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No. 120: Nov-Dec 1998

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The Force Is With Them

The communication loop between earthbased ground stations and interplanetary spacecraft allows extremely accurate measurements of the radial velocities of these distant man-made machines. As these spacecraft hurtle toward the fringe of the solar system, the visible sun dwindles to a small, bright point, and its gravitational field falls off as the inverse square of the distance. At least that is what is supposed to happen.

Four far-flung spacecraft, (Pioneer 10, Pioneer 11, Ulysses, and Galileo are experiencing a mysterious decelerating force not encompassed by the Law of Gravitation. It's a tiny force, but it seems to be real. Making it even more puzzling is the fact that it is decreasing according to the inverse of distance from the sun rather than the inverse square. Is it a non-solar force? Is it "new" physics? Or maybe just an artifact of the spacecraft and ground-based equipment?

The fact that four spacecraft feel its tugging suggests the force is real. But the motions of the distant planets do not seem to be affected by it. So, everyone is perplexed.

(Schilling, Govert; "Spacecraft Motions Puzzle Astronomers," Science, 281:1581, 1998. Seife, Charles; "If the Force Is with Them...," New Scientist, p. 4, September 12, 1998. Browne, Malcolm; "After Study, Mystery Force Remains One," New York Times, September 17, 1998. Cr. M. Colpitts.)

From Science Frontiers #120, NOV-DEC 1998. 1998-2000 William R. Corliss

Science Frontiers Sourcebook Project Reviewed in:


  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
  • "..the science is necessarily somewhat speculative, but Corliss's symthesis is based on reputable sources." -- "Corliss, William R. (Compiler). Lightning, Auroras, Nocturnal Lights, and Related Luminous Phenomena" reviwed by Joseph M. Moran, Univ. of Wisconsin in Science Books and Films, Sep/Oct 1983

  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987