NASA is tracking 93% of all giant asteroids that could smash into Earth

How do they come up with the estimated number of asteroids larger than 1 km? Is it based simply on sampling a volume of observed space and extrapolating? Or do they have some way of inferring based on gravitational effects and anomalies?

Is 93% really an acceptable amount of the 'giant asteroids that could smash into Earth' to track?<p>Clearly, its a better number than, for example, 50%; and its great that NASA is doing any tracking; but this is an species level existential threat that's being talked about.<p>I'd really prefer to live in a world whereby a joint international effort (ideally; or just anyone) was tracking 99.9999% of such asteroids, as a matter of priority.<p>I think that the LHC, or the moon landings, were great projects - but shouldn't we do this, first?