The interstellar asteroid 1I/2017 U1 (otherwise known as 1I/’Oumuamua) is fast receding into the distance, towards the constellation of Pegasus (1). The existence of this rocky visitor from the stars was announced last October (1). Its trajectory was too fast for it to be a solar system comet – even one from the furthest reaches of the Oort Cloud. That was an exciting discovery, because that meant that 1I/2017 U1 was the first confirmed observation of an object arriving in the solar system from deep space.
Although 1I/2017 U1 was initially considered to be an interstellar comet, that thinking changed when it failed to emit any gases as it performed its perihelion transit around the Sun (3). This barren rock, confirmed as an interstellar asteroid (4), is now speeding away from the Sun. It spent a relatively short time in the observation zone of professional telescopes, thanks to its great speed, but this was enough to reveal more weirdness (5). It is an elongated object spinning head over tip, doing cartwheels through the solar system. Some wondered whether it might be artificial, given the lack of coma as it traversed past the Sun. But attempts to pick up signals from the object came up blank (6). Still, its shape is nothing like any known body in our Solar System. If solar system asteroids resemble rocky potatoes, then 1I/2017 U1 is more like an interstellar carrot, spinning haphazardly through our system. To remain intact under these conditions, its internal structure must be robust (7).
The colour of our interstellar carrot is neutral with a reddish hue. The colouration may be patchy across its surface. Solar system minor bodies (asteroids, Kuiper Belt Objects, Trojans) vary in colour, often dependent upon which population group any particular object belongs to. Continuing my daft vegetable analogy, solar system potatoes come in different varieties. Many are neutral in colour, some are reddish, others distinctly red. Like comparing a Maris Piper to a King Edward. If we compare 1I/’Oumuamua’s colouration to those of various classes of solar system objects, then it seems to most resemble those of the dynamically excited populations of Kuiper Belt Objects. However, it is less red than the scattered Trans-Neptunian objects whose orbits extend beyond the heliopause (7).