S301 belongs to the family of S stars that swarm the centre of the Milky Way, and it is both the fastest moving and the closest-orbiting member found so far on a securely determined orbit. Its orbit is strongly elliptical: it takes 8.7 years to complete and brings the star within about 12 astronomical units of Sagittarius A*, a little further out than Saturn is from the Sun. At that point it is travelling at roughly 25,000 km per second. The measurement was made with GRAVITY+, an upgraded version of the GRAVITY instrument on the European Southern Observatory's Very Large Telescope Interferometer at Paranal in Chile, which combines light from four 8-metre telescopes to reach a resolution no single telescope could achieve. Felix Mang and Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics were among those who led the work, published in Nature.

The mass of Sagittarius A* is already known, but its spin is not, and spin is the last basic property needed to describe the black hole completely. A star this close and this fast is the instrument that can supply it. A rotating massive body drags spacetime around with it, an effect called frame dragging, and that dragging should slowly twist the orientation of S301's orbit. Tracking that twist over the coming years would both measure the spin and test general relativity in a strong-field, rotating regime that no previous stellar orbit has reached.