We live in a rough neighbourhood. We often think of our solar system as a vast and empty space populated by a few planets with their orbiting moons. But our solar system is besieged by debris left over from its birth. And we get periodic reminders of this. Much of the debris is small and harmless and is just interplanetery dust that frequently strikes the Earth and burns up in the upper atmosphere. We frequently see this burning up in the night sky as meteors. But there is much evidence to put us on our guard. There are more menacing intruders out there that have the potential to terrorize humanity. The probability of an asteroid or comet colliding with the Earth is small, but this thought should be put in the context of the consequences of a collision - which could be catastrophic. While both asteroids and comets pose a threat, asteroids are the greater threat due to their numbers, size, composition and proximity to our planet Earth.
Asteroid orbits can be very unpredictable as they can be easily deflected by the gravational effects of planets or the sun. Most asteroids reside in the asteroid belt between Mars and Jupiter. Their origin may well be to do with the strength of Jupiter's gravity which prevented material from condensing into a single planet in the early stages of the formation of the solar system. But from time-to-time these giant boulders of rock and metal break away from their normal orbit due to collisions with each other or the gravitional effects of Jupiter and Mars. The resulting new random orbits may pose a danger to us.
Asteroids range in size from a few meters to about a thousand kilometers (Ceres). Hundreds of thousands of asteroids have been discovered. Over 100,000 have orbits sufficiently well defined to have been catalogued and more than 10,000 have been named. But thousands more are suspected to exist. The threshold diameter for an asteroid to have serious global affects and maybe even cause a mass extinction is between 1km to 2km. Objects with diameters of tens of meters would cause severe local damage but no global threat.
The demise of the Dinosaurs in the Cretaceous Extinction 65 million years ago was one such catastrophy. Evidence for large impacts coincident with other mass extinctions remained elusive for years. But recently scientists are discovering evidence of ancient impact sites that could perhaps be ground-zero for some of the previous mass extinctions.
So while we are, of course, correct to worry about global problems of terrestrial origin we should also be aware that perhaps the greater risks come from a threat from outer space. We should be aware that somewhere in the solar system, orbiting quietly in the black darkness, lurks one of the biggest threats to the survival of the human species. Somewhere out there, perhaps closer than we think, there most likely is an asteroid, with our name on it! The question is not whether an impact will happen in the future, but when!
Evidence from the Past
Giant impact craters do exist on Earth. Scientists have found close to 200 craters of definate or probable impact origin. But this is far fewer than the number of actual impacts we would expect to have occurred throughtout Earth's history. We can estimate this from our knowledge of how the Solar System formed as well as the overwhelming evidence and abundance of impact craters on our nearest neighbour, the moon, as well as on many other fellow members of our solar system. So the actual number discovered is much less the the actual number of impacts (we would expect thousands of impacts over the course of 4.5 billion years of Earth's history).
We do know that many craters are covered over with thick layers of sediment. Many are obscured and unrecognisable due to the continuous process of erosion on the surface of the Earth. And many have disappeared completely due to plate tectonics and the continuous recycling of the Earth's crust.
In the last 600 million years, astronomers estimate that at least 60 large (greater than 5kms diameter) space rocks have collided with planet Earth. Again, many scientist's think this is a reasonably accurate estimate based on the number of large craters we observe on the moon.
An impact on land from a space rock 5kms in diamater would leave a crater about 100kms in diamater and release enormous amounts of energy. We should be in no doubt that massive impacts like this would be capable of triggering catastrophic changes to life on our planet.
So when and where will the next strike be? And how devastating will it be?
In 2002, an object estimated to be 10 meters in diameter collided with the Earth over the Mediterranean Sea. It exploded, and was vapourised in the upper atmosphere, with a force greater than the Hiroshima atomic bomb.
In 2001 and asteroid exploded high above the pacific ocean with the power of 10 Hiroshima bombs.
In January 2000, in Canada a fireball was spotted. It was a 200 ton asteroid exploding high in the Earth's atmosphere. An asteroid of that size hitting the earth would have the destructive power of an atomic bomb. But its angle of impact saved the local population this time as it exploded high above them.
In the early 1990s an object exploded high in the atmosphere that was of the order of 100 kilotons. The United States military thought it was a nuclear blast and was put on full alert. And President Clinton was woken up in Washington by senior advisors who feared the onset of a nuclear war!
In 1930, in Brazil, near the river Curuca, it is suspected that a cluster of three 10 meter diameter asteroids impacted in the upper atmosphere with an explosive force much greater than the Hiroshima bomb.
In 1908, Tunguska, Siberia, a 50 meter diameter asteroid exploded in the atmosphere about 3 miles high. Fortunately, because of its trajectory, it only grazed the atmosphere, but still produced an explosion greater than the Hiroshima bomb.
And most readers are familiar with Meteor Crater (Barringer crater), Arizona, where a 50 meter diameter Asteroid impacted 50,000 years ago.
These are just some reminders of the threat from outer space. So how much longer can the Earth's luck hold. How much longer until and Asteroid makes its way through the atmosphere and strikes a densly populted area. And if we spot and Asteroid on a collision course with Earth is there anything we can do to prevent an impact. What warning time would be needed?
In July 1994 the Schmaker Levy 9 comet changed the thinking of alot of people. The comet had an orbit around Jupiter.
And due to the strong gravitional effects of Jupiter, it's orbit was changed, putting it on a collision course with its adopted parent planet. The forces of gravity had broken it up and it was spread out into many different pieces, each about 2-3 kms in diameter.
It would be mankinds first time to witness such an impact (but not the last to be sure). Some of the blast clouds were bigger than the earth itself. This indeed was a huge wake-up call. Overnight the earth had become a more vulnerable place. If it could happen on Jupiter it could happen here. So the hunt began for these killers from space.