spaceSpace and Physics

'Oumuamua's Chaotic Spinning Suggests It Had An Incredibly Violent Past


Robin Andrews

Science & Policy Writer

An artist's impression of our first large interstellar visitor. ESO/M. Kornmesser

'Oumuamua, everyone’s favorite, difficult-to-pronounce space object, became our first ever interstellar asteroid last year. After a flurry of analyses, it was suggested that it wasn’t a comet (or, indeed, an alien spacecraft), but an asteroid, one that is oddly shaped like a cucumber.

The elongated entity features a glorious mixture of odd characteristics, including a dark, reddish hue that’s likely the result of cosmic rays blasting it as it traversed through the shadows. It also spins rather oddly, and now, a team led by Queen’s University Belfast (QUB) think they know why: it had a violent past.


Writing in the journal Nature Astronomy, the team note that “the discovery of 1I/2017 U1 (1I/‘Oumuamua) has provided the first glimpse of a planetesimal born in another planetary system,” before rather marvelously describing it as an “interloper.”

Although earlier research indicated that it spun on its axis once every 7.3 hours, this paper points out that various measurements of its bonkers ballet have been taken, and they’re all quite inconsistent. Using all the available imagery and data they have on this sneaky space tube to date, the team explain that the variations in brightness cannot be explained using just one rotational period.

That means that it’s spinning chaotically, or tumbling. Technically, 'Oumuamua is in an “excited rotational state undergoing non-principal axis rotation,” but tumbling will do just fine here.

There are a few different reasons posited as to why this spin is taking place. Impacts via a collision with another planetesimal, passing comets, gravitational nudges by far more massive celestial objects, or even the prolonged scattering of solar radiation on its surface are all possibilities.


Ruling out cometary influences and the solar radiation ideas due to its time in interstellar space, it’s likely that the tumbling began in 'Oumuamua’s original planetary system. Although not conclusive, the collisional hypothesis is the most probable here.

This, to be fair, is the usual explanation as to why celestial objects are spinning oddly or why their rotational axis is off to one side. Uranus, for example, has a rotational axis titled almost perpendicular to the Sun, and it’s thought that multiple impacts knocked it over.

“In all honesty, the evidence for which mechanism caused the tumbling is not that strong,” lead author Dr Wes Fraser, a research fellow at QUB, told IFLScience. Saying that, “collisions happen, and can happen frequently.”

“Just look at our asteroid belt for example; collisions happen there all the time. It is for this reason alone that I prefer the collisions idea over the others.”


He suggests that the original ejection of 'Oumuamua took place during the reorganization of the host system, when the planets “are in the last dregs of their growth phases, when lots of debris is left over.”

“The other methods are all viable, however unlikely, and we will never know which is the true one, because ‘Oumuamua is gone baby gone.”

Another idea, proposed in a recent arXiv pre-print, suggests that its origins was even more drama-filled than a collision or two. There’s a chance that it’s the remnant of a planet that got far too close to its host star, which triggered its fragmentation.

This paper suggests that this is a better explanation for its elongated shape than a collision between two asteroids, but this is fairly speculative at this time.


Whatever the cause, based on the rate of its spin, it’ll persist in its tumbling for at least 1 billion years until internal strain cancels out its rotational energy. Keep on truckin’, you cosmic cigar!


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