Pioneer spacecraft mystery may be laid to rest

New Scientist Space ^ | 15 April 2008 | Valerie Jamieson

[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.

[Swordmaker]

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.

If the impact of "heat" from the sun and the re-radiation of said "heat", plus heat generated from the radioisotopic generator, are the source of this thrust, then wouldn't all of it would be imbalanced, some coming at greater leverage and from larger areas than others, imparting an uncontrolled, erratic yaw on the vehicle?

[AndrewC]

If the impact of "heat" from the sun and the re-radiation of said "heat", plus heat generated from the radioisotopic generator, are the source of this thrust, then wouldn't all of it would be imbalanced, some coming at greater leverage and from larger areas than others, imparting an uncontrolled, erratic yaw on the vehicle?

They are spin stabilized, so no, I don't think that yaw would occur until the control systems run out of fuel.

[Swordmaker]

They are spin stabilized, so no, I don't think that yaw would occur until the control systems run out of fuel.

I know... but some of these spacecraft that are displaying this anomaly are far beyond their expected and designed life and probably have long ago exhausted the stabilizing fuel (it would be too expensive in Delta-V to provide a fuel supply that would exceed mission parameters, don't you think?) yet they are still sufficiently oriented toward Earth that we can receive their signal. Also, I think that the control people would have noticed an unusually high number of stabilizing maneuvers as the vehicle re-oriented itself if that were the case. It appears to me that whatever is applying a force to the vehicle is acting on the vehicle as a whole and not preferentially on any of its parts due to size or positional leverage.

[AndrewC]

It appears to me that whatever is applying a force to the vehicle is acting on the vehicle as a whole and not preferentially on any of its parts due to size or positional leverage.

Exactly.

[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.

[Lonesome in Massachussets]

Stimmt. AndrewC seemed to be poo-poo radiation pressure. The anomaly could simply be the difference between the model of radiation pressure and true radiation pressure. If you model, say, 0.001 kg-meter/sec/sec, plus or minus (say) 0.0001 kg-m/s/s, and the actual radiation pressure is 0.0005, you have an anomaly of 0.0005 kg-m/s/s “towards” the sun, which cannot be “explained” by your model.