Pioneer... neutrino layer?

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The Pioneer 10 and Pioneer 11 spacecraft, the ones which are mysteriously slowing down...

http://archives.cnn.com/2001/TECH/space/05/21/gravity.mystery/

 

Anyway, I was at the Mill Hill Observatory (I got to use the Radcliffe telescope: http://www.ulo.ucl.ac.uk/telescopes/radcliffe/ or image here: http://www.ulo.ucl.ac.uk/images/ulo_photos/rad_modern.jpg it's the only time it has ever been open to public (it was open because of national science week)).... back to the topic!... I was talking to a PhD astronomy/cosmolgy guy and asked him about what he thought was the 'mysterious force' acting on the Pioneers, he reckoned, off the top of his head, either mathematics error or there was a theory about a neutrino layer.

 

I was wondering, can anyone tell me about this 'neutrino layer' ?