Another blogger here, who is generally so far off the mark that I don't want to point to his earlier discussion, had some odd things to say about the "Pioneer anomaly," the unexplained deviation of the two Pioneer spacecraft from their predicted trajectory as they pass through the outer reaches of the solar system.

A much more cogent discussion has just appeared on the Planetary Society's website. This describes the challenging development of a mathematical model of Pioneer 10 that accounts for trajectory changes due to its reflection and absorption of sunlight and thermal effects.

That modeling effort explains enough of the anomaly to quiet those who are proposing more arcane solutions, such as unknown physics, but it leaves plenty of challenges for those who are proposing interactions with the solar wind or undiscovered interstellar materials--even dark matter.

It's the kind of open question that excites physicists like me. As I discuss in my history of 20th century physics, Physics: Decade by Decade (Facts On File, 2007), open questions can sometimes lead science in unexpected new directions or to new discoveries.

Stay tuned to Pioneer for more news from the edge of the Solar System. (And consider supporting The Planetary Society, as I do.)