While it is interesting to compute the efficiency with which a next generation survey such as PS1 can find impacting asteroids the reality of the situation is that it is unlikely that any large objects are on a collision course with the Earth in the next 100 years. The expected number of detected impactors, N(D), is simply the product of the efficiency, (D), for detecting impactors of diameter D and the size-frequency distribution of the population, n(D). The efficiency was already shown in Fig. 8 for D > 20 m. We use Brown et al. (2002)’s determination of the annual cumulative number of objects striking the Earth’s atmosphere: N(>D) = 37 (D/meters)2.7 which implies that objects in the size range of 2008 TC3 strike
the Earth about twice per year. Fig. 11 shows that unless the Earth is extremely unlucky PS1 will not detect a large (D > 20 m) impacting asteroid. A best case linear extrapolation of (D) to D < 20 m using just the 20 and 50 m diameter points suggests that the likelihood of obtaining three nights of detections in the discovery of even smaller impactors is extremely unlikely. The efficiency is decreasing faster with smaller diameter than the number of objects is increasing.