of looking at the same thing (much as we now know there is 1 electro-weak force in quantum theory, rather than 2 forces at work). With one dimension you have neither space nor time, but as soon as you have two or more dimensions you have space with time being the motion thru that space (hence the limit on the speed of light and why that limit exists, it’s govern by dimension). Gravity is a part of this dimensional fabric, not merely the result of it being warped by mass (mass is space too, energy is merely space in motion, etc.). If you think of space as being gravity a lot of things, like the issues in this article make sense.

  21. Ken Lozier says

    As a lay person my interpretation of these minute accelerations could be caused by the magnetic field of the spacecraft influenced by very small opposite polarity pushes from the planets magnetic field that cause a net effect when added in total – dependent on when the spacecraft intersects the magnetic field of the planet it is flying by. Even very, very small magnetic fields could have some effects on the spacecraft, but planets like the Earth and Jupiter would have much bigger sum total effects – again depending on when and at what angles the spacecraft intersects the magnetic fields. Has anyone looked into this as a possible cause of this ‘Unruh Radiation’ effect?

  22. Mickey Schmidt says

    It wasn’t clear if this accelleration occured only on near earth passages or whether any near planet encounted might contribute to the small but measurable boost. It has been proposed the massive stars can drag space-time around as the star rotates. It has been suggested that the sun and even the earth might do this to a small extent. If sowould a space craft be accellerated as it enters or leaves the region of space where the space time continuum is being modified by a rotating body? A small floating object just skimming a whirlpool could be accelerated.

  23. ed hammer says

    My prediction is this special acceleration will continue to increase if these space probes are approaching the horizon of another blackhole. if NASA is monitoring these replys, I would appreciate their comments.

  24. Kevin M. says

    My understanding of the pioneer effect was that it was a “negative” acceleration, or slowdown. On the wikipedia page for pioneer anomaly, there is a theory that this force would exactly negate the presumed accelerative effect being ascribed to “dark energy”.

  25. Lynd says

    That 13 mm/s doesn’t sound like much, but it equates to a difference of almost 255 miles over the course of a year. With travel to the outer planets taking years, that could be enough of an error to be catastrophic.

  26. Elijah says

    It’s called Mass Effect, my friends.

  27. bobi says

    My goodness !! With all these experts above, its a wonder that we are still earth bound. With the ‘expert opinions’ expressed above, we should be travelling to the stars on our weekends.

  28. Dewd says

    Maybe God Farted !

  29. Nasa Rep says

    ed hammer: Nasa here. Civilians are not allowed to know these things. Don’t say another word or we will shut you up! You’ve been warned.

  30. Nick says

    Aw, c’mon. The answer is as simple as an Einstein’s theories. The Universe is expanding while the robotic space probes are skidding its surface—-Just as a balloon’s surface would stretch the distance and time it would take an ant to reach point B on its surface from point A, the space probes are skidding along. You might suppose the skidding would appear to slow down the probes; Alas, you’d be mistaken if you did not take into account the time involved. On an earthbound balloon, using earhtbound measurements, the ant WOULD take longer. But in space travel, time shrinks some. So there. Furthermore, according to Nick’s Law, time will disappear the moment the Universe stops expanding., and thereafter will begin moving backwards as the Universe contracts back to where it came from. If you believe I’m kidding, just wait and see.

  31. jerry says

    You can count out relativity, and any of the usual suspects, such as solar wind pressure. We need a family of probes specifically designed to provide us with baseline gravity and acceleration measurements.

    Too many assumptions have been made in the past, and nothing will be resolved until it is fully understood what the molecular composition of the outer solar system is.

  32. Nat says

    Could it be something to do with the space warp as per Einstein’s theory.?. Also can’t comprehend the accuracy of measurements as claimed..

  33. Chad says

    Well if acceleration can occur in a manner that we don’t understand there are a couple potential consequences that pop into my mind (albeit my layman’s mind).

    1. Could this same effect also explain the rotation of galaxies without the need for dark matter?

    2. If we don’t understand acceleration completely then could it be that we are misinterpreting the red shifts we observe from far away galaxy clusters, and thus this could explain at least a portion of dark energy?

  34. N Stone says

    how do you measure the speed of something in space if there is no other matter to compare to?

  35. Matt D. says

    Perhaps it’s just God f’n with scientists.

  36. Seabear70 says

    It must be global warming 😛

  37. Seabear70 says

    “how do you measure the speed of something in space if there is no other matter to compare to?”

    It occurs to me that there would be three ways.
    1. compared to point of origin.
    2. compared to point of destination.
    3. measured through acceleration.

    Of the three, aceleration would likely be the most accurate though the least useful.

  38. Alex says

    Cosmology seems to think that you should invent something when you dont understand your data. And if you cant see that thing, you call it dark so that it both sound cooler, and also explains in a word why you cant find it and never will… : )

    I would really find it funny if half the probes lose speed and half gain it by a few mm-sec. Call it “dark measurement errors”.

  39. Malcolm says

    This May be going out on a limb a bit, but….
    It’s my understanding that we can not grasp the concept of where things go when they enter a black hole, so let’s say a few things I think I know and then get to the point:
    1. Things going in a black hole retain their same mass but are compressed greatly if not infinitely.
    2. According to human understanding what goes in must come out.
    3. There is a white hole theory explaining where the stuff goes.

    So here’s the point (and yet another theory)
    Things do escape a black hole, but only after it is compressed to a size that is no longer effected by gravity in the normal sense and simply floats away. Now this matter is smaller than any human instruments can measure. Most things will have some sort of half life, so what if after time one or more of these particles begin to de-stabalize generating a small gravitational field triggered by the acceleration of the craft, and after the craft passes because of the extreemly dense nature of the particle it restabilizes for now.

    Note: This theory has no support other than space is really big. And is based on the idea that large fields like earth and other planets cause further compression, where as the small fleeting field of the craft has a minor opposite effect.

  40. Daren says

    Would the Unruh radiation effect show up in orbits of comets? They accelerate nicely when doing a solar “flyby”.

    BTW, a good description of the Unruh effect can be found in Lee Smolin’s book Three Roads to Quantum Gravity.

  41. Required Reading says

    It’s an artifact of transmitted information over distance.

  42. N Stone says

    If mattter slows objects down with friction than in space, there could be varying amounts of antimatter that aren’t affecting the time it takes to get to one place but still affecting speed. like, if space was folded in, (kind of like a worm hole) (expressed as accelaration) but when it got unfolded then the space/time that it missed while it was accelarating would reappear around it increacing the distance.
    if it was deccelarating then the empty space (antimatter) would dissapear. (picture it in your head)

  43. Larry Webb says

    Well if this is true then could it account for the acceleration seen in the expansion of the universe instead of or perhaps in addition to “dark matter”?

  44. N Stone says

    I’ll have to read up on that

  45. N Stone says

    “Well if this is true then could it account for the acceleration seen in the expansion of the universe instead of or perhaps in addition to “dark matter”?” -Larry Webb

    the expansion of space might affect acceleration/decceleration but since the whole universe is being affected then universal time would slow down/speed up
    everything would slow down
    same if the universe was collapsing.
    I think dark matter is more of a type of gas or energy that is pushing on the edges of the universe; like helium filling up a balloon

  46. Maytag says

    Man, you people tossing out theories, like you know what you’re talking about, are ridiculous. There are some pretty dumb one- sentence ideas, but you people writing paragraphs are even more off base. Stop spouting like you have a clue.

  47. Jodeo says

    Look, space… it’s a vacuum, so I’m sticking with the Oreck derivative of the Hoover principal.


  48. Radtech says

    Perhaps the acceleration is due to gravitational attraction from otherwise undetectable massive objects__otherwise undetectable because the objects reside in a “neighboring” universe. Being as gravity is the only force that can pass between universes, such an explanation seems more reasonable than invoking less likely causes.

  49. Dregnaw says

    As the crafts flyby, are there frictional forces that could act like or produce a form of static electricity causing a minor form of repulsion? If so, then what would the friction be caused from? Atmospheric, probably not. Grafivity, or the electromagnetic field?

  50. Luke says

    Nobody like a smart arse, so I get along with most people but here goes.

    Galileo had accelerated 3.9 mm/s.

    Ten year mission error 1213.056 Km

    Cassini 13 mm/s Ten year mission error 4043.52 Km

    Huygens 0.11 mm/s Ten year mission error 34.2144 Km

    Rosetta 2 mm/s Ten year mission error 622.08

    I’m not keen on numbers.

    Don’t you just love me?


  51. Paul says

    Frame-dragging might explain this anomaly – look up T-machines. T for Tipler

  52. BillG says

    I have a gut feeling that the “Pioneer anomaly” is a relativistic/time-dilation effect.

    I have the same gut feeling with regard to galaxy movement and rotation. Scientists have invented mysterious”dark matter” and “dark energy” because they’ve had it beaten into their heads that relativistic effects are not important or detectable until an object reaches a speed approaching the speed of light. Not true.

    Case in point: the atomic clocks aboard GPS satellites will run 38 microseconds faster in earth orbit than if they were sitting on earth, which is entirely measurable. In fact, your GPS unit in your car wouldn’t work if relativity were not taken into account!! This relativistic effect must be taken into account for satellites moving at 17,000 MPH relative to us, so why would it NOT be important for space probes moving at 80,000 MPH (Pioneer 10) and 110,000 MPH (Pioneer 11) relative to us?!?!

    Forget silly theories about dark matter, dark energy, and radiation that has a wavelength greater than the size of the universe, all of which have never been quantified and there’s not a shred of evidence that they even exist. Albert Einstein had the answer 100 years ago, the scientists looking at the “Pioneer anomaly” just won’t look at what’s right in front of their face.

  53. Ingrid Eckhoff says

    Logically, if you remove every possibility that isn’t causing the effect, the remaining possibility(ies) however unlikely must be the answer. Is there something else orbiting around Earth that alters the gravitation just that much, perhaps operating at some unfamiliar wavelength, that we can’t see and don’t know is there?

    Just a possibility.

  54. Mike Millington says

    I think it’s a ripple in space/time where God’s finger sliced through our dimension on an earlier date.

  55. Janos Berta says

    Do you know any details when the satellites gain this extra speed? During the approach to the planet or at the phase when the satellite go away from the planet?

  56. WeeWeeJumbo says

    “January 21st, 2008 at 12:47 pm

    bobi Says:
    My goodness !! With all these experts above, its a wonder that we are still earth bound. With the ‘expert opinions’ expressed above, we should be travelling to the stars on our weekends.”

    I couldn’t have said it better myself. I’m actually embarrassed reading these comments. “It’s all so simple, why can’t these egghead researchers, with their advanced degrees and long lists of accomplishments and discoveries, figure it out? I’ve got the answer pegged.” Unbelievable!

  57. Oyéah says

    Maybe the melting poles and glaciers, have succeeded in making the Earth incrementally more massive over the last 30 years, and this increase in mass, influences the energy that satellites can amass.

  58. bdouds says

    Any chance that the fact that the earth is not a perfect sphere and that its gravitational field isn’t perfectly uniform would have enough of an effect?

  59. Let’s first agree on my solution of the Pioneer
    anomaly at

  60. robbb says

    jesus likes to ramp up the speed when he rides our hardware.

  61. Jim says

    Anyone know where I can find spacecraft info including flyby data, altitude, apogee, speed, lattitude entrance, exit info on all suspect craft ?

Comments are closed.