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Both probes are now travelling at 27,000 m.p.h. towards stars that they will encounter several million years from now. Scientists are continuing to monitor signals from Pioneer 10, which is more than seven billion miles from Earth. Research to be published shortly in The Physical Review, a leading physics journal, will show that the speed of the two probes is being changed by about 6 m.p.h. per century—a barely-perceptible effect about 10 billion times weaker than gravity. Scientists initially suspected that gas escaping from tiny rocket motors aboard the probes, or heat leak- ing from their nuclear power plants might be responsible. Both have now been ruled out. The team says no current theories explain why the force stays constant: all the most plausible forces, from gravity to the effect of solar radiation, decrease rapidly with distance. The bizarre behavior has also eliminated the possibility that the two probes are being affected by the gravitational pull of unknown planets beyond the solar system. Assertions by some scientists that the force is due to a quirk in the Pioneer probes have also been dis- counted by the discovery that the effect seems to be affecting Galileo and Ulysses, two other space probes still in the solar system. Data from these two probes suggests the force is of the same strength as that found for the Pioneers. Dr. Duncan Steel, a space scientist at Salford University, says even such a weak force could have huge effects on a cosmic scale. “It might alter the number of comets that come towards us over millions of years, which would have consequences for life on Earth. It also raises the question of whether we know enough about the law of gravity.” Until 1988, Pioneer 10 was the most remote object made by man—a distinction now held by Voyager 1. Should Pioneer 10 make contact with alien life, it carries a gold-plated aluminum plaque on which the figures of a man and woman are shown to scale, along with a map showing its origin that NASA calls “the cosmic equivalent of a message in a bottle.” 286 Appendix C: Suggested Areas of Research Atlantis, Alien Visitation, and Genetic Manipulation