Gravity Losing Its Pull?

This is an edited version of an L.A. Times story - for full version, go to the paper's website.

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. 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.
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.Today, after three decades, the difference is about 248,000 miles, the distance from Earth to the moon.Anderson, ever the cautious scientist, didn't tell anyone what he was seeing for a decade. Early on, the probes were still so close to the sun that he reasoned radiation and solar wind — streams of ionized gas spewing forth through the solar system — could be affecting them.The other possibility was a spacecraft "systematic" — an onboard mechanical problem. Prime suspects were gas leaks, along with releases of energy by the plutonium-powered radioisotope thermoelectric generators that provided electric power to the instruments.None of these candidates seemed capable of producing errors as large as Anderson was charting.There was one piece of evidence that seemed to support the idea that the anomaly could be real: It was almost exactly the same on both spacecraft. On the other hand, both Pioneers were built by the same company to identical specifications, so why shouldn't the same problem show up on both?As years passed, and the Pioneer probes moved away from the sun's influence, the anomaly didn't disappear — or change even one iota.Anderson was stumped. Unable to get the problem out of his head, he began spending his own time burrowing deeper into the numbers streaming back from space.He was still scratching his head when physicist Michael Martin Nieto at the Los Alamos National Laboratory in New Mexico called up one day in 1994 looking for material for an upcoming speech about new developments in physics. Anything interesting going on?"Well, I've got this thing with Pioneer," Anderson said."I almost fell off my chair," Nieto said.
That's when, for good or ill, the Pioneer Anomaly went public. Through all the years, Anderson has never permitted himself to hope he might have found something that could change the way people think about the cosmos. Or that his name would go down in history."I used to think the probability of making a fundamental discovery was pretty remote," Anderson said. "Now, I kind of wonder about it."Whatever the solution to the Pioneer Anomaly, it will be reached without any more help from the Pioneers. NASA lost contact with Pioneer 11 in November 1995. Pioneer 10 sent its last message home in January 2003.Anderson can't help feeling a touch of sadness. "You do feel a loss."The spacecraft are now ghostly twins speeding through interstellar space. Pioneer 10 could reach the star Aldebaran in the constellation Taurus in about 2 million years, scientists say. Unless, of course, some uncharted force brings it to a halt, or twists its course toward some dark corner of space.