S2's orbit is the epitome of extreme. In 2018, the star will reach the closest part of its 16 year journey around Sagittarius A*, coming as close as 17 light-hours of the black hole's event horizon. At this point, the star will be traveling 30 million kilometers per hour, that's 2.5 percent the speed of light! This makes S2 the perfect tracer for astronomers using GRAVITY to measure its position to extreme precision to check whether its motion agrees with predictions set out by Einstein's general relativity.
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"It was a fantastic moment for the whole team when the light from the star interfered for the first time -- after eight years of hard work," said Frank Eisenhauer, GRAVITY's lead scientist from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. "First we actively stabilized the interference on a bright nearby star, and then only a few minutes later we could really see the interference from the faint star -- to a lot of high-fives."
But GRAVITY's mission has only just begun. It will continue to study Sagittarius A*, eventually detecting and tracking flaring events near the event horizon, in real time, to measure the physics of the most extreme, strong gravity environment known.
GRAVITY isn't the only project in pursuit of getting up-close and personal with the supermassive black hole lurking in our galaxy's core. The Event Horizon Telescope, an interferometer composed of several radio telescopes around the globe, is also nearing completion and the first historic images of Sagittarius A*'s event horizon halo are only months away.