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Cosmic Confusion: Why the Pioneer spacecraft behaved strangely in space.

NASA’S TENTH AND ELEVENTH Pioneer Program spacecraft were launched, respectively, in 1972 and 1973. After reaching their primary goals, the planets Jupiter and Saturn, the two craft were designed to escape the gravitational pull of those solar system behemoths and journey out into the vastness of space.

This they did, continuing to communicate with earth-based scientists for years after developers thought they’d be kaput. Among the most interesting of the data collected during Pioneers’ long sojourn was evidence of a curious anomaly. Instead of acting in accordance with accepted notions of celestial mechanics, the two craft seemed to be slowing down more quickly than they should, a deceleration that flew in the face of gravitational force predictions put forward by Einstein.

For years, physicists puzzled over this “Pioneer anomaly.” Numerous theories were offered and rejected. Eventually, some scientists began to think the unthinkable: Could Einstein’s understanding of gravity be flawed?

This year two important insights, one of them from an MU physicist, have offered answers without jettisoning a century of astrophysics. The first, from a team of scientists led by Slava G. Turyshev at the Jet Propulsion Laboratory in Pasadena, Calif., showed that an effect called “thermal recoil force” — essentially unevenly distributed heat emission — was creating enough drag on the spacecraft to slow down its progress. Turyshev published his results, to wide acclaim, in April.

Not all physicists were satisfied, however, that Turyshev’s finding was sufficient. Sergei Kopeikin is an MU professor of physics whose gravity-related research was described in Illumination’s Spring 2003 issue. “Previous research has focused on mechanical explanations for the Pioneer anomaly, such as the recoil of heat from the craft’s electrical generators pushing the craft backwards,” Kopeikin says. “However that only explains 15 to 20 percent of the observed deceleration.”

The remaining 80-85 percent, Kopeikin argues in the journal Physical Review D, resulted from a previously incorrect calculation of the movement of photons that made up the light and radio wave used by the JPL to track Pioneer’s speed.

The universe is constantly expanding, he says, and earth-based observations that don’t fully account for this will appear to make an object traveling through deep space, like the Pioneer probes, appear to slow down. Physicists must be careful when dealing with propagation of light in the presence of the expansion of space, says Kopeikin. Light is affected by forces that are irrelevant in other equations; for example, the expansion of the universe affects photons, but doesn’t influence the rest of “local physics,” such as the motion of planets and electrons in atoms.

“Having accurate measurements of the physical parameters of the universe helps us form a basis to make plans for interstellar exploration,” says Kopeikin. “Discerning the effect of the expansion of the universe on light is important to the fundamental understanding of space and time. The present study is part of a larger, ongoing research project that may influence the future of gravitational physics.”

Colorful illustration of a Pioneer craft in space.

Image courtesy of NASA.

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