Interstellar asteroid’s tumble suggests a violent past
The strange interstellar visitor to our solar system – dubbed ‘Oumuamua by astronomers – is tumbling as it moves through space. A new study suggests its chaotic tumble is likely to continue at least another billion years, and it suggests ‘Oumuamua’s tumble is the result of a violent collision with another asteroid in the past. This collision might have knocked ‘Oumuamua out of its original solar system and sent it toward our solar system.
Astronomer Wes Fraser of Queen’s University in Kingston, Ontario, Canada, led this new research. Fraser and his team analyzed all the available data from optical photometry; that is, they looked at the data on how ‘Oumuamua’s brightness varied over time. Then they used computer modeling to understand why ‘Oumuamua varied in brightness, and to spin out reasonable possibilities for its past and future. Their study was published in the peer-reviewed journal Nature Astronomy on February 9, 2018. Fraser said:
Our modeling of this body suggests the tumbling will last for many billions of years to hundreds of billions of years before internal stresses cause it to rotate normally again.
While we don’t know the cause of the tumbling, we predict that it was most likely sent tumbling by an impact with another planetesimal in its system, before it was ejected into interstellar space.
‘Oumuamua’s tumbling isn’t new. The object was reported last fall to have a 7 or 8-hour spin period; different measurements showed slightly different results. The most likely explanation was that ‘Oumuamua’s spin isn’t regular, as is the spin of most (but not all) asteroids in our solar system. Instead, ‘Oumuamua tumbles.
Its complex tumbling motion means we see different views of the asteroid’s body at different times, and so the tumbling of ‘Oumuamua can explain the curious color change observed by astronomers. The cigar-shaped asteroid’s surface might be variously colored. Fraser said:
Most of the surface reflects neutrally, but one of its long faces has a large red region. This argues for broad compositional variations, which is unusual for such a small body.
It also settles the question of whether ‘Oumuamua is solid. Answer: it is. As Matija Cuk explained in an article for the SETI Institute, asteroids are often just piles of rocks and sand loosely held together by gravity. When this is the case, she wrote:
… internal motions of material inside asteroids … damp this tumbling relatively quickly (astronomically speaking), leaving only asteroids that suffered recent collisions as tumblers. ‘Oumuamua spent many millions of years in the interstellar void, so it should have damped its tumbling, but it apparently did not. This made planetary scientists conclude that ‘Oumuamua is likely a solid chunk of rock or metal, without any internal structure or loose material.
And so – despite its quick trip through our solar system – we continue to learn more about ‘Oumuamua. In case you weren’t following it last fall, some astronomers thought the object was a comet when the Pan-STARRS 1 telescope in Hawai’i first picked it up on October 19 as a faint point of light moving across the sky. Others thought it looked like a typical fast-moving small asteroid. As astronomers tracked its motion through space, they began to be able to calculate its orbit, showing beyond any doubt that this body did not originate from inside our solar system, like all other asteroids or comets ever observed.
Instead, this object was doubtless from interstellar space. It was the first such object known.
The question of whether it was a comet or asteroid was put to rest after astronomers looked at ‘Oumuamua’s location in the sky prior to its discovery and found no signs of cometary activity after the object passed closest to the sun in September 2017.
That’s when it was reclassified as an interstellar asteroid – the first ever observed – and named 1I/2017 U1 (‘Oumuamua). A statement from the Institute for Astronomy (IfA) at the University of Hawaii described the intricacies of naming this object:
Originally denoted A/2017 U1 (with the A for asteroid), the body is now the first to receive an I (for interstellar) designation from the International Astronomical Union, which created the new category after the discovery. In addition, it has been officially given the name ‘Oumuamua. The name, which was chosen in consultation with Hawaiian language experts Ka’iu Kimura and Larry Kimura, reflects the way this object is like a scout or messenger sent from the distant past to reach out to us (‘ou means “reach out for,” and mua, with the second mua placing emphasis, means “first, in advance of”).
Astronomers had been expecting to find interstellar asteroids. Before ‘Oumuamua’s discovery, they had estimated that an interstellar asteroid similar to this one passes through the inner solar system about once per year. Such objects are faint and hard to spot, and so they haven’t been seen before this.
But recent survey telescopes, such as Pan-STARRS, are powerful enough to discover them. Thus – although it’s the first – ‘Oumuamua is likely not the last known interstellar asteroid.
Bottom line: A study of ‘Oumuamua’s chaotic tumbling motion suggests it’ll tumble at least another billion years. It also suggests a violent past for this object – a collision with another object – which likely sent it careening toward our solar system.