Phil - According to our next guest, Bill Bottke from the Southwest Research Institute in Colorado that's exactly what happened to bring about the demise of our dinosaurs.
Chris - Hello, Bill. Where did this asteroid come from in the first place?
Bill - One of the interesting mysteries that we've had in the field of asteroid impacts and such is what kind of body produced the impact that killed the dinosaurs about 65 million years ago. In the process of doing a completely unrelated project we came across evidence for a very large asteroid breakup that happened in the asteroid belt between the orbits of Mars and Jupiter. This break-up event was about a 170km asteroid. We believe using different techniques we could tell its age, that it broke up 160 million years ago. Interestingly enough the fragments from this break-up event took a long time to get out of the asteroid belt into the inner solar system. We think there's a pretty strong chance that one of these fragments was actually the killer bullet that brought about the end of the dinosaurs 65 million years ago.
Chris - What were those asteroids doing there, next to Mars anyway?
Bill - The asteroid belt itself we believe is the leftover remnants of the original building blocks which formed the planet. At one point when the planets were forming in the solar system we had a lot of objects that looked like asteroids. We had bigger objects that ultimately became moon-size and ultimately we built the planets like Mercury and Venus and the Earth. There were some leftovers and these leftovers were caught in the somewhat stable region between Mars and Jupiter. The Asteroid belts in a sense are our ancestors. They tell what the original building blocks looked like.
Chris - How did they jostle together in such a way that one of them ended up on a collision course with Earth so recently? 68 million years ago in a solar system that's over 4.5 billion years old is really recent.
Bill - It indeed is. Something some of your listeners probably don't know but it's interesting to think about is, why do we even have asteroids hitting the Earth? Why should this take place? If you place asteroids on orbits close to the Earth and such eventually the Earth will clear them away and there'll be nothing left. It turns out that the asteroid belt has mechanisms take place. It's inside it which actually in certain interesting ways can cause small asteroids to move out of the asteroid belt and out on to orbits where they can cross the Earth's path. This has to do from a combination of effects. There's actually a player in this game how sunlight effects asteroids. Another player in this game is how the tiny gravitational kicks produced by Jupiter and Mars can actually move asteroids from the asteroid belt into the inner solar system.
Chris - The big question is, are there any more loitering out there that could do for us?
Bill - Certainly. Right now there's probably on the order of about 1000 asteroids larger than 1km in diameter that can either cross Earth's orbit or they can come very close to the Earth. Fortunately we've had astronomers from around the world searching for these asteroids and we've actually found, we believe, a good fraction of all the big ones. We've probably found around 70-80% of these kilometre size asteroids. None of them, fortunately, in a trajectory that will take them into the Earth. There are some others out there that we're still looking for and there's some smaller guys that perhaps won't cause global damage but could cause regional damage. We're going to continue to look for the next several tens of years until we find all the threatening asteroids.