The Pioneer Anomaly Revisited

Pioneer AnnomalyOne of the more interesting (read, fun) problems in astronomy (subset: cosmology) and physics (subset: gravitation) has been the one posed by the Pioneer and Voyager spacecraft.  After over 30 years and tens of billions of miles, these spacecraft are simply not where they should be.  In simplest terms, they’ve experienced a small but constant unexplained acceleration in the direction of the sun, enough to put them about 5000 miles off course every year.

Something similar has been noticed in the Galileo to Jupiter, the NEAR mission to the asteroid Eros, the Rosetta probe to a comet, Cassini to Saturn, and the MESSENGER craft to Mercury.

The “Pioneer Anomaly” has caused scientists, engineers and interested amateurs to search for an explanation.  Some of the more speculative have submitted that our understanding of gravity is incomplete, and modifications to both Newton and Einstein are warranted. The more mundane say “Naah! There’s some slow outgassing or uneven radiation pressure acting on the spacecraft.”

According to JPL researcher Slava Turyshev, most, if not all of the extra acceleration can be explained by how the spacecraft are dissipating heat.

The magnitude of the Pioneer Anomaly is so very tiny that it could conceivably result from the uneven radiation of heat from the spacecraft.  The Pioneers, like all spacecraft, were made from a wide variety of materials: aluminum, Teflon, Kapton, Mylar, aluminum-based paints, and so forth, all of which absorb, reflect, or emit radiation in different ways; and some materials, notably the plutonium in the spacecraft power supply, generate heat on their own.  To figure out in which directions the spacecraft radiates how much heat, Slava and his colleagues needed to start from scratch, building a CAD model of the spacecraft, covering the model with surfaces with the right thermal properties, plugging in the recovered spacecraft data on the temperatures measured at various points within the spacecraft, and then solving a beastly difficult set of differential equations to determine how heat conducts and radiates around within the spacecraft, and then in what direction it radiates once it exits the surface.

He’s had to use fairly simplistic models of the spacecraft to do this, and the rest of the acceleration would probably be explained if we had a better thermal model.

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