Three Spacecraft Reveal Unexplained Motion

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.

The research has been accepted for publication in Physical Review Letters.

"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.

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," the researchers wrote: 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. "Like a lot of problems in astronomy, many years of observation are needed," Anderson said.

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.

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

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

Other authors on the PRL paper were Philip Laing of The Aerospace Corp., Anthony Liu of Astrodynamic Sciences, and Eunice Lau and Slava Turyshev of JPL.

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.