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

I think I need to get some sleep.

[gcnaddict]

It's unlikely that this new finding will completely silence advocates of more erotic explanations, however.

Re:I think I need to get some sleep.

[sitharus]

That's what I read as well...

Re:I think I need to get some sleep.

[Anonymous Coward]

Heat erotically nudging man's most far-flung space missions off-course in an erotic fashion?

On the one hand, Man's exploratory bent is about more than sex.

On the other hand, it is *also* about sex.

Re:I think I need to get some sleep.

[chromas]

On the other hand—oh wait! That hand's full.

Re:I think I need to get some sleep.

[GooberToo]

erotic explanations

Zapp? Zapp Brannigan? Is that you?

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:d00d

[Anonymous Coward]

Yep. The article back then claimed Phong shading helped find this.

Re:d00d

[Jane Q. Public]

Mod up. I was going to say pretty much the same thing, but every time I have posted something like that I have gotten marked "troll". Go figure.

Re:d00d

[Anonymous Coward]

Not really, I see just a broken link from TFA to the actual paper of 14 July. Here it is

(that happens when you mess with slashes and dots...)

Re:d00d

[pz]

The editors (read: Timothy) should AT LEAST have included a link to the previous posting and a quick blurb on why this isn't a dupe. And at best rewritten the hype-filled and intentionally misleading summary.

Another clear failure of Slashdot editing.

Re:d00d

[Anonymous Coward]

That was awesome.

Re:d00d

[Anonymous Coward]

Oh yes I myopically watch the entire sum of slashdot history. I also slavishly watch every article that pops up. I get bitching about a dup from a few days ago, or the same day. But months???

It was new to me...

Re:d00d

[Lumpy]

Throwing nerf balls at the glass walls of CmdrTacos corner suite.

Re:d00d

[Jane Q. Public]

Not funny, folks. Obviously quite a few people on Slashdot have a pretty juvenile sense of humor.

What a Dune Coon!

[Anonymous Coward]

That's what I read as well...

Yeah "me too!" posts are always so useful.

Re:What a Dune Coon!

[derGoldstein]

Totally.

Re:What a Dune Coon!

[Anonymous Coward]

I agree.

Re:What a Dune Coon!

[Dunbal]

They are marginally more useful than posts pointing out how useless they are.

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

[Anonymous Coward]

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

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.

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

[Anonymous Coward]

The Kansas State Board of Education called. They said they miss you and want you back. Or as the windy city boys once crooned

As my life goes on I believe
Somehow something's changed
Something deep inside
Ooh a part of me

There's a strange new light in my eyes
Things I've never known
Changin' my life
Changin' me

I've been searchin'
So long
To find an answer
Now I know my life has meaning
Ow oh

Now I see myself as I am
Feeling very free
Life is everything
Ooh it's meant to be
When my tears have come to an end
I will understand
What I left behind
Part of me

I've been searching
So long
To find and answer
Now I know my life has meaning
Woah woah

Searching
Don't you know I'm here yeah
For an answer
To the question
Oh yeah
For our minds
Baby
Baby it's true
It's only natural
It's only natural baby, yeah
Good things
In life
Take a long time
yeah yeah

If you can think of it...

[Anonymous Coward]

... there is pr0n of it.

(I haven't seen any deep space probe pr0n yet though).

Re:If you can think of it...

[Opportunist]

I never wanted to know how V'Ger came into existence. There are certain things I do not want a Rule 34 for.

Re:If you can think of it...

[cpu6502]

According to Gene Roddenberry's novelization, V'ger was not a result of porn but the derelict probe falling through a wormhole and ending-up near a plant of "living machines". They captured the original Voyager 7(?) probe, and upgraded the technology so it could complete its task of exploration.

Re:If you can think of it...

[Opportunist]

They raped Voyager 7.

Again, please refrain from Rule 34'ing it!

Re:If you can think of it...

[Anonymous Coward]

(I haven't seen any deep space probe pr0n yet though).

Here ya go! No photoshop, it's the plaque we placed on the actual Pioneer space probes. (It's too early to dig out my copy of Murmurs of Earth to see what we put on the Voyager records.)

They found something

[future assassin]

and need to cover it up.

Re:They found something

[Opportunist]

Yeah, the probe collided with a super advanced alien probe and fused and now they're on their way back to Earth to meet their creator.

I think I should sell this novel script, I'll make millions. Maybe they even turn it into a movie.

Re:They found something

[Anonymous Coward]

I, for one, welcome our new V-Giny overlord!

So ...

[Mick R]

VGer poot'd. Must have been a relief.

Re:So ...

[Anonymous Coward]

VGer was a god probe created when the Voyager 6 probe fell down a black hole and ended up at the machine planet.

The Pioneer stumbled into the neutral zone was a destroyed by the Klingons along with the careers of everyone staring in that movie.

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

[Anonymous Coward]

Deceleration is always relative of something and is rarely used because to get it you need to know the context. Acceleration however doesn't need context.

Re:Deceleration

[Anonymous Coward]

This sounds wrong to me too, the probe isn't speeding up backwards, it's slowing down.

Re:Deceleration

[Anonymous Coward]

Deceleration is always relative of something and is rarely used because to get it you need to know the context. Acceleration however doesn't need context.

Yes it does, the context was "toward the sun"

Re:Deceleration

[Anonymous Coward]

Acceleration has a vector attached to it. Deceleration is a more lay term which tends to be used as a scalar - that means that deceleration is usually assumed to be in the opposite direction of the velocity.

Acceleration might be slowing it down, but it's also going to be changing the direction - it's basically moving slightly towards the sun. This gives us some more useful information that just being told it is slowing.

Re:Deceleration

[u38cg]

I'd imagine it's an artefact of vector analysis.

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

[Viperpete]

oops, sorry about responding to myself, but I messed up my link. I'm more of a hardware guy.

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

Re:Deceleration

[FrootLoops]

Deceleration means "a decrease in speed". If the probe is traveling directly away from the sun, and there are no other contributions to the probe's acceleration, a sunward acceleration causes a decrease in speed, and a decrease in speed causes a sunward acceleration. For deceleration to occur, you need the speed to actively decrease. If, for instance, there was a component of acceleration away from the sun overwhelming the sunward component, there would be no decrease in speed [as long as the velocity vector worked out correctly], so deceleration wouldn't make sense. I imagine the probe isn't traveling radially outward from our solar system, and that there are other contributions to the acceleration, so it's not clear to me if deceleration makes sense in this context.

Re:Deceleration

[CTachyon]

In physics, "deceleration" is just an informal shorthand way of saying "acceleration in the opposite direction of something", where the vector "something" is often "velocity" by default but can be anything else depending on context. Saying "Pioneer is decelerating" is not quite right, then: the Pioneer craft are traveling on hyperbolic paths that slingshot away from the Sun on a curve, not zipping away in straight lines, so an acceleration toward the Sun would not point in the opposite direction from the velocity. It would slow them down since the velocity-acceleration angle is obtuse, but not as much as an actual 180 degree acceleration would. (Perhaps the acceleration is Sun-ward instead of backward because the Pioneer craft aligned their spins to keep their radio dishes pointed toward Earth, and asymmetry makes them emit more RTG heat on the opposite side from the dishes? Pure speculation on my part.)

Re:Deceleration

[ceoyoyo]

Acceleration doesn't require a point of reference.

Re:Deceleration

[Anonymous Coward]

Since acceleration has directionality it always has a point of reference. Generally the point of reference is the object itself and the direction is the direction of the applied force, but there's nothing stopping you from having an external point of reference.

Re:Deceleration

[ceoyoyo]

Directionality doesn't require a point of reference either. The object itself doesn't count as a reference point.

Velocity requires a reference point - if you're floating in deep space with nothing around you, you can't tell if you have any velocity. The question itself doesn't make sense without some other object against which to measure your motion. Acceleration isn't like that.

Re:Deceleration

[Viperpete]

Following your logic acceleration would not be detectable either therefore pointless.

In order to detect acceleration, you must take at least 3 sample points of reference of the object in motion. The first to set a starting point, the second in order to set an velocity and the third in order to set a later velocity with this information you can detect the change in velocity.

Without these an object in empty space would never have velocity therefore no potential increase in velocity.

Re:Deceleration

[Anonymous Coward]

Directionality doesn't require a point of reference either. The object itself doesn't count as a reference point.

Velocity requires a reference point - if you're floating in deep space with nothing around you, you can't tell if you have any velocity. The question itself doesn't make sense without some other object against which to measure your motion. Acceleration isn't like that.

What is Acceleration? Well, "acceleration is the rate of change of velocity over time" according to the Pukey-Pedia.
So exactly how you plan on figuring acceleration without relying on velocity or time, both of which DO require a reference point, I'm not exactly sure.

But I do know that if you can provide a meaningful explanation for your logic, there's probably a Nobel Prize for Physics in it for you...

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

[Anonymous Coward]

In a nutshell: deceleration is a special case of acceleration.

Re:Deceleration

[BlackPignouf]

Sorry, but you're wrong.

In order to detect acceleration, you only need a pendulum, a glass of water or a faceplant against the spaceship hull. The pain you'll feel isn't relative to the frame of reference.

Re:Deceleration

[Viperpete]

That would be inertia.

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!)

Re:Deceleration

[Viperpete]

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 each time you note the position you create a new reference point for the following position.

Re:Deceleration

[BlackPignouf]

That would be linked to the spaceship's acceleration.

Re:Deceleration

[CrimsonAvenger]

Or not, since gravitational acceleration would act on both the spacecraft and your face, thus preventing the faceplant while still accelerating.

Re:Deceleration

[Lord Crc]

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

Generally yes, but not exactly, which is why there's a difference between deceleration and sunward acceleration. See this image.

Re:Deceleration

[Anonymous Coward]

Yes it does. How else do you calculate the direction of the vector measurement?

Re:Deceleration

[Anonymous Coward]

Actually Deceleration would mean a reduction in speed ( |v| ), while Acceleration would mean a change in velocity ( v ). Since you'r3e only decelerating if the magnitude of your velocity is reducing, than Deceleration is a pretty specific case of acceleration, and in practice it's just easier to always talk about acceleration and specify the direction you're accelerating. This doesn't track with "common sense" because most people move in very nearly one dimensional paths (roads are technically 2d but they barely use the second dimension).

Re:Deceleration

[thrich81]

One modern (well, early 20th century) modification to your description -- a tenant of General Relativity is that you cannot tell the difference between a force you detect due to gravity and a force due to acceleration without some outside reference. So, if you are in a train with no windows and felt a backward pull, there is no experiment you could do to determine if you were accelerating in an inertial frame or oriented such that gravity was trying to pull you back. After a while, of course, you could deduce that the train could not accelerate forever, but that is a characteristic of trains, not the physics involved. I probably messed something up in there...

Re:Deceleration

[robot256]

Actually Deceleration would mean a reduction in speed ( |v| ), while Acceleration would mean a change in velocity ( v ). Since you're only decelerating if the magnitude of your velocity is reducing relative to a specific inertial reference frame, then Deceleration is a pretty specific case of acceleration.

Furthermore, to specify this acceleration as a deceleration, you must assume that the velocity is directly *away* from the sun, which is likely not the case. So even if you did specify the reference frame (relative to the sun) and the initial velocity (away from the sun but not necessarily in a straight line), you cannot call it a deceleration and still convey the fact that the force is toward the sun.

And by the way, having just attended a lecture on spacecraft thermal design, I can say this is a perfectly reasonable theory. The thermal radiators used to dissipate excess heat into space necessarily need to be placed on the "dark" side of the spacecraft so they don't absorb energy from the sun. That means that the photons that radiate exert a force on the spacecraft pushing it back toward the sun, regardless of what direction it is moving. When the probe gets far enough from the sun, at some point the radiated heat will exceed the solar flux (which thins out as 1/r^2), and the net force will be toward the sun.

Re:Deceleration

[Anonymous Coward]

Do a basic physics course. It's one of the first things they teach you.

Re:Deceleration

[robot256]

Tell that to your face when it's planted on the floor of a Jupiter colony. Alternatively, amaze your friends and relatives by standing on the wall while your spaceship accelerates. Gravitational acceleration is no different from any other acceleration, only the magnitude and direction different.

Re:Deceleration

[robot256]

Yes exactly, you have to know position and time to know acceleration, this is why every Wiimote has a super duper GPS receiver that can resolve centimeter positions while indoors, which is why they all cost $10000000000 and are regulated by the military. What's that you say? They're only $20 and imported from China? Maybe that's because they use an ACCELEROMETER which senses the FORCE applied to a MASS whenever it gets ACCELERATED, without ever needing to know position OR time.

Re:Deceleration

[Dunbal]

Acceleration however doesn't need context.

Is this a fancy way of trying to get us to believe that acceleration is NOT a vector? Please define "context". Also considering that the definition of deceleration is acceleration in a direction opposite to velocity or even "negative acceleration" with the negative just being a flag for "the other way, dummy", I would say that your whole argument is on pretty shaky ground. If acceleration doesn't need your undefined, mystical "context" then neither does deceleration. Why don't you let real nerds do the nerding so you can avoid those headaches?

Re:Deceleration

[Dunbal]

This sounds wrong to me too, the probe isn't speeding up backwards, it's slowing down.

ie. Speeding up in the direction opposite to current velocity - especially if your frame of reference is the object. Humanities majors should not be allowed to post on slashdot.

Re:Deceleration

[Dunbal]

No? This may come as a surprise to you, but you are accelerating right now. In two directions at once, to be precise. I suggest you walk off a cliff to experience one of these accelerations. Maybe you will learn something about basic physics and vectors on the way down. The point of reference for "down" in this case being the center of the earth.

Re:Deceleration

[Dunbal]

where the vector "something" is often "velocity"

Just to nit-pick, you mean "the direction of movement". Velocity also implies the magnitude as well as the direction, and I don't see why we need to bring magnitude into the argument.

Re:Deceleration

[Johnny Mnemonic]

Or at least, they shouldn't be able to post questions on slashdot. God forbid they try to gain some understanding of things they don't know enough about.

And yes, I am an offended BA.

Re:Deceleration

[Anonymous Coward]

Sorry until I walk off the cliff I am not accelerating towards the center of the earth. I am being opposed by the ground underneath me which is pushing back with an equal and opposite force to gravity. Since the sum of the forces in that direction are 0, F=m*a, I have no acceleration in the center of earth direction. To be truly precise since forces are vectors and always sum around a body you can only move in one direction. You may have chosen to break the coordinate system into 3 directions but that is a notation issue not truly multiple directions.

Re:Deceleration

[waives]

jackass, the extra needed 'context' they are referring to is referenced in your own post: the current velocity.

Re:Deceleration

[Dunbal]

Velocity - another vector. Not a "context". Learn 2 physics.

Re:Deceleration

[Dunbal]

That's not correct. Mathematically your resultant acceleration is 0 because, as you pointed out, the forces are exactly opposite and equal. However the forces are there. You ARE accelerating both towards and away from the center of the Earth. Otherwise you have to start believing in magic - the magical earth "detecting" when you are no longer supported by a surface (and hence the normal force) and suddenly applying acceleration to you because of this.

While it's convenient to ignore the balanced forces because it makes the math easier and more legible, it doesn't mean that they're not there. They are ALWAYS there.

Re:Deceleration

[Anonymous Coward]

So how many meters per second per second is his velocity towards the center of the earth increasing?

Zero?

No acceleration, gotcha.

Force can cause acceleration, force does not equal acceleration.

Re:Deceleration

[Anonymous Coward]

"if you're floating in deep space with nothing around you"

CMBR, CMNR (although harder to detect)

"you can't tell if you have any velocity"

You look for anisotropies in the CMB or an angle-luminosity-redshift dipole for distant fixed objects...

Also, does one really "float" in space? On what? Quantum foam?

Re:Deceleration

[Altrag]

Only if you have the ability to leave a marker at that point. However, the marker you dropped off would be moving with approximately the same velocity you are, so you've lost the ability to measure velocity relative to the original object.

You might still be able to compute the velocity relative to the original object (just add up your velocity relative to the original object plus the velocity relative to the marker), but you can't measure it by using an object you yourself left in space.

Re:Deceleration

[Anonymous Coward]

Dammit, CNB (N=v) not "CMNR". Long day... The point is that unless you inhabit a de Sitter universe or have a completely obscured view, you almost certainly have a variety of frames of reference that seem like they would be commonly used by intelligent observers throughout the universe.

Re:Deceleration

[Altrag]

Only in theory. In practice, a real gravitational field is centered on a point, and you can use tidal forces to determine if such a point exists (and if you want, triangulate where it lies).

The theoretical concept that Einstein laid out is only true if you have a flat version of gravity. This can be approximated by simply having the gravitational field be so large relative to the sensitivity of the measurement device that the (truly spherical) surface appears flat. But all you need to break that scenario is a more sensitive measurement device.

Re:Deceleration

[ceoyoyo]

Sit down. Feel that on your ass? That's you detecting acceleration.

Re:Deceleration

[ceoyoyo]

Einstein tells us there's no such thing as gravitational acceleration. In general relativity the unaccelerated frame is the one where you fall freely under gravity's influence, NOT the one where you resist it.

Sitting on your couch, you are accelerating at 9.81 m/s^2. Yes it seems weird. Yes, it works out very well.

Re:Deceleration

[ceoyoyo]

Well, let's see. You can feel acceleration. If you pull down the window shades in a plane can you tell when it takes off? You could get a glass of water and observe the angle of the water in the glass (which is actually very appropriate because it's related to a famous thought experiment called Newton's Bucket). You could observe a swinging pendulum. You could use an accelerometer, solid state or gyroscopic, your choice.

There are LOTS of different ways to do it, and no, unfortunately, no Nobel prize.

Be very, very careful about taking a definition and making assumptions.

Re:Deceleration

[ceoyoyo]

The "if you're floating in deep space with nothing around you" is a thought experiment. Einstein put you in an elevator that you couldn't see out of. Newton stuck you in a universe with absolutely nothing in it, not even CMB. Or maybe you just don't have a handy radio telescope with you!

Quit trying to be clever and use your imagination.

Re:Deceleration

[ceoyoyo]

Ah, it's the know it all Slashdotter.

You might want to brush up on basic physics a little bit, or take a slightly more humble attitude.

Just because you've given an example of using a frame of reference to measure an acceleration doesn't mean you HAVE to have a frame of reference. Mistake #1.

Your example is actually wrong as well. According to general relativity, you are accelerating at this moment (in one direction, not "two directions at once"). That direction is commonly called "up." If you were to walk off a cliff, as you so generously suggested I do, then you would in fact NOT be accelerating. Surprising as it may seem to you, you would be experiencing unaccelerated motion while the rest of the planet accelerated towards you at 9.81 m/s^2. Yes, I know it sounds funny and your mastery of "basic physics" and "vectors" seems to tell you otherwise, but if you read a little bit more physics you'll be exposed to some of these funny ideas that seem to work out remarkably well.

Re:Deceleration

[ceoyoyo]

In front of me. Behind me. To my left. To my right. The coordinate system can be entirely defined by your own frame of reference and does not require another point.

Imagine you're in a completely empty universe. Nothing but you. Then you fire the rocket pack attached to your back. You'll know you're accelerating, and you'll be able to describe the direction, with no reference point besides yourself.

As another example, have you ever found yourself in pitch blackness? Did you have trouble figuring out where "down" was?

Re:Deceleration

[Anonymous Coward]

Acceleration needs a frame of reference? You mean general relativity is bogus?

Re:Deceleration

[waives]

learn to communicate like a human being - p.s. 'context' is not a meaningful physics term.

Re:Deceleration

[Anonymous Coward]

Deceleration means "a decrease in speed". If the probe is traveling directly away from the sun, and there are no other contributions to the probe's acceleration, a sunward acceleration causes a decrease in speed, and a decrease in speed causes a sunward acceleration. For deceleration to occur, you need the speed to actively decrease. If, for instance, there was a component of acceleration away from the sun overwhelming the sunward component, there would be no decrease in speed [as long as the velocity vector worked out correctly], so deceleration wouldn't make sense. I imagine the probe isn't traveling radially outward from our solar system, and that there are other contributions to the acceleration, so it's not clear to me if deceleration makes sense in this context.

You are misusing the term "causes". A decrease in speed does NOT "cause" a sunward acceleration.

Re:Deceleration

[Viperpete]

So you are saying that I can use an accelerometer in order to determine that bulk of mass in the cosmos is accelerating away from us? How do you propose to determine acceleration of an object that you do not have physical access to?

Re:Deceleration

[Viperpete]

So how many meters per second per second is his velocity towards the center of the earth increasing?

Zero?

No acceleration, gotcha.

Force can cause acceleration, force does not equal acceleration.

Precisely, the hangman put the noose on you and you have the force of gravity pulling you down, it is not until he opens the trap door that the force of gravity accelerates you downward to snap your neck.

Re:Deceleration

[Anonymous Coward]

Ok.

Einstein gave you a completely obscured view (although it's an open question about whether you could detect gravitational waves inside the elevator and whether one could erect a distance ladder using gravitational astronomy; this is interesting if you treat the elevator as an analogy to being surrounded by highly absorptive dust and gas such that optical and radio astronomy were largely unhelpful).

Newton gave you an approximation of a de Sitter universe. (So did Einstein in trying to resolve the hole argument).

My imagination tells me that if you have the technology with which to *be* in deep space in the first place, you probably have the ability to navigate using the awesomesauce view of the sky one would have there.

Additionally, you probably don't mean that "if you're floating in deep space with nothing around you" is a thought experiment, but rather that it's a deliberate simplification through the choice of an imaginary set of ideal conditions.

Your goal was to demonstrate that constant velocities are relative and that there is no preferred inertial frame of reference. Eliminating all[*] other points of reference does not really accomplish that, although it does serve to avoid considering the consequences of the universal invariance of c.

([*] Or practically all; how do you avoid an acceleration between you and the walls of the elevator in the Einstein example? Or is this the elevator which *is* accelerating, but you don't know whether it's because of gravitation or a thruster rocket, in which case velocity is not constant, and SR goes out the non-existing window :) [see also *** below])

Finally, I don't disagree with you that there is an enormous difference between being in free fall (or at a constant velocity in SR or Newtonian mechanics) in the weak field limit, and experiencing an acceleration. However the important -- or even obvious -- point isn't that you have no way to calculate relative velocities, or even that it's necessarily difficult to do so, but rather that you have *so many* things in view you can use to calculate your relative velocity, and that you might calculate a different velocity for each and every one of them. We say there is no preferred choice of reference because there is usually[**] no strong reason to choose any of them. (The *average* motions of elements of relic fields, however, seem like good choices for a variety of reasons).

([**] if you're on a collision course towards some object that will hurt you, you'd probably see that as a strong reason; we use "globally preferred" to avoid things like that, and then it's an open question as to whether the relic fields look much the same everywhere, which has been a biiig question since Dicke; if they do, on average, then they seem pretty globally preferable, but not to the point of making the choice of any other reference seem misguided -- moreover, the laws of physics behave the same to the elements of the relic fields (at least at and near t_0), so they are not *privileged* in that sense, at least in GR[***].)

([***] In SR and Newtonian mechanics, the set of all inertial reference frames are privileged because the laws of physics derived for inertial motion work in them (that's what's Special about SR) and are not guaranteed to work in non-inertial reference frames (i.e., in which there is acceleration)).

Re:Deceleration

[robot256]

Wait, what? No, that's not what I meant. It seemed the thread had diverged a bit from the OP, and your statement that you cannot "figure" acceleration without a reference point is patently false in the general case, so I took offense at your invocation of the Nobel Prize for something so simple. Forgive my juvenile use of sarcasm and capitalization, but I was merely defending a perceived slight to Newtonian physics.

I'm sure you remember from high school physics that there are two completely independent ways to compute acceleration: one using time and relative position, the other using force and mass. Any of those values can be directly measured in some cases, but in this case it is true that they are most likely tracking the satellite's position from afar and computing the force that would explain its trajectory over time. But if force and mass were measured, as they are in an accelerometer, and in fact could be on the spacecraft itself, no position reference would be required to compute your acceleration relative to the object's coordinate system.

I can go on to clarify that the statement "accelerating toward the sun" provides an external direction reference for the direction of the acceleration but not the velocity, leading once again to the conclusion that the word "deceleration" is inappropriate.

Re:Deceleration

[Viperpete]

You should reread the thread. It was not I who invoked the Nobel prize. I do not do ad hominems or taunts, I find rudeness interferes with the promulgation of knowledge.

Re:Deceleration

[CTachyon]

where the vector "something" is often "velocity"

Just to nit-pick, you mean "the direction of movement". Velocity also implies the magnitude as well as the direction, and I don't see why we need to bring magnitude into the argument.

No, I meant what I said. The noun phrase "<vector X> in the opposite direction of <vector Y>" makes sense for any vectors X and Y, even though it doesn't define a relationship between their magnitudes or otherwise mention them.

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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?

Re:It's the direction it is accelerating in

[Anonymous Coward]

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

To which an Intel Engineer would reply: 1.3337

Re:It's the direction it is accelerating in

[Anonymous Coward]

I think you mean accurate.

Re:It's the direction it is accelerating in

[Anonymous Coward]

It was a private contractor, not NASA, that screwed up the units.

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:VGer was a totally different design

[bcrowell]

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

I think you mean interstellar, not extragalactic.

The cost of Pioneer 10 was about $430 million in 2010 dollars. Since the Pioneer anomaly turned out to be a mistake, it is doubtful that it would be sensible to spend a similar sum on a follow-up. Furthermore, many of the systematic errors involved in measuring a spacecraft's trajectory come from parts of the tracking systems that are not aboard the spacecraft.

If the goal is simply to confirm by some independent technique that the effect is not gravitational, then that's already been done, and it didn't require spending hundreds of millions of dollars. Iorio has shown that if the Pioneer anomaly obeyed the equivalence principle, then we would see effects in the outer solar system that are not in fact observed.

Re:VGer was a totally different design

[nido]

I think you mean interstellar, not extragalactic.

Certainly, thanks for that. :)

it is doubtful that it would be sensible to spend a similar sum on a follow-up.

Did you know Explorer 1 was 12 minutes late? The twelve-minute hiatus of Explorer 1. I just found this response to that article, which concludes that something is indeed amiss...

Reading this article just confirmed my opinion that NASA is hiding something here. I don't think Hoagland knows what it is, but he is certainly correct that something is rotten. If the official story were true, then we would not have to see articles like this by Harris fifty years later, fudging equations and spinning furiously to create cover.
- http://milesmathis.com/pi4.html

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This is great news!

[lexsird]

You really have to love a mystery like this and especially when it's concerning something of this magnitude. The heat theory is seriously grasping at straws when they should be thinking outside the box on this one. Our human perceptive abilities are limited giving us naturally a skewed look at things. This might be a case of not seeing the forest for all the trees in the way.

Humor me please. The boys have done the long math and figured it wasn't from a gravitational effect. Groovy, but it's decelerating still, so WTF? Time to look at the really big picture. This probe is far enough out that it shouldn't be dicked with anything from the solar system. What is out there then? "Nothing" you say? I say there is no such thing as "nothing". Nature abhors a vacuum. We just can't see it.

I am talking about the fabric of space/time. If the probe is "catching" on the fabric, this is great news. This means we have a tangible effect from it that we can study. Considering if it's a kind of "fabric" while in the solar system where we have a sun and planets acting like an egg beater of sorts, trying to study the effects of the fabric would be like trying to study a fart in a tornado. But out in deep La La Land Space it's subtle effects become noticeable.

So, who cares? Effects are two way streets. If the fabric effects matter to the point of deceleration on our little pan-galactic hockey puck, this means it's tangible in some way. This opens up a big can of worms, meaning we can learn to effect it or just side step it, or fold it like a wash rag, who knows?

Consider this, if it is a space/time fabric decelerating it, it makes sense that it would feel the effects out of the solar system. Think of the egg beaters again, but upgrade that to Kitchen Aid blender and you are making icing. Where the beaters are whipping things the consistency of the mix is thin due to the agitation of the blades, but as you venture out from the blades the icing starts to take on a thicker consistency. This is why you have to move the bowl around or the blades around to speed up the process. If you just let it set, the current from it will eventually suck everything back into the beaters. This is probably the effect it's having on the probe. Sure it was easy to move through the mix while in the beaters, the icing was thin, but it's thicker away from the beaters and it has a current to it.

This is probably a great thing or else we would probably have a lot more crap whizzing through the universe, but if these things get stuck in the icing, they get sucked back into the beaters, burned up, etc. Anyway, I think this is great news!
 

Re:This is great news!

[Anonymous Coward]

Dear sir:

I regret to inform you that your attempt to be funny is overwrought and stupid.

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.

Re:This is great news!

[drinkypoo]

Science fiction is based on science; if it isn't, then it's fantasy.

Re:This is great news!

[lexsird]

Anyone who dares to dream or think outside the box must brace themselves for the snarky to angry remarks from the mental lemmings. Those who refuse to dream or think outside the box are doom to stay in it. Human intelligence is built upon previous works. We just don't have the lifespans to get very far alone. We have to pick up the torch via education, then run with it a ways further on our own. This helps progress us. If you haven't noticed, we need some progression. We have an overpopulated planet and all of humanities eggs in one basket.

Sorry, "but physics laws that we know is enough" aren't going to cut it. They weren't when we thought the world was flat, and they aren't going to cut it now. I know it's tough for some to still dream after the discipline of completing an education. It's a system that has fell in love with the sound of it's own voice, thinks that it's the high pentacle of knowledge, and it is the gatekeeper of high intellect. Sadly, it often reproduces, just what it is and nothing more. One can't blame the system completely, many just aren't suited for the task.

  Doesn't this hubris get popped like a balloon time after time in history? Thankfully yes, or else we would be still steering clear of the edge of the world.

There be monsters out there.

Re:This is great news!

[lexsird]

Thanks for not remarking like a condescending prick.

Pioneer is the first point of view that isn't taken from within our solar system. Besides, as far as being ruled out by these experiments, isn't the fact that they were conducted within the solar system's sphere of influence tainting them? Wouldn't we have to work out where Pioneer is with these experiments to really qualify them?

Also, pardon my 5 minute analysis of the presented information. A mystery such as what is up with Pioneer to me is exciting. If it's not reacting as predicted, this is always exciting, no? Even the "heat" theory could lead to something tangible. I am just the kind of person that if I think I see elephant toenails, I look for the entire elephant.

Re:This is great news!

[lexsird]

Ah yes, well lets hear your hypothesis? Or do you have the balls to put one out here? Or can you even formulate one? Even a extremely convoluted one? Anonymous Coward is rightly named.

Re:This is great news!

[Pieroxy]

Science fiction is based on science - check
Science fiction isn't science - check

Ok, we are in agreement here.

Re:This is great news!

[Pieroxy]

I'm not sure what your point is. I do agree with everything you wrote. I just think that we shouldn't spend too much time trying to find another explanation than the law of gravity to an apple falling off a tree.

Now, when we'll notice something that the law of gravity fails to explain exactly, then we'll theorize on another law, and I'm perfectly fine with it.

This is going ahead in my view. Trying to reinvent an existing theory based on nothing other than the will to reinvent it is fine. I just think we shouldn't spend too much time on that.

Re:This is great news!

[lexsird]

I am a habitual gun jumper, I will admit that. I like to leap on things, they tend to not squirm away as easy.

Ok, here is probably where I train wrecked. Pioneer is slowing down. That is a given, right? They are fielding this "heat" theory, but not quite 100% about it either, but it sounds good, right? Didn't the boys do the long math and say, "nope it's not perturbations of gravity?" Then didn't someone mention it was slowing at exponential rates? Exponential rates?? Wouldn't that mean its going to come to a stop soon then? Unless exponential has changed since I was a 5th grader, isn't that something to be a bit concerned with?

Granted, without doubt I am late to the party, but can I still ask questions and field an explanation of my own? Wacky and convoluted as it maybe, it still reminds me of that little lump of sugar that defiantly swirls around in the bowl, then gets sucked back into the beaters when I used to mix up home made frosting. Granted the model I present is goofy and doesn't represent the insane amount of variables and the dimensions and scope of it all. (Unless I drop the mixer in a giant bowl of frosting where it is suspended, running without the power cord to muck up the works.)

I thought it was something that we really didn't know and thus is an exciting opportunity to discover something fresh and new. Sorry, back to "meh" then.

Re:This is great news!

[marcosdumay]

"Then didn't someone mention it was slowing at exponential rates?"

Nope. Ok, somebody may have mentioned it, but he would be wrong. There are some good information at Wikipedia, the acceleration is 8.74(+-)1.33*10^(-10) m/s2.

Re:This is great news!

[waives]

I think the cause may be the gravitational influence of your balls, since you must have big brass ones to be repeatedly posting such drivel.

Re:This is great news!

[BJ_Covert_Action]

This probe is far enough out that it shouldn't be dicked with anything from the solar system.

You're wrong. The Pioneer is still within the Sun's gravitational sphere of influence. Good luck with your lunacy though.

Re:This is great news!

[lexsird]

I was married 13 years, /. flames bounce off my upper atmosphere after that kind of conditioning.

do you step on a car's decelerator pedal?

[decora]

technically, that's what you are doing. compared to some inertial reference frame, you are decelerating.

an easy frame would be to consider the earth, and consider that you drive from west to east. relative to its own axis, earth is spinning east to west. so, yeah. if you drive from los angeles to new york, what you are really doing is trying to 'decelerate' yourself for a couple of days in a row in order that new york can 'catch up with you'.

(yes i may have mixed east with west here... im too lazy to analyze it. just flip them if im wrong)

How does heat emission slow it down?

[Anonymous Coward]

What are the forces at work? Is it purely infrared radiation?

Re:How does heat emission slow it down?

[Dunbal]

Neutrons too, I imagine.

Heat to blame?

[aapold]

I knew it was LeBron's fault!!!!

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).

Re:Loss of mass

[robot256]

Actually, I'm more interested in the motive force produced by the radiation of all that energy in one direction. I forget the equations for the impulse imparted by a photon though, and we'd have to compare it with the force from the solar wind as well. Maybe I should RTFA and see if they did already.

Re:Loss of mass

[CesiumFrog]

That's 14 milligrams, not 13 micrograms. And it may not be negligible if the mass is ejected directionally.

Google "1 kW / c / 250kg". The momentum of 1kW of photons could accelerate the Pioneer by 10^-8 ms^-2, if it were all emitted in one direction.

The anomalous acceleration we want to explain is about an order of magnitude smaller than this figure. Hence, TFA.

Re:Loss of mass

[Dunbal]

Especially considering the distance. That (all over d^2) really has a major impact after a while...more than any petty decigrams of dust.

Re:Loss of mass

[treeves]

kW are not proper units for momentum.
In any case, the thing you're talking about is force, not momentum.
F (of the photon flux) = m (of the spacecraft) * a (of the spacecraft).
(I believe you know this, but for the sake of other physics-challenged /.ers...)
My question is, why would the photon flux have a preferred direction? Is the RTG shielded asymmetrically?

Re:Loss of mass

[Anonymous Coward]

Though E=mc2 does mean that a little mass is related to to a huge amount of energy (relative to everyday experience), the popular explanation that this is due to the speed of light being a huge number of meters per second is a fallacy. The size of the number is an artifact of the units we're using. If you express the speed of light in parsecs per second, it's not such a big number any more, but Einstein is still right. Don't overlook the units and dimensions and look just at the digits.

Not news to me

[tommy2tone]

They figured this out like 8 months ago...