Pioneer Anomaly Solved By 1970s Computer Graphics

Frans Faase updated us on a Pioneer Mystery we've been following for many years: something is tugging Pioneer 10 & 11. A few years ago a theory surfaced but now "A new computer model of the way heat is emitted by various parts of the Pioneer spacecraft, and reflected off others, finally solves one of the biggest mysteries in astrophysics. Previous calculations have only estimated the effect of reflections. A computer modeling technique called Phong shading was used to work out exactly how the the emitted heat is reflected (PDF) and in which direction it ends up traveling. Taking into account the reflections on the antenna seem to make the anomaly disappear."

Comments

To be fair...

[cobrausn]

The technique for Phong Shading was introduced in 1973 as an improvement to Gouraud Shading, but was too computationally intensive to be used for graphics back then. This is no longer the case.

Re:To be fair...

[BitterKraut]

It is also interesting to note that Phong shading is based on an empirical formula. That means it has not been derived from any known (i.e. accepted) "laws of nature". It is used in Computer Graphics because it can be calculated efficiently and approximates what we see or measure closely enough. Strictly speaking, it is not possible to scientifically explain any phenomenon by showing that Phong shading explains it. But as it seems, the whole scenario is so complex that showing its compatibility with the Phong model must already be regarded as a remarkable achievement.

Re:To be fair...

[Dr. Gamera]

It's not the radiation pressure from the sun that they're calculating, it's thermal effects from the on-board plutonium on the back of the antenna. So the source of the slowing is a very short distance away.

But... Phong is wrong

[pyalot]

everybody knows that. A much better aproximation to real life surfaces are the Oren–Nayar or Cook-Torrance models of the family of BRDFs.

Re:But... Phong is wrong

[fatphil]

And the earth isn't round either. It's just a closer approximation to reality than
saying the earth is flat. Or saying that Pioneer is a spherical cow. Scientists aren't looking for something that is right rather than wrong, they are looking for something that bounds the error term in a significantly tighter way. Phong apparently does this. Presumably any ad-hoc model that approximated reality closer than what was done before would have also decreased the error bounds.

And Oren-Nayar? Have you mistaken Pioneer for a slab of concrete?

Re:But... Phong is wrong

[tnk1]

They did explain why. Thermal effects. The only thing that Phong shading did was to remove an obstacle to that hypothesis that was merely due to built-in inaccuracies in how they accounted for those effects initially.

In other words, the shading increased the accuracy of the calculations and it was found that when it was applied the most likely solution became even more likely.

While it is true that the Phong solution is still likely "wrong" due to being not perfectly accurate, it's still a lot less wrong than thermal effects uncorrected, and much, much less wrong than assuming aliens or an entirely new discovery in gravitation.

Re:But... Phong is wrong

[Josef Meixner]

While it is true that the Phong solution is still likely "wrong" due to being not perfectly accurate, it's still a lot less wrong than thermal effects uncorrected

The main problem with Phong is, that it can create energy depending on the parameters. Meaning the emitted light can be stronger than the incident light and so in that calculation create thrust out of nowhere.

Additionally the Pioneer probes are made out of metal, Phong is derived from a model of plastic. The properties of those two materials are quite different, one being a conductor the other an insulator, so the Fresnel equation gives quite different values for the reflective properties, additionally metals are often anisotropic in their reflection capabilities. This has influence on the direction and form of the lobe for the first order effects. I also don't understand, why they didn't use one of the established BRDFs which are at least physically correct.

I looked through the paper and I see no prove, that the parameters they assume for diffuse and specular reflection don't violate the laws of physics.

Re:tao of physics??

[The Grim Reefer2]

So it seems, the mystery ain't a mystery after all...

That tends to happen when you solve them.

Re:tao of physics??

[Zephyn]

Because focus can now be placed elsewhere instead of continuing to investigate a red herring.

While it is sometimes disappointing that unknown effects don't always turn out to be from unknown causes, having the exciting new discoveries come from the basis of fact rather than imagination is the main difference between actual science versus everything else that claims to be science.

Here's the article ...

[Intron]

... from Slava Turyshev which describes what they did to model the craft and show that heat could be the culprit.

http://www.planetary.org/programs/projects/pioneer_anomaly/update_20080519.html

Re:Stupid scientists

[19thNervousBreakdown]

Fucking Phong Shading. How does it work?

Re:tao of physics??

[Kjella]

Sadly, unless it's one of the big unsolved problems or it takes a PhD to even understand the problem it's probably been solved before. We had a math book that so barely mentioned perfect numbers, I spent a lot of time reaching a result that I felt was "new". Eventually it turns out I had recreated a proof that Euler did in the 18th century. At least it wasn't the Greek, every time you feel bright then you learn someone already figured this out 2000 years ago.

Re:tao of physics??

[Chris Burke]

Well and every crackpot trying to overturn all of modern physics/cosmology without understanding it first. The number of times I've heard the Pioneer Anomaly brought up as evidence that modern physics was fundamentally broken and the Scientific Clergy refused to admit it is... very large. I think I've even heard EU morons claiming that their plasma cosmology explained the Pioneer Anomaly.

Of course nobody who latched on to the anomaly will be satisfied by this explanation. So it goes...

Re:tao of physics??

[Chris Burke]

Yes, he said "at least it wasn't the Greeks", because of how often he finds out that something he thought was novel was discovered by them. Which would be 2000 years ago. Merely 300 years ago is better. Thus "at least it wasn't the Greeks".

Got it now?

Re:Phong shading?

[camperdave]

They don't have to worry about the radiation of the universe. Pioneer has a radioactive battery which serves not only to power the on board electronics, but to keep them from freezing. The heat from the Warm Electronics Box is soaking through the craft and being emitted as infrared photons. These photons are bouncing off the back of the radio dish antenna and are slowing down the craft.

Re:Excuse my ignorance

[radtea]

The WORST thing is light, because light has so very little momentum for so much energy.

Unless of course the light is free. The solar constant is about 1 kW/m**2, so solar sails get 1 N per 150,000 m**2, or a circle of about 250 m diameter.