Comets vs. Asteroids – Instant Egghead # 7


Let’s say you’re a Hollywood screenwriter and you need some kind
of impending, apocalyptic disaster to provide the drama in your next movie. Why not an incoming space rock? After all,
it’s a tried-and-true plot device. In Deep Impact, it was a comet
bearing down on Earth. In Armageddon, it was an asteroid. What’s the difference? Well, Armageddon had
that melodramatic Aerosmith song… (Aerosmith’s song,
“I Don’t Wanna Miss a Thing”) What? Oh! Oh right… What’s the difference
between asteroids and comets? Well, both are made from scraps
left over from when a planet forms, but there is indeed a difference. It’s right there in the name. The word “asteroid” comes
from the Greek, for “star-like”. That’s because asteroids are just
little points of light in the sky, like very faint stars. “Comet” comes from the Greek word
meaning “long-haired”. Comets are icy and when they get close
to the sun, the ice heats up and turns to a dusty vapor. The gas and dust form a tail
and a miniature atmosphere called a Coma. This makes the comet look
sort of hazy or hairy. Comets are icy because they come from the cold,
outer reaches of the solar system; from the Kuiper Belt and the Ort Cloud. The Ort Cloud is especially far out. If the sun were a marble an inch across, the inner part of the Ort Cloud would be
about two to five miles away. Asteroids reside much closer to the sun, primarily in a belt
between Mars and Jupiter. At that distance, any ice
would have vaporized long ago. That’s why asteroids lack the tail
and the coma of a comet. But the story is a bit more complicated… In 1996, astronomers discovered a new kind
of object inside the asteroid belt. It has all the properties of an asteroid,
but also a tail. These objects are called
“main belt comets”. One theory is that they are asteroids where something has punched
through the surface and freed the ice trapped inside. Comets can masquerade as asteroids too. Comets can become dormant or extinct, meaning that they stop
giving off gas and dust. In other words, the line between comets
and asteroids is getting, well…fuzzy. So, if you’re a Hollywood screenwriter,
take your pick. Either one, bearing down on Earth,
packs plenty of drama. For Scientific American’s
Instant Egghead, I’m John Matson.