Gravity May Lose Its Pull

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.

For public consumption at least, Anderson and his close-knit group of researchers will not permit themselves the luxury of such grandiose speculation.

"I'm trying to stay away from" that kind of talk, said Slava G. Turyshev, a former Russian scientist who has been working on the anomaly for the last decade. "Even though I'm being dragged into it."

However it turns out, the episode offers a rare glimpse into the scientific process at its rawest, most inspirational and gut-wrenching.