Before getting into this, I just want to give you the best quote about physics from a physicist I’ve seen in a long time. In describing the phenomenon we are discussing here, JPL scientist Slava Turyshev says, “The effect is something like when you’re driving a car and the photons from your headlights are pushing you backward.”

I know, right? I hate when that happens!

Anyway, here’s the story. Pioneer 10 and 11 are space ships that were launched before most of you were born, in 1972 and 1973. They visited a bunch of places in the solar system, but are now heading out of our solar system with the distinct possibility (distinctly small, but still, distinct) of running into some other solar system and there, they will be encountered by intelligent life forms who will see this plaque, attached, and then come find us, hunt us down, and kill us because they misinterpret the symbol as a hostile act.

At the time the Pioneer craft were launched, it was assumed that a well meaning friendly peace-loving symbol like this would be correctly interpreted, or at worst, simply not understood. Since then the Internet was invented and we have learned that the tendency of intelligent life, such as it is, in the only form we’ve so far observed it, is to read the worst possible interpretation into any ambiguous symbol. So we now know that when highly advanced aliens far away in a different part of the Universe in the future encounter one of the Pioneer craft, they will take it as a terrible insult. At best, this will happen:

But it could be worse. They could start trolling us and never stop. Only time will tell.

Anyway, after the Pioneer craft were well on their way in their tour of our solar system’s planets, its drivers at JPL noticed that they were slowing down, as in pulling back a bit towards the sun, unexpectedly. They attributed this to some oddity in the fuel system. Like when your check engine light goes on for a while then goes off and you don’t really know what happened so you make up some story about the gas cap and go on with your life.

But then, later, when the Pioneer space ships were way farther out, approaching 30 years in space, careful measurements showed a continued deceleration, sufficient to suggest some interesting previously undiscovered physical phenomenon. A few years after that, in 2004, a research team led by Slava Turyshev started collecting data to model what was happening, and to try to figure it out. And now, a paper has come out that suggests what may be slowing down the Pioneer space craft.

Some of the data were stored on these:

Other data were stored on magnetic tape. I’ve not seen this confirmed but I’ve heard a rumor that the data from September 1978 through February 1979 were written on napkins from Beefsteak Charlies. The data was not all digital, it was in all sorts of formats, and it took a great deal of effort by the growing team of dedicated scientists to, well, put it all on one spreadsheet.

Then, things got interesting. For one thing, the slowing down effect with Pioneer was not happening with the other long range deep space proby things such as Voyager. They determined that the difference in the effect among spacecraft was determined by differences in shape and layout of the spacecraft.

The thing causing the space craft to slow down is heat coming out of themselves. Each craft has a small plutonium power source and some electrical circuits, all of which cause heat. This heat emanating from the craft pushes outward and causes slowing. Like your headlights slowing you down because the photons from your headlights are acting like very very weak reverse thrusting retrorockets.

Wouldn’t it be an interesting world if this effect, with heat and photons and such, was much much stronger. Flashlights would be weapons.

That would be cool.

Anyway, here’s the abstract from the paper so you know I’m not making most of this up:

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.

So there is sad news and there is happy news, and they are the same: Physics works as expected.

Note to self: If designing a deep space probe, put all the thermal radiation stuff on the back end so that the thing slowly speeds up forever instead of slowly slows down forever. The best way to get past those Klingons is to be going really fast.

Slava G. Turyshev, Viktor T. Toth, Gary Kinsella, Siu-Chun Lee, Shing M. Lok, & Jordan Ellis (2012). Support for the thermal origin of the Pioneer anomaly Phys. Rev. Lett, 108 (241101) DOI: 10.1103/PhysRevLett.108.241101

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