Research team appears to solve the Pioneer anomaly

Apr 18, 2012 by Bob Yirka weblog
An artist's concept of the Pioneer 10 Jupiter encounter. Image: NASA

(Phys.org) -- Back in the early 70’s NASA launched two exploratory spacecraft, Pioneer 10 and 11. Their missions were to gather information about the solar system as they made their way through it by flying farther and farther from the sun, until they eventually left altogether. Though neither craft has been heard from since 2003, both have confounded scientists since it was discovered in the 80’s that they were not accelerating at the rate that physicists had predicted, a phenomenon that has come to be known as the Pioneer anomaly. Now it appears a small team of dedicated researchers has figured out what is going on, and as they explain in their paper uploaded to the preprint server arXiv, it’s due to nothing more than the way the propulsion system onboard does its job.

The basic problem was that the spacecraft failed to accelerate at the rate predicted, and it did so at what at first appeared to be a constant rate. Subsequent research in the 90’s showed that whatever was preventing the craft from accelerating at the speed expected was lessening, which led some to believe it was tied to the radioisotope thermoelectric generator (RTG’s) onboard that created the electricity used by the craft’s electronic devices to take measurements and beam them back to Earth. venting could work against the direction of travel if it was being expelled in the opposite direction, which it was. The problem though, was that the plutonium used to power the RTG had a half life of 88 years while the accelerating lessening effect occurred at a half-life equivalent to just 22 years.

To solve the riddle the new research team built a computer simulation using any and all data that could be collected about the now decades old spacecraft. In doing so, they found that heat generated by the RTG likely wasn’t the culprit because it emitted heat in all directions. The electronics, on the other hand, because they were mounted on the back of the craft, away from the sun and dissipated heat mainly in that same direction, could indeed account for the discrepancy. But more importantly, they also found something prior researchers had not, and that was the decay in the efficiency of thermocouples that were converting heat to electricity which led to less and less heat being generated over time by the electronics, a rate that when combined with the radioactive decay of the plutonium, just happened to coincide with the rate at which the force that was causing the less than expected acceleration. And that they say, is why the craft have not accelerated at the rate calculated.

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More information: Support for the thermal origin of the Pioneer anomaly, arXiv:1204.2507v1 [gr-qc] arxiv.org/abs/1204.2507

Abstract
We investigate the possibility that the anomalous acceleration of the Pioneer 10 and 11 spacecraft is due to the recoil force associated with an anisotropic emission of thermal radiation off the vehicles. To this end, relying on the project and spacecraft design documentation, we constructed a comprehensive finite-element thermal model of the two spacecraft. Then, we numerically solve thermal conduction and radiation equations using the actual flight telemetry as boundary conditions. We use the results of this model to evaluate the effect of the thermal recoil force on the Pioneer 10 spacecraft at various heliocentric distances. We found that the magnitude, temporal behavior, and direction of the resulting thermal acceleration are all similar to the properties of the observed anomaly. As a novel element of our investigation, we develop a parameterized model for the thermal recoil force and estimate the coefficients of this model independently from navigational Doppler data. We find no statistically significant difference between the two estimates and conclude that once the thermal recoil force is properly accounted for, no anomalous acceleration remains.

via PhysicsWorld

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User comments : 22

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ECOnservative
1.7 / 5 (6) Apr 18, 2012
It's my understanding that both of the Pioneer craft have been 'coasting' since their earlier encounter with Jupiter and Saturn. What 'acceleration' is the article referring to?
Fleetfoot
5 / 5 (4) Apr 18, 2012
It refers to an observation popularly called the "Pioneer Anomaly". During the cruise, the craft seemed to be slowing down slightly slower than they should have due to the Sun's gravity. Look on arXiv for papers by Slava Turyshev.
Terriva
1 / 5 (3) Apr 18, 2012
the plutonium used to power the RTG had a half life of 88 years while the accelerating lessening effect occurred at a half-life equivalent to just 22 years
I don't get it. So is the electricity reason of the anomaly - or the radiation of heat, which is indeed independent to the effectiveness of thermocouple? And how it accounts to another fly-by anomalies?
Vendicar_Decarian
4.5 / 5 (8) Apr 18, 2012
Physics 1 anomaly 0.

Win: Physics.
tpb
3 / 5 (1) Apr 18, 2012
So, they appear to be saying that the electronics doesn't run with a constant power supply voltage and heat dissipation.
The only way I can make sense of this, is that the thermocouple output voltage is always greater than needed and the difference is dissipated in linear voltage regulators which are located with the rest of the electronics in the back of the craft.
nkalanaga
5 / 5 (8) Apr 18, 2012
Basically, what we have here is an extremely low efficiency photon drive. The electronics radiated heat (infrared photons) primarily in one direction, producing thrust, and they've now determined why the thrust has been reduced over time. Less electricity, less heat, fewer photons, lower thrust.
Tuxford
1.6 / 5 (9) Apr 18, 2012
Again, the effect is not limited to these spacecraft. JPL admits measuring the effect on recent Earth flyby's. I just hope they don't miss Pluto....

http://phys.org/n...ors.html

nkalanaga
5 / 5 (6) Apr 18, 2012
That's one reason they do midcourse corrections. And, yes, reflections from spacecraft surfaces will also have an effect. The Messenger mission to Mercury actually used the effect, using sunlight reflected from the solar panels to reduce the amount of maneuvering fuel needed. The reduction was very small, but every gram helps extend the mission for an orbiter. Thus, it was our first real "solar sail" spacecraft, albeit a very inefficient one. They didn't plan it before launch, but after noticing the effects, they learned to use them.
encoded
not rated yet Apr 19, 2012
That's one reason they do midcourse corrections. And, yes, reflections from spacecraft surfaces will also have an effect. The Messenger mission to Mercury actually used the effect, using sunlight reflected from the solar panels to reduce the amount of maneuvering fuel needed. The reduction was very small, but every gram helps extend the mission for an orbiter. Thus, it was our first real "solar sail" spacecraft, albeit a very inefficient one. They didn't plan it before launch, but after noticing the effects, they learned to use them.

citation needed
Fleetfoot
5 / 5 (2) Apr 19, 2012
So, they appear to be saying that the electronics doesn't run with a constant power supply voltage and heat dissipation.


Science instruments were switched off or on depending on the time into the mission.

The only way I can make sense of this, is that the thermocouple output voltage is always greater than needed and the difference is dissipated in linear voltage regulators which are located with the rest of the electronics in the back of the craft.


The power generated has to be enough at the end of mission but you can't turn down the decay of a lump of Plutonium, the power at the start was much more than needed. The thermocouples also degrade with time.

The fact that much of the power is used for the craft means the excess appears to halve in less time than the half-life of the Pu itself.
nkalanaga
5 / 5 (5) Apr 19, 2012
encoded: Here is your citation:

#43: "MESSENGER Sails on Sun's Fire for Second Flyby of Mercury". 2008-09-05.
http://messenger....?id=102.

"On September 4, the MESSENGER team announced that it would not need to implement a scheduled maneuver to adjust the probe's trajectory. This is the fourth time this year that such a maneuver has been called off. The reason? A recently implemented navigational technique that makes use of solar-radiation pressure (SRP) to guide the probe has been extremely successful at maintaining MESSENGER on a trajectory that will carry it over the cratered surface of Mercury for a second time on October 6."

http://en.wikiped...lar_sail
Short bloke
1 / 5 (5) Apr 19, 2012
In the reported reason for the unexpected slowing of the Pioneer spacecraft, the thermocouples and the plutonium would have been well shielded by metal and insulation from the cold of space. In that case, the heat produced would have been radiating spherically into space in all directions. How then is the very slight difference in radiation pressure in the direction of motion relative to that in the reverse of the spacecrafts motion going to account for the momentum (mass x velocity) of the spacecraft being decreased by 4oo kilometres a year. Also, the unexpected acceleration during the flyby of Jupiter remains to be solved.
Short bloke
1 / 5 (4) Apr 19, 2012
Sorry to have to correct but the above post should have read; radiating spherically into space equally in all directions. Also; How then can any very slight difference . . .
Tuxford
1 / 5 (5) Apr 19, 2012
LaViolette predicted the effect in the early 80's, and asked JPL scientists if they had yet detected it. They did not announce it until the late 90's, after having detected the acceleration for perhaps a decade. His photon-blue shifting prediction account for about half of the effect, with the rest attributed to spacecraft radiative effects, etc.

http://arxiv.org/.../0603191

So I wonder, did anyone else predict the effect before discovery? Was he just lucky?
nkalanaga
5 / 5 (4) Apr 20, 2012
Short bloke: "The electronics, on the other hand, because they were mounted on the back of the craft, away from the sun and dissipated heat mainly in that same direction, could indeed account for the discrepancy." You're right that the plutonium and thermocouples would have radiated spherically, and the researchers admit that was what they found in their new model. It was the electronics package itself causing the discrepancy, and it was designed to be shielded from the Sun, and thus radiate in one direction, for cooling purposes. The only effective way to cool anything is space is radiation, and it has to be where the Sun won't add heat faster than the electronics can get rid of it.
Terriva
1.1 / 5 (16) Apr 20, 2012
I wonder, did anyone else predict the effect before discovery? Was he just lucky

In dense aether model the red shif in visible light should be altered with blue shift at the microwave spectrum in analogy to the dispersion of ripples at the water surface: the long ripples are dispersed toward longer wavelength and vice-versa: short riples will be dispersed toward shorter wavelength. The treshold is the wavelength of CMBR. In this sense the Pioneer anomaly should be dark matter effect, not thermal effect. The fact that the plutonium used to power the RTG had a half life of 88 years while the accelerating lessening effect occurred at a half-life equivalent to just 22 years supports this view.
Sinister1811
1.6 / 5 (7) Apr 22, 2012
Who would've thought - something so simple could be the answer to this mystery.
Terriva
1.1 / 5 (9) Apr 22, 2012
The anomalous deceleration of Pioneer spaceprobes is equal just to the product of the speed of light and Hubble constant a = Hc = 8 x 10^{-10} meters/second^2. More precisely, the value of this acceleration is a=Hc - it is the Hubble constant multiplied by the speed of light, plus minus roughly ten percent. It indicates, the (seeming) expansion of Universe may be the main culprit here: the Universe expands during flight of Pioneer spaceprobe, which slows down its motion from perspective of terrestrial observer. Another feature of this value of a is that it seems to be the same acceleration as the critical acceleration relevant for the MOND theories that replace dark matter.
jcamjr
5 / 5 (2) Apr 22, 2012
Who would've thought - something so simple could be the answer to this mystery.

Occam!
julianpenrod
1.3 / 5 (6) Apr 22, 2012
Among the problems with the "analysis", if the electronics were ounts on the back of the spacecraft, they would not be exposed to space, to prevent damage from dust and such. They were in housings, which meant they would dissipate their heat to the spacecraft, which can release it in all directions. Also, if the electronics are releasing energy in the infrared area of the spectrum away from the direction of travel, the radio transmissions from the dish will be releasing energy in the radio part of the spectrum in the opposite direction, toward earth, which should offet at least some of the supposed energy loss by infrered radiation.
Neurons_At_Work
5 / 5 (1) Apr 23, 2012
According to the Pioneer design docs--

"The equipment compartments are insulated by multi-layered blankets of aluminized plastic. Temperature-responsive louvers at the bottom of the equipment compartment, opened by bi-metallic springs, allow controlled escape of excess heat. Other equipment has individual thermal insulation and is warmed by electric heaters and 12 one-watt radioisotope heaters, fueled with plutonium-238."

and...

"Excess power from the RTGs over that needed by the spacecraft is radiated to space thermally through a shunt radiator, or charges a battery which automatically supplies additional power needed for short periods when the spacecraft demands more power than the output of the RTGs."

I would be fairly certain that the fist paragraph addresses the source of the anomaly, and the second the source of the spherical emission from the RTG. Also, radio emission was of low power and sporadic at best, so it's doubtful that would have counteracted the effect mentioned.
Terriva
1.6 / 5 (7) Apr 23, 2012
Who would've thought - something so simple could be the answer to this mystery
Einstein: "Things should be made as simple as possible, but not any simpler."
I do perceive an apparent bias of mainstream physics to ignore all phenomena, which could threat the existing paradigms. Such tendency is nearly unconscious, but the more persistent throughout the mainstream science community. Everyone is looking for "new Physics" - well, proclamatively, but he tends to ignore all evidence for it at the same moment, when it could threat the religious pillars of existing science and his qualification for grant applications, indeed. This hypocritical stance leads to the paradoxical, if not comical situations, like this one.

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