The Pioneer 10 and Pioneer 11 space missions were launched in the 1970s with the goal of exploring the far reaches of the Solar System. Pioneer 10 was launched on March 2, 1972, and Pioneer 11 on April 5, 1973. Both carry plaques with messages explaining their origin to a possible extraterrestrial civilization, designed by the famous astronomer Carl Sagan.

Not only have both probes provided an invaluable wealth of scientific data, but they have also starred in one of the most intriguing enigmas of space exploration: the so-called Pioneer Anomaly.

This phenomenon, observed as an unexpected deviation in the accelerations of these spacecraft, kept the scientific community on edge for decades.

The plate with the message that the Pioneers carry on the outside
The plate with the message that the Pioneers carry on the outside. Credit: NASA / Public domain / Wikimedia Commons

The mystery began when both spacecraft, after surpassing 20 astronomical units (about 3 billion kilometers) from the Sun, began to exhibit peculiar behavior. Upon meticulously analyzing the navigation data, it was discovered that the Pioneers were decelerating slightly more than expected. This additional deceleration, extremely small and equivalent to a speed reduction of 1 km/h every ten years, corresponded to an acceleration towards the Sun of approximately (8.74 ± 1.33) × 10-10 m/s2.

Therefore, the calculated position of the Pioneers did not match the measurements based on the return time of the signals sent back from the spacecraft. They were found to be thousands of kilometers closer to the interior of the Solar System than they should be. The distance was increasing over time as measurements were taken, suggesting that whatever was causing the anomaly continued to act on the spacecraft.

The anomaly, first identified in 1980 and seriously investigated since 1994, gave rise to numerous theories. Explanations ranged from data errors to proposals involving a revision of gravitational laws, and even the possibility of new physics or unconventional gravitational effects was seriously considered. However, these models faced the difficulty of not being able to conclusively explain why the outer planets’ orbits of the Solar System did not show similar anomalies.

Trajectories of the Pioneers and the Voyagers
Trajectories of the Pioneers and the Voyagers. Credit: NASA / JPL / Public domain / Wikimedia Commons

From 1998 onwards, the hypothesis emerged that the cause could lie in anisotropic radiation pressure, that is, the effect of thermal radiation emitted by the spacecraft unevenly. The Pioneers, equipped with radioisotope thermoelectric generators (RTGs), could only dissipate heat through thermal radiation in the vacuum of space. If this radiation was emitted predominantly in one direction, it would produce an acceleration in the opposite direction due to the recoil of the thermal photons.

The precision in measuring this thermal force required the recovery and analysis of historical telemetry data, as well as the construction of detailed thermal models of the spacecraft. These complex and arduous tasks culminated in 2012 with a definitive study published by Slava Turyshev and collaborators in the journal Physical Review Letters. This work demonstrated that by adequately considering the thermal recoil force, there was no significant anomalous acceleration left. In other words, the anomaly could be completely explained by the thermal emission of the spacecraft, without the need to invoke new physical laws.

The peculiarity of the Pioneer Anomaly was identified thanks to these spacecraft traveling without additional course corrections, something that does not happen with many later space missions, such as the Voyagers, which require frequent attitude adjustments. More recent missions, such as Cassini and New Horizons, have provided data supporting the thermal explanation, although with different degrees of precision and operational circumstances.

Pioneer 11 and the rings of Saturn, on September 1, 1979
Pioneer 11 and the rings of Saturn, on September 1, 1979. Credit: Andrzej Mirecki / Public domain / Wikimedia Commons

The clarification of the Pioneer Anomaly not only reinforced our confidence in the known physical laws but also highlighted the importance of details in engineering and data interpretation.

The Pioneers have left the Solar System, and contact with both has been lost, possibly due to the exhaustion of the power source that drove them and the enormous distance they are at. The last signal from Pioneer 10 was received on January 23, 2003, when it was 12 billion kilometers from Earth heading towards the star Aldebaran, which it should reach in approximately 1,690,000 years.

The last signal from Pioneer 11 was received on November 24, 1995. On June 24, 2024, the spacecraft was 16.9 billion kilometers from Earth traveling at about 40,000 kilometers per hour towards the constellation Scutum. Along the way, it was overtaken and surpassed by the Voyager 1 probe, which is the human-made object currently at the greatest distance from Earth in deep space.

This article was first published on our Spanish Edition on July 8, 2024: La anomalía de las Pioneer, el misterio que desafió la ciencia espacial

SOURCES

John D. Anderson, Philip A. Laing, et al., Study of the anomalous acceleration of Pioneer 10 and 11. Phys. Rev. D 65, 082004. doi.org/10.1103/PhysRevD.65.082004

Slava G. Turyshev, Viktor T. Toth, The Pioneer Anomaly. arXiv:1001.3686, doi.org/10.48550/arXiv.1001.3686

John D. Anderson, Philip A. Laing, Eunice L. Lau, et al., Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an Apparent Anomalous, Weak, Long-Range Acceleration. Phys. Rev. Lett. 81, 2858, doi.org/10.1103/PhysRevLett.81.2858

Wikipedia, Anomalía de las Pioneer

  • Share on:

Discover more from LBV Magazine English Edition

Subscribe to get the latest posts sent to your email.

Something went wrong. Please refresh the page and/or try again.

Follow LBV Magazine on Google News