27.07.2022

Scientists hope a laser-reflecting sphere will produce the most accurate measure so far of how Earth’s rotation warps space.

The surface of LARES-2 is covered in hundreds of reflectors that will reflect laser pulses sent out by a global network of laser-ranging stations.Credit: CNES/ESA/Arianespace/Optique Vidéo CSG/P. Baudon

A newly launched satellite aims to measure how Earth’s rotation drags the fabric of space-time around itself — an effect of Einstein’s general theory of relativity — ten times more accurately than ever before.

The Laser Relativity Satellite 2 (LARES-2) launched from the European Space Agency (ESA) spaceport in Kourou, French Guiana, on 13 July. It was built by the Italian Space Agency (ASI) at a cost of around €10 million (US$10.2 million), and lifted off on the maiden flight of an upgraded version of the European Vega rocket, called Vega C.

The rocket’s performance was “spectacular”, says mission leader Ignazio Ciufolini, a physicist at the University of Salento in Lecce, Italy. “ESA and ASI put the satellite into its orbit with a precision of just 400 metres.” This precise positioning will help improve the quality of the researchers’ measurements, Ciufolini adds.

“I think this is a great step forward for measuring this effect,” says Clifford Will, a theoretical physicist at the University of Florida in Gainesville.

Reflective sphere

LARES-2’s structure is disarmingly simple: it is a sphere of metal covered with 303 reflectors, with no on-board electronics or navigation control. The disco-ball-like design is similar to that of its predecessor LARES, a general-relativity experiment launched in 2012, and of a probe called LAGEOS deployed by NASA in the 1970s, primarily for studying Earth’s gravity. (The Lares, pronounced LAY-reez, were deities in the pagan religion of ancient Rome.)

LARES-2 packs around 295 kilograms of material into a sphere less than 50 centimetres across. Its density minimizes the effects of phenomena such as radiation pressure from sunlight or the feeble drag from Earth’s atmosphere at high altitudes, says aerospace engineer Antonio Paolozzi of Sapienza University in Rome. After experimenting with custom high-density materials, the team opted for an off-the-shelf nickel alloy. This had acceptable density and enabled LARES-2 to qualify for the Vega C maiden flight without expensive flight-certification tests.

Using an existing global network of laser-ranging stations, Ciufolini and his colleagues plan to track the orbit of LARES-2 for several years. This kind of probe can continue to provide data for decades. “You can just sit back and send laser beams to it,” Will says. “In terms of cost it’s a cheap, good thing to do.”

According to Newtonian gravity, an object orbiting a perfectly spherical planet should keep tracing 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 were rotating, 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 make the plane of the orbit precess, or rotate, around Earth’s axis by 8.6 millionths of a degree per year.

In practice, Earth itself is not a perfect sphere, but “shaped like a potato”, Ciufolini says. The resulting irregularities in Earth’s gravitational field — the very things that LAGEOS was designed to measure — add some extra orbital precession that can make the relativistic effect harder to measure. But by comparing the orbits of two satellites, these irregularities can be cancelled out.

Ciufolini, who has worked on the LARES mission concept since his PhD thesis in 1984, first applied this principle in 2004¹ to measure frame dragging from a comparison of the orbits of LAGEOS and of LAGEOS-2 (a similar probe launched by ASI). He and his collaborator Erricos Pavlis, at the University of Maryland in College Park, claimed to have nailed down the effect with an accuracy of 10%.

Even though the result was still rough, the team managed to scoop an $800-million NASA experiment that had aimed to measure frame dragging with a different technique. The highly complex Gravity Probe B mission, launched in 2004, measured changes not in the spacecraft’s orbital trajectory but in the inclination of four rotating spheres, shifting by a tiny fraction of a degree per year. Unforeseen complications meant that Gravity Probe B could achieve an accuracy of only 20%, far from its initial goal of 1%².

Launch limitations

Ciufolini and his team subsequently improved their earlier result to an accuracy of 2% with LARES, the first probe explicitly designed for this kind of experiment³. But the limitations of the launch vehicle — the earlier Vega rocket — meant that LARES could reach an altitude of only 1,450 kilometres. LARES-2 is now at a more optimal 5,900 kilometres, where the irregularities of Earth’s gravitational field are dampened but the effect of frame dragging is still strong.

The mission aims to get to 0.2% accuracy, and the precise orbital injection should put that goal well within reach, Ciufolini says. This could enable the team to tell whether general relativity wins over alternative theories for space-time, he adds.

Thibault Damour, a theoretical physicist at the Institute of Advanced Scientific Studies (IHES) near Paris, praises the experiment’s low cost. “If one finds a deviation [from the theoretical prediction] this would be a major result,” Damour says. But he adds that there have been more stringent tests of general relativity in space. NASA’s Cassini mission to Saturn measured a different effect of the theory to an accuracy of nearly one part in 10,000⁴.

Although weak around Earth, the effects of frame dragging become gigantic when two black holes spiral into each other and merge. Gravitational-wave observatories might already have begun to detect such effects in the final orbits of some black-hole pairs: from the shape of the waves, they can calculate how fast the lighter black hole was precessing, and how fast the heavier black hole was spinning. With the detection of gravitational waves, understanding frame dragging “has become fundamental to astrophysics”, Ciufolini says.

Quelle: nature