Too bad they didn't pursue the question with the Pioneers. I've heard the reasons for the anomaly to be from the heat of the spacecrafts to solar cycles' effects on the heliosphere (inflating and deflating a balloon creating boundary changes...). There's also the mystery of interstellar space (ISM) and who knows what the effects of the bow shock plays. I prefer any one of those or a combination rather than the "dark matter" assumption.

[Data from the Galileo and Ulysses spacecraft suggest the anomaly may have affected them, too. But neither has been far enough from the Sun -- the dominant source of gravity in the solar system -- to firmly distinguish any possible discrepancy from noise in the data, Turyshev says. Galileo was crashed into Jupiter last year, and Ulysses will never go farther than it has.

That leaves two data points -- one from each Pioneer craft. Turyshev pointedly considers the pair as one data point, so as not to inflate the case for strange new physics. He looked at the two Voyager spacecraft, also exiting the solar system, but says their design involved "numerous attitude-control maneuvers" that "can overwhelm the signal of a small external acceleration."

NASA engineers have made their last communications with the Pioneer probes, so the two table-sized robots are carrying the unsolved mystery silently to the stars.]

Space.com