Everyone loves a good mystery. And, some science mysteries are so strange that they take on legendary status.
The so-called Pioneer Anomaly — which at first seemed to challenge the laws of physics — is a case study of when it’s best to bank on the simplest explanation for even the weirdest of observations.
Small, yet odd perturbations in the velocity of a pair of Pioneer spacecraft have spawned numerous science papers and conference discussions over the past two decades. In the end, it looks like the solution is rather mundane.
Nevertheless, the Pioneer spooky story became a magnet for exotic as well as plain kooky ideas. Commentary in some discussion groups have even tried to link it to Earth’s Ice Ages, and an ad hoc idea called “fractal gravity.” Creationists have glommed onto the mystery to try and demonstrate that “secular” scientists are wrong for ignoring so-called biblical cosmology.
The pair of Pioneer spacecraft, launched in the early 1970s to explore the outer solar system, are among an exclusive NASA fleet of five robotic “starships” that are moving fast enough to escape the sun’s gravitational pull and drift through our galaxy forever.
Pioneer has even made a cameo appearance in a Star Trek movie when the evil Klingons find it in interstellar space and shoot it for target practice.
Now over 7 billion miles from Earth (10 light-hours) Pioneer 11 and 12 serve as “test particles” for measuring the effects of gravity on manmade objects over very large distances. Such a test has never before been possible.
In the 1980s several research teams independently measured what was interpreted as an infinitesimal deceleration of both Pioneers, which are streaking away from us in nearly opposite directions. The amount was inconsequential by engineering standards, but a huge discrepancy in predictions made by the laws of gravity.
The direction of the anomalous force had also come under question: is it really in the sun’s direction, or Earth’s, or along the spacecraft’s spin axis or velocity direction?
Scientists began toying with the exotic theories for explaining the anomaly. Perhaps the laws of gravity needed to be modified. Was dark matter in our local neighborhood tugging on the Pioneers? Or did it have an even deeper implication for cosmology? One idea was that a localized blob of dark matter could be trapped in the sun’s gravitational field. Any effects from dark energy would be way to small for explaining the Pioneer motion.
The peculiar speed difference is nearly equal to the value calculated (in the same units) by multiplying speed of light by the expansion rate of the universe. Without any clear causal link, the mathematical tie can best be dismissed as coincidence. It’s just pseudo-scientific numerology. For example, the ratio of the perimeter to twice the altitude of the Great Pyramid of Ghiza is equal to the mathematical value for Pi. The height of the pyramid multiplied by 100 million yields the distance from Earth to the sun. So what?
Creation scientist, Russell Humphreys, has written extensively that the Pioneer Anomaly bolsters biblical scripture by demonstrating there really is a center to the universe, and that Earth must be near it. He reasons that the starbound Pioneers are being pulled back to the center of the universe, like a hiker struggling to climb up a steep slope. He envisions the space-time fabric of an 8,000 year-old universe relaxing like a worn bed mattress, and the Pioneer velocity change reflects this. However, a century’s worth of cosmological observation demonstrate that the universe has no center, and the idea is anti-Copernican to boot.
Simple explanations for the Pioneer Anomaly, dating back to the late 1990s, looked at non-gravitational forces produced by the spacecraft itself due to thermal and electrical sources. Heat from Pioneer’s electronics is 100 Watts. The heat from the radioactive plutonium-238 power source on Pioneer outputs 2.5 kilowatts. The nuclear “battery” is on a boom extending from one side of the 550-pound spacecraft. This would cause “thermal recoil” as one side of the vehicle was slightly warmed (thermal model above).
Slava Turyshev of NASA’s Jet Propulsion Laboratory proposed the recoil theory several years ago. Since then he has extract more archival data from Pioneer’s tracking. The smoking gun, as described in a recently published paper that the data show a drop in Pioneer’s anomalous motion. This is exactly what would be predicted if thermal heating is the culprit. The 10 pounds of plutonium aboard Pioneer cools as it decays exponentially.
Both Pioneer 11 and 12 would show exactly the same anomaly because they are identically built. But what about testing other spacecraft?
The New Horizons probe blazing its way to Pluto should also have peculiarities due to heat from its nuclear power source, though its tracking is not a precise as for the Pioneers. The two Voyager spacecraft are less sensitive to the effect seen on Pioneer, because their thrusters align it along three axes, whereas the Pioneer spacecraft rely on spinning to stay stable. Other solar system spacecraft are in the wrong orbit, have larger nuclear power sources, and do frequent maneuvers.
“For the foreseeable future Pioneer 10 and 11 remain the largest scale precision gravitational experiment ever conducted,” wrote Victor Toth (Perimeter Institute for Theoretical Physics, Waterloo, Ontario Canada) in 2011. “Far more likely this (Pioneer anomaly) was just a wild goose chase.”
Lessons learned are that there are limits to our tracking and navigational accuracy, it is critical to archive long-term data on spacecraft, and estimates of small forces acting on a spacecraft really need to be precisely done.
The Pioneer Anomaly is a case study where the easiest explanation for a phenomenon is usually the correct one, and there is no need for overturning the laws of physics.
This is a time honored maxim call Occam’s Razor: when you have two competing theories that make exactly the same predictions, the simpler one is the most likely solution.
Nobody said that all strange mysteries we uncover in the universe need to have, well, strange answers.
Finally, to borrow from Henry Wadsworth Longfellow:
Image credit: NASA, S. Turyshev