A newly launched satellite aims to measure how the Earth’s rotation drives the fabric of spacetime around it – an effect of Einstein’s theory of general relativity – ten times more precisely than ever before.
The Laser Relativity Satellite 2 (LARES-2) was launched from the European Space Agency (ESA) spaceport in Kourou, French Guiana on July 13. It was built by the Italian Space Agency (ASI) at a cost of around 10 million euros (US$10.2 million) and took off on the maiden flight of an upgraded version of Europe’s Vega rocket. , called Vega C.
The rocket’s performance was “spectacular,” said mission leader Ignazio Ciufolini, a physicist at the University of Salento in Lecce, Italy. “ESA and ASI placed the satellite in orbit with an accuracy of just 400 meters.” This precise positioning will help improve the quality of the researchers’ measurements, adds Ciufolini.
“I think this is a big step forward in measuring this effect,” says Clifford Will, a theoretical physicist at the University of Florida in Gainesville.
The structure of LARES-2 is disarmingly simple: it is a metal sphere covered with 303 reflectors, with no on-board electronics or navigation controls. The disco ball-shaped design is similar to its predecessor LARES, another general relativity experiment launched in 2012, and a probe called LAGEOS deployed by NASA in the 1970s, primarily to study Earth’s gravity. (The Lares, pronounced LAY-reez, were deities of the pagan religion of ancient Rome.)
LARES-2 packs about 295 kilograms of material into a sphere less than 50 centimeters in diameter. Its density minimizes the effects of phenomena such as radiation pressure from sunlight or the low drag of the Earth’s atmosphere at high altitudes, says aerospace engineer Antonio Paolozzi of La Sapienza University in Rome. After experimenting with custom high-density materials, the team settled on a standard nickel alloy. This had acceptable density and allowed the LARES-2 to qualify for Vega C’s maiden flight without costly flight certification tests.
Using an existing global network of laser ranging stations, Ciufolini and his colleagues plan to track LARES-2’s orbit for several years. This type of probe can continue to provide data for decades. “You can just sit down and shoot laser beams at it,” Will explains. “In terms of cost, it’s a good thing to do.”
According to Newtonian gravity, an object orbiting a perfectly spherical planet should continue to trace the same ellipse, eon after eon. But in 1913, Albert Einstein and his collaborator Michele Besso used a preliminary version of general relativity to suggest that if such a planet rotated, it should cause the satellite’s orbit to shift slightly. The precise mathematics of the effect was calculated in 1918 by Austrian physicists Josef Lense and Hans Thirring. Modern calculations predict that the Lense-Thirring effect, a kind of relativistic “frame dragging”, should cause the plane of the orbit to precess, or rotate, around the Earth’s axis, by 8.6 millionths of a degree per year.
In practice, the Earth itself is not a perfect sphere, but “potato-shaped”, says Ciufolini. The resulting irregularities in Earth’s gravitational field — the very things LAGEOS was designed to measure — add extra orbital precession that can make the relativistic effect harder to measure. But by comparing the orbits of two satellites, these irregularities can be canceled out.
Ciufolini, who has been working on the LARES mission concept since his doctoral thesis in 1984, applied this principle for the first time in 20041 measure frame lag from a comparison of the orbits of LAGEOS and LAGEOS-2 (a similar probe launched by ASI). He and collaborator Erricos Pavlis of the University of Maryland at College Park claimed to have determined the effect with 10% accuracy.
Although the result was still approximate, the team managed to win an $800 million NASA experiment that aimed to measure frame slip with a different technique. The highly complex Gravity Probe B mission, launched in 2004, measured changes not in the spacecraft’s orbital path but in the tilt of four spinning spheres, moving a tiny fraction of a degree per year. Unforeseen complications meant that Gravity Probe B was only able to achieve 20% accuracy, far from its original target of 1%2.
Ciufolini and his team subsequently improved their previous result to an accuracy of 2% with LARES, the first probe explicitly designed for this type of experiment.3. But the limitations of the launch vehicle – the old Vega rocket – meant that LARES could only reach an altitude of 1,450 km. LARES-2 is now at a more optimal distance of 5,900 km, where irregularities in the Earth’s gravitational field are smoothed out but the effect of frame slip is still strong.
The mission aims to achieve 0.2% accuracy, and precise orbital injection should make that goal within reach, Ciufolini said. This could allow the team to tell whether general relativity wins out over alternative theories for spacetime, he adds.
Thibault Damour, theoretical physicist at the Institut des Hautes Etudes Scientifiques (IHES) near Paris, praises the low cost of the experiment. “If we find a deviation [from the theoretical prediction] that would be a major result,” Damour says. But he adds that there have been more rigorous tests of general relativity in space. NASA’s Cassini mission to Saturn measured an effect different from theory with an accuracy of nearly one part in 10,0004.
Although faint around Earth, frame drag effects become gigantic when two black holes wrap around each other and merge. Gravitational-wave observatories could already begin to detect such effects in the final orbits of some pairs of black holes: from the shape of the waves, they can calculate the speed of precession of the lightest black hole and the speed of rotation of the heaviest black hole. . With the detection of gravitational waves, understanding frame slip “has become fundamental to astrophysics,” says Ciufolini.