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The Nation | COLUMN ONE

Gravity May Lose Its Pull

When conventional physics couldn't explain why space probes were acting strangely, one JPL scientist was determined to find the answer.

December 21, 2004|John Johnson | Times Staff Writer

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.

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.

Whether Anderson will be remembered as the man who changed history or the guy who spent decades chasing an illusion, all that's clear at this point is that he will be remembered.


For the record, gravity is one of the most closely studied forces in the universe.

Sir Isaac Newton first measured it in the 17th century. Every object in the universe attracts every other object, Newton determined, with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. That means the bigger things are, and the closer they are, the greater their gravitational pull.

In 1915, a former Swiss patent clerk named Albert Einstein refined the theory, arguing that gravity occurs when planets or stars warp the fabric of space around them, much like a bowling ball on a trampoline warps the surface of the trampoline. Instead of a sucking force, Einstein's general theory of relativity said small objects fall toward larger ones like a marble rolling down the slope of the trampoline to the bowling ball.

Einstein's theory has been successfully tested again and again. Without it, complex space missions such as Pioneer 10 and 11 would have ended in disaster, either by missing their targets -- in this case fly-bys of Jupiter -- or by crashing.

Anderson was already an experienced space hand when the Pioneers were launched in 1972 and 1973. Having worked on the Mariner missions in the 1960s, he was chosen to be principal investigator for gravity research on both Pioneer missions.

It would prove to be a surprisingly long ride. The TRW-built Pioneers performed so well that after the initial two-year mission ended, NASA decided to send them on a new mission to explore the solar system's outer planets.

They were the first spacecraft to travel through the asteroid belt, which some scientists at the time thought could be as dangerous as a field of icebergs. Pioneer 10 was first to pass the orbit of Pluto. For many years, until overtaken by the speedier Voyager 1, the Pioneers were the farthest venturing man-made objects in space.

By 1980, the vehicles were still zipping through space in fine shape -- when Anderson stumbled upon the unexpected.

"I started plotting this anomalous acceleration toward the sun," Anderson said. In space science-speak, that meant the spacecraft were improbably slowing down.

To be sure, the anomaly was small, just 8 X 10--8 centimeters/second2. That amounted to about 8,000 miles a year, a tiny fraction of the 219 million miles the spacecraft covered annually. The anomaly is about 10 billion times weaker than the Earth's gravity.

But over time, even inches and meters add up.

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