According to Newtonian gravity, an object orbiting a perfectly spherical planet should traverse the same ellipse over and over for eons. But in 1913, Albert Einstein and his collaborator Michele Besso, using a tentative version of general relativity, suggested that a rotating planet should cause a slight shift in the satellite’s orbit. The exact mathematical calculation of the effect was carried out in 1918 by the Austrian physicists Josef Lense and Hans Thirring. Modern calculations predict that the lens thirring effect, a kind of relativistic “frame dragging,” should cause the plane of the orbit to rotate about the Earth’s axis by 8.6 millionths of a degree per year.
In practice, the earth itself is not a perfect sphere, but “shaped like a potato,” says Ciufolini. The resulting irregularities in the Earth’s gravitational field – which is exactly what LAGEOS was designed to measure – lead to an additional precession of the orbit, which can complicate the measurement of the relativistic effect. However, these irregularities can be compensated for by comparing the orbits of two satellites.
Ciufolini, who has been working on the concept of the LARES mission since his PhD in 1984, first applied this principle in 2004 to measure the orbit shift by comparing the orbits of LAGEOS and LAGEOS-2 – a similar probe launched by ASI. He and his colleague Erricos Pavlis of the University of Maryland, Baltimore County, claimed to have determined the effect with an accuracy of 10 percent.
Although the result was still very inaccurate, the team managed to thwart a $800 million NASA experiment that tried to use a different technique to measure frame shift. The highly complex Gravity Probe B mission, launched in 2004, measured not the changes in the spacecraft’s trajectory, but the inclination of four rotating spheres, which shifted by a tiny fraction of a degree per year. Unforeseen complications meant that Gravity Probe B was only able to achieve an accuracy of 20 percent, far from the original goal of 1 percent.