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I once read somewhere that the forces acting on the spacecrafts might be due to the fact that they are metal objects passing through a magnetic field (the Sun's) and so, through 'induction', a force is exerted onto them. But I don't think they mention this in the article below.

[Induction: "The generation of an electromotive force in a closed circuit by a varying magnetic flux through the circuit."]

The Problem with Gravity: New Mission Would Probe Strange Puzzle

By Robert Roy Britt
Senior Science Writer
posted: 18 October 2004

Imagine the weight of a nagging suspicion that what held your world together, a constant and consistent presence you had come to understand and rely on, wasn't what it seemed. That's how scientists feel when they ponder gravity these days. For more than three centuries, the basics of gravity were pretty well understood. Newton described the force as depending on an object's mass. Though it extends infinitely, gravity weakens with distance (specifically, by the inverse square of the distance). Einstein built on these givens in developing his theory of relativity. Then more than a decade ago a researcher noticed something funny about two Pioneer spacecraft that were streaming toward the edge of the solar system. They weren't where they should have been.

Something was holding the probes back, according to calculations of their paths, speed and how the gravity of all the objects in the solar system -- and even a tiny push provided by sunlight -- ought to act on them.

Now scientists have proposed a new mission to figure out what's up with gravity.

Staggering possibilities

Pioneer 10 and 11 launched in 1972 and 1973. Today each is several billion miles away, heading in opposite directions out of the solar system.

The discrepancy caused by the anomaly amounts to about 248,500 miles (400,000 kilometers), or roughly the distance between Earth and the Moon. That's how much farther the probes should have traveled in their 34 years, if our understanding of gravity is correct. (The distance figure is an oversimplification of the actual measurements, but more on that in a moment.)

Scientists are quick to suggest the Pioneer anomaly, as they call it, is probably caused by the space probes themselves, perhaps emitting heat or gas. But the possibilities have been tested and modeled and penciled out, and so far they don't add up.

Which leaves open staggering possibilities that would force wholesale reprinting of all physics books:

Invisible dark matter is tugging at the probes

Other dimensions create small forces we don't understand

Gravity works differently than we think

lot's more at:
http://www.space.com/scienceastronomy/mystery_monday_041018.html