one of the asteroids, 2008 TC3, was detected in advance, some 19 hours before exploding in the atmosphere. As was the case with the 2013 Chelyabinsk meteor, no warnings were issued for any of the other impacts. At the presentation, alongside former NASA astronauts Dr. Tom Jones and Apollo 8 astronaut Bill Anders, Foundation head Ed Lu explained that the frequency of dangerous asteroid impacts hitting Earth was from three to ten times greater than previously believed a dozen or so years ago (earlier estimates had pegged the odds as one per 300,000 years). The latest reassessment is based on worldwide infrasound signatures recorded under the auspices of the Comprehensive Nuclear-Test-Ban Treaty Organization, which monitors the planet for nuclear explosions. Dr. Brown's UWO study used infrasound signals generated by asteroids that released more than a kiloton of TNT explosive force. The study suggested that "city-killer" type impact events similar to the Tunguska event of 1908 actually occur on average about once a century instead of every thousand years, as was once previously believed. The 1908 event occurred in the remote, sparsely populated Tunguska area of Siberia, Russia, and is attributed to the likely airburst explosion of an asteroid or comet that destroyed some 80 million trees over 2,150 square kilometres (830 sq mi) of forests. The higher frequency of these types of events is interpreted as meaning that "blind luck" has mainly prevented a catastrophic impact over an inhabited area that could kill millions, a point made near the video's end. 99942 Apophis During the first decade of the 2000s, there were serious concerns the 325 metres (1,066 ft) wide asteroid 99942 Apophis posed a risk of impacting Earth in 2036. Preliminary, incomplete data by astronomers using ground-based sky surveys resulted in the calculation of a Level 4 risk on the Torino Scale impact hazard chart. In July 2005, B612 formally asked NASA to investigate the possibility that the asteroid's post-2029 orbit could be in orbital resonance with Earth, which would increase the likelihood of a future impact. The Foundation also asked NASA to investigate whether a transponder should be placed on the asteroid to enable more accurate tracking of how its orbit would be changed by the Yarkovsky effect. By 2008, B612 had provided estimates on a 30 kilometers-wide corridor, called a "path of risk", that would extend across the Earth's surface if an impact were to occur, as