In October 2017, astronomers using the Pan-STARRS telescope in Hawaii discovered a small object passing relatively near the Earth (33 million km, 0.22 astronomical units (AU), or about 85 times farther away than the Moon). Initial attempts to fit an orbit to its path, tracked by a series of observations failed. It was realised that the object is on a strongly hyperbolic orbit and is not gravitationally bound to the Sun: dynamically, it is not a part of the solar system—it is an interstellar object, the first to be observed, just passing through. It was first considered to be a comet, but extended observations by large telescopes failed to detect any of the emissions of dust and gas which one would expect from a comet, especially one making its first close approach to a star in many millennia, and perhaps ever. It was then re-classified as an asteroid. Finally, it was given the designation 1I/2017 U1 and the informal name `Oumuamua, which means “scout” or “messenger” in the Hawaiian language.
Further observations deepened the mystery: `Oumuamua was discovered after it had made its closest approach to the Sun on September 9th, 2017 at a distance of 0.25 AU (inside the orbit of Mercury), and as it receded from the Sun, careful tracking of its position indicated it was not following a trajectory as would be expected from Newton’s laws, but rather losing velocity slower than gravitation would account for (or, in other words, it had an outward acceleration added to the deceleration of gravity). This is often the case for comets, whose emission of gas and dust released due to heating by the Sun acts like a rocket to propel the body away from the Sun. But that conflicts with the failure to detect any such emissions from `Oumuamua by telescopes and instruments with more than adequate sensitivity to observe emissions which could account for the acceleration.... [Read More]