In late 2017, an interstellar asteroid named ‘Oumuamua swept past Earth, looped around the Sun, and departed— the first recorded visit from beyond our solar system (see my “Interstellar Visitor: The Strange Asteroid from a Faraway System, News and Comment, SI, March/April 2018). Its orbit was unique but so was its shape as revealed by its light curve—its variation in brightness as it rotated. ‘Oumuamua was highly elongated, with a length at least five times greater than its width. Because it was so small, perhaps 200 meters long, actual images could not be obtained.
By the time of its discovery it was already moving away at high speed, so there was no hope of launching a spacecraft mission for a closer look. Despite the meager data, this unique object has proved an ideal subject for speculation.
The first question was whether it was an asteroid or a comet. The distinction is that a comet outgases volatiles as it approaches the Sun, while a rocky asteroid does not. Telescopic images showed no tail or wisps of gas, so it was classified as an asteroid. However, there is a second way to detect escaping gas: from changes in the orbit. If jets of gas are strong enough, they can accelerate the comet. Careful tracking of ‘Oumuamua showed a slight acceleration; on this basis, some scientists began to call it a comet, although a very strange one.