Heat 'Most Likely Cause' of Pioneer Anomaly

astroengine writes "Everything from clouds of dark matter, weird gravitational effects, alien tampering and exotic new physics have all been blamed for the 'Pioneer Anomaly' — the tiny, inexplicable sun-ward acceleration acting on the veteran Pioneer deep space probes. However, evidence is mounting for a more mundane explanation. Yes, it's the emission of heat from the spacecrafts' onboard radioisotope thermoelectric generators (RTGs), slowly nudging the Pioneers off course, that looks like the most likely culprit. It's unlikely that this new finding will completely silence advocates of more exotic explanations, however."

Comments

d00d

[Mana Mana]

hate to tell you this but this is a dupe from like 6 months ago. Next time search the /'s archive.

Re:d00d

[SnarfQuest]

This is slashdot. The editors have better things to do than search for dups, fix typos, and check content. Don't know what they are, but it must be important.

Re:d00d

[scdeimos]

Sort of. The /. story from six months ago was about Frederico Francisco's arXiv paper. What's new in TFA is confirmation by JPL's Slava Truysev. That barely gets a paragraph, though, after summarising the previous research.

Re:FACT IS, NO ONE KNOWS AND NO ONE EVER WILL !!

[causality]

Put that on a sandwhich at eat it all up !! Costs you nothing so why not believe it ?!

If it turns out to be wrong (specifically the "no one ever will" part) then it costs me my chance to know the real answer.

If "no one knows" then you don't really know whether or not it's truly knowable, so by your own rules, please shut up.

The rest of us will find purpose in searching.

Deceleration

[Johnny Mnemonic]

What's the difference between "sunward acceleration" and deceleration?

I mean, isn't the probe generally traveling away from the sun?

Re:Deceleration

[Viperpete]

The AC who responded to your comment is completely wrong.

While deceleration is used in common speech to indicate a reduction of velocity, in physics there is no deceleration only acceleration in the opposite direction of the trajectory. Both concepts, acceleration reverse acceleration, require a point of reference, in this case it is the sun.

I would have been disappointed if /. used deceleration, particularly on a space article.

http://en.wikipedia.org/wiki/Acceleration

Re:Deceleration

[mhotchin]

What? No, both are vector quantities. One is just the negative of the other, they each have just as much 'context'.

Re:Deceleration

[Pieroxy]

Science classes might have been optional for some...

Re:Deceleration

[Altrag]

Constant velocity is not detectable without a frame of reference. This is relativity (extremely simplified of course!)

Change in velocity (ie: acceleration) IS detectable. You can detect forces acting upon you and therefore compute acceleration (F=ma). If you can measure the force acting on you (which you can if have the right equipment), and you know your mass, then its pretty trivial to calculate your acceleration without needing any external reference frame.

For a real world example, go ride a train (preferably between two stops seperated by a relatively straight run of track.) You definitely feel a backward "pull" as the train speeds up, and a forward "pull" as the train slows again for the next stop (plus some sideways pulls if the track curves, but for the sake of simplicity lets assume it doesn't).

During the middle of the trip -- when the train is maintaining a constant velocity -- you don't feel any different than you do when you're standing on solid ground, give or take a factor of imprecision such as a rough track or the operator not maintaining exactly constant speed.

Your entire knowledge of motion is based on a) looking out the window and b) previous experience with trains -- what they sound like, what they look like, how they move relative to the earth (which is the frame of reference you generally care about if you're taking a train somewhere) and so on. None of these factors have anything to do with the train's frame of reference however.

As for creating a frame of reference, you only need two points. Yourself (the observer) and a target (reference point) that you assume to be fixed (or you can consider yourself fixed and the target as moving -- the math is the same, you just get an extra minus sign).

You just continually monitor the distance between yourself and the target and can compute both your speed and your acceleration by comparing the distances over specific time intervals. As you take the interval times to zero, you get better and better approximations of your exact acceleration curve (that's pretty standard calculus -- sample and integrate.)

And finally, for an object in empty space. You're kind of correct. Its not so much that it doesn't have a velocity as much as velocity is simply undefined. You can still have an acceleration (F=ma as above) but what speed you accelerate from and what speed you accelerate to both have absolutely no meaning without a point of reference.

Of course in the real universe, forces (at least the ones we know about) are actions between objects, so the fact that you have an acceleration implies that there's something around that could be used as a reference point (but you have to be able to find it to use it!)

It's the direction it is accelerating in

[Sycraft-fu]

Remember that any change in velocity over time is an acceleration in the proper sense, and also remember velocity has both a speed and direction component. You accelerate a car to a stop, and you accelerate around corners when you change direction.

I understand that in regular speech it just means "going faster" and the direction component is dropped. Understand that NASA is full of scientists and they may use science terms in a more precise manner.

Re:It's the direction it is accelerating in

[Gadget_Guy]

I understand that in regular speech it just means "going faster" and the direction component is dropped. Understand that NASA is full of scientists and they may use science terms in a more precise manner.

Quite right. It is just the units of measurement that NASA does not always get right. Miles, kilometers; what's the difference?

VGer was a totally different design

[nido]

The Pioneers were spin-stablizied (like tops), whereas Voyager was 3-axis stabilized (with thrusters).

The first probes fired at the moon were also spin-stabilized. Both the US probes and the Soviet probes missed, by large margins. The Russians were the first to hit the moon - I guess they loaded extra propellant to perform course corrections.

The proper thing to do is launch another spin-stabilized probe on an extragalactic trajectory. I wonder how much that would cost.

What is the "Pioneer Anomaly"... <snip>
Is the same effect seen with the Voyager spacecraft?
The Pioneers are spin-stabilized spacecraft. The Voyagers are three-axis stabilized craft that fire thrusters to maintain their orientation in space or to slew around and point their instruments. Those thruster firings would introduce uncertainties in the tracking data that would overwhelm any effect as small as that occurring with Pioneer.

This difference in the way the spacecraft are stabilized actually is one of the reasons the Pioneer data are so important and unique. Most current spacecraft are three-axis stabilized, not spin stabilized.

- http://www.planetary.org/programs/projects/innovative_technologies/pioneer_anomaly/update_20050720.html

Re:This is great news!

[Pieroxy]

You should know that science fiction is not science at all. You can theorize all you want, but is there a point when a good old phenomenon based on physics laws that we know is enough to explain the phenomenon?

Re:This is great news!

[Anonymous Coward]

What was that you were saying about grasping at straws?

Re:This is great news!

[Altrag]

Any sort of Aether/"fabric" theory has been pretty much completely ruled out by experiment over a century ago.

There are lots of other explanations proposed however, though the Wikipedia page doesn't list any of the more crackpot theories like alien tampering.

One of these is dark matter, which could somewhat sound like what you're suggesting, but DM is definitely not a "fabric" of spacetime in any sense. Its "normal" matter that happens to not interact with the electromagnetic, weak or strong forces. That leaves gravity as its only interaction and we're just barely cracking the surface of gravitational telescopes. Once those have got a decent resolution though, DM should be confirmed or denied once and for all. In the meantime its just a theory that happens to fit certain data sets.

People studying Pioneer would prefer a more concrete solution that doesn't rely on unproven physics.

Loss of mass

[necro81]

I started wondering if the radioactive decay in the RTGs would have resulted in a significant loss of mass, and if that could have any effect. I am sure that JPL and others have looked at it in detail, and would have accounted for it if it were significant. Still, I was curious...

It's a bit tough to estimate, because the power output of the RTGs has diminished over the years, and I'm not interested in doing integrals this early in the morning. Their electrical output at launch was about 155 W, meaning that the heat output was probably more than 1 kW. Because it's an easy number to work with, let's estimate using 1 kW average thermal output over the mission life:

1 kW * 60 sec/min * 60 min/hr * 24 hr/day * 365.25 day/yr * 39 yr = 1.2e12 Joules

As a lovely demonstration of just how big a number the speed of light is, using E=mc^2 equates that energy to a whopping 13 micrograms.

So, yes, they have lost measurable mass. But, no, it is probably insignificant to the orbital mechanics at work. The rest of Pioneer weighed over 250 kg at launch. It probably picked up more than 13 ug in dust and solar wind.

Re:Loss of mass

[mark_osmd]

Changing the mass of an object in orbit has no effect on the trajectory due to the orbital mechanics, at least in this case where the object (Pioneer) is negligible in mass compared to the main object in the system (Sun).