In the end of January, a 100 m class asteroid called 2007 TU24 will pass near earth, somewhat beyond moon’s distance. It’s not visible with the naked eye, but only with reasonable size amateur telescopes. JPL has more.
This was only detected in November 2007. If some other big one was headed towards earth one day, it’s possible there wouldn’t be much time for warning…
Here is a nice calculator to estimate the impact effects. The energy equivalent of a 200 m low velocity asteroid is about 100 megatons TNT or twice the biggest hydrogen bomb ever. At 100 km distance, a loud bang would be heard.
The best and most cost effective way to currently work with the asteroid threat is extensive sky surveys, which are already being carried out. Later, something more is required.
Of course, humanity has many other threats facing its comfort (an asteroid wiping out humanity to extinction is a nonexistent threat) and if those are more probable, they need to command more attention than asteroids.
That is, if rational resource allocation is used.