Gravitational Force X

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Projected Path Of Pioneer Space Probes

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 than 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.

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."

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."

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 to be available on the Internet scientific archive site of the Los Alamos National Laboratory.

On September 24, 1998 a team of planetary scientists and physicists at Los Alamos identified a tiny, unexplained sunward acceleration in the motions of the Pioneer 10, Pioneer 11 and Ulysses spacecraft. The anomalous acceleration, about 10 billion times smaller than the acceleration we feel from Earth's gravitational pull, was identified after detailed analyses of radio data from the spacecraft.

The research team, led by John Anderson of NASA's Jet Propulsion Laboratory and including Michael Nieto of The Department of Energy's Los Alamos National Laboratory, considered and ruled out many possible causes for the perturbation in the spacecrafts' motions. The team expects the explanation, when found, will involve conventional physics and understanding, but the team has also considered what implication the anomalous motion has for some new physical effect. The accelerations are so persistent that they could be pointing to some relevant physics that's been overlooked in trying to explain the motions of bodies in the universe.

"In order of decreasing probability the possible causes are some systematic effect associated with the spacecraft; some subtle effect associated with our tracking systems, which would be important to know for space navigation; or some manifestation of 'new physics,' " Nieto said. "By looking at the third possibility we can examine how well 'normal' physics works, which in itself gives you further insight into the universe.

The researchers analyzed signals sent from Earth that were actively reflected by a transponder on the spacecraft. The resulting Doppler shift in the signal was used to calculate the motions. NASA's Deep Space Network sent and received the signals. Pioneer 10 was officially tracked until March 1997, when it was some six billion miles away from the sun. (Pioneer 10 is still transmitting, and occasional additional radio Doppler data is provided to the research team.)

Pioneer 11, due to a radio failure, last sent useful radio Doppler transmissions in October 1990, when it was less than three billion miles from the sun. Ulysses has been tracked on its looping flight out of the ecliptic and around the sun's poles. The researchers also examined Galileo data from that craft's journey from Earth to Jupiter. Measurements of both Pioneer craft and Ulysses gave approximately the same answer for the strength of the anomalous acceleration. Galileo, too, yielded a similar value, but its flight was so close to the sun the researchers could not rule out the effects of solar radiation pressure.

Pioneer 10

Newton's laws of gravity alone, with the sun providing the dominant gravitational force, are good enough for NASA to send spacecraft on planetary rendezvous with near-pinpoint precision. But the anomalous motions of these spacecraft are so small that the researchers had to consider numerous possible causes: perturbations from the gravitational attraction of planets and smaller bodies in the solar system; radiation pressure, the tiny transfer of momentum when photons impact the spacecraft; general relativity; interactions between the solar wind and the spacecraft; possible corruption to the radio Doppler data; wobbles and other changes in Earth's rotation; outgassing or thermal radiation from the spacecraft; and several others. The researchers have so far not found that any of these effects can account for the size and direction of the anomalous acceleration.

After exhausting the list of possible "normal" explanations, the researchers looked at possible modifications to the attractive force of gravity or the possible influence or non-ordinary matter, or "dark" matter. The dark matter explanation didn't hold up because so much matter would have been required to create the measured spacecraft acceleration it would have affected motions of other bodies in the solar system.

Looking at other mathematical representations for gravitational interactions also "come up against a hard experimental wall", namely that the gravitational effect would also be seen in planetary motions, especially for Earth and Mars. "If the anomalous radial acceleration acting on spinning spacecraft is gravitational in origin, it is not universal," the researchers concluded. It would have to affect bodies massing a thousand kilograms or so more than bodies the size of planets.

Nieto has long been interested in the possibility that gravity works differently on antimatter than on the familiar matter that makes up our everyday world. This led him to consider how well we understand gravity's influence on normal matter and whether studies of the motions of comets or spacecraft could be used to identify any deviations from the expected influence of gravity.

Meanwhile, John Anderson and his JPL colleagues had for years puzzled over "residual errors" between the calculated and measured positions of the Pioneer spacecraft. Anderson first saw the effect in 1980, but until he had accumulated data over the next 15 years, he could easily dismiss it as systematic errors. After Anderson and Nieto hooked up the group redoubled its efforts to analyze the spacecraft motions and possible contributing perturbations. An independent analysis of the motions using a computer program developed by The Aerospace Corp. ruled out errors in JPL's orbital determination software as the source of the anomalous acceleration. "Like a lot of problems in astronomy, many years of observation are needed," Anderson said.

Images and data courtesy of the Jet Propulsion Laboratory, NASA,
and the Los Alamos National Laboratory