Pioneer puzzle pinned on thermodynamics

New study declares long-standing mystery solved, but experts are skeptical

A puzzling deceleration of the Pioneer 10 and 11 spacecraft as they drift toward the edge of the solar system is almost certainly due to forces generated by onboard heating elements rather than an exotic force or modification of gravity, a new study concludes. But other researchers say the study, posted April 1 at, doesn’t fully explain the anomalous motion.

SLOWING AHEAD A new study suggests that the unexplained deceleration of both Pioneer 10 and 11 (Pioneer 11 shown in this illustration) can be accounted for by heat reflected off the crafts’ components, but some researchers aren’t convinced. JPL-Caltech/NASA

Researchers first reported in 1998 that the sun’s pull couldn’t fully account for the deceleration of the craft, which were launched in the early 1970s and are now drifting in opposite directions well beyond the orbit of Pluto. Some theorists invoked new physics to explain the tiny slowdown — an average deceleration of just 8.74 x 10-10 meters per second squared, or about 10 billion times smaller than the tug of Earth’s gravity.  Other scientists suggested that infrared radiation, or heat, emitted and reflected by different parts of the craft might account for at least some of the deceleration (SN online: 4/24/08). Because of the craft’s orientation relative to Earth, the net recoil of infrared photons reflected from different parts of the craft would create a force that would slow the craft down.

Using a new computer model, Orfeu Bertolami of the Instituto Superior Técnico in Lisbon and the University of Porto in Portugal and his colleagues say they have done a more careful job than previous attempts of calculating the amount of heat emitted and reflected by the crafts’ components, which are powered by tiny nuclear generators. The team adapted a technique called Phong shading, developed in the 1970s to model how objects reflect visible light, to determine how parts of the two craft would reflect heat.

The researchers find that the net forces generated by the emission and reflection of heat are enough to account for the deceleration. “With the results presented here … the puzzle of the anomalous acceleration of the Pioneer probes can finally be put to rest,” the team concludes.
But John Anderson, a retired NASA scientist who published the first study of the crafts’ unaccounted-for deceleration 13 years ago, says that an analysis of heat emitted by the two probes doesn’t explain all of the anomalous motion.

Anderson suggests that because of uncertainties in modeling the heat emitted and reflected from the probes, the acceleration question may not be settled until another spacecraft is carefully monitored as it travels to the edge of the solar system.

Theorists have variously suggested that Einstein’s theory of gravitation needs modification, that the outer reaches of the solar system are subject to a new gravitational force, or that the deceleration of the craft is an illusion due to variations in the speed of light.

Viktor Toth, a software developer based in Ottawa who has studied the Pioneer puzzle for several years, agrees that forces generated by thermal effects will end up accounting for much of the deceleration. But he believes the problem can be solved only by taking the material properties of the spacecraft surfaces and structures into account and building a detailed computer model that accounts for the heat generated by all the craft’s components. Undertaking all that would require time and money. “However frustrating the wait is — and believe me, it is frustrating — jumping the gun will not serve anybody,” Toth says.

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