## Heat 'Most Likely Cause' of Pioneer Anomaly 133 133

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."*
## d00d (Score:5, Informative)

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

## Re:d00d (Score:2, Informative)

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 (Score:5, Informative)

## Re:Deceleration (Score:0, Informative)

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:If you can think of it... (Score:2, Informative)

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:Deceleration (Score:4, Informative)

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 [slashdot.org]

## Re:Deceleration (Score:4, Informative)

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'sframe 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

somethingaround that could be used as a reference point (but you have to be able to find it to use it!)## Re:Deceleration (Score:0, Informative)

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).