If im not wrong a couple a years ago a Astroid or Meteor passed the earth very close we detected it after it has passed us.
Scientists are actively in the process of cataloging the largest
asteroids. As a rule of thumb, the larger the asteroid, the easier it
is to spot as it nears Earth (and by astronomically 'near', it can
still be decades away from possible impact). So, for a very large
asteroid, the odds are quite good that would know about a possible
impact well in advamce -- enough time to devise a defensive strategy.
However, though the odds are good, they are not 100%. A very large
asteroid could, conceivably, sneak through our observation net.
Source:
http://answers.google.com/answers/threadview?id=723742