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Bug NASA Space Science

Heat 'Most Likely Cause' of Pioneer Anomaly 133

Posted by timothy
from the that-dog's-not-so-shaggy-after-all dept.
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."
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Heat 'Most Likely Cause' of Pioneer Anomaly

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  • d00d (Score:5, Informative)

    by Mana Mana (16072) on Tuesday July 26, 2011 @01:01AM (#36880182) Homepage

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

  • by Mick R (932337)
    VGer poot'd. Must have been a relief.
    • by nido (102070) <nido56.yahoo@com> on Tuesday July 26, 2011 @03:16AM (#36880712) Homepage

      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 [planetary.org]

      • by bcrowell (177657)

        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,

        • by nido (102070)

          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 [thespacereview.com]. 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 equatio

  • by Johnny Mnemonic (176043) <mdinsmore.gmail@com> on Tuesday July 26, 2011 @01:50AM (#36880362) Homepage Journal

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

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

    • by u38cg (607297)
      I'd imagine it's an artefact of vector analysis.
    • by Sycraft-fu (314770) on Tuesday July 26, 2011 @02:29AM (#36880530)

      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.

      • 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:Deceleration (Score:4, Informative)

      by Viperpete (1261530) on Tuesday July 26, 2011 @02:29AM (#36880534) Homepage

      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]

      • 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 [wikipedia.org]

      • by ceoyoyo (59147)

        Acceleration doesn't require a point of reference.

        • by Dunbal (464142) *
          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.
          • by ceoyoyo (59147)

            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 wa

    • 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 vecto

    • by CTachyon (412849)
      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 v
      • by Dunbal (464142) *

        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.

        • by CTachyon (412849)

          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.

    • 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 analyz

    • by Lord Crc (151920)

      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 [wikipedia.org].

  • I knew it was LeBron's fault!!!!
  • by necro81 (917438) on Tuesday July 26, 2011 @08:54AM (#36882224) Journal
    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 [wikipedia.org], 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.
    • by mark_osmd (812581)

      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).

      • by CesiumFrog (41314)

        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.

        • by treeves (963993)

          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?

      • by Dunbal (464142) *
        Especially considering the distance. That (all over d^2) really has a major impact after a while...more than any petty decigrams of dust.
    • by robot256 (1635039)
      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.
  • They figured this out like 8 months ago...

What this country needs is a good five dollar plasma weapon.

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