The Pioneer anomaly

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It was in 1980 that John Anderson first wondered if something funny was going on with gravity. The Jet Propulsion Laboratory physicist was looking over data from two Pioneer spacecraft that had been speeding through the solar system for nearly a decade.

Only something was off base. The craft weren't where they were supposed to be.
Rather than traveling at a constant velocity of more than 25,000 mph toward the edge of the solar system, Pioneers 10 and 11 were inexplicably slowing down. Even factoring in the gravitational pull of the sun and its other planets couldn't explain what he was seeing.
How could that be?

At first, Anderson figured there must be a simple explanation. Maybe there was a malfunction on board the spacecraft. Maybe his calculations were wrong. Shy, bookish and soft-spoken, Anderson was not the type to call a news conference to announce that two U.S. spacecraft appeared to be disobeying the physical laws of the universe. "I assumed something was going on that I didn't understand," said Anderson, now 70. "So I just kept at it."

For years. It was a lonely, often comfortless pursuit. Some critics pounded away at him for daring to question the conventional wisdom about the force that keeps our feet on the ground and the stars on their appointed rounds. Others questioned his math.

Two decades later, Anderson's work on what is now called the Pioneer Anomaly may finally be paying off. In October, a European Space Agency panel recommended a space mission to determine whether Anderson had found something that could rewrite physics textbooks. Some cosmologists even speculate the Pioneer Anomaly might help unravel some of the thorniest problems in theoretical physics, such as the existence of "dark matter" or mysterious extra-dimensional forces predicted by string theory.
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"There are two possible explanations," Turyshev said. "The most plausible is systematics."
The second possibility is new physics. "If it's new physics, the implications are truly tremendous," he said.
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So what would be the implications?

One possibility is that invisible, so-called dark matter is holding the spacecraft back. Some cosmologists believe that dark matter exists because only 10% of the expected mass of the universe has been found. If 90% of the universe's mass and energy is invisible, maybe it could exert gravitational pull on spacecraft.

Another possibility, even more fanciful, is that invisible dimensions of space are tugging at the Pioneers. This idea has its origin in string theory, an idea that suggests we are surrounded by far more than the three dimensions we know about. Some versions of string theory suggest there may be as many as 11 dimensions, most of which are curled up and hidden from us.

As with dark matter, no hard evidence has been found proving the existence of vibrating strings far tinier than the smallest known particles.

A third possibility is that gravity has been hiding secrets that three centuries of research have failed to uncover.