A "small" comet impact on Earth is the most likely extraterrestrial (地球外的) object to kill millions of people and it could happen at any moment, according to a British expert.
Dr. Matt Genge, from London’s Natural History Museum, has investigated the damage that a wide range of small meteorites (陨星) and comets would have, if they struck our planet. He identifies comets between just 50 and 100 meters wide as the most terrifying destructive, with massive heat and shock waves burning people and crushing buildings. He explains that while meteorites bigger than two kilometers could wipe out humanity, these hits are expected only once every million years or so. Smaller ones could still lead to the deaths of tens of millions and arrive much more frequently. These are therefore much more erous, he said. A 60-meter-wide comet exploded over Siberia in 1908 with 600 times the energy of the bomb dropped on Hiroshima. It laid waste to a 40-kilometer-wide patch of forest, but, fortunately, the area was unpopulated. Comets of this size are expected to strike the Earth every 100 to 300 years.
If the 1908 comet had arrived just eight hours later, it would have struck London, killing everyone and destroying everything as Dr. Genge described.
Dr. Genge looked at the physical properties of meteorites and comets to identify the Siberian-type strike as the most erous. Ironically, it is because the 50to 100-meter-wide comets are so weak that they are so erous. They break up into fragments which explode just a few kilometers above the ground--"the optimum (最优) altitude for maximum devastation (毁灭)," said Dr. Genge. Stone and metal meteorites of similar size are much stronger and do not break up and explode. However, 100-meter-wide metal meteorites would create a blast area of 60 kilometers across compared to just two kilometers for a stone meteorite of the same size. The metal’s higher density is to blame.
Dr. Genge believes that this type of ysis will help to decide what level of er an Earth-bound object would pose and aid decisions about what course of action to take.
The 50to 100-meter-wide comets are more erous than stone and metal meteorites because______