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Number 391, September 15, 1998 by Phillip F. Schewe and Ben Stein
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.)
THE MOST ACCURATE MEASUREMENT YET OF THE PLANCK CONSTANT, the number which describes the bundle-like nature of matter and energy at the atomic and subatomic levels, has been carried out by NIST physicists, instantly improving the accuracy of related fundamental constants (such as electron mass, proton mass, and Avogadro's number) and paving the way for a quantum-based definition of mass. Carrying out an experiment first proposed by Brian Kibble of the National Physical Laboratory in England (011-44-171-594-7845), a NIST group (Edwin Williams, 301-975-4206) determined Planck's constant, otherwise known as h, by using a "moving-coil watt balance," an apparatus with a kilogram mass connected to a metal coil in a magnetic field. Injecting a current through the coil created an upward magnetic force which exactly balanced the downward force of gravity on the mass. In a second step, the group allowed the coil to move downward, measuring its velocity and the voltage it generated. In both steps, the electrical power associated with the mechanical motions of the system contained quantities proportional to Planck's constant, allowing the researchers to extract the value of h while cancelling out factors such as the geometry of the setup. The team calculated a value for h of 6.62606891 x 10-34 Joule-seconds, with an uncertainty of 89 parts per billion, two times better than previously published measurements. Their watt-balance setup ultimately promises to lead to a definition of the kilogram based on quantum units, rather than one based on the stalwart physical artifact currently stored in France. (Williams et al., Physical Review Letters, 21 September 1998; figure at Physics News Graphics.)
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