The Pioneer effect just gets weirder and weirder

Last week I wrote about the Pioneer effect, an unexplained acceleration we’ve spotted in many of our far-traveling spacecraft. The Economist has a good article on it, and the additional details it reveals are fascinating. A team at JPL has put together a simple equation (unfortunately not in the article) that solves for the discrepant acceleration. It’s just that there’s no known force in physics that could be causing it. So here we have a mathematically predictable, yet unexplained, effect! This is huge! This is how many new laws of physics have come to us in the past.

The Economist also notes that the discrepancy isn’t observed in objects traveling in elliptical orbits (which is basically everything in the solar system except our space probes). There’s something about the hyperbolic orbits our space probes are in that exposes them to additional unexplained accelerations. There’s a huge mystery just waiting to be unraveled here, and I’m hopeful that when it is, we will emerge having learned a new fundamental truth of the universe.

3 Responses to “The Pioneer effect just gets weirder and weirder”

  1. arensb Says:

    The Economist article suggests two differences between the probes and planets: the probes are small, and they travel in non-elliptical orbits. There are thousands of small objects in elliptical orbit that are monitored closely (lest they fall onto Indonesia or something), so it should be fairly easy to test the hypothesis that smallness alone causes the Pioneer effect.

    Would comets be useful in testing whether the orbit causes the effect? I suspect that in principle, yes, but in practice, the mass of any useful comet is so uncertain that the Pioneer effect would be dwarfed by measurement error bars.

    Of course, it may simply turn out that God’s universe simulator uses a Pentium FPU :-)

  2. Cyde Weys Says:

    Well, keep in mind that the mass of an object is irrelevant in determining its orbit. A gigaton asteroid revolves in the exact same orbit around the Sun that a one gram fleck of dust would (and yes, I know that this doesn’t scale up to planets because they are able to pull the Sun back, but that is a polynomial effect). I suspect the error bars are large but for different reasons, not least of being how in the world do you try to look for Doppler shifts in a radar signature bouncing off an irregular object much, much larger than the scale of the effect (a few mm/sec of velocity discrepancy). Also, I don’t think we know any asteroid/comet velocities to within partial millimeters of a second.

    I suspect, but cannot prove, that the effect has nothing to do with the size of the object, and that it relies solely on the shape of the orbit. The space probes already analyzed were of different masses and shapes, yet the discrepancies could be predicted in a formula based off orbit alone. That seems to say it all to me.

  3. arensb Says:

    I suspect, but cannot prove, that the effect has nothing to do with the size of the object, and that it relies solely on the shape of the orbit.

    You’re probably right, though if it turned out to be some bizarre new physical effect that affected small objects differently from large ones, that would obviously be tremendously exciting. But I was just thinking of ways to isolate two possible causes: if satellites (small objects in an elliptical orbit) behave differently from the Pioneer probes (small objects in a hyperbolic orbit), then the size is probably the wrong thing to look at.