Cosmos - Carl Sagan [43]
In the vast spaces between the planets there are many objects, some rocky, some metallic, some icy, some composed partly of organic molecules. They range from grains of dust to irregular blocks the size of Nicaragua or Bhutan. And sometimes, by accident, there is a planet in the way. The Tunguska Event was probably caused by an icy cometary fragment about a hundred meters across—the size of a football field—weighing a million tons, moving at about 30 kilometers per second, 70,000 miles per hour.
If such an impact occurred today it might be mistaken, especially in the panic of the moment, for a nuclear explosion. The cometary impact and fireball would simulate all effects of a one-megaton nuclear burst, including the mushroom cloud, with two exceptions: there would be no gamma radiation or radioactive fallout. Could a rare but natural event, the impact of a sizable cometary fragment, trigger a nuclear war? A strange scenario: a small comet hits the Earth, as millions of them have, and the response of our civilization is promptly to self-destruct. It might be a good idea for us to understand comets and collisions and catastrophes a little better than we do. For example, an American Vela satellite detected an intense double flash of light from the vicinity of the South Atlantic and Western Indian Ocean on September 22, 1979. Early speculation held that it was a clandestine test of a low yield (two kilotons, about a sixth the energy of the Hiroshima bomb) nuclear weapon by South Africa or Israel. The political consequences were considered serious around the world. But what if the flashes were instead caused by the impact of a small asteroid or a piece of a comet? Since airborne overflights in the vicinity of the flashes showed not a trace of unusual radioactivity in the air, this is a real possibility and underscores the dangers in an age of nuclear weapons of not monitoring impacts from space better than we do.
A comet is made mostly of ice—water (H2O) ice, with a little methane (CH4) ice, and some ammonia (NH3) ice. Striking the Earth’s atmosphere, a modest cometary fragment would produce a great radiant fireball and a mighty blast wave, which would burn trees, level forests and be heard around the world. But it might not make much of a crater in the ground. The ices would all be melted during entry. There would be few recognizable pieces of the comet left—perhaps only a smattering of small grains from the non-icy parts of the cometary nucleus. Recently, the Soviet scientist E. Sobotovich has identified a large number of tiny diamonds strewn over the Tunguska site. Such diamonds are already known to exist in meteorites that have survived impact, and that may originate ultimately from comets.
On many a clear night, if you look patiently up at the sky, you will see a solitary meteor blazing briefly overhead. On some nights you can see a shower of meteors, always on the same few days of every year—a natural fireworks display, an entertainment in the heavens. These meteors are made by tiny grains, smaller than a mustard seed. They are less shooting stars than falling fluff. Momentarily brilliant as they enter the Earth’s atmosphere, they are heated and destroyed by friction at a height of about 100 kilometers. Meteors are the remnants of comets.* Old comets, heated by repeated passages near the Sun, break up, evaporate and disintegrate. The debris spreads to fill the full cometary orbit. Where that orbit intersects the orbit of the Earth, there is a swarm of meteors waiting for