DynaSoar writes "On Friday November 13th, ESA'a Rosetta probe will get its third and final gravity assist slingshot from Earth on its way to its primary targets, the asteroid Lutetia and Comet 67P/Churyumov-Gerasimenko. 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. More troubling than mere speed increase is the inconsistency of the effect. While Galileo and NEAR had appreciable speed increases, Cassini and Messenger did not. Rosetta itself gained more speed than expected from its 2005 fly-by, but only the expected amount from its 2007 fly-by. Several theories have been advanced, from mundane atmospheric drag to exotic variations on special relativity, but none are so far adequate to explain both the unexpected velocity increases and the lack of them in different instances. Armed with tracking hardware and software capable of measuring Rosetta's velocity within a few millimeters per second while it flies past at 45,000 km/hr, ESA will be gathering data which it hopes will help unravel the mystery."
Never try to teach a pig to sing. It wastes your time and annoys the pig.
-- Lazarus Long, "Time Enough for Love"