Free Astronomy Magazine September-October 2019

32 SEPTEMBER-OCTOBER 2019 SMALL BODIES It will therefore be essential to con- tinue and intensify the search for smaller and smaller asteroids, until a complete census of those whose orbits can intersect the Earth’s orbit will be available. Many telescopes are already engaged in this type of survey and others are about to start oper- ations both on the ground and in orbit. Also, the knowl- edge of the struc- ture and composi- tion of the small bodies of the Solar System is important for a correct modeling of Tunguska-like events, and in this context, automatic mis- sions such as Hayabusa and Rosetta are crucial. No effort will be superfluous in the attempt to avoid the only foreseeable and predictable natural disaster. ! deduced that if the Tunguska asteroid was larger than previously believed, events of this type must neces- sarily be less frequent than expected, since the number of exist- ing asteroids de- creases as the diame- ter increases. And, in fact, combining the results obtained by the Ames models with the most recent esti- mates of asteroid pop- ulations, researchers concluded that the av- erage interval be- tween Tunguska-like impacts should be on the order of millennia and not centuries, as hypothesized in the past. This data is cer- tainly reassuring, but since human beings al- ready inhabit around 10% of the Earth’s surface, the probability that the next Tun- guska event will occur above a built-up area, with obvious consequences, is far from negligible. L ake Cheko is the only prob- able trace left by a fragment of the Tunguska aster- oid. On the left, a three-dimensional computer simula- tion of the lake immediately after the impact, recon- structed based on topography and bathymetry data. The water level is kept about 40 meters lower than actuality for a better view of the bottom shape. [Institute of Marine Sci- ences, CNR] Below is how it looks today. [V. Romeiko]