The Pioneer Anomaly

The Pioneer anomaly, or Pioneer effect, has been puzzling scientists for decades—and has had some pretty interesting explanations to go along with it, from radiation pressure to changing the very known laws of the universe. Here's an overview:

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.

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The Pioneer anomaly was first detected with the radio Doppler 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.

Many of the possible explanations for the slowdown depend 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.

Some people have put forth that there has been some slight, constant error made in measuring and computing the Pioneer anomaly. Computing errors don't seem too likely considering that people have checked and rechecked this calculations over the years with this already in mind.

However, there is a possibility that some of the Pioneer effect is 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.

There are a number of explanations that insist that the Pioneer effect is really just real deceleration that scientists didn't account for in their predictions. Some of these have merit, some don't.

That there might be 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, is a common explanation. 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 seems unlikely.

Deceleration from drag by the interplanetary medium is another proposal. However, the measured density of this dust, solar wind, cosmic rays and the like is simply too small to make as big of an effect as the discrepancies require.

Drag from radiation pressure is a similar explanation—and one that is 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 have suggested that as much as a third of the Pioneer anomaly might be due to radiation pressure and heating.

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 has led to some speculation that the Pioneer effect might have something to do with this expansion of the universe. However, this idea can be dismissed when one observes 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.

Another rule-breaking theory: some people have 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, proposes that gravity changes on very low accelerations. This may also be interpreted as a modification to theories of inertia, and then applied to the Pioneer effect.

Another theory that's a little more out there, but impossible for the moment to either accept or refute, is the tired light explanation. The Pioneer effect is measured by the Doppler effect and the radio frequency, with acceleration either appearing as an unexpected blue or redshift. This theory suggests 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 is not a commonly accepted theory, however, and has many errors.

...we don't know! In all likelihood, the Pioneer effect is caused by a variety of small things that we just haven't accounted for or thought up quite yet. Observational errors and drag from radiation pressure, two of the possibilities discussed, could together account for a considerable fraction of the Pioneer Effect. However, there's no clear, complete answer yet agreed upon. Stay tuned on the Pioneer Effect for something a little more definitive. A good place to keep up on news regarding Pioneer anomaly theories is the Planetary Society's Pioneer anomaly project webpage.