Mysterious Pioneer Anomaly May Finally Be Solved
A flurry of recent papers could lay to rest once and for all a longstanding mystery in astrophysics: the so-called "Pioneer anomaly," an as-yet-unexplained deceleration of NASA's Pioneer 10 and Pioneer 11 spacecraft in their wanderings beyond our solar system.
NASA launched Pioneer 10 in 1972 and Pioneer 11 the following year, part of a mission to explore the asteroid belt, Jupiter, Saturn, and their respective moons.
Those "flyby" missions were completed within a few years, but the plucky little Pioneers kept going, eventually traveling beyond our solar system, carrying messages for any aliens they might happen to encounter along the way. And all along scientists at the Jet Propulsion Lab have been tracking their progress through Doppler radio transmissions.
The debate has been raging ever since an astronomer named John Anderson first noticed the anomaly in 1980. Anderson created an impossibly complicated algorithm so that he and other JPL scientists could use the radio transmission data to study gravitational effects in the outer solar system.
But it didn't seem to work. Or rather, he noticed a small discrepancy between the Doppler shifts predicted by his algorithm, and the actual shifts being measured in the radio signals coming from the Pioneer spacecraft.Dark Matter Drag?
The discrepancy is 10 billion times smaller than the acceleration due to gravity, but it was unmistakably there in Anderson's calculations. (The canonical number, for those who care, is 8.74 x 10-10 m/s2.)
What could be causing this discrepancy? Theories have abounded over the years. One popular hypothesis is that there are huge quantities of dark matter -- as yet undetected -- hanging around in the universe and exerting a little extra drag on the spacecraft, thereby slowing them down.
Another possible explanation is that gravity doesn't follow an inverse square law as formulated by Isaac Newton; this falls under the rubric of MOND (MOdified Newtonian Dynamics), the focus of a flurry of technical papers over the last 10-15 years. It's among the alternatives to dark energy to explain the fact that the expansion of our universe is accelerating.
Evidence for this hypothesis appeared to be bolstered in 1994, when Los Alamos cosmologist Michael Martin Nieto noticed that the value for the deviations of the Pioneer spacecraft was almost exactly equivalent to cosmic acceleration (the speed of light multiplied by Hubble's constant).
For Nieto, and others who noticed this strange coincidence, "The Pioneer anomaly could be the first evidence that gravity deviates from an inverse square dependence," he told Popular Science last year. "It could be huge."Radiating Heat?
On the more mundane side of things, there was always the possibility that the culprit might just be heat. Specifically, heat from the plutonium inside the spacecrafts' generators, some of which got converted into electricity while the rest of it radiated into space. If it did so unevenly, radiating more heat in one direction than in another -- only a 5 percent difference is required -- that might be sufficient to give rise to the Pioneer anomaly.
Okay, the heat explanation lacks the cool panache of exciting new physics, but the fact is that if dark matter or MOND is causing the anomaly, then the outermost planets' motion should be affected as well -- and so far there just isn't any observational data supporting that. Unfortunately, if heat were the culprit, then the effect should fade over time as the plutonium decayed, and when JPL scientists spent a few years investigating, they found this wasn't the case. So it was back to the drawing board yet again.
Still with me? Good, because now we're getting to the latest findings. First, there is the work of Viktor Toth (a software developer based in Canada) and JPL scientist Slava Turyshev, who wrote a comprehensive review paper in April 2010, claiming that, in fact, the most likely culprit was heat. As Natalie Wolchover wrote in her Popular Science article last December:
Using the telemetry data, the two scientists created an extremely elaborate “finite element” 3-D computer model of each Pioneer spacecraft, in which the thermal properties of 100,000 positions on their surfaces are independently tracked for the duration of the 30-year mission. Everything there is to know about heat conduction across the spacecraft’s surfaces, as well as the way that heat flow and temperature declined over time as the power of the generators lessened, they know.
The results of the telemetry analysis? “The heat recoil force accounts for part of the acceleration,” said Turyshev. They wouldn’t tell me how significant a part. (Turyshev: “We’d like to publish that in the scientific literature.”) But according to Toth, “You can take it to the bank that whatever remains of the anomaly after accounting for that thermal acceleration, it will at most be much less than the canonical value of 8.74 x 10-10 m/s2, and then, mind you, all those wonderful numerical coincidences people talk about are destroyed.”
And that's not all! Last year, Italian researcher Lorenzo Iorio published a paper on the arXiv adding further to the evidence that the Pioneer anomaly is not likely to be due to modified gravity, based on his analysis of the orbits of three moons of Neptune: Triton, Nereid and Proteus. None showed evidence of similar perturbations.
Finally, we come to the recent work by Portuguese scientist Federico Francisco and colleagues at the Instituto de Plasmas e Fusao Nuclear in Lisbon, claiming those original thermal calculations -- the ones that ruled out heat as a possible explanation for the anomaly -- were wrong. Yep, we're back to the heat theory again!
Specifically, Francisco et al. reworked the calculations taking into account not just how heat is emitted, but also how it gets reflected off various parts of the spacecraft (pictured here). Per Technology Review:
Previous calculations have only estimated the effect of reflections. So Francisco and co used a computer modeling technique called Phong shading to work out exactly how the the emitted heat is reflected and in which direction it ends up traveling.
Phong shading was dreamt up in the 1970s and is now widely used in many rendering packages to model reflections in three dimensions. It was originally developed to handle the reflections of visible light from 3D objects but it works just as well for infrared light, say Francisco and co.
In particular, Phong shading has allowed the Portuguese team to include for the first time the effect of heat emitted from a part of the spacecraft called the main equipment compartment. It turns out that heat from the back wall of this compartment is reflected from the back of the spacecraft's antenna. Since the antenna points Sunward, towards Earth, reflections off its back would tend to decelerate the spacecraft. "The radiation from this wall will, in a ﬁrst iteration, reﬂect off the antenna and add a contribution to the force in the direction of the sun."
When you do that, say Francisco et al., the anomaly magically disappears. His conclusion: "With the results presented here it becomes increasingly apparent that, unless new data arises, the puzzle of the anomalous acceleration of the Pioneer probes can ﬁnally be put to rest."
So, is the issue finally, truly resolved once and for all? I'm guessing only in the minds of some. In fact, I expect the debate will continue to rage on for a few more years now, as would-be challengers re-crunch the data and do their own calculations. Only after the latest analyses pass this rigorous scrutiny, can we truly say the debate has ended.