Every day of our lives we are in danger of instant death from small high-speed missiles from space—the lumps of rocky or metallic debris which continuously bombard the Earth. The chances of anyone actually being hit, however, are very low, although there are recorded instances of "stones from the sky" hurting people, and numerous accounts of damage to buildings and other objects. At night this extraterrestrial material can be seen as "fireballs" or "shooting stars", burning their way through our atmosphere. Most, on reaching our atmosphere, become completely vaporised.
The height above ground at which these objects become sufficiently heated to be visible is estimated to be about 60-100 miles. Meteorites that have fallen on buildings have sometimes ended their long lonely space voyage incongruously under beds, inside flower pots or even, in the case of one that landed on a hotel in North Wales, within a chamber pot. Before the era of space exploration it was confidently predicted that neither men nor space vehicles would survive for long outside the protective blanket of the Earth’s atmosphere. It was, thought that once in space they would be seriously damaged as a result of the incessant downpour of meteorites falling towards our planet at the rate of many millions every day. Even the first satellites showed that the danger from meteorites had been greatly overestimated by the pessimists, but although it has not happened yet, it is certain that one day a spacecraft will be badly damaged by a meteorite.
The greatest single potential danger to life on Earth undoubtedly comes from outside our planet. Collision with another astronomical body of any size or with a "black hole" could completely destroy the Earth almost instantly.
Near misses of bodies larger than or comparable in size to our own planet could be equally disastrous to mankind as they might still result in total or partial disruption. If the velocity of impact were high, collision with even quite small extraterrestrial bodies might cause catastrophic damage to the Earth’s atmosphere, oceans and outer crust and thus produce results inimical to life as we know it. The probability of collision with a large astronomical body from outside our Solar System is extremely low, possibly less than once in the lifetime of an average star. We know, however, that our galaxy contains great interstellar dust clouds and some astronomers have suggested that there might also be immense streams of meteorite matter in space that the Solar system may occasionally encounter. Even if we disregard this possibility, our own Solar system itself contains a great number of small astronomical bodies, such as the minor planets or asteroids and the comets, some with eccentric orbits that occasionally bring them close to the Earth’s path.
What is the greatest danger to life on Earth？（）
A．Collision with small high-speed missiles.
B．Collision with an astronomical body.
C．Collision with stones from the sky.
D．Collision with spacecrafts.
