[Lonesome in Massachussets]

It's those heavy heat particles.

I can't tell if you're kidding, man. Radiation pressure is real, heat causes thermal radiation.

(Einstein derived the formula E = mc2 as an addendum to his 1905 paper on relativity, actually reasoning that the mass of radiation is related to its energy, m = E/c2 from considerations of radiation pressure.)

Most of the radiation pressure on objects in space is caused by reflection of solar radiation. Reflection actually imparts twice the impulse that absorbtion does. Absorbed radiation is ultimately reradiated, but perhaps not as directionly, some component of the scattered velocity in the same direction as the incident, thereby partially cancelling the effect of absorbtion.

Radiation pressure is never more than a few millionths of the effect of gravity, but integrated over time, the effect is measurable.

[AndrewC]

I was kidding, but I don't think that the probes have an inadvertant heat thruster pointed towards the sun. I also would guess that the thermal radiation from the sun along with whatever solar wind remains at that distance would overwhelm the contribution that chance heat would impart. (the variances of the two mentioned sources I would expect to be larger than the total directional output from heat.)

p=h/λ is rather small for "heat"

[AndrewC]

Looking further into the RTG source, it sure seems unlikely that it would cause the effect under study.

The dish antenna would be pointed towards the sun. The RTG would then be perpendicular to the spacecraft sun axis. This same configuration applies to the Ulysses and Voyager spacecraft.

[Southack]

"Most of the radiation pressure on objects in space is caused by reflection of solar radiation."

Sure, insofar as reflecting infrared light (heat) is pressure...but the Pioneer anomaly is pushing P1 and P2 toward the sun, not away from it.