Gravity, Working As Usual

By Julianne Dalcanton | March 31, 2011 10:43 pm

I am in absolutely no position to judge the technical execution of this work, but a group has posted a possible solution to the “Pioneer Anomaly” on the Physics/Astronomy ArXiV server (

For those who haven’t been following along at home, there appear to be subtle unexpected Sun-ward accelerations (i.e. higher than expected decelerations) seen in the Pioneer 10 and 11 spacecraft while leaving the Solar System. Sean posted an earlier discussion of the anomaly and a possible reported solution that does not involve modifying gravity. However, this latest paper is by a different group, and posits that reflections within the spacecraft are enough to explain the discrepancies. There’s a nicer write-up than this one at the Technology Review Physics ArXiV blog.

FYI, it was posted a few days ago, and is not an April Fool’s posting.

Also FYI, my kids just got passports, and did you know that there’s an image of Pioneer 10 on the inside back cover? Beamjockey did some nice sleuthing a few years back and dug up the source image!

CATEGORIZED UNDER: arxiv, Science, Space
  • Jonathan Burns

    If this is the explanation, it ought to be marked in history as the first photon drive.

  • AnotherSean

    I can’t imagine what the crack-pots will do without the Pioneer Anomaly. Maybe its time to revisit the Lochness monster, I heard they found its relative in a lake in New Mexico.

  • Mandeep

    Julianne- yes, i was psyched about that image when i first saw it (when i got a new passport last year). I also had time (once when in waiting room for the Federal Police office in Rio — getting a Brazilian work visa, thankfully) to go through and read all the quotes. And i must say, i liked some of them especially — they really moved me. It’s nice to have a passport that one likes like this, to carry around all the time while travelling. :->

  • Ron


    When Pioneer behaves in a manner that is not predicted, it is either due to an outside force or a basic misunderstanding of gravity. I’d say it’s rather important to know which. (Is Lochness an adjective?)

  • Bill Higgins– Beam Jockey

    Thanks for the kind words about my sleuthing. I still wish I knew who created the Pioneer artwork on the U.S. passport. I never did track down the answer, although someone at the State Department must know.

    I did ask NASA’s history and graphics people, both at Johnson Space Center and at Ames Research Center (where the book containing the original, and quite good, Pioneer painting was published). Nobody at NASA remembers who painted the original. I also had no luck with e-mail to a number of prominent astronomical artists; they were sympathetic but they didn’t know either.

    One thing is clear: The NASA artist demonstrates a much better understanding of astronomy than the passport artist. (Could the latter have been ordered to portray the Earth lit from an impossible angle? )

    Bill Higgins

  • AnotherSean

    Ron: Outside force or incorrect understanding of gravity? Why not just the disippation of heat?

  • Torbj√∂rn Larsson, OM

    RTG rocketry.

  • Thomas Smid

    I have suggested already a few of years ago that the anomaly is simply explained by the inaccuracy with which the earth’s rotation rate is known. This produces an apparent acceleration of the spacecraft. See for more.

  • Ron

    @AnotherSean (#6),

    Yes, a non-gravitational force/acceleration due to the directional radiation of heat. So, if that explains the anomaly, there is no indication of a problem with the current understanding of gravity.

    I don’t mean to imply that we truly understand gravity. I’m just saying that it’s not a “crackpot” endeavor to try to explain what caused the anomaly. By “outside force” I meant other than gravity, such as collisions with particles, radiative pressure, magnetic fields, etc.

  • Eric Habegger

    A lot of people prematurely linked the Pioneer anomaly with what is happening at the galactic level with dark matter. Things are never that simple, especially if all other objects in orbit around the sun at that distance don’t act like Pioneer. Now, what WOULD be interesting is to see of there is some correlation between the temperature of space at the galactic periphery and what is observed as dark matter. It just may not be cold enough within our solar system for those effects to take hold.

  • AnotherSean

    Ron: I agree with you that it was not a crackpot endeavor to understand the anomaly either. I was just amazed that many people took very small deviations from the pattern of a complicated object that isn’t in thermal equilibrium as evidence in a break down in our understanding of gravity or a new force. Calling them crackpots was probably rash, but no more so than their conclusions. For the record, I do expect eventual evidence of this sort, but not at at these distances or energies.

  • jick

    Wow, I liked the way the physics arxiv blog talked about Phong Shading as if it’s an esoteric technique developed for special applications. In fact, you can learn about it in any decent undergrad-level computer graphics textbook.

    WIN for interdisciplinary study. :)

  • Richard

    Julianne or Sean,

    Do you know if f(R) gravity disagrees with any observational evidence thus far? Are there any deep problems with the theory, or is regular GR safe?


  • Sean

    Richard– a “generic” f(R) model is ruled out by tests of GR. But you can make models that are not ruled out, by carefully choosing the function f. See e.g.

  • Richard

    Could gravitational waves from merging binary black holes, in principle, rule out or confirm f(R)? I assume higher precision observations of the CMB could also rule out or validate f(R).

  • doyle featherston

    Any time an object moves through a medium it is slowed by drag

    Pioneer 10 is an object, dark matter is a medium


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