Asteroid or comet? Find out below. Photo: NASA, ESA, K. Meech and J. Kleyna (University of Hawaii), and O. Hainaut (ESO)

Asteroid Impersonates a Comet on Path to Self Destruction

Comets and asteroids share some characteristics, both being leftovers from the formation of the solar system 4.6 billion years ago, but in many ways they’re as different as dirty snowballs and solid rocks. Usually.

Comets are far out, having formed in the outer regions of our solar system. They’re made of lots of ice, some rock and dirt and other materials. When one ventures into the inner solar system, it’s prone to shedding some of that stuff as its surface is heated by the sun. When we see a comet in the night sky, we’re not seeing its surface, or nucleus. Instead we see a kicked-up cloud of gas and dust called the coma — and, on rare occasions, a tail or two.

The spectacular Comet McNaught seen over the Pacific Ocean from Paranal Observatory in Chile in January 2007. Photo: ESO/Sebastian Deiries

Asteroids likely formed inside Jupiter’s orbit. They are made mostly of rock and metals, often in a loosely packed conglomerate held together tenuously by gravity. Smaller ones come in all sorts of shapes, including dog bones. Some asteroids are huge: Vesta is about 330 miles wide.

Vesta has a rocky outer shell and a core of iron and nickel, suggesting it’s more like an unformed planet than other asteroids. Photo: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA