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Challenge Einstein, and you’d better be prepared to lose.
There was the recent case of the neutrinos that now appear not to exceed the speed of light. And now a puzzling quirk in the trajectories of two 1970s-era spacecraft appears not to challenge what we know about physics.
The Pioneer 10 and Pioneer 11 probes have been traversing the solar system for some 40 years. But as they venture outward, they’ve been slowing down a bit more than would be expected from the sun’s gravitational pull.
It was thought that the discrepancy, known as the Pioneer anomaly, could reveal a flaw in Einstein’s general relativity, the reigning theory of gravity. But a new analysis offers a more mundane explanation.
Pioneer 10 and 11 are slowing down due to a small-but-ever-present thermal recoil. Both spacecraft give off heat from their electronics and from the radioactive decay of their plutonium fuel, and that’s enough to impart the measured deceleration. The researchers liken it to photons from a car’s headlights pushing gently back on the vehicle. The analysis is in the journal Physical Review Letters. [Slava G. Turyshev et al., "Support for the Thermal Origin of the Pioneer Anomaly"]
So the Pioneer anomaly probably does not mean general relativity is broken. Score another one for Einstein.
—John Matson
[The above text is a transcript of this podcast.]
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22 Comments
Add CommentIt's a photon drive!
Reply | Report Abuse | Link to thisBut the thrust is a bit low.
But wouldn't they give off heat in all directions, so that the backward push would be counteracted by a forward one?
Reply | Report Abuse | Link to thisThe heat would be given off equally (isotropy) in all directions if Pioneer 10 was a sphere. Since it is actually asymmetric, the radiation is given off unevenly (anisotropy)with more heat being emitted in the direction away from the sun.
Reply | Report Abuse | Link to thisAlternatively, there is more mass in the Solar system than just the Sun. While the Sun contains 99.86% of total Solar system mass, its great distance from the Pioneer spacecraft significantly reduces it gravitational influence. Other objects with relatively miniscule masses are now much closer to the spacecraft an could impart more significant gravitational effects. One such set of objects that may impart consistent gravitational effects on the two spacecraft, perhaps as a composite object, is the Oort cloud...
Reply | Report Abuse | Link to thisKeep in mind that the Earth has much greater gravitational influence on our moon than does the Sun, despite its relatively enormous mass.
BTW, I'm very confident that Einstein's general relativity can successfully predict the observed motion of the two spacecraft - as long as the effective distribution of relevant masses are properly represented.
Reply | Report Abuse | Link to this
Reply | Report Abuse | Link to thisMy favorite crackpot explanation for the Pioneer anomaly was always that the heliosphere affected spacial density (ie, that space was slightly denser outside significant gravitational fields, or less creatively, that the cold and lack of radiation somehow mad even the vacuum slightly denser.)
Aren’t The Voyagers also showing this weird slowing down?
Is the anisotropic forward thermal radiation a sure thing, or just another guess?
As I suggested above, proposed explanations include http://en.wikipedia.org/wiki/Pioneer_anomaly#Gravity
Reply | Report Abuse | Link to this"It is possible that deceleration is caused by gravitational forces from unidentified sources such as the Kuiper belt or dark matter. However, this acceleration does not show up in the orbits of the outer planets, so any generic gravitational answer would need to violate the equivalence principle (see modified inertia below). Likewise, the anomaly does not appear in the orbits of Neptune's moons, challenging the possibility that the Pioneer anomaly may be an unconventional gravitational phenomenon based on range from the Sun."
The skepticism expressed above based on the effect not being detected in the orbits of the outer planets is likely invalid, since they are located within the Kuiper belt and, unlike the Voyager spacecraft, are encompassed by the nearly all Oort cloud objects. Please see http://en.wikipedia.org/wiki/Oort_cloud
It seems that the more we discover about the physical conditions of the Sun's domain within our galaxy, the more realize how wrong we've been. Please see http://en.wikipedia.org/wiki/Heliosphere
- which includes a recent "Gallery of out-dated models" along with more current information.
Reply | Report Abuse | Link to thisNope, it seems the Voyagers are not useful to study the same phenomenon. Retroboosters.
Wikipedia? We can do even better than that.
Let’s take a look at the reviwed paper:
http://arxiv.org/pdf/1204.2507.pdf
I’m not sure I buy it. Look at the key data, the graph on page 2. I am not sure the push’s forward-facing anisotropy is significant “Spacecraft exterior (blue −155◦ C, red −108◦ C)” Note the small contrast, [47◦ C]
compared to the sunlit antennas “Entire spacecraft (blue −213◦ C, red +136◦ C).” Like the author says, “The uncertainty of the thermal recoil force estimate is large.” Even if they were bouncing off the antenna, like some say. I think it is one scale of magnitude too large for that.
But I would go neither for the humdrum thermal explanation, nor the fringy gravitation-is-wrong, sci-fi, lookit-Einstein-poking-his-nose stuff, such as in:
http://www.sciencedirect.com/science/article/pii/S1875389211005888
Could the density of space be slightly greater? And this not affect heavy orbital objects? Beats me. It would however account for the tired-solar-wind phenomenon the Voyagers are getting, which is the closest corresponding phenomenon we are getting from the Voyagers:
http://science.nasa.gov/science-news/science-at-nasa/2006/21sep_voyager/
The only thing we can do, really, is keep an eye on New Horizons as it passes Saturn. The PA may be just a glitch (but it would be far more interesting otherwise).
No need to dismiss Wikipedia - you might have found the Turyshev et al letter's abstract and reference there - please see
Reply | Report Abuse | Link to thishttp://en.wikipedia.org/wiki/Pioneer_anomaly
section "Explanation - heat".
The Wikipedia entry, "Voyager_program" section "Voyager Interstellar Mission" states:
"Gyro operations are used to rotate the probe 360 degrees six times per year to measure the magnetic field of the spacecraft, which is then subtracted from the magnetometer science data."
However, it's stated in
http://en.wikipedia.org/wiki/Pioneer_anamoly#Indications_from_other_missions
"The Voyagers flew a mission profile similar to the Pioneers, but were not spin stabilized. Instead, they required frequent firings of their thrusters for attitude control to stay aligned with Earth. Spacecraft like the Voyagers acquire small and unpredictable changes in speed as a side effect of the frequent attitude control firings. This 'noise' makes it impractical to measure small accelerations such as the Pioneer effect; accelerations as large as 10^−9 m/s^2 would be undetectable."
For whatever reason the Voyagers are rotated, if only the resulting velocity 'noise' could be precisely identified the proposed thermal explanation could be tested, since the rotation events would alter the direction of any thermal acceleration.
Another Wikipedia reference,
Reply | Report Abuse | Link to thishttp://www.ihes.fr/~vanhove/Slides/iess-IHES-jan2011.pdf
(unnumbered page 21 of 53, "Pioneer anomaly - Facts") provided me with some new information: the 'unmodeled' deceleration only begins about 9 AU (as you mention, as a spacecraft passes Saturn's orbital radius - regardless of where Saturn might be in its orbit at that time). The deceleration increases significantly up to a distance of ~13 AU and then appears to generally stabilize out to the most distant observation shown at >45 AU. Their is no planet or significant mass whose orbit corresponds to ~13 AU, although planetary proximity to a spacecraft varies with orbital position as well as radial distance from the Sun.
However, the Voyager 10 mission used thrusters to reach Jupiter (5.2 AU), while Voyager 11 visited both Jupiter and Saturn (9.5 AU). While the "Facts" chart unfortunately does not distinguish between Voyager 10 & 11, it seems to suggest that the anomalous deceleration begins only after the thrusters are no longer used for navigational maneuvers. A nearly constant deceleration factor may have been erroneously attributed to thruster velocity changes prior to around 9 AU.
This leaves the questions of why the "unmodeled" deceleration is shown to increase significantly from about 9 AU to around 13 AU (it is also thought to begin at 20 AU), and what might explain the small variations in the generally constant velocity attributed to "Unmodeled" deceleration.
Assigning those factors to unexplained analytical errors, a thermal thrust internally produced within the spacecraft, preferentially directed by reflection off the back of its antenna could produce the otherwise constant "Unmodeled" deceleration.
Other potential causes such as space density or gravitation would almost certainly not produce a constant deceleration effect over a spacecraft traversal distance of more than 25 AU.
(continued)
Reply | Report Abuse | Link to thisThere is one other possible explanation - erroneous attribution of velocity changes produced by "Solar Pressure". My experienced eyeballs detect a likely correlation between the curvature of the "Unmodeled" deceleration curve and the rapidly diminishing effect of "Solar Pressure". If this apparent correlation is the product of a causal relationship, I suggest it is most likely an error in the measurement of spacecraft velocity and/or attributing acceleration/deceleration effects to modeled contributing factors.
If this error case is correct and NASA uses the same methodology to evaluate spacecraft distance and resulting speed for the New Horizons and other spacecraft, they will also appear to exhibit the effects of the erroneously identified "Pioneer Anomaly" - of similar magnitudes. I also understand that the New Horizons spacecraft may produce thermal radiation effects similar to the Pioneer spacecraft...
Yup, let's hope New Horizons doesn't need retroboosters going off near the Saturn orbit distance or planetary swinging, or anything, and can just coast smoothly in that part of the trajectory. It's our best shot at figuring this out.
Reply | Report Abuse | Link to thisThe "Pioneer anomaly" might be due to photon-photon attraction. Photons have gravity, and so as they travel great distances their mutual gravity keeps them from diverging, as they "should", so that they do not follow straight lines exactly.
Reply | Report Abuse | Link to thisThis photon-photon gravity effect also means that the universe is not expanding, and that the big bang is an illusion.
I'm merely a lay person, but I expect you base you conclusion that photons 'have gravity' on general relativity's confirmed prediction that light will be effectively curved when traversing spacetime distorted by sufficiently massive objects. Note that the mass of the Sun was necessary to produce a detectable effect on light from background sources. The 'gravitation' of the photons produced no detectable effect. I can't do the math, but I suspect that any gravitational effect produced by the effective mass of photons, even when they are not just 'ships passing (briefly) in the night' (i.e. exposed to each others effects only when they are exceedingly close to each other), is essentially non-existent.
Reply | Report Abuse | Link to thisPhotons exert "radiation pressure". Although photons do not have mass they do exert pressure, which is difficult to explain although this property enables them to affect other particles. Photons cannot escape a black hole, which means that gravity acts on photons. Logically, if photons are affected by gravity they must also generate gravity. This means that light exerts gravity, and so light acts like dark matter, and photon-gravity might explain part of what dark matter really is.
Reply | Report Abuse | Link to thisFrom Microsoft's Encarta Encyclopedia: "Relativity, however, holds that gravitational attraction depends on the energy of an object and that mass is just one possible form of energy. Einstein was proven correct in 1919, when astronomers observed that the gravitational attraction between light from distant stars and the Sun bends the path of the light around the Sun (Gravitational Lens)."
An explanation of why "they’ve been slowing down a bit more than would be expected from the sun’s gravitational pull" is that photon-photon gravity-distortion makes the spacecraft appear to be further out in space than they really are. Since they are actually going slower than their observed speed, their speed-change will be even more distorted.
Reply | Report Abuse | Link to thisThanks for explaining. I don't agree that it necessarily follows that, since gravity affects photons that photons must produce gravitational effects. I guess that photons' momentum/relativistic mass (if I'm using correct terms) explains why they are affected by very large gravitational fields, but even if their relativistic mass distorted spacetime wouldn't its effect be negligible? At any rate, GR explains the Sun's gravitational effects as a curvature in an abstract system of spacetime coordinates; that light merely follows a straight path through curved spacetime.
Reply | Report Abuse | Link to thisAs I understand, any photon-photon gravity distortion would be concentrated in the close proximity actual light wave as it propagates through spacetime. If I understand you, opposingly directed photons passing each other in the darkness would only be near enough to each other to produce any effect for an exceedingly brief time, over the distance of >45 AU...
I suppose I'm misunderstanding something...
A "quirk in the trajectories of two 1970s-era spacecraft appears not to challenge what we know about physics" is true. The challenge is making people understand that relativity holds that gravitational attraction between photons depends on the "energy of the photons" and not on their "mass".
Reply | Report Abuse | Link to thisPhotons coming to us from the spacecraft diverge less than they "should", so that the spacecraft appear to us to be further from Earth than they really are. Photons diverge less than they "should" because the photons attract each other due to the "energy of the photons", which causes them to attract each other with gravity. Since change in speed is calculated using change in energy divided by the spacecrafts momentum, the spacecraft's change in velocity appears smaller than calculated. Calculations must include photon-photon gravity to be correct.
As stated in http://en.wikipedia.org/wiki/Pioneer_anomoly
Reply | Report Abuse | Link to this"Upon very close examination of navigational data, the spacecraft were found to be slowing slightly more than expected."
This seems to imply that spacecraft distance is determined from some transmitted navigational data, not the luminosity or density/divergence of any emitted or reflected light.
Actually, photons coming to us from the spacecraft do not diverge less than they "should". This is because photons attract each other equally, so that their mutual attraction only tends to decrease their energy as they “fight” against their mutual gravity as the photons escape from each other, as they spread out into space. Therefore, the only change should be in the frequency of the radio transmissions from the spacecraft.
Reply | Report Abuse | Link to thisA red shift occurs from very distant objects in space. The red shift is due to photon-photon gravity, and not the expansion of the universe.
The sun emits photons that add to the total gravity of the solar system. That extra gravity might explain the extra slowing of the spacecraft.
My previous comment was intended to explain that the Pioneer distance determination is not based on any characteristics of the EM radiation received but from digital navigation data. However, the same Wikipedia entry ("Description" section) indicates that:
Reply | Report Abuse | Link to this"...it is possible to characterize the density of the solar medium by its effect on the spacecraft's motion. In the outer solar system this effect would be easily calculable, based on ground-based measurements of the deep space environment. When these effects were taken into account, along with all other known effects, the calculated position of the Pioneers did not agree with measurements based on timing the return of the radio signals being sent back from the spacecraft."
Also:
"As the anomaly was growing, it appeared that the spacecraft were moving more slowly than expected. Measurements of the spacecraft's speed using the Doppler effect demonstrated the same thing: the observed redshift was less than expected, which meant that the Pioneers had slowed down more than expected."
We can explain the "quirk in the trajectories of two 1970s-era spacecraft" by including photon gravity. All the photons that the Sun has emitted for the past year have gravity, gravity that acts upon the spacecraft more than normal since the spacecraft are so far away from the Sun.
Reply | Report Abuse | Link to thisFor example, if one measures gravity at the center of the Earth one would measure zero. Similarly, measuring gravity of all the photons that have left the Sun for a year will result in a sum of nearly zero, if measured here on Earth. But the gravity of all those photons will not be zero at the distant spacecraft.