This is a youtube video with history of discoveries https://www.youtube.com/watch?v=xJsUDcSc6hE , http://neo.jpl.nasa.gov/stats/ - has more up to date info. There are two projects to find almost all asteroids in comining decades http://en.wikipedia.org/wiki/Large_Synoptic_Survey_Telescope https://en.wikipedia.org/wiki/Pan-STARRS Pan STARRS already works to some degree see http://neo.jpl.nasa.gov/stats/ again, Those projects, which work now, are in process of upgrade http://www.dailymail.co.uk/sciencetech/article-2117062/Nasa-boosts-funds-telescope-team-hunting-dangerous-asteroids.html and then there will be http://en.wikipedia.org/wiki/Asteroid_Terrestrial-impact_Last_Alert_System Europeans too have project to deploy some telescopes http://belissima.aob.rs/Conf2012/Milani_2012.pdf and Russians think of this too. There are also satellites which look for asteroids like http://en.wikipedia.org/wiki/Near_Earth_Object_Surveillance_Satellite there are pending projects in Europe http://www.dlr.de/fa/Portaldata/17/Resources/dokumente/abt_17/projekte/Handout_Asteroid_Finder.pdf ( I think it can be resumed later ) and in Russia. There are consents http://b612foundation.org/sentinelmission/ and http://en.wikipedia.org/wiki/Near_Earth_Object_Camera of satellites with infrared telescopes. combined we have: we know almost all big asteroids > 1 km ( 95% now ) , so probability the Earth is hit hard is less, than, say 30 years ago - because we know 95% of big asteroids are already do not hit us in near future ( btw asteroid which caused dino extintion was several km wide, we know maybe 99.9% of all such asteroids now). Currently we have quite a high rate of discovery ( which will be much bigger in 2020s due to planned big asteroid hunting telescopes ) so in 30 years - we have only unknown asteroids few meters wide ( similar to than in Chelyabinsk ), we could be faster if mentioned satellites are launched and they work as expected. But even if we keep just today's rate of discovery the worst we could unexpectedly get - is a destruction of a city, in 30 years even with the current rate ( given planned improvements though ) of discovery we will have very low probability to have even this unexpected event.