[Cc J. as FYI]
It's been known for a long time that the Pioneer spacecraft are slowing more than General Relativity predicts. Some people have wondered if, perhaps, this means that gravity doesn't fall off with distance quite as rapidly as we think it does. (I.e. there may be a linear term as well as a quadratic term.)
Another possibility is that it's caused by the spacecraft design emitting more heat in front than in back, thus creating a tiny bit of excess thrust to the front, and slowing the craft down.
How do you answer a question like this? Well, you could try modeling the spacecraft in detail, and comparing your calculations of its heat radiation to changes in the unexplained deceleration over time. Somebody did this, and it looks like the second alternative (anisotropic heat emission) is probably the cause.
Hahahahaaaa!!! That is ME laughing at YOU, cruel world.
I could not love thee, dear, so much,
Loved I not Honour more.