The Pioneer anomaly is an unexplained deceleration of the Pioneer 10 and 11 spacecraft that seems to be acting on them as they head out of the Solar System. This deceleration is tiny: just (8.74±1.33)×10^−10 ms^−2. The big question is where does it come from.

One possibility is that the deceleration is the result of some long range gravitational force that is not observed on Earth. But if that’s the case, then this force should act on all of the many objects in the outer Solar System.

Earlier this year, we looked at one study of the orbit of Pluto which was unable to rule out the possibility that a Pioneer-like force acts on it because Pluto’s orbit is so poorly known.

Today we look at a similar study in which the objects of attention are the major Neptunian satellites: Triton, Nereid and Proteus. Earlier this year, astronomers published the result of a new analysis of the motion of these bodies taken over several orbits.

Now Lorenzo Iorio, at the Istituto Nazionale di Fisica Nucleare in Pisa, Italy, has analysed these orbits and concludes that a Pioneer-like force cannot be acting on Triton, Nereid and Proteus because the resulting anomalous perturbations would be too large to have escaped detection.

“The possibility that the Pioneer anomaly may be an exotic gravitational phenomenon seems to be challenged,” says Iorio.

This work is part of a growing body of evidence that the Pioneer anomaly is not a gravitational effect.

That’s a puzzle. If not gravitational in origin, what kind of force is acting on the Pioneer spacecraft?

Ref: arxiv.org/abs/0912.2947: Does the Neptunian System of Satellites Challenge a Gravitational Origin For The Pioneer Anomaly?