This is a fun one.  Pioneer 10 was launched in 1972 and contact was maintained until 2003.  Pioneer 11 was launched in 1973 but had troubles and contact was lost in 1995.  Starting in 1980, it was noticed that there was an anomalous acceleration toward the sun of both spacecraft, amounting to 8.5E-10 m/s^2. That's about a ten billionth of a gee.  The truly startling thing is that the acceleration has remained constant since then.  It's not some mechanical fluke of the Pioneers, either, caused by fuel leaks, radiation pressure, etc., or tracking errors, because this anomalous acceleration also affected the Ulysses and Galileo spacecraft.   You have 4 spacecraft on very different orbits, of 3 different designs, exhibiting the same behavior.

What can be causing this?  There an explanation involving MOND, or MOdified Newtonian Dynamics, which suggests that once gravity drops to a_0, the universal critical acceleration (the product of the Hubble constant and the speed of light (2.3E-18 seconds times 3E8 m/s = 6.9 E-10 m/s, ) it begins to follow an inverse-distance law.  Where stars in spiral galaxies begin to showe constant rotation speeds the acceleration toward the center is about a_0.  MOND is an attempt to remove Dark Matter (DM) from the equations that describe the movements of stars in a galaxy or galaxies in a cluster but it doesn't seem to have much support.  Is it just a coincidence that a_o is so similar to the PA?  There's also an explanation involving string theory, which is untestable as far as I know.  There are others, of course, but I won't list them here.

Could DM be the culprit?  A thin disk of rotating DM centered on the sun and  extending well past the Pioneers and with a near-constant density would produce a slight attraction that would be constant.  From 88 au, Pioneer 10's current position, it would be about .001 times the mass of the sun.

Obviously there are a lot of questions to answer.  We assume DM forms a sphere centered on each galaxy, so does DM also cluster around stars?  If it did, would it spin and form a disk?  Would that spin cause it to be less dense than it is in galaxies? 

Or am I just baying at the moon?

I'd appreciate any questions, comments, or criticisms.  I don't care if I'm wrong, I just want to know more about this stuff.

Is that an acceleration torward the SUN or the Center of the Galaxy?

morbas wrote:

Is that an acceleration torward the SUN or the Center of the Galaxy?

It is traveling away from the sun. At last contact it was more than 8 million miles away. The lost contact in Feb 2003 did not get any data in return. The power source has decayed and The last time it sent any data in return back in April of 2002 with a very weak signal. They no longer plan any more attempts at contact.

Here's a link to a great many more links regarding the Pioneer 10 anomaly. 

http://www.space-time.info/pioneer/pioanomlit.html

I liked

'Study of the anomalous acceleration of Pioneer 10 and 11'; John D. Anderson et al.

Finally, we observe that if no convincing explanation is to be obtained, the possibility remains that the effect is real. It could even be related to cosmological quantities, as ha]s been intimated. [See Eq. (16) and Sec. XI, especially the text around Eq. (57).] This possibility necessitates a cautionary note on phenomenology: At this point in time, with the limited results available, there is a phenomenological equivalence between the aP and at points of view. But somehow, the choice one makes affects one’s outlook and direction of attack. If one has to consider new physics one should be open to both points of view. In the unlikely event that there is new physics, one does not want to miss it because one had the wrong mind set.

After all is said and done, we form a hypothesis, a conjecture. More importantly we conclude with a question. We are not afraid to challenge our own work. We post apposing arguements and stur the pot so to say.

morbas wrote:

Is that an acceleration torward the SUN or the Center of the Galaxy?

The Pioneer Anomaly is an acceleration toward the Sun.  There is a similar anomaly  which accelerates stars toward the centers of spiral galaxies.  The cause of the latter is presumed by most to be dark matter.  The cause of the former is so far unknown.  We just don't have enough data.

There's some enthusiasm here and abroad to launch a probe specifically to test the PA.  The probe would mainly measure its own position with great accuracy and report back.  Unfortunately deep space probes are expensive and so far nobody wants to cough up the money for one.

Personally I'd like to see 2 probes --- one straight out in the plane of the Solar System, the other high above or below.  If they show the same results then my DM disk idea is wrong.  And I definitely agree with what one of  you said, that ideas need to be presented and tested with as little prejudice as possible.  It would be fun to learn that there is a disk of DM around the Sun, but just as much if there's some new principle of physics responsible for the PA.

Thanks for the URL.  I'll check it.  Has anybody tried to figure out how a DM disk might form?