Pioneer spacecraft a step closer to being boring (APS April 2008)
April 13, 2008 | 9:51 am
For many years, scientists have known that the Pioneer spacecraft have not been exactly where they thought they should be. Each year the spacecraft falls behind where it should be by about 5000 km. The spacecraft seem to have been undergoing a very small acceleration toward the Sun and, so far, scientists haven’t been to explain it.
Explanations for this have ranged from the prosaic (heat is being radiated into space and providing an acceleration) to the speculative (gravity might not act the way we expect). One thing is certain. The debate about the cause of the Pioneer anomaly, as it is known, has been raging for years.
Newly released telemetry data, incorporating over 100 measured properties including the temperatures of many points on the spacecraft, have been released. At the APS April meeting in St. Louis, Slava Turyshev from the Jet Propulsion Laboratory described his group’s efforts to build a very detailed computer model of the spacecraft geometry and heat flow, and showed the comparison of the model to the new data.
Their model manages to match the measured temperatures of Pioneer to within 3 degrees Celsius at every measured point, which Turyshev seemed extremely pleased with. Having a good thermal model meant that the scientists could start to really ask whether thermal effects could account for the anomaly.
Indeed, when Turyshev’s team calculated the emissions from the Pioneer spacecraft, it found that heat is given off in some directions preferentially, enough to account for 28-36% of the anomalous acceleration.
So what does this result now mean? It weakens some previous claims that the thermal emissions weren’t significant, an argument that not many physicists really believed anyway. However, there is still two thirds of the anomaly to account for? Could there be mysterious physics hiding in the gaps? Turyshev thinks that there is a lot more to take into account such as whether the optical properties of the spacecraft have changed over time-perhaps there is a layer of dust on some surfaces now, for example.
In May, new data about the speed of the spacecraft will be released and that could further clarify the situation, or just add to the debate!
I find the fact that this argument has received so much attention quite amusing. After all, nobody is going to really believe that the laws of physics are different, based on interpretation of Pioneer’s flight. And the immense amounts of work that have to go into trying to model the system properly is quite incredible. Scientists need to dig out information from decades ago to try to get everything they need and there are a lot of uncertainties. Turyshev quipped, “It’s like being on CSI.”
The exercise is certainly improving scientists modeling skills, which could then be used for much more practical purposes like building structures or vehicles on earth. It could even be quite useful in future space missions, although the problem will always be much easier in the future as the engineers will have better data about any spacecraft they send up.
Perhaps the story just reflects human’s unending fascination with the exploration of space and a desire to be part of that exploration, in whatever form it can take.
See all posts from the American Physical Society April 2008 conference here.
Note: This post has been edited since it was first posted to correct the distance Pioneer is falling behind each year.
David Harris
Posted in APS April 2008 |
April 13th, 2008 at 2:59 pm
Surely the “1 millimeter out of place” bit is a misprint? I’m fairly certain we don’t have the ability to measure distances to that precision out past the edge of the solar system. Furthermore, according to this article from last year:
http://www.planetary.org/programs/projects/pioneer_anomaly/
they are falling behind by roughly 5,000 km/year.
April 13th, 2008 at 3:04 pm
Quite right, Mike. That was a pretty bad slip I put in there! I must have had too much LHC physics in my mind where 1 mm is a long way! I’ve corrected the story. Thanks for catching me!
April 13th, 2008 at 5:07 pm
Hey, no problem! As a former Fermilab employee (Computing Division 1991-1996, worked on the ACPMAPS project), I always click when I see a link that mentions the lab. Fascinating stuff. In many ways, I still miss working there…
April 14th, 2008 at 1:01 am
Al the work on the anomaly makes a nice point about falsifiability. Especially in space, away from all those butterflies flapping their wings and messing up sensitive experiments, physics is supposed to WORK, damn it.
But I like to think it’s justified because of a comment Isaac Asimov once made: “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’ (I’ve found it!), but ‘That’s funny…’”
I think that describes the situation here quite nicely.
April 14th, 2008 at 3:31 am
Isn’t it the case that the anomalous acceleration is seen in both Pioneers? I know they are basically identical, and so a design-based explanation would likely be equally applicable to both, but I just wanted to clarify what the new evidence shows.
April 14th, 2008 at 7:36 am
You say that “after all, nobody is going to really believe that the laws of physics are different”…
Well believe it or not, if I dare say: in the last call for mission ideas from the European Space Agency here, we indeed found one dedicated “Pioneer Anomaly Mission” proposal, along with two or three others that featured dedicated “Pioneer” measures as “valuable” add-ons. (out of some 70 total IIRC)
Needless to say all of these were filtered in the complex, peer-reviewed elimination process that led to the present set of tentative ESA missions; still, it tells a lot on the (poor) level we are now in physics, er, credulity level…
Hervé
April 14th, 2008 at 8:29 am
Willis: The anomalous acceleration is seen in both Pioneers and possibly in other long-range spacecraft. However, it is too hard to determine whether there is an unexplained acceleration in those other spacecraft. So for now, the studies essentially have to be conducted with the Pioneers.
The anomalous acceleration is about one ten-billionth of the gravitational acceleration at the earth’s surface so we are talking about a tiny tiny effect, and a significant number of unknowns about the condition of the spacecraft.
April 14th, 2008 at 8:34 am
Hervé: The idea certainly has a lot of appeal to some people, but I still think that the physics community will not believe that there is some exotic physics going on solely based on the Pioneer anomaly remaining unexplained. It doesn’t surprise me that there are proposals for investigating the anomaly. It is a low probability, high payoff experiment. Given that the more detailed the analysis gets, the more of the anomaly disappears, I wouldn’t want to bet on new physics at all.
But is it really worth the cost of doing it when that would mean a lot of other more important science couldn’t be performed? I suspect we probably agree on the answer to that.
April 18th, 2008 at 3:24 am
[...] Das hat jemand bei der APS-Konferenz berichtet, einen Bericht
April 19th, 2008 at 4:31 am
Harris;
Since you are a science reporter, you might point out that the Pioneers are spin-stabilized, making them excellent test platform for probing gravity over long periods, unlike the Voyagers.
Now, as for the tenor of this report, I find it astonishing, particularly comments made on behalf of “the physics community”. Those people in fact believe string theory - “believe” I say, because there is no theory to know - here, we are talking about real numbers from a real thing regarding a real aspect of nature. The Pioneer anomaly is not only real, it is probably the most interesting thing to come out of observational gravitation since the Mercury perihelion anomaly, which as we know was finally resolved by general relativity.
I will not ever bother mentioning the relevant papers - find them yourself. Suffice to say that this one report makes not the slightest difference as to the reality of this effect. After what was done with, and to, the WMAP data, these people are not to be trusted.
-drl
April 19th, 2008 at 5:10 am
Danny,
I could indeed point out that the Pioneers are spin-stabilized but I’m not sure what the point is. They are better than the Voyagers, true, but the orientations of the Pioneers are really not all that well known. It is one of the sources of uncertainty in the measurement, as pointed out in the conference session by the speaker.
“The physics community” doesn’t believe in string theory. There are some physicists who think it is the best candidate for quantum gravity and the dedication of some to the theory could be called belief. But the whole physics community? No way. There are aggressive and common debates on this topic within physics.
You claim the Pioneer anomaly is real, yet you provide no evidence for that claim. The anomaly is real in the sense that is is unexplained for now, but do you mean it is a sign of new physics rather than unexplained effects? For some reason you seem more willing to believe that the anomaly is a sign of new physics but want to ignore studies about it, like the one discussed here. That doesn’t seem to be a very scientific attitude, so if you have extra evidence that members of physics community doesn’t know about it, I’m sure they’d be keen to hear it. But dismissing studies that provide more information doesn’t seem like a strong argument for supporting the existence of new exotic physics.
The main conclusion of the talk was that the more the anomaly is studied, the more it seems that the unexplained effect can be accounted for by non-exotic means. The speaker himself pointed out that one of the largest factors still hasn’t been taken into account: the changing optical properties of the Pioneers as they travel. This could be enough to explain the anomaly quite well.
As for your comment about the WMAP data, I’m not sure what you are talking about. Care to explain?
April 19th, 2008 at 9:26 am
Harris,
Read the paper by Anderson, Lang et. al. on the archive as gr/0104064 (extensive bibliography at the end). Some of these people, as well as those whom they consulted, were on the original Pioneer science and engineering teams - no one in the world knows the spacecraft systems as well as they do. The most critical factor in rejecting the heat flow hypothesis as the origin of the anomaly is that the decay of the on-board supply of fuel in the RTG should have been seen by now, and is not.
Furthermore, the authors did not, as you put it, claim that thermal emission was “not significant”. On the contrary, they analyzed the problem very thoroughly, and stated flatly among their conclusions that “it is hard to resist the notion that [RTG thermal emission] must somehow be the origin of the effect”. Also, the debate is not “raging” - real science is about neither fashions nor fads, and those who do it are not given to “rages”.
But the most interesting statement, and in hindsight the one that rings most true, is this, in reference to a possible cosmological origin of the anomaly - “This possibility necessitates a cautionary note on phenomenology: At this point in time, with the limited results available, there is a phenomenological equivalence between the aP and at points of view. But somehow, the choice one makes affects one’s outlook and direction of attack. If one has to consider new physics one should be open to both points of view. In the unlikely event that there is new physics, one does not want to miss it because one had the wrong mind set.” It is precisely this “wrong mind set” that, these days, has allowed theoretical particle physics to be dominated by a non-theory that is patent nonsense as science, and in past days, caused Poincare and Hilbert to miss relativity, Lorentz to indulge himself with deformable electrons, etc. etc. The Mercury perihelion anomaly was itself tiny almost beyond measure - 43″ per century. Almost, but not quite - and some people trusted their instruments more than their models, and believed what they were being told.
Today, it seems that instruments can never be trusted - that instead, we must ignore what is real in favor of what is fashionable. The last thing people need now, it seems, is new physics, when the existing models at all scales are already under attack from so many directions. Every time some new effect is discovered, whether this tiny anomaly painstakingly recorded and reduced, or the clumping of galaxies, or the essential role of non-linearity in general relativity, the orthodoxy are all too ready to leap on the heretics while they apply patch upon patch to the decadent models. The real issue in physics is one of mind-set - one must know history, and have faith in his own eyes and brain.
As for the Pioneer anomaly, paraphrasing someone - “and yet it accelerates”.