‘Oumuamua: Comet, asteroid or other?

Discovered in October last year, the interstellar object 1I/2017 U1 (more widely known as ‘Oumuamua) was initially thought to be a comet, only then to be categorised as an asteroid.

It made history when it was reclassified yet again as a ‘class I’ or an ‘interstellar object’ – the first object confirmed to have entered our Solar System from beyond.

‘Oumuamua was confirmed as an interstellar object because of the eccentricity of its orbit – ‘eccentricity’ in this context being a mathematical term, rather than the synonym for ‘strange’ (although it is rather weird!) Eccentricity describes how close an orbit is to a circular shape. Zero is perfectly circular, while values greater than zero but less than 1 describe an imperfectly circular orbit – like one that mimics an ellipse or an oval. The greater the eccentricity, the more bulged and imperfect the ellipse becomes. At an eccentricity of 1, the shape fails to close up entirely. Rather, the orbit becomes a parabola. The gravity of the orbited object – such as the Sun or a planet – can pull in the orbiting object, but as it is moving too fast to be trapped, it will then simply slingshot away. ‘Oumuamua’s orbit is like this – it’s so far from a closed loop that it can’t possibly have come from the Solar System.

However, based on the recent scientific findings of Micheli et al. presented in Nature, ‘Oumuamua is actually physically still a comet. Originally, as it lacked a coma – a cloud of gas which is formed when the comet is warmed by the sun and the ice on the surface sublimates – there were doubts surrounding this conclusion. However, the paper argues otherwise. Rather than directly investigating its composition, the scientists instead observed ‘Oumuamua’s motion. Objects within our solar system are accelerated by the gravitational force exerted by the Sun. But, it was found that if the attraction of the Sun was subtracted from the observed acceleration, there was still some ‘leftover’ – meaning that the source of some of the acceleration is coming from elsewhere. Their best explanation for this: ‘Oumuamua releases gas that acts as a propellant. If it releases gas, even if there’s not enough for a visible coma, then it is a comet.

What will ‘Oumuamua be classified as next?! Unfortunately, the window to observe it is just about gone – it’s hurtling away from our Solar System.

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