I know this is an old topic. Not sure about the policies here regarding bringing back to life one that has gone on before. Since this was posted we've had a few noticed that have come fairly close to the earth without actually effecting us.
One of the things that makes this so hard to detect incoming is that the sun's glare prevents us from seeing them directly and it is usually after it passes and we see them by the reflection of light from the sun that we know such are in our neighborhood.
NASA has put into orbit a couple of satellites to correct this so we can see them coming before they get past us. Not long enough of a time span to actually do a lot but some warning.
Sooner or later one of these has our name on it, just like the dinosaurs found out. It's not if but rather when. Both the
Oort Cloud and the
Kuiper Belt have left over remnants of the making of the solar system. The Oort Cloud extends so far out that it is possible that Alpha Centari may well swap material in the outer fringes of it since the gravity between the two systems is near equalized. It wouldn't take much to bump a new comet into moving towards either system.
Many of these are long term orbits and it is a near certainty we haven't discovered all that orbit since we've not had telescopes, much less ones to reach out far distances towards the outer planets and planetoids that long to cover them.
Even if we did, there are other
gravitational bodies beyond Pluto. We are fairly certain of this because new comets keep showing up from time to time. A fairly recent news article claims to have found a new body out beyond Pluto's orbit and one unknown to us if it is true. We know of at least 4 orbiting planetoids which have been named but this one is a new one we didn't know of.
It is such as these that can disturb the ice bodies orbiting out on the fringes and send them our way. There is a practically unlimited supply of them considering the solar system sets within a spherical shell of them. So sooner or later, one has our number. Not if but rather when.