Under the Microscope/Prof William Reville: The threat of Earth colliding with an asteroid is very real, even though the probability of a major collision in the near future seems very low.
It is certain that future collisions will occur, some disastrous for life on Earth, unless we prevent them. Developing the technology that will protect us is a long-term project, but now that we understand the threat, we have a solemn obligation to future generations to develop this technology as fast as possible. R.L. Schweickart and others describe one promising approach in the November 2003 edition of Scientific American.
Millions of small particles from space enter our atmosphere daily but they burn up quickly and never reach the surface of the Earth. When somewhat bigger objects enter our atmosphere however, they explode rather than vaporise. In 1908 an asteroid 60 metres in diameter exploded 6 km above ground in the Tunguska region of Siberia, flattening an area the size of metropolitan New York city.
The Tunguska explosion had the power of 10 megatons of TNT and experts calculate that there is a 10 per cent chance of a Tunguska-like event occurring within the next 100 years. An asteroid about 100 metres in diameter would penetrate much deeper into the atmosphere and possibly hit the surface of the Earth causing an explosion equivalent to 100 megatons of TNT. There is a 2 per cent chance that such an event will occur within the next 100 years.
Asteroids with a diameter of 1 km or greater are called killer-asteroids. Such bodies would hit the Earth with an energy equivalent of 100,000 megatons of TNT, much more than the combined energy of all nuclear weapons on Earth. Such an impact could destroy life on Earth and there is one chance in 5,000 that such an event will occur within the next 100 years.
Killer-asteroids have hit Earth in the past. There is good evidence that such an event happened 65 million years ago and that it caused a mass extinction of life, including the dinosaurs. It is highly likely that such an event will happen again in the future. In order to protect ourselves we need to do two things: (a) identify any asteroid that is on a collision course with Earth decades before it arrives here, and (b) develop a technology that will deflect the incoming asteroid so that it misses the Earth.
In 1998 NASA began a programme of detecting near Earth objects (NEOs) larger than one km in diameter. Over 660 NEOs, and more than 1,800 smaller bodies, have been found. These asteroids orbit the sun, as does the Earth. NASA checks each NEO to see if its orbit crosses the Earth's orbit around the sun, and, if it does, NASA calculates forward to see if the two will ever collide. Luckily, any asteroid destined to hit the Earth will probably make many close orbital passes, perhaps thousands, before finally striking us. Therefore, NASA will probably detect any NEO destined to strike Earth many decades before it actually hits us. NASA plans to extend this asteroid hunt to search for NEOs as small as 200 metres in diameter.
The first solution that occurred to people when they began to ponder how to deal with an incoming asteroid was to destroy it using nuclear weapons. On reflection however, this option is seen to be highly unpredictable. A nuclear explosion might break an incoming asteroid into several fragments that would continue on to hit the Earth, perhaps with greater consequences than if the asteroid had remained unbroken. Most of the other technologies under consideration depend on detecting the NEO on collision course with Earth decades before the collision occurs. The idea then is to gently deflect the asteroid so that when it eventually crosses Earth's orbit it misses us.
If an asteroid is on course to collide dead-centre with the Earth, then it would miss the Earth if, by the time it arrived, the Earth had moved one-half its diameter further. This would happen if the speed of the asteroid had been slightly slowed down. The asteroid would also miss the Earth if its speed was increased slightly so that when it crossed the Earth's orbit the position of the Earth was slightly over one-half its own diameter away.
The Earth moves in its orbit at a speed of 29.8km per second and the diameter of the Earth is 12,800km. The Earth takes 215 seconds to move half its diameter. Therefore if an asteroid on a collision course with Earth is speeded up or slowed down so that it crosses Earth's orbit at least 215 seconds before or after it otherwise would, it will safely miss the Earth.
The technology advocated by Schweickark and his group is an unmanned nuclear-powered space tug that would lock on to an incoming asteroid and exert a gentle push. Schweickark believes that the feasibility of his proposal could be demonstrated by 2015.
If a killer-asteroid with our name on it was discovered tomorrow, destined to reach us in five year's time, there is nothing we could do about it except to blast it with nuclear weapons when it got close in. We have no guarantee that a killer-asteroid will not appear soon. We therefore have the most solemn obligation to make the development of technology to deal with killer-asteroids our number one space priority. We can, of course, continue to do other things in space, but devising a plan to save human civilisation would seem to merit top priority.
William Reville is associate professor of biochemistry and director of microscopy at University College Cork.