If the force is with them . . .
By Charles Seife GRAVITY may not be working as advertised. Spacecraft hurtling through the Solar System have been behaving so bizarrely that some scientists wonder whether our theories of gravity are wrong. “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,