ANOMALOUS ACCELERATION. Data from several spacecraft, including Pioneer 10 and 11, Galileo, and Ulysses, provide evidence for an unexplained, weak, long-range acceleration, a new report shows. Position and velocity information is derived from radio signals sent from the craft to the Deep Space Network back on Earth. Any change in velocity over time can be ascribed to a variety of known sources: the sun and planets, the solar wind, the Milky Way, the Kuiper belt, etc. But even after taking this all into account, as well as other possibilities such as the presence of dark matter in the solar system (only a millionth of a solar mass of dark matter could reside within the orbit of Uranus, it is estimated) or gas leakage from the vehicles themselves, a small acceleration in the direction of the Sun---8 x 10^-8 cm/sec² for Pioneer 10---remains unaccounted for. Signs of this anomaly first appeared in the Pioneer tracking as long ago as 1980; Pioneer 10 was launched in 1972 and is presently 70 astronomical units from Earth. Now six space scientists, armed with many years of Pioneer data, supplemented with trajectory information from Galileo and Ulysses, have carried out the first thorough analysis of the problem and find the anomaly to be as persistent as ever. (The Voyager spacecraft are less useful for determining acceleration anomalies.) The researchers doubt but do not rule out the possibility of a novel gravitational effect or other kind of new physics. Alternative explanations include subtle systematic errors in the data analysis or unexpected aspects of space navigation. Further work on this problem may extend to the observed motions of planets, comets, and the proposed Pluto Express craft. (John D. Anderson et al., Physical Review Letters, 5 October 1998; contact John Anderson at JPL, 818-354-3956, john.d.anderson@jpl.nasa.gov; or Michael Nieto at Los Alamos, 505-667-6127, mmn@mmn.lanl.gov; journalists can obtain copies of the article from AIP Public Information.)