A joint Fermilab/SLAC publication

Pioneer spacecraft a step closer to being boring (APS April 2008)


For many years, scientists have known that the Pioneer spacecraft have not been exactly where they thought they should be. Each year the spacecraft falls behind where it should be by about 5000 km. The spacecraft seem to have been undergoing a very small acceleration toward the Sun and, so far, scientists haven't been to explain it.

Explanations for this have ranged from the prosaic (heat is being radiated into space and providing an acceleration) to the speculative (gravity might not act the way we expect). One thing is certain. The debate about the cause of the Pioneer anomaly, as it is known, has been raging for years.

Newly released telemetry data, incorporating over 100 measured properties including the temperatures of many points on the spacecraft, have been released. At the APS April meeting in St. Louis, Slava Turyshev from the Jet Propulsion Laboratory described his group's efforts to build a very detailed computer model of the spacecraft geometry and heat flow, and showed the comparison of the model to the new data.

Their model manages to match the measured temperatures of Pioneer to within 3 degrees Celsius at every measured point, which Turyshev seemed extremely pleased with. Having a good thermal model meant that the scientists could start to really ask whether thermal effects could account for the anomaly.

Indeed, when Turyshev's team calculated the emissions from the Pioneer spacecraft, it found that heat is given off in some directions preferentially, enough to account for 28-36% of the anomalous acceleration.

So what does this result now mean? It weakens some previous claims that the thermal emissions weren't significant, an argument that not many physicists really believed anyway. However, there is still two thirds of the anomaly to account for? Could there be mysterious physics hiding in the gaps? Turyshev thinks that there is a lot more to take into account such as whether the optical properties of the spacecraft have changed over time-perhaps there is a layer of dust on some surfaces now, for example.

In May, new data about the speed of the spacecraft will be released and that could further clarify the situation, or just add to the debate!

I find the fact that this argument has received so much attention quite amusing. After all, nobody is going to really believe that the laws of physics are different, based on interpretation of Pioneer's flight. And the immense amounts of work that have to go into trying to model the system properly is quite incredible. Scientists need to dig out information from decades ago to try to get everything they need and there are a lot of uncertainties. Turyshev quipped, "It's like being on CSI."

The exercise is certainly improving scientists modeling skills, which could then be used for much more practical purposes like building structures or vehicles on earth. It could even be quite useful in future space missions, although the problem will always be much easier in the future as the engineers will have better data about any spacecraft they send up.

Perhaps the story just reflects human's unending fascination with the exploration of space and a desire to be part of that exploration, in whatever form it can take.

See all posts from the American Physical Society April 2008 conference here.

Note: This post has been edited since it was first posted to correct the distance Pioneer is falling behind each year.

Latest news articles


This is an important milestone on the way to research at the first light-source laboratory in the Middle East.

Washington Post

Rubin’s groundbreaking discoveries revolutionized the way scientists observe, measure and understand the universe.


The measurement is the result of over 20 years of work by the CERN antimatter community.


Particle physics is petrolhead science—a particle-revving, high-octane demolition derby near the speed of light.