Aerospace Analysis Helps Confirm
Unexplained "Pull" on Spacecraft

EL SEGUNDO, Calif. (10/26/98) -- The Aerospace Corporation has helped confirm a tiny and unexplained acceleration of NASA spacecraft toward the sun and ruled out a number of possible causes. The principal investigator at Aerospace also has explained what the company believes to be the most likely cause.

The anomalous acceleration, resulting from a pull about 10 billion times smaller than the acceleration we feel from Earth's gravitational pull, was confirmed through detailed analyses of radio data from Pioneer 10, Pioneer 11 and Ulysses spacecraft.

The Aerospace Corporation was brought into an investigation of the phenomenon by researchers at JPL and Los Alamos National Laboratory. Preliminary data examined by these researchers needed to be confirmed. A JPL researcher, John Anderson, first saw the effect in 1980, but until he had accumulated data over the next 15 years, he could easily dismiss it as the result of systematic errors. "Like a lot of problems in astronomy, many years of observation are needed," Anderson said.

Another researcher, Michael Nieto of Los Alamos, 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.

Anderson and Nieto began working together along with colleagues from their labs. About three years ago the researchers redoubled their efforts to analyze the spacecraft motions and possible contributing perturbations by asking Aerospace to check their work.

Philip Laing, engineering specialist and principal investigator, working with consultant Anthony Liu of Astrodynamic Sciences Co., Los Angeles, under a NASA Office of Space Science grant, conducted an independent analysis of data using a computer program developed by Aerospace. Laing said they been have able to rule out these possible causes of the anomalous acceleration.

• Errors in JPL's orbital determination software
• Gravity of the Kuiper belt and gravity from the galaxy
• Errors in the planetary ephemeris
• Errors in the values of the Earth's orientation, precession, and nutation
• Nongravitational effects from solar pressure and attitude control maneuvers
• Solar wind and interplanetary medium
• Nominal thermal radiation and plutonium half-life
• Drifting clocks, general relativity, and the speed of gravity
• Hardware problems at the tracking stations

After exhausting the list of explanations deemed most plausible, the researchers examined possible modifications to the force of gravity as explained by Newton's Law--with the sun being the dominant gravitational force--or the possible influence or nonordinary, or "dark" matter. The dark-matter explanation falls short, because so much matter would have been required to create the accelerations it would have affected the motions of other bodies in the solar system.

The researchers concluded that if the anomalous radial acceleration acting on spinning spacecraft is gravitational in origin, it is not universal. It would have to affect bodies with masses of a thousand kilograms or so more than bodies the size of planets.

The researchers noted that NASA's planned mission to Pluto, which would include more accurate tracking systems, may provide additional, high-quality data for investigating this mystery. In addition, Pioneer 10 is still a potential source of data because its transmitter is still functioning. The team also is conducting more detailed analyses of Ulysses' swing around the sun. The Pluto mission is scheduled for early in the 21st century.

"Clearly, more analysis, observation, and theoretical work are called for," the researchers concluded.

Laing said the research has generated considerable interest among physical scientists and that a number of these have offered to participate in the research. They are intrigued by the fact that while the acceleration is tiny and has no significant effect on NASA missions, it holds great interest because no explanation based on conventional physics and understanding has been found. The effect is so persistent that it could indicate some physics not considered in previous attempts to explain the motions of bodies in the universe.

The research by Laing, Liu and their colleagues at JPL and Los Alamos was published Oct. 5 in Physical Review Letters, a publication of the American Institute of Physics. Reports also have been published in

The New York Times and New Scientist magazine.

In an interview with Scientific American Oct. 13, Laing stated that the The Aerospace Corporation's view is that the anomalous acceleration is most likely due to gas leaks. This possible cause he attributed to imperfect valves. Laing explained that this conclusion is supported by tracking data not available in earlier studies. Thermal radiation or new physics are possible but less likely causes, Laing said.