EXCERPTS: The US-based group, B612 Foundation, which includes a number of former Nasa astronauts, campaigns on the issue of space protection. It hopes a new visualisation will press home the idea that impacts are more common than we think. The presentation leans on data collected by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). The CTBTO operates a network of sensors that listens out for clandestine atom bomb detonations. Between 2000 and 2013, this infrasound system catalogued 26 major explosions on Earth. None were caused by A-bombs; they were all the result of asteroid strikes. They ranged in energy from one to 600 kilotons. By way of comparison, the bomb that destroyed the Japanese city of Hiroshima was a 15-kiloton device. Fortunately, most of these space rocks disintegrated high up in the atmosphere and caused few problems on the ground. A few, people will have heard about, such as the 20m-wide object that ripped across the sky above the Russian city of Chelyabinsk last year. But many will have gone unseen because they occurred far out over the oceans. And just one of the 26 events was detected in advance, and then by only a matter of hours. ..... The foundation says the CTBTO data would suggest that Earth is hit by a multi-megaton asteroid - large enough to destroy a major city if it occurred over such an area - about every 100 years. Remembering the Tunguska event of 1908 - it was fortunate that that object, thought to be about 45m wide, struck a very remote part of the globe. ..... Previous surveys have suggested that we have probably found a little over 90% of the true monsters out there - the objects that could lead to extinction if they struck the Earth. And the good news is that none look like they will hit us anytime soon. But data from Nasa's Wise telescope suggests that the population of objects in the 100-1,000m size range may number close to 20,000, and the vast majority of these have yet to be identified and tracked. ..... BBC News - Asteroid impact risks 'underappreciated'