Astronomers haven’t taken their eyes off the interstellar asteroid better known as ‘Oumuamua since its discovery in late October. Now more than twice the distance of Earth from the sun and ultra-faint at magnitude 27, it continues its headlong flight out of our solar system. From whence it came we don’t know, but its boomerang-shaped orbit proves it originated in another solar system and had been traveling through space for untold years before it dropped by our neighborhood en route to its next rendezvous, wherever that might be.
Since its discovery, recent scientific papers (here, here and here) based on new observations have shed more light on the object’s character including the intriguing possibility that it’s not an asteroid after all but rather a comet.
Here’s what we know. Examining sunlight reflected from ‘Oumuamua with an instrument called a spectrograph, astronomers have learned that its surface is light red, the same color seen on asteroids in the outer belt beyond Neptune. The red color comes from carbon-rich ice that slowly turns red when bombarded by energetic cosmic radiation (mostly protons) bounding around the galaxy. Since there’s no evidence in its spectrum for rocky materials, there’s a good possibility that we’re seeing an icy crust rich in organics darkened by cosmic “space weathering.”
For all we know, it’s ice all the way down, but that begs a question: why astronomers didn’t see a hazy coma around ‘Oumuamua when it passed just 22.2 million miles (35.7 million km) from the sun. That’s closer than Mercury! For a time, temperatures at the object soared to about 620° F (327° C), plenty hot enough to vaporize ice. All they can figure is that the crust had been thoroughly baked by its own sun and thick enough to insulate the object’s icy core. 20 inches (0.5-meter) of hard-baked, carbon crud would do the job.
Unlike most solar system bodies but like Halley’s Comet, ‘Oumuamua tumbles irregularly, indicating it may have had a rough-and-tumble time during the event that forced its departure from its own solar system.
Finally, there had been some speculation that this asteroid / comet might possibly be a spaceship from another star system because of its unusual orbit and shape. So Breakthrough Listen, a SETI-like initiative to monitor stars for signals of intelligent life, used the Robert C. Byrd Green Bank Telescope in West Virginia on December 13 to scan billions of channels across four radio bands looking for signals of artificial origin. Drum roll! … at least so far, nothing has been detected. Quiet as an inert comet out there.
The Hubble Space Telescope is also being used to take a series of photos of ‘Oumuamua to determine its precise position over time in the hope of getting a better idea from whence it came and where it’s headed.