What are the types of impactors with Earth?

A 1992 NASA-sponsored symposium examined the relationship between hazard and the size of impacting objects, the result of which was the Custodial Investigations in Space: a Report of the NASA International Symposium on the Detection of Near-Earth Objects, edited by David Morrillin, chair of the conference. According to the report, hazards caused by projections can be categorized into three main groups based on the size or function of the impactor.

Category 1: Impactors 10 to 100 meters in diameter?

Impacting objects in this category usually break up before they reach the ground and are between 10 and 100 meters in diameter. The explosion of the object produces a function equivalent to 5 to 10 tons of TNT (based on an entry velocity of 20 km/s into the atmosphere), and the function of an impactor of this size is completely consumed in the atmosphere.

Objects in the smaller end of this size range appear to collide with the Earth once every 10 years. 10-meter-sized impactors rarely make it through the atmosphere to produce craters, and only iron- or iron-stone-like objects, which are similar to the well known type of meteorite, persist. However, such occurrences are rare. In fact, stony objects don't survive either; they break up and slow down, causing the rock mass to slow to free-fall velocity, and the kinetic energy is converted into atmospheric shock waves. Some of this shock wave energy is transformed into mechanical waves; some is transformed into sudden bursts of light and heat often thought to be fireballs. The explosions it triggers tend to occur at considerable altitudes in the atmosphere. There is little or no such occurrence on the ground or in the oceans. As a result, when it passes over an observer, the fireball is usually seen to emit a cracking sound and the vibrations from the shockwave are heard.

The impact gets worse when the size of the object approaches the 100-meter standard. Such collisions occur a few times every thousand years on Earth, and all are equivalent to 100 million tons of TNT. such an actual projection has a better chance of reaching the ground, or because of its size it can reach a lower part of the atmosphere and dissipate.

Because of the lower location of the explosion, the energy converted into a shock wave is correspondingly larger. If the pressure of the shock wave and the shock radiation energy (mostly in the form of excess heat) are combined, the consequences would be extremely destructive. The best example of such a critical state occurred in 1908 in the Tunguska explosion, which released a huge amount of energy and destroyed more than 2,000 square kilometers of wilderness in Siberia.

What would an impact of one of the larger asteroids in this category mean for Earth? The results would be localized, though still devastating to the area around the impact site. Stormy fires will be prevalent, ignited by the asteroid and its impact-burning sparks. Although nitrogen oxides and dust would not affect the globe, they could travel a long way through the atmosphere. As with volcanic eruptions, these gases and dust can color sunsets for months. If the object travels through the atmosphere and hits the ground, the natural changes in the area are obvious - the holes in the ground and everything that is crushed and buried beneath the surface. If it were to appear over a town or city, buildings within a 20-kilometer radius would be knocked down, and oil and gas pipelines would catch fire from the heat generated by the impact. Thousands of people would be killed by the crushing impact or the shockwave or the ejecta - the natural bombardment slamming material into everyone around the location of the impact. If the same impact had occurred in the ocean, the vapor from the hot object hitting the water would have rolled upward, causing short-term climatic changes due to the massive increase in moisture in the atmosphere.

The second category: impactors 100 to 1,000 meters in diameter?

In appearance, the second type of impactor ranges from 100 to 1,000 meters in diameter, and it collides with the Earth on average about once every 5,000 years. Its best case description for us on Earth is a comet of water, ice and volatiles. Scientists believe that such an object would break up and disintegrate in the Earth's thick atmosphere before it actually hit the Earth. The impact of such a comet would cause the greatest locational damage. It would come primarily from the atmospheric shock explosion of the disintegrating object as it passes through the atmosphere. This type of impact comet would cause damage similar to that caused by the Tunguska explosion.

But if an asteroid were to drop an object of the same size on Earth, it might raise more concerns: a metallic asteroid might reach the surface and produce a small impact crater, while a stony asteroid would have to be about 150 meters or more in size to cause the same crater.

At the smaller end of that scale (nearly 100 meters in size), the impact would cause localized damage, and similar to the effects of asteroids at the high end of the first category, the energy of the impactor would be absorbed by the ground during crater formation.

For larger impactors (closer to 1,000 meters), especially on land, the splash of the excavated material would cover an area of about 10 kilometers in diameter. Like slamming a rock into the dirt, the ejected material would have to splash in a particular direction (depending on the angle of impact) and cover everything, spilling out in all directions. Damage can also be produced by a deep crater caused by the impact. The object would have a diameter of about 2 kilometers, and it would destroy everything that previously existed below the impact. Moreover, this destruction would not be limited to the point of projection of the explosion. The atmospheric explosion of the impactor would destroy buildings, forests, and much of the natural environment within a few hundred kilometers. Estimates suggest that such an impact would include an entire state or an entire country. If such an attack occurred in a densely populated area, the death toll would be in the tens of millions.

If the larger asteroid were to hit the ocean, the undamaged object would wreak havoc. The massive object would create a wall of water, throwing vapor and seawater high into the sky. The result would be localized changes in weather (especially rainfall), which would affect the operation of other systems around the world. The impact wave would be so powerful that it would set off huge waves similar to the seismic tsunamis caused by seismic activity. Like a pebble dropped into a pond, the ripples will move outward in concentric circles, and it could inundate nearby shorelines. The waves thus cause intense shoreline erosion. But the greater damage would be to wipe out entire seaside cities and towns, killing thousands of people along the densely populated coastline.

Scientists believe the overall impact of an asteroid at the high end of the second category would be similar to that of a nuclear-style explosion, on a scale closer to the smaller end of the megaton scale of nuclear weapons tests over the years. However, the actual impact of an impact of such a nearly 1,000-meter-sized object is purely speculative, and we have no real way to determine its actual impact. Scientists can only guess that it is based on localized effects from impacts that have already landed on Earth in recent years.

Category 3: An impactor 1,000 to 5,000 meters in diameter?

One of the most frightening scenarios has to come from the third category of impacts. These impactors range from 1,000 to 5,000 meters in diameter and travel at tens of kilometers per second. They were active in the early days of the solar system, as seen on the Moon, Mercury, Venus, and in many of the larger impact craters on our own planet, but that doesn't mean they no longer exist. The rate of such large rocks hitting the Earth to form craters is low, and in the case of land, they occur about once every 300,000 years.

How does the Earth contend with such impacts? In general, craters produced by Type III impactors are about 10 to 15 times the size of the projectile. For example, a 10- to 15-kilometer crater would be caused by a 1-kilometer asteroid, and a 50- to 75-kilometer crater would be caused by a 5-kilometer asteroid. While they may seem like small numbers, they are not. A 15-kilometer-sized crater would wipe out everything in a circle that could match the diameter between Los Angeles International Airport and Florence, California, and the largest type of crater in an impact would be as large as a circle with a diameter of the distance between Baltimore, Maryland, and Washington, D.C., in the United States. These impacts would have local effects, and the actual damage would be worldwide.

While the exact size is not known, an impactor of 1,000 meters or more would push the planet into a global catastrophe. If such an impact were to occur on land or in the oceans, the result would be a disruption of the Earth's integrated equilibrium system. A cratering impact would spread dust across the globe, enough to produce significant short-term changes in the world's climate, combined with the effects of a devastating explosion in the impact area.

So what happens when a large asteroid hits the Earth? In the case of the smaller asteroids in the third category, the damage would be infinitely greater. As for the larger impactors in that class series, human civilization itself would be threatened, if not all but wiped out, if they were to hit Earth. Fortunately, as we know so far, the chances of a collision of a larger asteroid of about 5,000 meters in diameter with the Earth are astronomically small.

But the likelihood of a collision with an asteroid several kilometers in diameter has increased. Simply put, such an impactor would be accompanied by a massive explosion large enough to shatter and partially defuse projectiles and sites on the ground below the impactor. After about half an hour, the high-velocity projectiles from the impactor would generate enough heat to burn everything alive around it and create a stormy conflagration that would engulf everything in its vicinity (triggered by self-scorching and descending incendiary debris), which would then spread rapidly across the continent. Many lakes, rivers, construction sites, and some ocean surfaces would become acidic as nitric acid from the impactor's fireball entered the atmosphere and covered parts of the surface.

The main problem would originate with dust and rock debris entering the upper atmosphere (stratosphere). This dust will be carried around the world by prevailing winds, which spread out and block out a large portion of the sunlight. Sunsets and sunrises will appear amazingly scorching, and the dust moves like a screen during the day. The lack of sunlight would cause temperatures to drop tens of degrees Celsius (according to many climatologists studying global warming, a drop of a few degrees in temperatures around the world is capable of causing dramatic climatic and natural changes that would lead to an increase in the ice that makes up the polar ice caps). This would shorten the growing season, or even miss one or more growing seasons, causing massive worldwide crop reductions.

All this catastrophe won't be over soon. In a few months, the effects will switch, and water vapor and carbon dioxide will increase, pushing the greenhouse effect to the global warming juncture that everyone knows about. That's when temperatures rise, perhaps by 10 degrees Celsius. The greenhouse effect is evident when the warming of the ground increases the temperature of the troposphere (the lowest layer of the atmosphere, where we live). This in turn enters the dreaded cycle that when the oceans are warmed, it releases carbon dioxide, thus increasing the global greenhouse warming effect. Such an Earth cycle makes the biosphere overused and depressing, reducing the chances of all living organisms surviving.

In the middle of the tragedy, humans will be pushed to the limits of trying to stay alive. The cold weather after impact would freeze crops, with the corresponding result being a lack of food, including the loss of livestock and wildlife (which in many cases became competitors for food) for lack of feed. The little food that would be available would then cause a worldwide famine. Simultaneously occurring will be the spread of disease, not only from famine, but also from the decay of organisms that die on impact. Fuel will be valued and in high demand, and as more fuel is consumed, these resources will be taxed at extreme levels, not only with actual taxes, but possibly with taxes from the political structure that remains, which requires us to pay taxes for the difficulty of finding and transporting fuel resources. The uninterrupted dimness of the daytime hours will also contribute to this situation, the lack of warm sunlight grinding away any hope of rebuilding.

After a few months, greenhouse gases increase, and the warmer climate it creates only serves to exacerbate an already devastating scenario. The warming would continue for decades, causing just the opposite effect; the ice caps would melt, flooding seaside towns and cities, and populations would move inland. This heat increases the burden on the atmosphere, causing drought or rain in many areas. The world's water vapor would increase and much of the remaining land would be transformed into tropical-like jungles. The additional atmospheric water also increases the intensity of storms around the world, with winds and heavy rains causing widespread flooding and destruction. Humans and wildlife will again be forced to flee, fighting for the best land, seeking those places with enough food, shelter and water. The total number of organisms will plummet along with the scramble for shrinking supplies.

Even a smaller impactor would cause effects that would not go unabated for centuries, or it could even change the course of the Earth's future geologic activity, as in the case of the Cretaceous-Tertiary boundary. In the above scenario, the impact changed the course of planetary evolution in favor of mammals controlling the Earth as the masters of living organisms. If such a large impact were to occur today, most of the eradication might be mammals, and does anyone know what species would escape such a cataclysmic event to re-dominate the planet?