recorded by infrasound sensors designed to detect detonation of nuclear devices. It shows that the success rate is increasing over time, but that the vast majority are still missed. One problem with assessing effectiveness this way is that the missed asteroids tend to be small. Missing small asteroids is unimportant as they generally do very little damage (the mid-size unpredicted Chelyabinsk meteor being a notable exception). However, missing a large day-side impacting asteroid is highly problematic. In order to assess the effectiveness for detecting larger asteroids, a different approach is needed. Another way to assess the effectiveness is to look at warning times for asteroids which did not impact Earth, but came reasonably close. Looking at asteroids which came closer than the Moon, the below diagram shows how far in advance of closest approach the asteroids were first detected. Unlike actual asteroid impacts where, by using infrasound sensors, it is possible to assess how many were undetected, there is no ground truth for close approaches. The below chart therefore does not include any statistics for asteroids which went completely undetected. It can be seen however that about half of the asteroids that were detected, were not detected until after they had passed Earth. That is to say, if they had been on an impact trajectory, they would have been undetected before impact. This includes larger asteroids such as 2018 AH, which wasn't detected until 2 days after it had passed, and is estimated to be around 100 times more massive than the Chelyabinsk meteor. It is worth noting that the number of detections is increasing as more survey sites come on line (for example ATLAS in 2016 and ZTF in 2018), and that approximately half of the detections are made after the asteroid passes the Earth. The below charts visualise the warning times of the close approaches listed in the above bargraph, by the size of the asteroid instead of the year they occurred in. The sizes of the charts show the relative sizes of the asteroids to scale. For comparison, the approximate size of a person is also shown. This is based the absolute magnitude of each asteroid, an approximate measure of size based on brightness. Abs Magnitude 30 and greater (size of a person for comparison) Abs Magnitude 29–30 Absolute Magnitude 28–29 Absolute Magnitude 27–28 Absolute Magnitude 26–27 (probable size of the Chelyabinsk meteor) Absolute Magnitude 25–26 Absolute