for National Geographic News
Earth-bound asteroids grab newspaper headlines for good reason. Scientists say the fallout of an asteroid several city blocks wide smacking into the planet would be catastrophic. Mass extinctions, runaway infernos, erratic climate fluctuations, and devastating impacts on human civilization are just some of the scenarios imagined.
Why, then, does the threat of a comet impact with Earth—potentially as dire if not worse than an asteroid—rarely leak onto the pages of the popular press?
"Primarily because the rate of comet impacts on Earth is not as great as the rate of asteroid impacts," said Daniel Durda, a senior research scientist at the Southwest Research Institute in Boulder, Colorado.
Most comets, and potentially some asteroids, have orbits that bring them close to Earth only once every 200 years or longer. Such bodies are known to astronomers as long-period objects.
The rate of long-period comet impacts on Earth is on the order of one every 32 million years, whereas the rate of comparably-sized asteroid impacts is more like one per every 500,000 years.
"When—note that I do not say if—we find a comet which has some potential to hit Earth, it might cause an even bigger sensation than potential asteroid impactors," said Robert Jedicke, an asteroid expert at the University of Arizona in Tucson.
The Threat
The consequences of comet and asteroid impacts on Earth are roughly comparable. Both would cause widespread destruction and loss of human life, said Jedicke.
"Big chunks of rock with a little ice, an asteroid, or big chunks of ice with a little rock, a comet, create a lot of damage when they impact Earth," he said. "[It's] like getting hit on the head by a stone with an icy coating or an iceball with a lot of rock in it—it's going to hurt your head."
A key difference is that long-period objects, like comets, will impact Earth with much greater speed than short-period objects, said Dan Mazanek, an engineer at NASA's Langley Research Center in Hampton, Virginia.
"If we happen to come across a long-period object that is dense, it would not have to be large to produce the same kinetic energy of a one-kilometer [0.6-mile] near-Earth asteroid," he said. "To me, that seems like something worthwhile to investigate."
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