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.
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.
Although it was too distant for imaging by even large telescopes, its size and shape could be estimated from its brightness and large light fluctuations. Remarkably, ‘Oumuamua is highly elongated, with an approximately cylindrical shape. The nominal dimensions are about 200 meters in length and only thirty-five meters across, the most extreme of any natural object known. Large objects such as planets and moons are pulled by their own gravity into roughly spherical shape, and even small asteroids and comets (often described as “potato shaped”) rarely have irregularities of more than a factor of two. The extreme shape of ‘Oumuamua is as unique as its orbit.