Ulysses and the 'Mystery Force'
15 October 1998Recent reports have created a stir among scientists studying the effects of gravity. A team leadby John Anderson, a planetary researcher at the Jet Propulsion Laboratory in Pasadena, hasbeen conducting an experiment in celestial mechanics using the radio signals from spaceprobesfar from the Earth, including Pioneers 10 and 11, and Ulysses. In all cases, Anderson and hiscolleagues found that an unexplained Sun-directed force appears to be acting on the spacecraft.
This apparent deviation is tiny - equivalent to an acceleration of 120 billionths of a centimetre per second squared in the case of Ulysses - and has no impact on spacecraft operations. If a force of this size were applied to a car moving at 60 km/hr, it would take 650 years to come to a standstill! Nevertheless, for scientists studying the detailed aspects of gravitational theories, it presents a puzzle.
As John Anderson himself admits, despite a very thorough analysis, the most likely explanation is a systematic effect in the radio data that has been somehow overlooked. If a real "mystery force" is involved, it should also affect the orbits of the planets, and this can be almost certainly ruled out. Another exotic candidate, dark matter, is equally unable to account for the observations.
Until someone can come up with definitive proof that the data are in error, the possibility remains that Anderson and his team have uncovered a new physical phenomenon. "There's a small probability that it's very important," says Anderson. [Extracted from New Scientist, 12 September, 1998].