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Pioneer Anomaly Solved By 1970s Computer Graphics

Posted by CmdrTaco on from the whats-your-vector-victor dept.

Frans Faase updated us on a Pioneer Mystery we've been following for many years: something is tugging Pioneer 10 & 11. A few years ago a theory surfaced but now "A new computer model of the way heat is emitted by various parts of the Pioneer spacecraft, and reflected off others, finally solves one of the biggest mysteries in astrophysics. Previous calculations have only estimated the effect of reflections. A computer modeling technique called Phong shading was used to work out exactly how the the emitted heat is reflected (PDF) and in which direction it ends up traveling. Taking into account the reflections on the antenna seem to make the anomaly disappear."

3 of 169 comments (clear)

  1. But... Phong is wrong by pyalot · · Score: 3, Interesting

    everybody knows that. A much better aproximation to real life surfaces are the Oren–Nayar or Cook-Torrance models of the family of BRDFs.

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    Experiments and other stuff
    1. Re:But... Phong is wrong by Josef+Meixner · · Score: 3, Interesting

      While it is true that the Phong solution is still likely "wrong" due to being not perfectly accurate, it's still a lot less wrong than thermal effects uncorrected

      The main problem with Phong is, that it can create energy depending on the parameters. Meaning the emitted light can be stronger than the incident light and so in that calculation create thrust out of nowhere.

      Additionally the Pioneer probes are made out of metal, Phong is derived from a model of plastic. The properties of those two materials are quite different, one being a conductor the other an insulator, so the Fresnel equation gives quite different values for the reflective properties, additionally metals are often anisotropic in their reflection capabilities. This has influence on the direction and form of the lobe for the first order effects. I also don't understand, why they didn't use one of the established BRDFs which are at least physically correct.

      I looked through the paper and I see no prove, that the parameters they assume for diffuse and specular reflection don't violate the laws of physics.

  2. Re:To be fair... by BitterKraut · · Score: 5, Interesting

    It is also interesting to note that Phong shading is based on an empirical formula. That means it has not been derived from any known (i.e. accepted) "laws of nature". It is used in Computer Graphics because it can be calculated efficiently and approximates what we see or measure closely enough. Strictly speaking, it is not possible to scientifically explain any phenomenon by showing that Phong shading explains it. But as it seems, the whole scenario is so complex that showing its compatibility with the Phong model must already be regarded as a remarkable achievement.