An asteroid on a collision course for Earth would be a pretty obvious threat.
Not necessarily so. A preventable collision requires an object far enough from the Earth so that it wouldn't be visible to the naked eye. It's not like we're going to see a second moon-sized object until it's too late.
Of course, the object would be visible by telescope and it's trajectory could be plotted. So a small subset of people could see it.
But that raises an important question -- do we trust the people telling us the world is going to end?
* This "Near Earth Object" might not even exist.
* Or, say it does exist, the likelihood of it striking the Earth is infinitesimal.
* I've read on a blog that it's really just the size of a grapefruit, so there's really no consensus on the so-called danger. It's really much closer and smaller and most professional astronomers can't discern the difference between small and far away.
* Are we really trusting these "scientists" to handle facts, figures, and calculations? It's like a religion, I tell you, where we're being force-fed their point of view.
* Astronomers are just creating a panic to ensure they have more funding. Most observatories are run by universities and that's how academia works.
* This whole project to "save the Earth" is just another political boondoggle to take money out of the hands of hardworking taxpayers to prop up an ailing aerospace industry.
* And even if we are doomed, it's not as if we could do anything in the face of such a impossible task.
* I'll be dead by the time it hits the Earth anyway...