As data came in it became apparent that this is no ordinary rock. Large variations in the object’s brightness suggested that ʻOumuamua was rotating and tumbling and that its shape was highly elongated; with as much as a 1 to 10 (diameter to length) aspect ratio i.e. think ‘long grain of rice’. When ʻOumuamua was pointing its long axis directly at Earth it’s brightness dropped significantly, when the flat side faced Earth the brightness was highest. A highly elongated shape was the only model fit that could explain the variation in brightness.
No object of such a description has been observed in our solar system before, calling in to question the object’s origin. It is dark red, showed no signs of a comet’s tail (although jets of gas have been observed emanating from it’s surface), has a high metallic content and is moving very fast. So fast, that it could not have originated in (or near) our solar system. SETI even spent time looking for radio signals down to the strength of one tenth of one mobile phone and heard nothing...
Furthermore, infrared imaging can give information regarding the objects temperature, from which its shape can be indirectly inferred. NASA’s infrared Spitzer Telescope was pointing towards ʻOumuamua for more than 30 hours but could not detect it, suggesting that the object was cooler (or more reflective) than expected. How so?
It is suspected that as the object passed close to the sun it could have developed an icy coating, removed dust and refreshed the surface which would have increased it’s albedo (basically its reflectance), making it undetectable to Spitzer. This is highly unusual, although sometimes detected in comets (although ʻOumuamua has no comet tail, despite coming close to the Sun). Rocky bodies like this, travelling for (possibly) millions of years unimpeded will be very cold in interstellar space, possibly only a few degrees above absolute zero. In fact, the likelihood of even coming close to our sun is quite stupendously low, hence, astronomers being rather excited*! Another observed phenomenon was outgassing which is often seen in comets. This is when gas is released through the surface of the object as it heats up as pressure builds up within the object (like a pressure cooker releasing steam). Is it a comet or an asteroid or a space-faring alien-craft hell bent on invasion? Nobody can say for sure. However, ʻOumuamua has undergone somewhat of an identity crisis as scientists still aren’t quite sure how to classify it.
And as quickly as it came, ʻOumuamua is disappearing. This odd object is gravitationally misbehaving i.e. it’s not following it’s predicted gravitational trajectory. A number of theories have been suggested to explain this, such as outgassing jets that push the object around and/or solar wind (essentially photons) being responsible for the added acceleration. All this adds to the mystery of this most intriguing object! The likelihood is that soon it will be lost forever… au revoir ʻOumuamua.
*I make note here that alien enthusiasts have had a very exciting time following ʻOumuamua’s discovery. You've got to love alien enthusiasts (keep up the good work!)
Photo from NASA Getty images.