http://www.foxnews.com/story/0,2933,334090,00.html


Mysteriously, four spacecraft that flew past the Earth have each displayed unexpected anomalies in their motions.

These newfound enigmas join the so-called "Pioneer anomaly" as hints that unexplained forces may appear to act on spacecraft.

A decade ago, after rigorous analyses, anomalies were seen with the identical Pioneer 10 and 11 spacecraft as they hurtled out of the solar system.

Both seemed to experience a tiny but unexplained constant acceleration toward the sun. In other words, they were moving just a little bit slower than they should be, and by the same amount.