A lone researcher working with borrowed data may have beaten a $700 million NASA mission to the finish line by being the first to measure an obscure subtlety of Einstein鈥檚 general theory of relativity.
The phenomenon in question is the Lense-Thirring effect, a small force produced as the fabric of space-time gets twisted by a spinning mass such as a rotating planet. The force will drag the point at which a polar-orbiting satellite crosses the planet鈥檚 equator by a small amount each year. In April 2004, after 40 years of funded development, NASA launched Gravity Probe B to measure the effect of this force on precision-engineered gyroscopes. A final analysis of the data it collected is still to come.
Now Lorenzo Iorio of the University of Bari in Italy says he has found evidence of the effect in data prepared by Alex Konopliv of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, and his colleagues. To showcase just how sophisticated NASA鈥檚 grasp of Mars was becoming, Konopliv鈥檚 team modelled the orbit of the Mars Global Surveyor craft, accounting for such factors as atmospheric drag and solar radiation pressure. They then compared the model with the real orbital data and produced a graph of 鈥渞esiduals鈥�, which quantify the difference between the real and modelled orbits.
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鈥淲hen I saw that graph I realised that I could look for the Lense-Thirring effect in the residuals,鈥� Iorio says. It was the only gravitational effect that Konopliv鈥檚 team had not modelled. After grinding through the mathematics, Iorio found that in five years the Global Surveyor鈥檚 orbit had been dragged around by 1.6 metres. It looked like a clear signature of the Lense-Thirring effect, though slightly smaller than predicted by theory (). Iorio is unconcerned by this discrepancy, which he believes is probably caused by small errors in Konopliv鈥檚 original model rather than by some exotic new physics.
The result has intrigued the Gravity Probe B team. James Overduin, a team member at Stanford University, California, likes Iorio鈥檚 idea but remains sceptical about the details. 鈥淓xperimental claims of this importance need to be supported by rigorous error analysis, and it鈥檚 far from clear that this new [work] by Iorio meets that standard,鈥� he says. 鈥淎 more serious treatment would be of significant interest.鈥�
鈥淐laims of this importance need to be supported by rigorous error analysis, and it鈥檚 not clear this standard has been met鈥�
Iorio believes that new tests of the Lense-Thirring effect will soon be possible thanks to the precise orbital data that NASA is collecting for the Mars Odyssey spacecraft.
