Pioneering Gas Leak?; December 1998; Scientific American Magazine; by Musser; 2 Page(s)

Scientists are victims of their own success: as theories improve, it becomes harder to distinguish genuinely new phenomena from boring experimental errors. The recent announcement of discrepancies in the motions of distant space probes is a case in point. When Pioneer 10 and 11--launched in 1972 and 1973 to visit Jupiter and Saturn-- ventured beyond the realm of the nine planets in the early 1980s, researchers began monitoring their orbits for evidence of the long-hypothesized Planet X. They found no such planet, in accordance with later observations, but they did notice that the Pioneers have been slowing down faster than predicted by Einstein¿s general theory of relativity. Some extra tiny force--equivalent to a ten-billionth of the gravity at Earth¿s surface-- must be acting on the probes, braking their outward motion.

"I started out looking for Planet X but stumbled on this instead," says John D. Anderson of the Jet Propulsion Laboratory in Pasadena, Calif. In 1994 Michael Martin Nieto of Los Alamos National Laboratory and his colleagues suggested that the anomaly was a sign that relativity itself had to be modified.