Forces that mould the earth - Asteroids
Two centuries since the first asteroid, Ceres, was spotted, space rocks are becoming identified as forces that mould our planet, wiping out species or crowning them, in a brutal act of death and creation.
It is popular knowledge that an asteroid or comet slammed into modern-day Mexico around 65 million years ago, leaving behind a gigantic crater and extinguishing much of the life on Earth, notably ending the long reign of the dinosaurs.
But it is now clear that this was not a one-off. Scientists have evidence of four other cataclysms that have occurred over the last quarter of a billion years, and conclude that space plays a key role as kingmaker of the species.
A study published last month by University of Washington paleontologist Peter Ward suggests a mass extinction suddenly occurred 200 million years ago, destroying the mammal-like reptiles that dominated the planet and handling the throne to T rex and his ilk.
"Something suddenly killed off more than 50 percent of all species on Earth, and that led to the age of the dinosaurs, " says Ward. His team's research is based on a count of fossilised marine plankton that died en masse because of rapid climate change. Ward suggests asteroid impact was the likely cause but does not rule out other factors, such as surge of volcanic activity.
In their hurtling track around the Sun, comets or big asteroids, say 10 kilometers across, can unleash enormously destructive energy if they smack into the Earth. They can release a shockwave equal to thousands of Hiroshma bombs and kick up a dust cloud that is borne by wings around the planet, filtering out the sunlight and changing weather patterns.
In this "impact winter," plants shrivel and die from lack of photosynthesis, denying food to life farther up the food chain. The extinction studied by Ward occurred in less than 10,000 years, "the blink of an eye, geologically speaking."
Benny Peiser, a social anthropologist at Liverpool John Moores University, says the emerging importance of asteroids as changers of planetary life poses a challenge to evolutionists.
Man, he argues, rose not as a gradual process of selective fitness and genetic supremacy but simply because he was lucky. "The reason that Homo sapiens have survived in spite of these global disasters has little to do with the traditional explanations give by neo-Darwinists, " says Peiser.
"It is sobering to realise that we are alive due to cosmic luck rather than our genetic makeup. After all, the populations of hominids and early modern humans were extremely small. Had any of these impacts occurred in the proximity of these population groups, we might also have gone the way of the dodo."
For all their destruction, asteroids may also have provided the seed for life on Earth billions of years ago, according to some minds. Under this "panspermia" theory, asteroids or a comet delivered the chemical building blocks for DNA when the planet was in its infancy.
The first sighting
In 1801, an Italian astronomer named Piazzi spotted an object in space through his telescope. At first, he thought it was a small planet. Piazzi named his discovery Ceres.
Soon, other astronomers noticed similar objects. They shone at night like faint stars, so they were named asteroids, which means "like stars".
The first close-up pictures of an asteroid were taken in 1991, by the space probe Galileo. It photographed the asteroid Gaspra. The images showed Gaspra to be more than 20 km ( 12 miles ) across and irregular in shape. It is grooved, and pitted with craters.
Very rarely, asteroids collide with the Earth, leaving large craters. Scientists think that a massive asteroid slammed into the Earth 65 million years ago, causing more damage than thousands of nuclear bombs.
It is possible that the force of this asteroid caused tidal waves in the sea and fires over the land that blotted out the sunlight for years. Many types of plants and animals, including the dinosaurs, died out.
Different types of asteroids
Although there are many thousands of asteroids, they fall into three main groups depending on what they are made of. The groups are carbonaceous (like Ceres), silicaceous (like Gaspra) and metallic asteroids.
Carbonaceous asteroids are common. They are stony and darker than coal.
Silicaceous asteroids are bright, stony bodies which contain metal.
Metallic asteroids may be the exposed, metallic cores of much larger bodies.
Jupiter holds clusters of asteroids in its gravity. As it spins around the Sun, some asteroids sit in front of its orbit, while others sit behind. These asteroids are called the Trojans.
asteroids, known as the Apollo asteroids, sometimes cross the Earth’s path.
Their usual orbit, however, is farther away from the Sun.