The Pioneer Anomaly: Spacecraft or Spacetime?

By AJ Klesman

First off, you might be asking yourself, “What is the Pioneer Anomaly?” Basically, it’s this: there’s something strange showing up in the recorded flight paths of the Pioneer 10 and 11 spacecraft as they hurtle through (supposedly) empty space. To start at the beginning, the Pioneer spacecraft look like this:

See all 2 photos

The Pioneer spacecraft under construction on the ground.

Source: The Internet Encyclopedia of Science

The spacecraft weigh 259 kg apiece, or 571 lb - not exactly tiny by everyday standards, but hardly noticeable to the rest of the solar system. The missions were launched in 1972 and 1973, respectively, with one-way tickets out of the solar system. They were given opposite trajectories with respect to the galactic center, meaning Pioneer 10 is heading away from the center of the Milky Way and Pioneer 11 is heading towards it. Traveling at 11-12 km/s, they travel about 2 AU per year, or twice the distance between the Earth and the Sun. As of 2003, we’ve actually lost contact with the spacecraft entirely, and now we’re left to analyze all our recorded data.

Actually, let’s stop for a minute and define “the solar system” - because I’ll bet it’s a lot bigger than you think! The current picture of the solar system basically looks like this:

The inner and outer solar system.

Source: NASA/JPL

It’s not just the planets that make up the solar system - there’s also a ring of rocky, icy objects called the Kuiper Belt (which contains dwarf planets bigger than Pluto) and the spherical Oort Cloud, which encloses the solar system and is where most of our long- period comets come from. Anyway, all of that is fine, now that we know what’s out there scientists can calculate how much the sun and the gravity from all of these other objects would affect the Pioneer flight paths, right? All is well.

But wait - all is not well! There’s a discrepancy: in looking at the tracking data from both the Pioneer 10 and 11 spacecraft, scientists have discovered that both are experiencing an unexplained and constant acceleration of ∼8.74 ± 1.33 x 10⁻¹⁰ m/s² towards the sun. That means if you were to calculate where the spacecraft should be at the end of one year and then actually measure its position, you would find it to be almost 400 kilometers closer to the sun than you expected! Stranger still, the magnitude of this effect is very close, numerically, to what you would get if you multiplied the speed of light by something known as the Hubble Constant, which measures the expansion of the universe. But objects within the solar system aren’t really part of the expansion of the universe - the scale is too small for it to matter. In fact, the scale of the whole Milky Way, which is 100,000 light-years or 9 x 10¹⁷ km, is still too small for it to matter! It’s only when we start talking about galaxies and the vast distances between them that the expansion of the universe really starts to become important. So then is this number important, or is it a coincidence? The current answer is, perhaps frustratingly, we don’t know.

Of course, scientists have been trying to work out what is causing this anomaly. Obvi- ously the first conclusion was that it was a tracking, computer, or calculation error. But this acceleration has been confirmed by seven separate groups to date - that makes it pretty unlikely that it’s a mistake somewhere in the works. What about the possibility that there’s something wrong with both spacecraft? Gas leaks (such as helium found in the spacecrafts’ generators) have been suggested, as well as electromagnetic forces that might result if the spacecraft had somehow become charged. What about a heating differential, due to the fact that the side of the spacecraft pointing towards the sun is hotter than the side that’s not? This could account for up to one third of the effect, if it’s true. But investigations into whether it’s caused by something on the spacecraft have been inconclusive, and most scientists believe this acceleration is being caused by some force outside the spacecraft; therefore, we want to know what it is, because if it’s an outside force then it’s something we’ve missed up until now. Learning what’s causing it will tell us something about the neighborhood of the solar system, which is important to astronomers, as that’s pretty close to home!

So what could be affecting the spacecraft from the outside? It could be some kind of drag due to collisions with dust particles, the solar wind (which is really a stream of particles emitted by the sun), or cosmic rays. But the densities of these things should be too small to matter to the Pioneer spacecraft. We know that all radiation can actually exert pressure on objects (called, predictably, radiation pressure), but not only should it be too small at the distances of the Pioneer spacecraft from the sun to matter, but it would push the spacecraft away from the sun, not pull it back towards us. And it’s probably not something like gravitational pull from the Kuiper Belt or any dark matter floating around, because we’d see the same effect in the orbits of the outer planets and we don’t - and we’ve measured their orbits pretty precisely, using orbiting and flyby spacecraft measurements.

But if it’s not any of these things, what else could it be? People have proposed a number of explanations that invoke some new law or effect of physics that we haven’t described yet, such as a clock acceleration between time on the Earth and time on the moving spacecraft (a little like relativity, only something that might occur at speeds much less than that of the speed of light). Another explanation invokes something called MOND, which stands for MOdified Newtonian Dynamics, which says that gravity acts differently at very low accelerations ∼1.0⁻¹¹ m/s² from the traditional Newtonian value. MOND can also be described as a modification of inertia, which simply put is how well an object can resist changes in its motion. Maybe there’s some kind of interaction with vacuum energy that’s changing the spacecrafts’ inertia. (Now we’re really getting into some interesting terms: vacuum energy is defined as the “underlying background energy that exists in space even when devoid of matter.” It’s due to something called virtual particles, which I’m not going to explain here but you can certainly do some research of your own if you're interested.)

There are a couple other theories as well, mostly involving photon energy and how it changes depending on how we think the universe might be shaped or how it might be expanding. All of these “new physics” explanations are basically dependent on the model we choose to use to describe the universe - something that scientists are definitely not unanimously agreed on, even today. While there are a lot of things we can measure, there are a lot of different theories that can work with the measurements we do have to produce the universe we see today. Sometimes the differences between models are very small, and our observational errors are still too large to definitively discriminate between them. And because we only have one universe to measure, it actually makes it hard to say a lot of things for sure!

Whatever the case, the Pioneer Anomaly still remains unexplained - while things like tracking errors have been ruled out, it may be something as simple as a gas leak or heat differential on the spacecraft, or it may be something as complicated as a new concept in physics that we haven’t incorporated into our general picture of the universe just yet. It’s pretty interesting stuff, and we haven’t even really left the solar system yet!