Saturday, November 14, 2009

Pioneering Misinformation

Slashdot is running a story about a physics experiment to be performed with the Rosetta spacecraft,

...but the slingshot itself will allow ESA scientists to examine the trajectory for unusual changes seen in several other probes' velocities. An unaccountable variation was first noticed as excess speed in Pioneers 11 and 12, and has since been called the Pioneer Anomaly.

Uh, no, sorry, that would be Pioneer 10 and 11, not 11 and 12. I should know, I've done a little work on the subject.

The so-called flyby anomaly that would be measured with Rosetta is quite distinct from the "Pioneer Anomaly." Both are unexplained discrepancies between measured Doppler shift data and currently understood theory, but the Pioneer Anomaly pertains to unexplained gradual velocity shifts of spacecraft cruising through deep space, while flyby anomalies pertain to sudden impulses as a spacecraft swings by the earth. Both discrepancies have been observed. In all likelihood, these experiments are telling us that our models of the classical physical forces affecting these spacecraft are not complete. Perhaps, on the odd chance that there is "new physics" involved, both anomalies are related somehow. But they definitely not the same observed effect.

Thankfully the original ESA press release gets these points correct. The more subtle points seem to have gotten lost in translation on the way to publication in Slashdot.