[meteorite-list] Earth Impact: Are Comets a Bigger Danger Than Asteroids?

[Ron Baalke]

http://www.space.com/26264-asteroids-comets-earth-impact-risks.html

Earth Impact: Are Comets a Bigger Danger Than Asteroids?
By Mike Wall
space.com
June 18, 2014

Discussions about "death from above" scenarios usually center on asteroids, 
but a comet impact could be far more devastating than a space rock strike.

Near-Earth asteroids (NEAs) have Earth-like orbits, so their collisions 
with Earth tend to be glancing blows from behind or from the side. But 
comets travel around the sun in more random paths and can thus slam into 
the planet head-on, with potentially catastrophic results, researchers 
say.

"It would be a much bigger explosion, a much bigger crater, much more 
damage," impact expert Mark Boslough, of Sandia National Laboratories 
in New Mexico, said on June 5. He made the comment during a webcast produced 
by the online Slooh community observatory, which previewed the June 8 
Earth flyby of the asteroid 2014 HQ124. 

In fact, comets can be traveling up to three times faster than NEAs relative 
to Earth at the time of impact, Boslough added. The energy released by 
a cosmic collision increases as the square of the incoming object's speed, 
so a comet could pack nine times more destructive power than an asteroid 
of the same mass.

The speed of comets also means that a dangerous one could be nearly upon 
Earth by the time scientists detect it.

"They come in fast," Bill Ailor, principal engineer with the Center for 
Orbital and Reentry Debris Studies at The Aerospace Corporation, said 
in March during a presentation with NASA's Future In-Space Operations 
working group. "In some cases, people have said that we may have two years' 
or so warning in the best case on something like that."

Two years may sound like a lot, but scientists and engineers would prefer 
even more lead time to keep Earth out of harm's way.

For example, one of the most promising deflection strategies envisions 
launching a robotic probe to rendezvous with and fly alongside of the 
incoming object, nudging it off course via a slight but persistent gravitational 
tug. This "gravity tractor" method obviously cannot work overnight.

Adding to the intrigue and the danger is the unpredictability of cometary 
orbits. The icy objects begin spouting gas as they near the sun and heat 
up; these gas jets act like little thrusters, making it tough to forecast 
exactly where a comet is going to go.

Despite all of these factors, however, the focus on asteroids as Earth's 
primary impact threat is not misplaced, Boslough and Ailor said. The reason 
is simple: numbers.

"I'm more worried about asteroids than I am comets, because there are 
so many more asteroids," Boslough said. "The likelihood of an impact from 
an asteroid is probably 100 times the likelihood of an impact from a comet 
of the same size."

There are probably trillions of comets out there, but the vast majority 
of them reside at the extreme outer edge of the solar system, in a shell 
of icy bodies known as the Oort Cloud. Near-Earth space, meanwhile, is 
dominated by asteroids. Scientists think millions of NEAs exist, but only 
about 11,000 have been discovered and tracked so far.