Oumuamua...

Illustration of `Oumuamua, the first-known interstellar asteroid. Its unusual shape and color offer cryptic clues about the nature of objects from other solar systems. The challenge now is to find more of these messengers from the stars.
(Credit: ESO/M. Kornmesser)

Topics: Astronomy, Astrophysics, Space Exploration

It isn’t aliens. It’s never aliens.

That’s the only sensible answer whenever astronomers spot something truly weird in space. That unusual radio blip from the planet Ross 128b? Not aliens. Potential SETI signal SHGb02+14a? Not aliens. The mysterious ‘alien megastructure’ star? Probably not aliens, either. There are so many unexplored natural explanations for unusual phenomena, and so many ways to make errors, that the starting assumption has to be no, no, a thousand times no, it is not aliens.

Then astronomers observed `Oumuamua, the first known interstellar asteroid, as it raced out of the solar system. Its wildly elongated shape resembles that of a rocket stage or–even more enticingly–the interstellar ship from Arthur C. Clarke’s science-fiction novel Rendezvous with Rama. Soon sober-minded reporters (including this one) were exchanging curious messages: Could this ‘asteroid’ actually be an alien artifact? How would we know?

Deep breath. Let’s take this one step at a time. On October 19, the automated Pan-STARRS 1 telescope (which is primarily intended to scan the sky for potentially hazardous, Earth-approaching asteroids) detected an unusual object. It was originally regarded as a possible comet, catalogued as C/2017 U1. By the end of the month, though, astronomers could clearly see that it was something much more remarkable.

First, the ‘comet’ had no fuzz; it was clearly not a comet but rather a fast-moving asteroid. It got a new designation, A/2017 U1 (A for asteroid). Much more intriguing, though, was its orbit. It was moving past the sun on a hyperbolic path, a trajectory indicating that it originated from beyond our solar system. It got another new designation, introducing a naming scheme never used before: 1I/2017 U1 (I for interstellar).

The Pan-STARRS team quickly picked a more apt name for such an important object. It’s now known as `Oumuamua (pronounced ‘oh-oo-moo-ah-moo-a’), a Hawaiian word that translates roughly as ‘messenger from the distant past.’

That Interstellar Asteroid is Pretty Strange. Could It Be…?
Corey S. Powell, Discover Magazine