[jlogajan]

Hmm, they didn't mention drag due to interstellar gas, did they? That would seem to be a candidate, would it not?

[Calvin Locke]

...drag due to interstellar gas...

My initial thought too. Perhaps related to the same force that initiates/starts up/creates comets as well?

[mrsmith]

"drag due to interstellar gas"
Impact and accretion of cosmic debris would about fit their measurements IMHO.

"Look here brother-who you jiving with that cosmik debris?
Now is that a real space probe or is that a Sears space probe?"

[Yeti]

Hmm, they didn't mention drag due to interstellar gas, did they? That would seem to be a candidate, would it not?

My thoughts exactly.

Funny that these guys would assume that our understanding of the laws of physics is way off before they consider the possibilty that we may have slightly mismeasured or miscalculated the density of matter in the region. Or the gravitational pull of the solar system en masse as the vehicles become more distant. Or the flux of matter moving into the solar system.

One ten-billionth the force of gravity is more likely rounding error in computations than anything else. Or how about accumulation of mass on the probes due to collisions with microparticles?

[malakhi]

Hmm, they didn't mention drag due to interstellar gas, did they? That would seem to be a candidate, would it not?

That is the first thing that occurred to me, too.

[litehaus]

NAH! It's a magnet being held the wrong way by the PROJECTIONIST who is on the OTHER side of the screen that we all think is the sky!!