The Disco Ball satellite will put Einstein’s theory to the toughest test yet


The surface of the LARES-2 is covered with hundreds of reflectors that will reflect laser pulses transmitted by a global network of laser range stations.Credit: CNES / ESA / Arianespace / Optique Vidéo CSG / P. Boudon

A newly launched satellite aims to measure how the Earth’s rotation is pulling the fabric of spacetime around itself – an effect of Einstein’s general theory of relativity – with ten times greater accuracy than ever before.

The Laser Relativistic Satellite 2 (LARES-2) was launched from a 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 about 10 million euros (US$10.2 million), and took off on the first flight of an upgraded version of Europe’s Vega rocket, called the Vega C.

The rocket’s performance was “amazing,” says mission leader Ignacio Ciofolini, a physicist at the University of Salento in Lecce, Italy. “The European Space Agency and the US Space Agency put the satellite into orbit with an accuracy of only 400 metres.” This precise positioning will help improve the quality of the researchers’ measurements, Siofolini adds.

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

reflective field

The structure of the LARES-2 is very simple: it is a metal ball covered with 303 reflectors, without electronics or navigation control. The disco ball-like design is similar to that of its predecessor LARES, another experiment of general relativity launched in 2012, and a probe called LAGEOS deployed by NASA in the 1970s, primarily to study Earth’s gravity. (Laris, pronounced LAY-reez, were deities in the pagan religion of ancient Rome.)

LARES-2 packs approximately 295 kilograms of material into a ball less than 50 centimeters in diameter. Its density reduces the effects of phenomena such as radiation pressure from sunlight or weak clouds from the Earth’s atmosphere at high altitudes, says aeronautical engineer Antonio Polozzi of Sapienza University in Rome. After experimenting with high-density custom materials, the team selected a ready-to-use nickel alloy. This intensity was acceptable and enabled LARES-2 to qualify for its first Vega C flight without expensive flight certification tests.

Using an existing global network of laser rangefinding stations, Ciufolini and colleagues plan to track the orbit of LARES-2 for several years. This type of sensor can continue to provide data for decades. “You can just sit back and send lasers to her,” Will says. “In terms of cost, that’s a cheap and good thing.”

According to Newtonian gravity, an object orbiting a perfectly spherical planet should continue to follow the same ellipse, eon after eon. But in 1913, Albert Einstein and his collaborator Michel 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 exact mathematics of the effect was calculated in 1918 by Austrian physicists Joseph Lins and Hans Thiering. Recent calculations predict that the Lense-Thirring effect, a kind of relativistic “frame drag”, should make the plane of the orbit move, or rotate, about the Earth’s axis, by 8.6 millionths of a degree per year.

Practically speaking, the Earth itself isn’t a perfect sphere, but rather “in the shape of a potato,” says Siovolini. The resulting irregularities in Earth’s gravitational field – the very things that LAGEOS is designed to measure – add some extra orbital motion that can make the relativistic effect more difficult to measure. But by comparing the orbits of two satellites, these irregularities can be canceled out.

CSG Vega-C VV21 Launcher Launched

LARES-2 was launched on July 13 aboard a Vega C rocket.Credit: CNES / ESA / Arianespace / Optique Vidéo CSG / S. Martin

Ciufolini, who has worked on the concept of the LARES mission since his doctoral thesis in 1984, first applied this principle in 2004.1 To measure frame drag from a comparison of the orbits of LAGEOS and LAGEOS-2 (a similar probe launched by ASI). He and his co-author Erikus Pavlis, at the University of Maryland in College Park, claimed to have nailed the effect with an accuracy of 10%.

Although the result was still approximate, the team was able to get hold of an $800 million NASA experiment that was meant to measure tire drag with a different technique. The highly complex Gravitational Probe B mission, launched in 2004, measures changes not in the spacecraft’s orbital trajectory but in the inclination of four rotating fields, shifting by a fraction of a degree per year. An unexpected complication meant that Gravity Probe B could only achieve an accuracy of 20%, far from its initial 1% goal.2.

launch restrictions

Ciufolini and his team subsequently refined their previous results to an accuracy of 2% using LARES, the first probe designed explicitly for this type of experiment.3. But the limitations of the launch vehicle – the former Vega missile – mean that LARES can only reach an altitude of 1,450 km. LARES-2 is now 5900 km long, since the irregularity of the Earth’s gravitational field is reduced, but the effect of tire drag is still strong.

The mission aims for 0.2% accuracy, and micro-orbital injection should make that target within reach, Ciufolini says. He adds that this could enable the team to see if general relativity outperforms alternative theories of spacetime.

Thibault Damour, a theoretical physicist at the Institute for Advanced Scientific Studies (IHES) near Paris, praises the experiment’s low cost. “If one finds a deviation, [from the theoretical prediction] “This will be a major finding,” Damour says, but adds that there have been more rigorous tests of general relativity in space. NASA’s Cassini mission to Saturn measured a different effect of the theory to an accuracy of about one part in 10,000 .4.

Despite the weakening around Earth, the effects of frame-pulling become massive when two black holes spiral into each other and merge. Gravitational-wave observatories may have already begun to detect such effects in the terminal orbits of some pairs of black holes: from the shape of the waves, they can calculate how fast the lighter black hole spins, and how fast the heavier black hole spins. . With the discovery of gravitational waves, Siovolini says, understanding frame drag “has become fundamental to astrophysics.”

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