An orbiting disco ball gave Einstein’s theory its most precise test yet - Ars Technica

Albert Einstein’s general theory of relativity predicts that a rotating mass like the Earth pulls the fabric of space and time around with it in a perpetual swirl. This phenomenon is known as frame dragging or the Lense-Thirring effect, after the two physicists who modeled it back in 1918. Frame dragging becomes more significant with larger masses and faster rotation, so we’ve mainly observed it around huge black holes.

Measuring how much the Earth twists spacetime as it rotates has been much more challenging because our pale blue dot of a planet is millions of times lighter than a typical black hole and rotates rather slowly.

But now, a team of astronomers led by Ignazio Ciufolini, a physicist at the Wuhan Institute of Physics and Mathematics in China, reports the most accurate measurement of the terrestrial Lense-Thirring effect to date. Their work brings our uncertainty down from a few percentage points to just 0.2 percent. And they did it with a satellite that looks like a cross between a golf ball and a disco globe.

The disco globe satellite that Ciufolini and his colleagues use in their experiment is called LARES-2 (Laser Relativity Satellite 2) and has been developed by the Italian Space Agency. It’s a solid sphere of Inconel 718, a dense nickel-chromium alloy, covered with 303 corner-cube retroreflectors and measuring a bit over 40 centimeters across. It has no thrusters, no solar panels, and no electronics of any kind. It weighs 294.8 kilos. That combination of small size and large mass gives it the lowest area-to-mass ratio of any satellite in medium-Earth orbit.

This was exactly what the scientists needed, since it helped them minimize the impact of other forces.

“The idea is that we want to measure gravitation,” Ciufolini said. “We have non-gravitational effects like photons impinging on the satellite and pushing it. So, the mass must be very large and the cross-section of the satellite very small, so the acceleration induced by photons is very, very small.” In theoretical physics, satellites of this kind are called test particles, meaning an object whose motion is governed almost entirely by the gravitational field. LARES-2 was placed in orbit at an altitude of roughly 12,265 kilometers by a Vega-C rocket in July 2022.

Once the LARES-2 was in position, the researchers started shooting it with ground-based lasers.

The retroreflectors on LARES-2 are designed to reflect a beam of light exactly in the direction this beam came from. When Ciufolini and his colleagues fired short laser pulses at the satellite, they could pinpoint its position down to roughly 1 millimeter based on the light that came back. About 200,000 such observations, spanning July 2022 to June 2025, formed the dataset the team used to measure Earth’s frame dragging.

But even such precise positioning was not enough to achieve the accuracy the team wanted.

The problem with measuring frame dragging using Earth-orbiting satellites is that the Earth is not a perfect sphere. Its equatorial bulge produces classical Newtonian forces on satellite orbits that are orders of magnitude larger than the frame dragging signal. The solution Ciufolini proposed decades ago while working with physicist John Archibald Wheeler was to use two satellites in supplementary orbits, meaning with orbital inclinations that sum to 180 degrees.

“Suppose we have a satellite orbiting around a perfectly spherically symmetric object—the orbit of this satellite would act like a gyroscope,” Ciufolini said. Under ideal conditions, the orbital plane and its orientation in space would remain fixed, and the only thing altering this orientation should be frame dragging.

“But the Earth is not spherically symmetric,” Ciufolini said. “It is oblate, and this oblateness produces a change in the orientation of the orbital plane.” With two satellites at supplementary inclinations, the Newtonian perturbations are equal and opposite in the two orbital planes and cancel each other out. The Lense-Thirring effect, which pushes both orbital planes in the same direction, adds algebraically—the noise vanishes and the relativistic signal survives.

That’s why LARES-2 was working in synchrony with its older and larger cousin called LAGEOS, a NASA satellite designed exclusively for high-precision laser-ranging, launched in 1976. The orbital inclinations LAGEOS and LARES-2 summed up to 180.01 degrees, which the team considered close enough.

But the Earth’s irregular shape was not the only challenge.