No. 120: Nov-Dec 1998
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.)