Most of the time, they’re harmless. Innocuous, really. They tumble along, minding their own business, not hurting anybody. But every once in a while–BOOM!
“They” are asteroids, and when they go boom, it’s because they’ve run into something. When that something is Earth…well, you’ve got trouble with a capital T, and that rhymes with E, and that stands for “extinction.” Probably, 65 million years ago, of the dinosaurs; possibly, next time, of us.
Asteroids are rocky and/or metallic objects that orbit the Sun, just like the Earth does, but are too small to be considered planets. They’re also known as “minor planets.” They can be as small as a pebble or as large as Ceres, which has a diameter of 1000 kilometres. (There are actually quite a few very large ones; we’ve found 16 with a diameter of 240 kilometres are greater.)
The majority of the asteroids in the Solar System orbit in the main asteroid belt, between the orbits of Mars and Jupiter, but they’ve been found from inside Earth’s orbit to outside Saturn. Worse, many of them have highly eccentric orbits that cause them to cross Earth’s orbit from time to time. Occasionally, an earth-crossing asteroid and Earth happen to be in the same place at the same time–and then, BOOM!
How did asteroids end up littering the Solar System in the first place? A popular theory at one time was that the asteroid belt was the remnants of a planet that was somehow shattered. (1930s and ’40s science fiction had great fun with this theme, with intrepid space explorers sometimes discovering the remnants of a great civilization among the fragments.) However, it’s generally thought now that asteroids are simply material that, during the formation of the Solar System, never coalesced into a planet. There’s just not enough of it: if the estimated total mass of all the asteroids was gathered into a single object, that object would be less than 1500 kilometres across–less than half the diameter of the Moon.
Most of what we know about asteroids comes from the ones that have hit the Earth–small ones, fortunately. An asteroid on a collision course with Earth is known as a meteoroid, or, more popularly, a meteor. Most meteors are very small and burn up in the atmosphere, but a few survive their fiery descent to impact the Earth. Meteors that survive are called meteorites.
Of all the meteorites examined, 92.8 are composed of pretty ordinary stone, 5.7 percent are composed of iron and nickel, and the rest are a mixture of the stone, iron and nickel. Presumably, this holds true for asteroids in general.
Small meteors and meteorites cause a spectacular streak of light through the atmosphere and occasionally minor damage on the ground. But what happens when a large meteorite hits the ground?
Well, as you’d expect, the results become more and more spectacular as the meteor increases in size. According to one recent study, an asteroid with a diameter smaller than about 650 metres, would cause blast damage, earthquakes and fires over an area between 1000 and 10,000 square kilometers–not too different from other natural disasters we face from time to time. (That’s assuming it hit on land. If it hit the ocean–which is much more likely, since there’s more water than land on Earth–the resulting tsunami could cause flooding up to a kilometer inland on all coastlines bordering the ocean where it hit.)
An asteroid 1.5 kilometers in diameter would cause damage unprecedented in all of human history, affecting the climate by injecting water vapor and dust into the atmosphere, and possibly severely damaging the ozone layer that protects the Earth from ultraviolet radiation (because the impact creates a vast plume of nitrogen oxide).
An asteroid larger still, say three kilometres in diameter, could completely destroy the ozone shield and cause a deadly cooling of the planet, due to smoke and to the release of sulfur from inside the impacting asteroid. And an asteroid larger still, up to 6.5 kilometres in diameter, could darken the sky so much photosynthesis would become impossible and plants would die. Fragments of the asteroid and the Earth hurled back into space by the impact would rain down all over the planet, setting forest fires. The resulting smoke would further darken the atmosphere, until permanent night settled over the planet. The temperature would plunge. Tsunamis 100 metres high could flood 20 kilometres inland, if the impact was in the water. Suffice it to say it would not be pleasant.
Just ask the dinosaurs. There’s a vast, buried crater in the Yucatan peninsula that dates back to about 65 million years ago–just about the time that the dinosaurs disappeared. Many scientists believe the impact played an important role in their extinction. It’s easy to see why: according to scientists, several nasty things happened when the asteroid hit, with an energy 10,000 times greater than all the world’s nuclear weapons being exploded at once–equivalent to 10 trillion megatons of TNT. (That, by the way, is considered a conservative estimate.) First, the asteroid blew a hole in the air. Three second later, it plowed through a kilometer of ocean and into the Earth’s crust, generating earthquakes at fault lines all over the world. Within an hour, 30-metre-high walls of water flooded coastal plains in what is now the Gulf of Mexico. Within that same time frame, the billions of tons of debris that had been hurled into space hurtled back down again, red-hot, turning the sky into a big, glowing sheet of rock, in the word of one researcher, and heating the air hot enough to ignite paper. The warmth was short lived; the sun went out, blocked by soot, and didn’t show it’s face again for at least a year, possibly longer. The global temperature dipped far lower than during the depths of the Ice Age, and stayed that way for a decade. And creatures all over the world–those that survived those first horrendous few hours–died.
In the aftermath, new life forms came to the fore: a new study by Dr. David Jablonski fo the University of Chicago, released last week, says that after the asteroid hit, there was a real bloom of new life forms as survivors diversified and filled suddenly vacant ecological niches. He compared it to weeds moving in after a forest is cut down. Interestingly, this bloom of new species happened within one million years in North Amercia, closest to the impact site; it took much longer across the Atlantic, where five million years passed before clams and snails managed to get back to where they were before the extinction.
The asteroid was estimated to be 10 kilometres in diameter, but you don’t need anything that large to wreak substantial havoc. In 1908, something blasted into the atmosphere above the Tungaska forest in Siberia and exploded, incinerating hundreds of square kilometres of forest and knocking down thousands of square kilometres more. It’s estimated that that same object, had it appeared in the skies over Europe, would have killed 500,000 people. Today, the death toll would be much higher.
You don’t even have to go to the Gulf of Mexico or Siberia to see evidence of asteroid bombardment. Deep Bay, off Saskatchewan’s own Reindeer Lake, is a large, circular bay completely out of character for lakes in the North. It’s thought to be the crater left behind by an asteroid impact around 100 million years ago (give or take 50 million years). It’s 13 kilometres in diameter.
The odds of a major asteroid impact are low–about one in 10,000 over the next century. But it has happened, and it probably will happen again. Worse, it could happen at any time. Every now and then, astronomers report discovering that an asteroid just missed the Earth by a few hundred thousand kilometres. There’s no guarantee we would see a killer asteroid coming. If we did see it coming, we might be able to deflect it with nuclear weapons, but we would need years’ advance warning. Without that, we’d be sitting ducks.
Governments have been asked to fund scientific surveys of asteroids that cross Earth’s orbit, but so far, they haven’t seen the threat as real. Some privately funded work is being done, but the fact remains, that out of an estimated 2000 Earth-orbit-crossing asteroids, we’ve only actually located 200. It’s estimated that advances in astronomical imaging systems could allow us to catalog all asteroids larger than one kilometre in diameter within 10 years for just $50 million.
Ironically, within the next few months, Hollywood will release two big-budget disaster films about asteroids colliding with Earth. For the cost of making either one of them, we could be taking measures to avoid the fate of the dinosaurs.
Someday soon, maybe we will.
