The statistical chance of a large asteroid striking the Earth in any given year is very small--in fact almost statistically insignificant--since such events occur on average only once in more than 10,000 years. However, the consequences if such an event were to occur would be tremendous--perhaps even an "extinction level event." Recent advances in astronomy have led to the realization that a large number of Near-Earth Objects (NEOs) that have true "Earth-crossing" orbits was far greater than was originally estimated. See: http://cfa-www.harvard.edu/cfa/ps/mpc.html
The Tunguska, Siberia event in 1908 and the Curucá incident in Brazil in 1930 (in both of which it is suspected that a small asteroid vaporized before striking the Earth) are indicative that he asteroid threat is real. The1997 asteroid XF11's near miss (see: http://ssd.jpl.nasa.gov/ca_97xf11.html) should have served as a wake-up call, but politicians tend to be complacent about subtle threats unless they suddenly become non-subtle and a goodly number of registered voters get obliterated.
Here is another interesting site to peruse: http://www.pibburns.com/catastro/impacts.htm
It is not reassuring to read that a number of asteroid "near misses" (in terms of astronomical units) were detected only after the asteroid had passed.
A recent article in England's Guardian newspaper outlined
the threat posed by the 390 meter wide Apophis asteroid, which could
strike the Earth
in 31 years. An asteroid that large could represent the risk
of major climate change but not quite an extinction level event. (That
would probably take an asteroid more than a kilometer in diameter.)
In part, the article stated: "NASA
has estimated that an impact from Apophis, which has an outside chance
than 100,000 times the energy released
in the nuclear blast over Hiroshima. Thousands of square kilometers would be
directly affected by the blast but the whole of the Earth would see the effects
of the dust released into the atmosphere." To read the entire article, see: http://www.guardian.co.uk/space/article/0%2C14493%2C1660485%2C00.html