A staple of science fiction has always been aliens from other worlds. As science has revealed the inhospitableness of planets such as Mars and Venus, the source of these fictional visitors has increasingly moved to deep space. Now it has happened—the first visit from beyond. In late 2017, an interstellar asteroid named ‘Oumuamua swept through the solar system. Inevitably, it has stirred speculation that it could even be a spacecraft, a messenger from beyond our solar system.
An artist’s conception of the highly elongated interstellar asteroid. There is no telescopic image resolving the fast-moving object.
Illustration: M. Kommesser, European Southern Observatory
This visitor was discovered on October 19, 2017, with the Pan-STARRS optical telescope on Haleakala, Hawaii, a part of the NASA-supported Spaceguard Survey of asteroids that come close to the Earth. At the time of its discovery, it was 33 million kilometers from Earth and had already passed its closest point to both the Sun and the Earth. As astronomers followed up on the discovery, it quickly became apparent that the orbit of ‘Oumuamua was unlike anything seen before. All members of the solar system are gravitationally bound to the Sun, with orbits that are ellipses. This object, however, was travelling far too fast to be part of the Sun’s family. Its orbit is a hyperbola, and when discovered it was already rapidly leaving the inner solar system, having passed close enough to the Sun to bend its orbit dramatically.