Flyby Anomalies Explained?

Article written: 18 Sep , 2008
Updated: 24 Dec , 2015

Several different spacecraft have exhibited unexplained changes in speed during gravity assists when flying by Earth. First there was Galileo in 1990 and 1992, NEAR, which flew by Earth in January 1998, and then Cassini in August of 1999. Rosetta — the ESA spacecraft that recently flew by an asteroid – swung by the home planet in March 2005, followed by MESSENGER in August of the same year. All these probes showed an expected change in speed during the flyby. The largest anomaly was recorded for NEAR, whose velocity changed 13 millimeters per second more than it should have. Earlier this year, a group of JPL researchers that had been working on the problem for years basically threw up their hands, saying they hoped other physicists could come up with a solution. They had concluded the anomaly was too large to be explained by known effects related to Einstein’s general theory of relativity. But a new paper proposes that Special Relativity may explain everything.

The speed of the spacecraft is measured by the Doppler shift in radio signals from the spacecraft to the antennas of the Deep Space Network. In a very short and concise paper, (reading it is like watching Will Hunting solve the MIT professor’s equation), Jean Paul Mbelek from CEA-Saclay in France says that the relative motion of the spacecraft and the spinning Earth have not been properly accounted for. When a well known but overlooked effect of Special Relativity is taken into account, where the transverse Doppler effect of the Earth’s spin and the velocity of the craft are factored in, there is no flyby anomaly. “Thus, GR (General Relativity) does not need to be questioned and the flyby anomaly is merely due to an incomplete analysis using conventional physics,” says Mbelek.

flyby-anomaly.  credit:  arXiv blog

flyby-anomaly. credit: arXiv blog

Other explanations had proposed dark matter or “Unruh radiation” could be the answer. But Mbelek says we just haven’t been doing the physics right. He concludes that spacecraft “flybys of heavenly bodies may be viewed as a new test of Special Relativity which has proven to be successful near the Earth.” He proposes a follow-up of tracking the spacecraft trajectories beyond just the probes’ closest approach to Earth to test this hypothesis further.

Sources: arXiv, arXiv blog

18 Responses

  1. Astrofiend says

    Beautiful. Although it was always likely to be a result of something like this, it’s fantastically ironic to see that an effect that some thought may occur as a result of incomplete or new physics is quite likely in fact to actually be a highly exacting confirmation of established theories of physics. These sort of complexities can be easily overlooked, even by top brains.

    The commenter ‘w. scott’ almost hit it on the head in his comment on the ‘Unruh effect’ link posted in this article. Among other things, he said “…we could be missing a scalar component of acceleration which affects the flow of time at far lower speeds. After all, we are talking about passing by a large, spinning gravity well.” and that “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.”

    Not quite on the money, but insightful and prescient none-the-less for a throwaway comment.

  2. Graham says

    Does this still leave us with the Pioneer anomaly?

  3. Nigel says

    On the transverse Doppler effect, I have never been able to settle for a purely radial motion for the large scale, cosmological, space-induced motion of galaxies. How big a transverse velocity is required (e.g. for quasars) before this starts to measurably affect our interpretations of (space-induced) red-shift?

  4. Mike McCulloch says

    I do not think the flyby anomalies can be any sort of Doppler effect, because, although they are seen in the Doppler data as mentioned, they are also present in the ranging data. This implies it is not just a frequency shift, but a real velocity and position anomaly.

  5. RL says

    But, of course! (said with my bad fake French accent). It would be interesting to hear JPLs reaction. I hope someone puts this to a test.

  6. BadGuy1081 says

    gravity field drag? if you orbit with the spin you pick up more velocity?

  7. jerry says

    It is silly to suggest that there is a special relativity explanation when general relativity fails. Special relativity is applicable in linear systems only. Even if there is a term missing from the GR equations; such as a failure to properly account for the angular momentum of the earth, the effect would be small relative to the measured errors. If there is an ‘unaccounted for’ property of GR; that would be new physics, not SR.

  8. WooferHound says

    These spacecraft are not just passing the Earth on their flybys, they are flying by the Earth/Moon system. so there is combined mass of these two put together. Also the Moon will have different effects on the spacecraft depending on the location of the Moon in it’s orbit around the Earth.

  9. troy says

    “13 millimeters per second more than it should have” That calculates to .03 mph. The precission in measurements and calculations are impressive.

  10. watchful stone guardian says

    or 46.8 metres per hour (0.047 km/h)

  11. Steven says

    For Graham…
    “Does this still leave us with the Pioneer anomaly?”

    Yes it does, but I think that has been expalined by leaking heat form the RTG’s.

    Still all in all I was hoping for mirco wormholes or something crazy.

  12. Carl says

    What I love most about this story (actually, this whole site) is the comments by people who think they have a better handle on this than actual physicists, especially the ones who throw out some plausible sounding jargon without any actual calculations to back up their guesses. Hilarious.

  13. JamesB says


    Why would anyone who COULD understand the math come to this kind of website for this kind of news? They dumb it down for us so we don’t need to understand the math.

    And knowing that something is wrong doesn’t imply that you know the right answer yourself. Somethings are just counterintuitive or outright ridiculous at the most cursory glance (“dark energy” for one)!!

    I personally think that it’ll be something small like this which will eventually undermine our understanding of cosmology. And while I also think it’ll be a new branch of physics you could describe as “dimensional physics” that’ll be needed, it’s up to the guys who DO understand the math to work out the details!!

  14. JamesB says

    PS- it’s like a guy going to a biker bar and complaining about all the leather…

  15. quantum_flux says

    Well, at least that’s 1 mystery solved.

  16. RetardedFishFrog says

    Is the transverse Doppler effect related to frame dragging?

  17. HolyAvengerOne says

    “Is the transverse Doppler effect related to frame dragging?”

    Nope. Not the same branch of physics there.

  18. General Zod says

    All branches of physics, I think, apply to all things all the time. It’s just a matter of how much you want to nit pick.

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