FOR RELEASE: Wednesday morning, March 24
Martin Nieto (505) 667-6127,
mnm@pion.lanl.gov)
Los Alamos National Laboratory
Los Alamos, NM
Popular Version of Paper
[OA03.02]
Wednesday morning, March 24
APS Centennial Meeting, Atlanta
In tracking the Pioneer 10 (and later Pioneer 11) spacecraft, it became clear around 1980 (when Pioneer 10 was 20 AU from the Sun)* that there was an apparent extra force acting on both Pioneer 10 and 11. As they escape from the solar system the spacecraft seem to be slowing down at a slightly greater rate than expected. When expressed as an acceleration, this extra slowing is about 12 billion times smaller than the acceleration of gravity on the surface of the Earth. Furthermore, this unexplained acceleration appears to be constant, to high accuracy, between 1987 and 1998. (Pioneer 10 was between 40 and 71 AU.)
At the current distance of Pioneer 10 from the Sun (10 billion kilometers) the gravitational pull of the Sun is over 1500 times larger than the unexplained acceleration. Since the Sun's pull weakens with the inverse square of the distance, it will equal the unexplained acceleration when Pioneer is at a distance of 2700 AU.
The anomalous acceleration seems directed at the Sun, but we cannot say for sure. As viewed from Pioneer 10, the Earth and Sun are not very far apart. Both Pioneers are spinning with their spin axes directed towards the Earth. It is quite possible that forces generated by the spacecraft themselves (these are called "systematics") cancel out in all directions except the one direction along the spin axis. The spacecraft are identical twins, so it is not surprising that they would generate the same force.
Various systematics have been proposed to explain the effect, such as 1) radiant heat from the power sources (Radioisotope Thermoelectric Generators), 2) heat from the instrument compartment being vented from the rear of the craft, 3) computer program errors, 4) gas leaks from the attitude-control thrusters, and others. However, the size and constancy of the force present the real difficulties for a precise systematic explanation. Indeed, we find that none of these can, as yet, adequately explain the effect.
Our conservative estimation is that the cause of the effect is most likely some undiscovered or not understood systematic. However, if further analysis indicates that the force is not generated by the spacecraft, and that it is not a result of systematic errors in the data, then we must consider the small probability that nature is saying something new. If so, it is strange.
The effect is not observed in planetary or asteroid orbits. What is needed is a force that affects the Pioneers, but not planets or asteroids; a tall order. Besides their small mass relative to large solar-system bodies, the only thing particularly unique about the Pioneers is the fact they are on escape trajectories. No comets or other bodies have been observed, to this accuracy, on escape trajectories from the solar system. So this is something new. But we do not understand how it could be a factor. Even so, there exist a number of interesting speculations.
Whatever the cause of the anomalous Pioneer force, we will continue to work on the problem, not only with the Pioneers but with other spacecraft as well. For example, a Pluto Kuiper-Belt mission would explore the outer regions of the solar system, but with far superior instrumentation to the 1969 technology available to Pioneer. As a start, we have met with JPL engineers and scientists designing the mission to let them know we hope for a probe with as good an understanding of spacecraft-generated forces as possible.
*[An AU, the Sun-Earth distance, is about 150,000,000 km.]