Hi Laura
One thing worth considering at the outset is that space is really very empty. If the inner solar system (within the orbit of Jupiter where most asteroids are found) were the size of a football field, then the Earth would be the size of a pea. Now a pea in a football field really isn't a very big target.
As the Ask an Astrobiologist page says, NASA is working its way through the asteroids from largest to smallest and it's true that there may be some small ones not detected yet which could take out a city. In time, NASA will work their way down to finding these, if they exist, as well as increasing the probability that they will be found many years in advance.
If they are found in advance, there have already been plans formulated to deflect them. In the case of Asteroid 99942 Apophis, due to pass close to us in 2029 and 2036, there is mention of a plan to send a spacecraft to cover part of the surface with reflective material in order to alter the amount of solar energy absorbed and deflect the object. This will almost certainly not now be needed as this asteroid currently has a 1 in 250,000 chance of hitting us and future observations will very likely confirm a miss by a wide margin. However, the principle of asteroid deflection is certainly viable: http://neo.jpl.nasa.gov/apophis/
To return to probability. There was an explosion in Tunguska, Siberia in 1908 thought to have been caused by an asteroid or comet. This object was estimated to be about 60 metres in diameter, yet studies have shown that such an event might be expected once in 1,000 years: http://www.space.com/scienceastronomy/asteroid_exploding_021120.html
As you move up in size, the probability of an impact decreases because there are fewer of them around. The chances really are extremely remote and in a few years, NASA will have all the potentially dangerous ones under observation. It isn't something you should lose any sleep over.
Laura (guest) 29 May 2010 19:22
