October 26, 1998
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.
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.
14 OCTOBER 1998
In September, John Anderson of NASA's Jet Propulsion Laboratory near Los Angeles announced that the spacecraft -- Pioneer 10, Pioneer 11, Ulysses and perhaps Galileo -- were slowing down faster than expected as they travelled away from the Sun. Physicists wondered if this meant they would have to rewrite the equations of gravity (This Week, 12 September, p 4). But now two scientists have suggested an alternative solution.
The spacecraft have plutonium-based radioisotope thermoelectric generators (RTGs) to power them. Resistance in the spacecraft's circuits turns some of the electrical power produced by the RTGs into heat. To get rid of it, the spacecraft are fitted with louvred fins that open when they get hot and radiate the heat away, according to Edward Murphy, an astronomer at Johns Hopkins University in Baltimore, Maryland.
The radiators face away from the Sun, so most radiation is emitted in this direction. Murphy says the departing photons give the spacecraft a small push in the opposite direction, towards the Sun, slowing them down. He believes the amount of radiation leaving the spacecraft could easily account for the observed push. "It's pretty close, and within observational errors," he says.
Jonathan Katz of Washington University in St Louis, Missouri, also blames heat -- in this case, the heat wasted because of the RTGs' inefficiency at turning thermal energy into electricity. He points out that the satellites have large antennas that point to the Earth, and that the RTGs sit just off to the side. "The radiation can bounce off the back of the antenna and push the spacecraft towards Earth," he says.
Both Katz and Murphy have submitted their calculations to Physical Review Letters. But Anderson, who had last month ruled out a heat effect as the cause of the deceleration, is still unconvinced by the new arguments. "You can't get the force you need," he says.
Author: Charles Seife
New Scientist issue 17th October 1998
9 SEPTEMBER 1998
"We've been working on this problem for several years, and we accounted for everything we could think of," says John Anderson, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena.
In 1972, NASA launched Pioneer 10 in the direction of Jupiter. For a quarter of a century, radio signals have been beamed to the spacecraft and reflected back to Earth as it continued its odyssey to the outer Solar System and beyond. By studying the red shift of the returning radio waves -- how "stretched out" they are -- NASA scientists have been able to work out how fast the probe is travelling. Pioneer 10 seems to be slowing more quickly than it should.
The signals bouncing back from Pioneer 10 are far from clean. The Earth revolves around the Sun, stretching and compressing the radio waves periodically. The probe also occasionally corrects its course so that its antenna remains pointing towards the Earth. But scientists have a good handle on these effects, and can cancel them out.
That is, they thought they could, until Anderson's team started analysing Pioneer 10 data collected since 1987. They found a systematic anomaly, as if Pioneer 10 were receiving an extra tug from the Sun's gravity. The disagreement is 80 billionths of a centimetre per second squared, a tiny rate of deceleration that would take more than 650 years to bring a car travelling at 60 kilometres an hour to a halt. But to scientists used to working with absolute precision it is a glaring discrepancy.
What could be to blame? A fuel leak was quickly ruled out - Pioneer 10's gauges show no unexpected loss of fuel. Aerodynamic drag from the interstellar medium also couldn't be involved, as there just isn't enough material to account for the effect. Thermal radiation from the spacecraft's batteries would also be too puny, and would be emitted in all directions rather than pushing the probe towards the Sun. An unknown asteroid couldn't be responsible, either. "We ruled out other sources of gravitation," says Anderson.
If just one spacecraft were being affected, the discrepancy would be infuriating, but certainly not enough to start questioning current theories of gravity. But Pioneer 11, launched in 1973 towards the other end of the Solar System (see Diagram), is also slowing at about the same rate. The Ulysses probe, launched in 1990 towards Jupiter, before swinging into an orbit that took it over the Sun's poles, had an even larger anomalous pull towards the Sun. Data from Galileo, now orbiting among Jupiter's moons, appear to show the same effect.
Other researchers aren't ready to abandon cherished ideas about gravity on the basis of the data gathered by Anderson's team. "They're extremely good at what they do," says Clifford Will, a physicist at Washington University in St Louis. "But I think there's some kind of systematic effect that has corrupted the data." John Ries, a planetary scientist at the University of Texas at Austin, says he can't believe a new gravitational force is involved, because that should affect the motions of the planets.
Anderson and his colleagues are similarly cautious. "It's likely that it's some systematic error," says Michael Nieto of the Los Alamos National Laboratory in New Mexico, a member of the team. But until someone can identify an error in the data, outlined in a paper to be published in Physical Review Letters , the possibility that the team has broken new ground in physics remains. "There's a small probability that it's very important," says Anderson.
Author: Charles Seife
NEW SCIENTIST MAGAZINE - ISSUE 12 SEPT 1998, PAGE 4