Mother Earth is located in a cosmic shooting gallery. Too many of the asteroids, comets and long-term comets have orbits that cross the orbit of our home planet. These are called Near Earth Objects or NEO's. Close asteroids are also known as Potential hazardous Asteroids or PHA's. Congress used the term large Earth-orbit crossing asteroids ECA's. The most Dangerous may be Earth-orbit Crossing Comets or ECC. The more we look for these objects the more we find.
Here is the resource for the congressional report on ECA's:
http://www.ssi.org/earth_orbit_crossing_asteroids.html
Here Is a reference to the group that records all Asteroids or minor planets:
http://cfa-www.harvard.edu/iau/NEO/TheNEOPage.html
Here are several NASA reference:
http://impact.arc.nasa.gov/
http://nssdc.gsfc.nasa.gov/planetary/planets/asteroidpage.html
Here is a National Geographic Reference site:
http://www.nationalgeographic.com/asteroids/
Now, it is suggested that some of these objects, specifically long-term comets, have an albedo of .1%. This means that they can't be seen with optical instruments. They can be "seen" with infrared instruments. These Comets are found in the Oort belt located in the region beyond Pluto and out half way to the next solar system. There are an estimated 100,000,000,000 long-term comets and at least 1% are estimated to be potentially NEO. That is 1,000,000,000 comets.
See the story:
http://www.spacedaily.com/news/asteroid-04o.html
Here is an article from citizens concerned about NEO impact hazards:
http://www.space.com/scienceastronomy/neo_letter_030709.html
This is a website for the letter signed by 100 scientists and another organization that is concerned about the NEO hazard.
http://www.space-frontier.org/Projects/TheWatch/20030708NEOletter.html
Here is what NASA Apollo 9 Modular Pilot, Astronaut Rusty Schweickart had to say about this.
Please note his references to a group called B612 and their plans to move an asteroid by 2015:
http://www.voanews.com/english/2004-12-17-voa3.cfm
Now I will go out on a limb and suggest that the reason that humans have never found intelligent exterrestial life.
Civilization never gets a chance to survive long enough to keep ahead of the catastrophes that work to destroy it. This could be called celestial darwinism or astroevolution. Dr Drake fails to include this in his famous formula see:
http://www.seti.org/site/pp.asp?c=ktJ2J9MMIsE&b=179073
How stuff works SETI explaination:
http://tinyurl.com/4jb43
I realize that my statement violates
the ARGUMENT AD IGNORANTIUM (Argument From Ignorance). Where a statement must be true because it has not been shown to be false. Or, must be false because it has never been proved to be true. (Assuming false is the scientific principle.)
Isn't the issue of keeping mother earth safe for civilization, too important, despite the yearning that we not be the only civilization, to allow a wait and see logic?