Thanks RS, but I prefer Bill Napier and Victor Clube's hypothesis concerning comet injection from the Oort Cloud: that the gravitational perturbation of our solar system passing through the plane of our galaxy, the Milky Way, dislodges them and sends them inbound. Bill and Victor were the first to note the impacts of fragments of Comet Encke in the recent past.
The Nemesis hypothesis was developed by Walter Alvarez' s colleague Richard Muller, and later asdopted and supported by NASA's David Morrison, a pioneer impact researcher who worked with Eugene Shoemaker.
Rick Firestone has proposed that the explosions of nearby suprnovas has injected comets. Rick was the first to spot the new physical evidence of 10,900 BCE impacts.
(I wrote my book 2000-2005, in complete ignorance of Rick's work. Benny Peiser had sifted the Cambridge Conference eniterly over to AGW scepticism in January, 2004. I am of the opinion that neutrons are released in some large hypervelocity impacts, and this causes the data Rick Firestone first observed.)
For Bill Napier's analysis of the YD impacts, and his physical evidence tying them to the initial disintegration of Comet Encke, visit the Cosmic Tusk
http://cosmictusk.com
By the way, President Obama was stunned by the unforeseen impact of a comet with Jupiter last year. Thanks to Obama and NASA Administrator Bolden, the NASA detection budget has risen from $4 million to $20 million our of NASA's budget of $17,000-18,000 million. Of course, you could add the cost of the WISE space infrared observatory to that number.
We're getting closer to having a handle on this hazard, but still have a way to go.