I am writing a blog post on the threat of asteroids on earth. I have read a number of Science and Nature articles on the matter with the general consensus being that the probability is really low of being hit by a highly destructive 300-meter and above asteroid but that if we were to find an asteroid on a collision course for earth, the current ideas of how to deflect it would highly depend on the composition but also are largely untested. Could you please comment on whether you think funding (NASA or others) should be going towards locating smaller (<1km) objects that are not often seen if they are very close to the sun OR whether the budget should be expanded to test the deflection methods on a variety of non-dangerous asteroids within reach?
The probability of an impact with an asteroid large enough to cause large scale destruction is indeed rather small. Generally we tend to focus on objects 1km or more in size as those that are particularly dangerous, since although smaller objects could cause a lot of damage it would be more localised (a single continent say) rather than global. Roughly speaking we think that 1km size asteroids hit Earth about every 500,000 years and the impact rate falls off rapidly for larger objects (since there are less of them), impacts with 5km size objects happen only every 10 million years or so. The largest impact event in recorded history was the Tunguska event in Siberia in 1908, which involved an object that was probably about 60m across. The most potentially dangerous asteroid we know of at the moment is 1.1-1.4km (29075) 1950 DA, which has a probability of up to 0.33% of colliding with Earth in 2880, so we have a good while to work out how to deal with it.
There are a number of ongoing projects searching for near Earth asteroids, such as LINEAR and the Catalina Sky Survey. Also, at the beginning of last year the European Union, in collaboration with the United States and Russia, set up the NEOShield project whose remit is to study appropriate deflection methods, including how the choice of method depends on composition. One of their aims is also to test some of the proposed deflection technologies, both in the laboratory, and in space. Both aspects are important, since if we don't know the asteroid is heading our way we clearly can't do anything about it, but it isn't much use knowing about the deadly asteroid if we don't have some way of deflecting it.