At the periapsis (closest-in point) of its orbit, it is thought to come within 5 billion kilometres of the Schwarzschild radius of the supermassive blackhole, being the boundary beyond which light can no longer escape – and a point we might loosely regard as the surface of the black hole. S2 travels at an orbital speed of about 5,000 kilometers per second – which is nearly 2% of the speed of light. In both Mercury’s and S2’s cases, these apparently anomalous effects are predicted by Einstein’s theory of general relativity, as a result of the curvature of spacetime caused by a nearby massive object – the Sun in Mercury’s case and the black hole in S2’s case. S2’s orbital dynamics can be expected to differ from what would be predicted by Kepler’s 3 rd law and Newton’s law of gravity, by an amount that is three orders of magnitude greater than the anomalous amount seen in the orbit of Mercury. S2, a bright B spectral class star, has been closely observed since 1995 during which time it has completed over one orbit of the black hole, given its orbital period is less than 16 years. These days it’s possible to debunk (or otherwise) your own pet theory by predicting on paper what should happen to an object that is closely orbiting a black hole – and then test those predictions against observations of S2 and perhaps other stars that are closely orbiting our galaxy’s central supermassive black hole – thought to be situated at the radio source Sagittarius A*. Many an alternative theory of gravity has been dreamt up in the bath, while waiting for a bus – or maybe over a light beverage or two.

Hole 7 gravity lab