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And it is 910 kilometers in diameter, which is quite a bit less than 600 miles.
It is highly unlikely any rocks much bigger than 10 miles in diameter have hit the Earth since the solar system reached it's modern state. Maybe when our planet was newly formed and hadn't formed a solid crust yet we were hit by some bigger, the solar system was a LOT more crowded back then.
Much bigger than 10 miles and there is a good chance it would get through the crust, and the signs of that would be very obvious even billions of years later.
A lot of paleontologists don't believe that a single asteroid or cometary impact could have wiped out the dinosaurs. Some believe that we were hit by several comets over a few hundred thousand or million years, which would tie in with the hypothesis of the sun having a brown dwarf companion that swings by every 20 million years or so, pulling in swarms of comets from the Oort Cloud into the inner solar system. Some think the asteroid impact that created the iridium layer at the K-T boundary did no more than finish them off. See, before the way they determined where the Cretaceous ended and the Tertiary began was there were dinosaurs in the Cretaceous, and none in the Tertiary. After they discovered the iridium layer was world-wide, they realized that on some continents they were off by up to 5 million years - the iridium layer was some distance above what they thought was the K-T boundary (Cretaceoud-Tertiary, they use K for Cretaceous because C is taken by Cambrian). Dinosaurs had been extinct on all continents except the Americas long before the asteroid hit (well, some think the iridium layer may be due to some unexplained geological event). Even in the Americas, towards the end there was very little diversity of species - something like 90% of the large herbivores were Triceretops. They were dying out for some other reason - my pet theory is that a technological civilization developed from some undiscovered species. Bakker thinks it might have been disease. We will probably never know for sure, which is why a lot of people like to go with the asteroid impact since we have some evidence that it happened, although not as much tying it to the extinctions.
No matter what kind of catastrophe hit, it is almost guarenteed that humans would outlast all the other vertebrates, we would be the last to go. We are simply more adaptable than any other species on the planet, nothing else can live in as many different habitats. I still think we need to stop keeping all our eggs in one basket and get some self-sufficient habitats in space started ASAP. It's scary that every human shares the same life support system, and though your typical 'once in 100 million years' asteroid or cometary impact would probably not wipe us out, there are other possible things that could. What if somehow one of the big asteroids WERE to somehow get nudged out of orbit and hit us, or some rogue extrasolar planet hit our sun? Either would probably wipe out everything except the bacteria living miles under the surface of our planet. Maybe a disease that only 1 in 10,000,000 were resistant to would show up - humans would need a minimum of 300 survivors and a carefully controlled breeding program to be able to build our population back up without inbreeding killing us off in a few generations - if there were only 600 people left, widely spread across the Earth, most of them would probably die and the rest wouldn't be able to get together and get organized quick enough. Maybe technology might kill us off. Modern nuclear weapons probably would leave enough survivors to build back up again in a millenium or two, but we might develop something more powerful, like antimatter bombs or a nanotechnological plague. Who knows?
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