Asteroid with unexplained orbital shift turns out to be a 'dark comet'

Typically, astronomers identify comets by their distinct atmospheres, or comae, and their tails, which are created as the sun vaporizes a comet's ice.

These features emit a glow when sunlight bounces off dust and water droplets.

As gas escapes the body of the comet, it also acts as a tiny thruster, subtly changing the comet's path. ...