The Planetary Society has a project called the Apophis Mission Design Competition. This is a project which, if it proceeds as planned, will award $50,000 for the best design for a space mission to put a transponder on or near the asteroid Apophis. The goal is to constrain the asteroid's orbit to determine whether or not it threatens the Earth. Donations to help fund the prize will be matched $1 for every $2 donated.
A while ago I had some thoughts about prizes and the asteroid/comet impact threat. The prize I imagined would be for actually detecting a natural object whose orbital parameters are determined with a high degree of certainty to result in an impact with the Earth. There are all sorts of variations on the rules that could be made for such a prize. Would there be a bonus for detecting the object sooner (years or decades in advance so the orbit can be shifted), or at least soon enough that evacuations can occur? Would there be a bonus based on the estimated mass of the object (mountain-sized wins a much greater prize than room-sized)? If multiple people detect the object, how do you judge who won? If an object is found but its orbit isn't determined until later by a different party, who wins? Can an astronomical group define the likelihood of a strike independently and fairly? Would there be a bonus for detecting small impacts, or impacts to other solar system bodies, in advance so the impact can be monitored by scientific instruments?
The motivation for offering the prize would not only be to encourage detection of impactors that threaten life or property (whether all of civilization, a small city or port, or a remote farm), but also to increase knowledge of the population of asteroids and comets. (The Planetary Society already has grants that do this, but as you can see in some of the papers I have linked, both grants and prizes have their strengths and weaknesses). In addition, the prize should encourage improvements in technologies like image processing software, telescopes, and sensor electronics. It would also provide focus for interested astronomy clubs and individual amateur astronomers. A sophisticated professional search might be more likely to win, but a dedicated and resourceful amateur astronomer with a modest budget would have a chance, too.
As you can see, a simple prize concept can quickly get complicated. It's not necessarily easy to come up with prize rules that are fair, easily managed, and reasonably sure to encourage results the prize sponsor wants.