It's of no consequence, but the Pioneer 10 spacecraft launched on my 9th birthday. Although I don't recall if I noted the event while in the 3rd grade, that was the year I remember becoming interested in astronomy. That year I did a big report on "space" with my then best friend. (I still wonder if there are any remnants of the spray paint we used on the schoolyard blacktop to create our impressive outer-space backdrop for our model solar system.)

During the following 3½ decades, Pioneer 10 and its twin, Pioneer 11, blazed a trail out to Jupiter and Saturn, and then continued onward, well beyond the orbit of Pluto. They eventually fell silent — Pioneer 11 faded out in 1995 and Pioneer 10 lasted until 2003.

But during the mission's many years of operation, tracking specialists noticed something odd: neither craft was exactly where it was supposed to be. After crossing billions of miles in space, they were instead thousands of miles closer to the Sun than expected — a tiny fraction of the distance traveled, but nevertheless significant. Significant enough to have some people wondering if it was a sign of some unknown refinement needed to the theory of gravity, a bedrock of modern physics.

The latest examination of this curiosity concludes that no exotic explanations are required. According to Slava Turyshev (Jet Propulsion Laboratory), some of the retarding force is due to uneven emission of heat radiation from the spacecraft. He described his preliminary findings at a meeting of the American Physical Society earlier in the week, summing up work he's been doing for 15 years.

Turyshev has spent the last several years retrieving archival tracking records from obsolete storage media (those classic magnetic tapes, some of them corrupted) as well as detailed specifications of the Pioneer spacecraft itself from 40 years ago. He likens his searching and discovery to rooting around a dusty attic. "No one told me what I'd be getting into," he says. The Pioneer missions lasted so long that they outlived programming languages and data formats. (The Pioneers were launched in the days of punched cards.)

After piecing together the tracking history, Turyshev and his colleagues created a computer model of the Pioneer spacecraft and determined that they had warm and cool spots not previously modeled well. The uneven emission of infrared photons from the craft's radioactive power sources could account for at least a third of Pioneer 11's anomalous acceleration. (Photons, like other particles, carry a bit of momentum.)

What's still unclear is how the warm surfaces may have physically changed during the decades in space. Space weathering by dust particles, solar wind, and ultraviolet sunlight could have altered the thermal properties of the surfaces significantly, possibly enhancing the effect enough to account for the entire "Pioneer anomaly." The researchers hope to get clues to this by running various computer simulations of their model and by examining 40 gigabytes of telemetry data, which will indicate the state of many systems on the spacecraft, and thus how much power they actually used and how much heat they gave off.