Fig. 8 shows that the efficiency of the PS1 survey at detecting impactors increases as a function of time and impactor diameter. In just fouryearsof PS1operations it couldidentify85%of all1kmdiameter objects thatwillimpact the Earthinthenext100years. Interestingly, it has a 74% chance of identifying a 1 km diameter that would impact during the 4 year time span of the survey. Atfirst glance this is surprising since PS1 can detect a 1 km diameter asteroid at opposition at a geocentric distance of2.67 AU.The objects shouldbe visible longbefore impact in the large volume of space surveyed by the PS1 system. We will explore the reasons for the ineffectiveness of the survey at detecting these large hazardous objects below.