The Answer to The Pioneer Anomaly - Unexpected Deceleration in Unmanned Spacecraft

The Answer to The Pioneer Anomaly - Unexpected Deceleration in Unmanned Spacecraft
Page content

Editor’s Note: This article was originally published June 25, 2009 by Ally Chevalier. It has been recently updated by RC Davison to include new information about the resolution of the Pioneer Anomaly.

The Pioneer Anomaly is as much a testament about the wonders of Nature as it is about the importance of paying attention to the smallest of details. It is also a good example of Occam’s Razor, paraphrased here: the simplest solution is the most probable.

What Is The Pioneer Anomaly?

We’ve launched a number of unmanned space probes out into the fringes of our solar system… but they haven’t behaved entirely as expected. As they’ve gotten further and further away from the gravitational influence of the Sun, they’ve been slowing down subtly, but more so than predicted.

The Pioneer anomaly was first detected with the radio signals that scientists use to track the movements of Pioneer 10, the first probe to visit the planet Jupiter. Since then, it has been detected on a variety of unmanned, far-out space probes, including Pioneer 11, Voyagers 1 & 2, Galileo, and Ulysses.

There is no universally accepted explanation for this slight slowdown, which vary from the mundane to the out-of-this-universe.

Previously Proposed Explanations for the Anomaly

Same Effect, Different Spacecraft

Many of the explanations for the slowdown depended on the specific spacecraft, for instance, on a gas leak or a malfunctioning thruster from the particular design of the particular spacecraft. However, the Pioneer anomaly has been observed by all such spacecraft, so having different explanations for the individual spacecraft for a single effect doesn’t make a whole lot of sense.

Observational Errors

Some people had put forth that there may have been some slight, constant errors made in measuring and computing the Pioneer anomaly. Computing errors don’t seem too likely considering that people have checked and rechecked these calculations over the years with this already in mind. However, the possibility existed that some of the Pioneer effect was due to measurement error. The various spacecraft are all difficult to track in their different ways, on top of their great distance and small size. Acquiring position and velocity data to the degree of accuracy that the Pioneer effect requires isn’t possible all the time for all the spacecraft because of noise in the transmission and other little things.

Real Deceleration

There were a number of explanations that insisted the Pioneer effect is really just real deceleration that scientists didn’t account for in their predictions. Some of these had more merit than others.

A common explanation focused on the belief that other gravitational influences on the spacecraft that we did not consider or know about, perhaps an undiscovered pocket of dark matter or a hefty new planet, caused the effect. However, seeing that other far-off objects, such as the outer planets, do not seem to be under the influence of any mysterious, unknown gravitational forces, this theory seemed unlikely.

Deceleration from drag by the interplanetary medium was another proposal. However, the measured density of this dust, solar wind, cosmic rays and the like was simply too small to make as big of an effect as the discrepancies required. Drag from radiation pressure was a similar explanation — and one that was also too small to account for much of the Pioneer anomaly. The pressure of sunlight is simply too small at so great a distance. However, some estimates suggested that as much as a third of the Pioneer anomaly might be due to radiation pressure and heating.

Hubble Constant & the Expansion of the Universe

The precise magnitude of the Pioneer effect happens to be rather close to the product of the speed of light and the Hubble Constant, by which the expansion of the universe is measured. This led to some speculation that the Pioneer effect might have something to do with the expansion of the universe. However, this idea could be dismissed when one observed that our local gravitationally bound objects—such as all objects in the solar system—are not included in this effect, as proved by theory and by direct measurement.

MOND: Modified Newtonian Physics

Some people had proposed, using everything from the Pioneer anomaly to reinterpretations of dark matter as supporting evidence, a few tiny alterations to the theory of gravity. MOND, or Modified Newtonian Dynamics, proposed that gravity changed on very low accelerations. This may also be interpreted as a modification to theories of inertia, applied to the Pioneer effect.

Tired Light

This final theory, the tired light explanation, was a little more “out there” than the others, but impossible to either accept or refute. This theory suggested that light will simply lose energy over time. For the Pioneer anomaly, this just results in a case of miscommunication between Earth and our spacecraft. For the rest of the universe… well, let’s just say that Big Bang didn’t happen quite according to mainstream theory. Tired light was not a commonly accepted theory, however, and had many errors, as pointed out here.

And the Answer to the Pioneer Anamoly is…

The cause of the anomaly is light pressure. Light in this case is infrared light—heat—generated by the radioisotope thermoelectric generators (RTG) used to power the probe and the electronics within Pioneer.

The RTGs radiated almost 2500 Watts into space at the beginning of the mission, and some of this infrared radiation bounced off the back of the large parabolic communication antenna that was constantly pointing toward Earth. This imparted a retarding force to the probe’s forward motion, and was about half of the force needed to slow down the probe to match the measured degradation in velocity. Note that this is the same affect that is used to propel solar sails through the solar system, like Japan’s IKAROS mission in 2010 ( These satellites move by nothing more than the force of the light bouncing off their reflective sails.

The other component of the retarding force came from the electronics within Pioneer. This heat energy was radiated out through louvers in the back of the probe, which pointed away from Earth. By Newton’s Third Law of Motion—for every action there is an equal and opposite reaction—this imparted a force toward Earth that added to the RTG’s force contributing to the slowing of Pioneer. The following picture shows the forces imparted to the probe by the infrared radiation emitted from the RTGs and electronics.

This conclusion was the result of hours of painstaking analysis of over 30 years of data from both Pioneer 1 and 2 by Slava G. Turyshev from JPL and Viktor T. Toth.

The analysis showed that there has been a slight decline in the slowing of the probe over the years, which correlated to the the RTGs losing power as their plutonium-238 was being depleted. This reduction in the speed wasn’t seen before in an initial study of the problem, which lead to a dismissal of the idea that the anomaly was related to thermal events. The analysis of the data was coupled with new 3D-modeling of the probe to assess its thermal properties and emission profile, and this lead to the realization that the on-board electronics contributed significantly to the overall force slowing the probe.

The end result is a lesson in subtlety. The smallest of forces, insignificant on Earth, can, over time, become a measurable disturbance in the vastness of space.


  • Published Paper:  Support for the thermal origin of the Pioneer anomaly
  • Picture credit: Image courtesy of NASA, annotations by RC Davison
  • “Finding the Source of the Pioneer Anomaly”: IEEE Spectrum, [](/tools/“Finding the Source of the Pioneer Anomaly”: IEEE Spectrum, http:/