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.
Thanks to radio tracking data, received at an Australian tracking station, by the ever receding Pioneer 11, this vexing anomaly has become a fascinating research problem for astrophysicists.
Some physicists attribute the Pioneer slowdown to the gravitational effects of the Sun, some to the effects of so-called mirror matter or antimatter, others to spacecraft problems like undetected hydrazine thruster venting that is affecting velocity.
While common sense tells us the effects of the Sun's gravity on a spacecraft leaving our solar system should grow weaker (not stronger), the opposite seems to be the case. As a result of the gravity field change, the Pioneers are not to be found in the spot in deep space they're supposed to be at-they're lagging behind.
According to NASA-JPL's John Anderson, who has been studying this anomaly since the 1980s, "Something strange is happening in the outer reaches of our solar system. The Pioneer 10 and 11 spacecraft are not where they are supposed to be. These missions, launched in 1972 and 1973, have covered hundreds of millions of kilometers, heading toward the edge of our solar system. But something is holding them back. Each year, they fall behind in their projected travel by about 5,000 kilometers (3,000 miles)."
Anderson and his team of researchers haven't figured it out-yet.
"No spacecraft behavior or previously unknown property of the outer solar system can explain the deceleration of the Pioneer spacecraft," says the Planetary Society's website about the problem. "Scientists are being forced to consider the unthinkable: something may be wrong with our understanding of the laws of physics. An important line of inquiry will be to study mounds of Doppler (velocity) data and spacecraft status data (like temperatures) that have been unavailable to researchers-but that is about to change."
Thanks to NASA and a project spearheaded by the Planetary Society, Anderson said "We have now recovered data covering nearly 30 years to help solve the mystery, in addition to recovering never before analyzed spacecraft status data that will be valuable in assessing possible spacecraft influences on the Anomaly. Analysis of the recovered data is now in progress."
Stay tuned. Anderson and company may yet help revise the physics textbooks.
What's in the Sky: Southeast after sunset Jan. 23 look for Messier object 24, a huge star cloud within our Milky Way, spread across thousands of light years; it's a part of a spiral arm of our galaxy. M24 is easily located with the unaided eye.
Louis Varricchio, M.Sc., was a senior science writer at the NASA Ames Research Center in California. He is a member of the NASA-JPL Solar System Ambassador program in Vermont. He is the recipient of the U.S. Civil Air Patrol's Chuck Yeager Achievement Award in Aerospace Education.