Astronomy Picture of the Day

Explanation: Why is 'Oumuamua differing from its expected trajectory? Last year, 1I/2017 U1 'Oumuamua became the first known asteroid from interstellar space to pass through our Solar System. Just over a year ago, this tumbling interstellar rock even passed rather close to the Earth. The asteroid's future path should have been easy to predict given standard gravity -- but 'Oumuamua's path has proven to be slightly different. In the featured animation, 'Oumuamua is shown approaching and exiting the vicinity of our Sun, with the expected gravitational and observed trajectories labelled. The leading natural hypothesis for this unexpected deviation is internal gas jets becoming active on the Sun-warmed asteroid -- but speculation and further computer simulations are ongoing. 'Oumuamua will never return, but modern sky monitors are expected to find and track similar interstellar asteroids within the next few years.