Puzzling hyper-gravity proves weighty mystery
By Richard Stenger
(CNN) -- An unknown force seeming to pull on a pair of distant space probes has left astronomers with a weighty mystery, one that appears to defy the conventional laws of physics.
The Pioneer 10 and Pioneer 11 spacecraft, which for decades have steadily traveled in opposite directions in the solar system, have covered significantly less space then they should have, astronomers said.
A team of NASA researchers has systematically attempted to determine what has slowed the sibling NASA robot ships, to no avail.
"Something is slowing down the spacecraft. And we have not been successful in finding the source of that. There is more slowing than you would expect from Newtonian gravity," said John Anderson, a senior scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.
Mystery force a real drag
The probes have traveled far beyond Jupiter since their launch in the early 1970s. But astronomers have been able to measure with great precision the trajectories and distances of the pair.
Noticing that Pioneer 10 was unexpectedly lagging on its journey away from the sun, they speculated that an unknown object could have been exerting an influence.
But they had to revise that theory when they realized that a mysterious force was acting in an identical manner on Pioneer 11, which was on the other side of the system.
"It's the same magnitude and the same direction, namely pointed toward the sun. The force points to the sun in both cases," said Anderson.
Astronomers studied the Doppler shift of the radio signals to help calculate the distances of the probes. After extensive analysis, they dismissed instrumentation error, propellant leaks and minor heat emissions as causes of the negative thrust.
Perhaps the spacecraft inadvertently produced an unknown force that is not yet understood, Anderson said. Perhaps scientists will have to reconsider basic assumptions about the laws of physics.
"No one has come up with a conventional explanation," he said. One possible reason "is that it is a modification of gravity."
Pioneer sends shocking signal
Launched in the early 1970s, the Pioneers were the first probes to explore the outer solar system, astounding the world when they sent home flyby images of giant planets like Jupiter, Saturn and their moons.
The resilient Pioneer 10, now far beyond the orbit of Neptune, surprised astronomers in April when it managed to send a transmission back to Earth as directed. Radio communications with Pioneer 11 ceased in 1995.
The scientists were unable to calculate the effects of distant gravity on other deep space probes, like Voyager I or Voyager 2, because they employ a different kind of orientation and propulsion system, Anderson said.
Anderson and his colleagues have submitted their work to the journal Physical Review D. Their findings are currently available on the Internet scientific archive site of the Los Alamos National Laboratory, http://www.xxx.lanl.gov.
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