A BRIEF VISIT FROM THE FIRST INTERSTELLAR ASTEROID REVEALS A RED EXTREMELY ELONGATED BODY

Until recently, none of the ~750,000 known asteroids and comets were thought to have originated outside our solar system. Many decades of asteroid and comet characterization have yielded formation scenarios that explain the mass distribution, chemical abundances and planetary configuration of today’s solar system, but up until now there has been no way to tell if our solar system is typical. Solar system formation models suggest that orbital migration of the giant planets as they formed ejected a large fraction of the original planetesimals into interstellar space(1). The predicted interstellar number density(2) of icy interstellar objects of 2.4 × 10(−4) au(−3) suggested that these should have been detected by surveys, yet none had ever been seen. Here we report on the discovery and characterization of 1I/2017 U1 (‘Oumuamua), the first object known to originate outside our solar system. Follow-up observations and subsequent analysis verified the extrasolar trajectory of ‘Oumuamua. Our observations reveal the object to be asteroidal, with no hint of cometary activity despite an approach within 0.25 au of the Sun. Spectroscopic measurements show that the object’s surface is consistent with comets or organic-rich asteroid surfaces found in our own solar system. Light-curve observations of ‘Oumuamua indicate that the object has an extreme oblong shape, with a 10:1 axis ratio and a mean radius of 102±4 m assuming an albedo of 0.04. Very few objects in our solar system have such an extreme lightcurve. The discovery of ‘Oumuamua suggests that previous estimates of the density of interstellar objects were pessimistically low. Imminent upgrades to contemporary asteroid survey instruments and improved data processing techniques are likely to produce more interstellar objects in the upcoming years, creating opportunities to interrogate the mineralogical, elemental or isotopic composition of material from other solar systems.