An unexplained force is pulling on distant spacecraft. It could be an unnoticed effect in the spacecraft themselves, but scientists feel it may be the first hint that we don?t fully understand the force of gravity.

?It is almost as if the probes are not behaving according to the known law of gravity,? says Dr. John Anderson of NASA. ?We?ve been working on this problem for several years, and we have accounted for everything we could think of.?

The unexplained force is tugging at 4 deep-space probes scattered around the solar system. For example, Pioneer 10, which was launched in 1972 and is now well beyond Jupiter, is slowing down more quickly than it should.

The mystery deepened when it was discovered that its sister spacecraft, Pioneer 11, launched in 1973, was also slowing down. But Pioneer 11 is on the opposite side of the solar system from Pioneer 10, meaning this can?t be due to the gravitational effect of some unseen body. There is also evidence that the same thing is happening to the Galileo spacecraft that is on its way to Jupiter, as well as the Ulysses space probe that is circling the sun.

Scientists have discovered that the strength of the slowing effect seems to be related to the speed of light and the speed of the expansion of the universe. Others have argued that if the anomaly indicated a change in our understanding of gravity, it would be revealed in the orbits of the planets around our sun?which it isn?t.

Since the 4 probes that are showing the effect will never return to earth, it remains a mystery for now.

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