S2, also known as S0–2, is a star in the star cluster close to the supermassive black hole Sagittarius A* (Sgr A*), orbiting it with a period of 16.0518 years, a semi-major axis of about 970 au, and a pericenter distance of 17 light hours (18 Tm or 120 au) – an orbit with a period only about 30% longer than that of Jupiter around the Sun, but coming no closer than about four times the distance of Neptune from the Sun.

[citation needed] Its changing apparent position has been monitored since 1995 by two groups (at UCLA and at the Max Planck Institute for Extraterrestrial Physics) as part of an effort to gather evidence for the existence of a supermassive black hole in the center of the Milky Way galaxy.

[7] A team of astronomers, mainly from the Max Planck Institute for Extraterrestrial Physics, used observations of S2's orbital dynamics around Sgr A* to measure the distance from the Earth to the Galactic Center.

[17] In 2018, S2 made its closest approach to Sgr A*, reaching 7,650 km/s or almost 3% of the speed of light, while passing the black hole at a distance of just 120 AU or about 1,400 times its Schwarzschild radius.

[18][19] S2 reached its pericenter on May 19, 2018, while its velocity in the line of sight from Earth peaked in April, and later hit its minimum in late August and early September.

Additional analysis has revealed a Schwarzschild precession of 12 arcminutes (0.2 degrees) in S2's orbit caused by the close passage, fully consistent with general relativity.