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ASTEROID THREAT?
By Duncan Steel
Comets and asteroids that could hit the Earth are grouped together under the term 'Near-Earth Objects' [NEOs].
Increasing numbers of Near-Earth asteroids are being discovered by dedicated searches in particular in the USA, such as LINEAR, NEAT and Spacewatch. There are two thousand Near-Earth asteroids ranging in size to some tens of kilometres and thousands more await discovery.
There are growing calls for an international 'Spaceguard' programme aimed at determining whether there is an NEO due to hit the Earth soon. The odds of a calamitous event within the next century, are slim – but the consequences are so huge that the scientists involved take the threat very seriously.
To date, we know the accurate orbits for over 40,000 asteroids, and several hundred thousand others have been catalogued. Most of those are in the main belt, where some millions of asteroids larger than 1 km in size are thought to circuit the Sun. Using large telescopes astronomers have found almost a thousand minor planets beyond Neptune [and some argue that Pluto is simply the largest of these]. It is anticipated that there are billions of smaller asteroids in this region, but they are mostly too faint to be detected using current telescopes.
THE CHANCES OF AN ASTEROID HITTING THE EARTH
Predicting the path of an asteroid is not an easy task. Even with the best equipment available today, we can’t measure the position or direction of an asteroid exactly. So when we try and work out its future path, there is always a degree of uncertainty. However, the more measurements that we are able to get, the smaller this uncertainty becomes.
Whenever we discover a new Near Earth Object, we can only make a very rough estimate of its future path. This is why the news sometimes reports that there is a risk of an asteroid hitting Earth. Of an asteroid’s many possible paths, some could result in collisions with our planet. However, as we collect more data, we are usually able to eliminate the chances of the body hitting Earth in the near future.
There are many asteroids and Near Earth Objects of which we have no knowledge. Based on the recent evidence, we can estimate that the chances of an asteroid doing catastrophic damage in the next century are approximately one in 5000. But unless we make an effort to keep track of as many Near Earth Objects as we can, we can never be sure exactly when the next impact will be.
STABLE ASTEROIDS?
Most asteroids are in stable orbits around the Sun, and have stayed in much the same place since they were formed 4.5 billion years ago, when the Sun and the planets were formed. However, Jupiter's gravitational field can sometimes pull bodies out of orbit, and send them off on wayward trajectories through the Solar System. Asteroids in the main belt can also collide with each other, and this is thought to be the biggest reason why asteroids come into the inner Solar System, posing a potential hazard to life on Earth [Just ask the dinosaurs]!
Trojans are a type of very stable asteroid. There are two groups of Trojan asteroids, both sharing Jupiter’s orbit. They take the same length of time to orbit the Sun, but half remain 60 degrees in front of the giant planet and half 60 degrees behind. They are safe, and astronomers believe they have been there since the Solar System began. Recently several asteroids have been discovered in the same relationship with Mars. If Earth Trojans exist then they would be very valuable, because they would be the easiest objects to visit in space, but so far none have been found despite several intense searches.
There are also many asteroids known in the region inhabited by the gas giant planets Jupiter, Saturn, Uranus and Neptune. These are called Centaurs. Such asteroids are unstable, and have orbits that are changed markedly every time they come too close to one of those big planets. Some of the smaller moons of these planets are thought to be asteroids captured from Sun-centred to planet-centred paths.
ASTEROID CLASSIFICATION
Astronomers often classify asteroids according to their chemical make up. Broadly-speaking their compositions can be deciphered by comparing their colours [seen through a telescope] with the characteristics of different types of meteorite. There are three main classes:
Stony asteroids [S-Type]:
These are the most common, although they come in various sub-classes. Asteroid 433 Eros is a stoney asteroid that was recently photographed by the Near-Earth Asteroid Rendezvous [NEAR] mission. Some stoney asteroids are bright, reflecting up to 40 percent of the sunlight that hits them [for example the large asteroid Vesta], while others are darker, reflecting only 10-15 percent of sunlight. Most meteorites are chips off stony asteroids.
Metallic asteroids [M-Type]:
These are composed mostly of nickel-iron. Although they are bright like stony asteroids, these have slightly different colours. Radar echoes from these metallic asteroids are also strong, indicating that they are made of electrically conductive material [i.e. metal]. Grouped with iron meteorites, these are pieces from the cores of fledgling planets destroyed by collisions when the Solar System formed.
Carbonaceous asteroids [C-Type]:
These are dark and difficult to spot because they contain a great deal of carbon and tar-like substances. However, to scientists they are some of the most interesting targets. Only a handful of fragments of them – carbonaceous chondrite meteorites – have ever been found on Earth, the most recent falling in Canada in January 2000. The two moons of Mars, Phobos and Deimos, are actually carbonaceous asteroids, captured in a Martian orbit.
In the outer Solar System and the asteroids/minor planets tend to be redder in colour because they contain more organic material. They also contain a great deal of ice. Since some of the organic complement is actually alcohol, all the ingredients are available for a scotch on the rocks if you get the chance!
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