Its been a bad year to bet against Albert Einstein. In the spring physicists had to withdraw a sensational report that the subatomic particles known as neutrinos were going faster than light, Einsteins cosmic speed limit; they discovered they had plugged in a cable wrong. Now scientists from NASAs Jet Propulsion Laboratory have reported that they have explained one of the great mysteries of the space age, one that loomed for 30 years as a threat to the credibility of Einsteinian gravity.