## Pioneer Anomaly Solved 147

First time accepted submitter gstrickler writes

*"After years of work recovering and analyzing old mission data and vehicle schematics, a just published analysis(Pdf) provides strong evidence for anisotropic thermal radiation being the source of the slowing of the Pioneer 10 and 11 spacecraft. The theory isn't new, but the recovered data and new analysis provide solid evidence that at least 80% of the deceleration is accounted for by anisotropic thermal radiation. Members of The Planetary Society were instrumental in recovering the data and helping fund the analysis. The lesson is, in space, it matters what direction your heat radiating surfaces point."*
## Re:Slowing? (Score:4, Informative)

Acceleration can be positive or negative.

## Oops. More specifically... (Score:5, Informative)

Sorry to self-correct - acceleration is a vector, it has both magnitude and direction. By summing all of the acceleration vectors, you get a resultant which determines the rate of change of your velocity.

## Re:Slowing? (Score:5, Informative)

its accelerating backwards... ie, it's effectively got its "engine" (ie the heat radiating surface) pointing in the direction its heading, and this is slowing it down. I guess its still got a lot of velocity, but this is being reduced by the lack of anything pushing it in the right direction.

In other words, in space, your hot arse is a form of propulsion!

## Re:This just in (Score:4, Informative)

Faster, slower - just depends on where the heat is being emitted - either towards where the craft is going (mainly the case here) or back towards where it came from.

## Re:Slowing? (Score:3, Informative)

Usually my hot arse propels people away from ME~

## This was already solved by a portuguese in 2009 (Score:5, Informative)

A portuguese aeronautics engineering student from Instituto Superior Técnico [ist.utl.pt] already figured this out way back in 2009 in his masters thesis, available here [ist.utl.pt].

## Old News (Score:4, Informative)

## Re:Oops. More specifically... (Score:5, Informative)

There's an acceleration vector for every force vector and a net acceleration and a net force vector. They're mathematically equivalent. You could also look at it as a sum of momentum delta vectors. How is the electromagnetic force conveyed? By a photon exchanging momentum. So is force something that only exists as an effect of adding up all the changes in momentum, or vice versa? Neither, both are correct viewpoints. Have a nice day.

## Re:Mass?? (Score:5, Informative)

Does it have mass?

In a sense yes. Radiation and all other forms of energy have relativistic mass, as in the m in E=mc^2, as in things that have more energy (moving objects, high-energy states of atoms and molecules, systems in general) have more mass as it applies to inertia and gravity.

Heat radiation as in (mostly infrared) photons don't have

restmass. That's the m0 in E = root(pc^2 + m0^2*c^4). So they don't have mass in the sense ofmatteras you usually think of it. But it turns out the way you usually think ofmassis not equivalent to matter. Even though the usual way you think about it is that they are.Hope that clears things up. :)

## Re:This just in (Score:5, Informative)

Am I arrogant for saying "wasn't this obvious?"

Not really as long as you realize that some things seem obvious once you know they're true... Or as long as you just mean "obvious

possibility".It's not like they didn't know that if there was a favored direction for the emission of radiation that this would affect the velocity of an object. The concept of a photon drive existed for decades before the Voyagers were launched. It's just that it was though that whatever net force there was would be essentially zero. Assume a uniform, spherical Voyager craft...

This has been a long-standing possible, and then probable, explanation for the anomaly. Seems to have taken quite a bit of effort to figure out what the actual value of the force would be with sufficient precision. I remember what seems like a long time ago an article posted to /. about someone calculating the effect of heat radiation using Phong shading, the 3D graphics technique, as an approximation and got pretty good agreement.

Going all the way to a complete finite element analysis, using multiple methods to come up with the coefficients for the model, and getting a result that leaves only a noise-level signal is pretty impressive. And not what I'd call obvious.

So despite maybe feeling like it, it's not exactly a case of research by the Maximegalon Institute of Slowly and Painfully Working Out the Surprisingly Obvious.

## Re:Mass?? (Score:5, Informative)

I thought that E=mc^2 meant that E could me converted to M not that all it was both energy and mass at the same time.

So did I! But it's not a conversion, it's an equivalence! Energy and mass -- relativistic mass, the quantity that informs our notions of gravity, weight, and inertia -- are really, always, the same thing just in different units. If there's more energy in a system, then it weighs more on a scale. Water weighs less than two hydrogen and one oxygen because it's at a lower energy state. This of course

includesthe energy that is in the form ofrestmass.Rest mass is a form of energy. It can be converted into other forms of energy at a rate equal to m0*c^2. Energy and relativistic mass are always related by the equation E=mc^2.

## Re:This just in (Score:2, Informative)

Faster, slower - just depends on where the heat is being emitted - either towards where the craft is going (mainly the case here) or back towards where it came from.

In space, speed is a meaningless attribute without a point of reference. As well, space is non-euclidean in nature -- you can travel in a straight line and wind up in the same place you started. It's better to speak in terms of vectors and delta...

## Re:This just in (Score:4, Informative)

I wouldn't worry about that seeing as its velocity is enough to escape our solar system [wikipedia.org] altogether. It's more likely to get closer to Aldebaran, a star currently 68 light years away, than our own Sun... given 2 million years or so that is.