The Ultimate Human Nemesis:
Near Earth Asteroids
The average persons knowledge of a near-Earth asteroid comes from movies such as Armageddon or Deep Impact if they are even aware of such a thing. Usually in the Hollywood version a group of improbable heroes swoop in at the last minute and save Earth from a catastrophic event. This is far from realistic and the goal of this paper is to show the true nature of what a near-Earth asteroid is, why they are important, and what can be done, if anything, to prevent an impact with the Earth.
According to A Dictionary of Astronomy a near-Earth asteroid is defined as "Any asteroid belonging to the *Apllo, *Amor, or *Aten groups. Such asteroids have perihelion distances of less than 1.3 AU."(Ridpath 321) The perihelion of an object is the position in its orbit that it is closest to the Sun. Earth has an average distance from the sun of 1 AU. The difference between the two is .3 AU which is roughly 45 million kilometers. This is a huge distance by Earth standards but by Astronomical standards it is a very small distance that separates Earth from these Asteroids. From time to time Earth and one of the asteroids is in the same place at the same time resulting in an impact.
Throughout history Earth has crossed paths with asteroids many times. While some of these impacts have been harmless because the asteroid burned up in our atmosphere many others have hit and caused tremendous damage. It is widely believed that the extinction of the dinosaurs and most of the life on Earth was due to an asteroid impact 65 million years ago. A layer of rock known as the K/T boundary provides a sharp division between the age of dinosaurs and the age of mammals(Encyclopedia of World Biography 336). Even if this event was not completely responsible for the mass extinction the fact that evidence of this event exists around the world means that the impact must have been absolutely massive.
A more recent event occurred in Siberia, when an object, assumed to be an asteroid exploded above the Tunguska Valley. The explosion was enormous and destroyed forests over an area of hundreds of square miles("Gene Shoemaker" 338). By comparison the nuclear detonation at Nagasaki incinerated everything within .25 miles(Lee 208).
One statistic that no one seems to question is that there is a one hundred percent certainty that the Earth will be hit in the future. If you were to take a look at the moon it is easy to see all of the impacts that have taken place there. There are two things that stop the Earth from having the pock marked surface of our Moon. The first is the Earth's atmosphere. This causes most objects to burn up before ever touching the planet. Larger objects that make it through the atmosphere devastate the landscape but because of the Earth's weather the evidence of most of these impacts is covered up over time.
Since the possibility of an impact in the future is a certainty it is important to consider the consequences of such an impact. For this purpose we will consider three types of possible impacts. While these categorizations are arbitrary it is an interesting way to view the consequences to the human population. The categories we will look at will be a localized impact, a regional impact, and an extinction level impact. These three categories are actually a subset 11 levels of the Torino Scale. They represent levels eight, nine, and ten which are all certain impacts of varying levels of severity ("The Torino Impact Hazard Scale").
A localized impact would be something on the scale of the Tunguska Valley explosion. It could level a city if it were to hit land and could cause a Tsunami if it were to impact the Ocean close to the coastline. Events such as this occur once every 50 to 1000 years. While this seems catastrophic it gets much worse when looking at levels nine and ten.
A regional impact occurs once every ten thousand to one hundred thousand years. An impact on this scale would be capable of devastating entire continents in the event of a land impact. Massive tsunami would occur if it hit land. A tsunami on this scale could put entire coastal cities under water.
An extinction level impact could end all human life, along with many other species, on planet earth. The one bit of good news here is that these happen less than once every hundred thousand years. That good news will not help humanity if this year happened to be one of them. An impact on this scale would kick so much dust into the atmosphere, blocking the Sun, that whatever life survived the impact would be in serious and immediate jeopardy as plant life around the planet died. With no food this would lead to massive starvation of anyone and anything left alive.
With stakes this high at this point the important question would seem to be, "What is being done?" Efforts to locate near-Earth asteroids have increased in recent years. In 1995 only 18 near-Earth asteroids were discovered. By the year 2000 this number had grown to one hundred sixty-one near-Earth asteroids discovered. In just the first four months of 2007 two hundred eleven have already been discovered. An interesting note is that while Asteroid discoveries have been on the rise, the discovery of large asteroids has been following a downward trend since 2000(Chamberlin).
While finding these asteroids is clearly important, the next step was to visit one. The NEAR, Near Earth Asteroid Rendezvous, mission aimed to do just that. The goal of this mission was to send a spacecraft to orbit a near-Earth asteroid. The NEAR Shoemaker spacecraft was launched on Feb. 17, 1996 and eventually settled into an orbit around Eros in August of 2000("NEAR Mission Timeline"). Although not part of the original official mission the team would eventually land the spacecraft on Eros. This mission was important because regardless of the plan implemented to either divert or destroy an asteroid on a collision course it will be crucial to know the make-up of the asteroid. According to the NEAR mission website the reasons Eros was chosen were that it is close, it is big and its location was convenient based on the timing of the launch of NEAR Shoemaker("NEAR Mission FAQ").
Over the years there have been different ideas related to either diverting or destroying any near-Earth asteroid that poses a threat to Earth.
Early plans leaned towards the Hollywood route of destroying the asteroid with Nukes. This plan was later discounted as it would require building the largest nuclear device ever. The possibility of failure or sabotage and the consequences are far to grave for this to be anything other than a last resort.
More recent plans include deflection using various methods. The benefit of deflection is that the amount of deflection needed if done well in advance is very small. Again the possibility of a nuclear blast has been raised. This time not to destroy the asteroid but rather to give it a nudge in the right direction. It has been estimated that a 20 megaton blast in close proximity to an asteroid could deflect an asteroid("Killer Asteroid"). The problem with this theory is that not all asteroids are created equally. For the detonation plan to work the asteroid would have to be a solid mass and many asteroids are actually closer to a pile of rubble. A blast would have a much smaller effect on an asteroid of this type. As mentioned in the previous paragraph this would also risk the accidental detonation of a twenty megaton dirty bomb in our atmosphere.
Another plan to focus sunlight into an intense beam heating part of the asteroid seems to be the best plan of all those considered in writing this. Unlike using nukes or rockets to push it out of the way the fuel for the deflection would come from the Sun. The basic plan would be to fly a spacecraft into orbit around a near-Earth asteroid. Once in orbit the spacecraft would shine a beam of sunlight so intense it would heat up a small section of the asteroid. As the area is heated it would release gasses acting as a small propulsion system. While this would not work if an asteroid were discovered that would hit within years given a decade or more this small continuous push could change the orbit of the asteroid to reduce the danger of it impacting Earth.
So in conclusion it would seem that there is good news and bad in the consideration of near-Earth asteroids. The good news is that we know about a lot of them and we at least have some ideas about what to do to avoid an impact. The bad news is that the ideas we have are untested and they all hinge on one important detail. The asteroid would need to be discovered in time to launch a mission to deflect it, assuming a better idea does not come about. Unlike in the movies it isn't realistic to detect an asteroid, fly a quickly assembled team into space and blow it up. In the real world it could take decades to do something about an asteroid heading on a collision course with Earth.
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