S301 Hits 56 Million MPH, Could Reveal Milky Way Black Hole’s Spin

Fifty-six million mph is the peak speed of S301, a newly characterized star racing around Sagittarius A*, the supermassive black hole at the center of the Milky Way. That is more than 8% of the speed of light and fast enough to make S301 the fastest known star in our galaxy.

The larger scientific opportunity comes from where that speed is reached. S301 passes within about 12 times the Earth-sun distance of the black hole and completes its highly elliptical orbit in just 8.7 years. The peer-reviewed study published in Nature on August 19, 2026, concludes that this compact orbit could make the star sensitive to spacetime distortions caused by the black hole’s rotation. Measuring that effect may provide the first direct determination of Sagittarius A*’s spin, but researchers have not completed that measurement.

A short orbit creates a faster test

The 8.7-year period is central to S301’s value. Astronomers looking for small changes in a stellar orbit need repeated passes, and stars with longer periods can require decades of monitoring before enough motion accumulates. Researchers have already reconstructed one S301 orbit from 19 observational datasets spanning more than eight years, including archival detections from 2017 and 2021 and dedicated observations beginning in 2023.

Those measurements came from the European Southern Observatory’s Very Large Telescope Interferometer in Chile. Its GRAVITY instrument combines light collected by four large telescopes, providing the angular resolution needed to separate a faint star from the crowded and much brighter environment around Sagittarius A*. Multiple image-reconstruction and point-source fitting methods returned consistent positions for S301, an important cross-check when the target sits near the practical limits of the observations.

The star’s path is already subject to the familiar relativistic effect produced by the black hole’s mass: its orbit gradually changes orientation rather than repeating as a fixed Newtonian ellipse. The proposed spin measurement targets a smaller additional effect called frame dragging. Under general relativity, a rotating black hole twists the surrounding spacetime, slightly altering the orbit of a sufficiently close object.

For S301, the predicted spin-driven change is small compared with the orbital shift caused by the black hole’s mass. The study calculates an in-plane frame-dragging contribution of up to roughly 0.11 degrees per revolution, depending on the black hole’s spin and its orientation relative to the star’s orbit. Detecting that increment requires precision across repeated observations rather than simply confirming the star’s speed or closest approach.

The measurement still has important constraints

Current observations do not include S301 radial-velocity measurements, leaving two possible orientations for its three-dimensional orbit. Position measurements show its motion across the sky, but spectroscopy is needed to establish the line-of-sight component and reconstruct the full trajectory. That distinction matters because the apparent spin signature depends partly on the geometry between the black hole’s rotation and the stellar orbit.

Nearby objects also complicate the analysis. A population of stars and stellar-mass black holes around Sagittarius A* can gravitationally perturb S301 and produce orbital changes that may overlap with the relativistic signal. Simulations indicate those disturbances are generally smaller than the expected frame-dragging effect for moderate-to-high black-hole spins and favorable orientations, but they cannot simply be ignored. Robust analysis will have to model both the relativistic terms and the surrounding mass distribution.

The researchers’ simulated observing campaign extends through 2035 and assumes improved position measurements plus future radial-velocity data. Under an optimistic case involving a rapidly spinning black hole and favorable alignment, the model indicates that the spin could be constrained within about a decade. That is a forecast based on assumed precision and geometry, not a detection already in hand.

One proposed explanation for S301’s location is that it once belonged to a binary-star system disrupted by Sagittarius A*. In that scenario, S301 was captured while its companion was expelled. The star’s extreme eccentricity is consistent with that mechanism, but the disruption itself was not observed and remains an inferred origin.

The next hardware step is as important as the orbital clock. ESO expects its Extremely Large Telescope to begin scientific observations in December 2030, with its MICADO instrument anticipated to add the sensitivity needed for better velocity measurements. S301 is expected to make another closest passage in 2031. That pairing a new telescope entering service as the star returns to the most revealing section of its orbit could determine whether 56 million mph becomes more than a speed record and turns into a direct measurement of a black hole’s rotation.

By David Whitaker — Associate editor for AMI’s aerospace and drone systems desk, translating flight systems, aircraft programs, spaceflight, and UAV developments into accessible technical stories.