The Pioneer Anomaly

A space mystery solved with help from The Planetary Society

At a Glance

  • Something was slowing down NASA's Pioneer 10 & 11 spacecraft on their journey out of the solar system. This effect was called the Pioneer Anomaly.
  • The Planetary Society and its members funded a team of scientists to restore and analyze critical spacecraft data to provide crucial clues to this mystery.
  • In 2012, the scientists concluded that non-uniform heat emission from the spacecraft was acting to slow their speed, thus solving the Pioneer Anomaly.

Something strange was happening in the outer reaches of our solar system.

The Pioneer 10 and 11 spacecraft were not where they were supposed to be. These missions, launched in 1972 and 1973, have covered hundreds of millions of kilometers, heading toward the edge of our solar system. But something was holding them back. Each year, they fell behind in their projected travel by about 5,000 kilometers (3,000 miles).

Jet Propulsion Laboratory scientist John Anderson and his colleagues began searching for an explanation in 1980 but had found nothing conclusive. No spacecraft behavior or known property of the outer solar system could explain the deceleration of the Pioneer spacecraft.

Scientists began to consider the unthinkable: something may be wrong with our understanding of the laws of physics.

Pioneer 10 exiting the solar system
The Pioneer Spacecraft After their flybys of Jupiter and Saturn, the twin spacecraft were speeding out of the solar system slower than originally predicted. Why? NASA

Despite this, NASA declined to fund the critical data recovery and analysis necessary to shed light on this mystery. So Anderson and his colleague, Slava Turyshev, reached out to The Planetary Society for help.

The Planetary Society asked its members to fund efforts to save old Pioneer data from destruction and  support the complex analyses necessary to understand the problem. Our members and supporters responded with unprecedented enthusiasm, provided the resources required to attack the problem.

"Without the rescue of the Doppler data, we would have been blind, never able to claim the quantitative data we need to solve the anomaly. The recovery of Doppler and telemetry data and the entire effort in thermal analysis would not have happened without the Planetary Society."

-Dr. Slava Turyshev

In the end, after the recovery of the data and years of painstaking work, the researchers concluded that anisotropic thermal radiation (heat radiating from the spacecraft unevenly in many directions) explained the mystery of the Pioneers' deceleration. No revolution in physics was needed.

Pioneer Anomaly Solved!

With the latest piece of the puzzle just published in a scientific journal, a solar system mystery that has perplexed people for more than 20 years has been solved, truly thanks to the support of Planetary Society members.

Planetary Society Statement on the Pioneer Anomaly

Planetary Society Members have been supporting Slava Turyshev and his colleagues in their scientific quest to solve the famous Pioneer Anomaly. They have just published a peer-reviewed paper that reports on what may be the last step toward the solution.