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The Pioneer anomaly

NASA's atomic-powered robot Pioneer 10 and 11 sister spaceships were the first artificial objects to leave our solar system. Both probes explored the outer solar system after being launched from Cape Canaveral, Florida, during the early 1970s.

Both interplanetary Pioneer probes have since transitioned to interstellar space objects. And that's why they are best remembered for the unusual communiques they carry: attached to both spacecraft platforms are etched, metallic plaques with messages of peace from the planet Earth.

The illustrated Pioneer messages, showing the location of our solar system and Earth, as well as the probes mostly hairless bipedal creators, are meant for the eyes of extraterrestrials-that is, those future alien astronauts who might encounter the Pioneers adrift in interstellar space. Similar messages are onboard the Voyager 1 and 2 spacecraft which have already passed Pioneer 10 and 11 into interstellar space.

In 1983, Pioneer 10 was the first terrestrial object to leave the solar system. After three decades of tracking the Pioneers across the outer solar system, mission controllers lost radio contact with Pioneer 11 first. Then, Pioneer 10 fell silent in 2003. At the time its radio voice faded away, Pioneer 10 was 7.6 billion miles away from Earth.

Those of us who remember the exciting Pioneer 10 and 11 flybys of the giant planets Jupiter and Saturn during the 1970s may not be aware that Pioneer 10 left the science community with a space mystery to solve-a mystery now known as the Pioneer Anomaly.

To understand the complex mathematics behind the Pioneer Anomaly would require a deep study of gravitational and Doppler physics, but the easiest way to explain the Pioneer mystery is to describe it simply this way:

When a spacecraft leaves the solar system - in our case, Pioneer 11- it slows down. It's not supposed to do that according to the way physics is understood today. So, before Pioneer 11 exited our solar system in 1983, physicists and space mission planners had expected just the opposite.

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