Imagine hearing news reports that a large, newly discovered comet from deep space is found to be on an intersecting orbit with the Earth. Astronomers are stunned to discover the 7-kilometer wide object, comprised of ice and rock, has a high probability of colliding with the Earth within the next two months. To the horror of the world, further calculations indicate the comet will strike in the Pacific Ocean near coastal California in less than one month—far too little time to mount an effective defense strategy. The impact occurs 150 miles from the coast of California in the Pacific Ocean as predicted. Traveling at 72 kilometers per second at the point of impact, the comet’s velocity has kinetic energy equal to 1.11 x 108 Megatons of TNT, far more than the combined potential energy of the world’s stockpiles of nuclear weapons. The blinding flash is seen beyond the horizon. At 150 kilometers from the impact site, coastal California is consumed by the fireball, which ignites vegetation, plywood constructions, clothing, and exposed skin. The ejecta reaches sizes of 150 feet wide and returns to Earth to further devastate the landscape. The pressure wave kills nearly every living thing in its path, collapses buildings, bridge structures, and flattens forest regions to the ground. The final crater in the Pacific measures nearly 70 miles in diameter and creates a series of tsunamis that will devastate coastal regions thousands of miles away.
While the preceding scenario is unlikely to happen in our lifetime, it is not impossible. Unlike asteroids in stable paths around the sun, comets are unpredictable due to their highly elliptical orbits that bring them into the Earth’s path from the frozen, outer regions of the solar system with little or no warning. Hale Bopp, for example, was discovered in July of 1995 as it approached its perihelion—its closest approach to the sun—on April 1, 1997. The object’s brightness was due to its immense size, and although determining its precise dimensions were difficult, estimations gauged Hale Bopp’s nucleus to be around 25 miles across at its widest point. Further studies indicated that when the comet reached its perihelion, it would pass as close to the Earth as 122 million miles. If Hale Bopp had been found to have an Earth intersecting orbit, there would have been little time to do anything about it. At 25 miles across with a velocity of more than 70 kilometers per second, the impact would have been a catastrophe the likes of which have not occurred since the early times of the Earth.
A year earlier in July of 1994, Astronomers had the unique opportunity to witness an impact event when Comet P/ Shoemaker-Levy 9 collided with Jupiter. The resulting impact plume that was observed rising from the upper atmosphere of the giant planet was larger than the Earth and prompted speculations of what an impact of this magnitude would mean for humankind closer to home. The answer to this question was terrifying—if the initial fragment of Comet P/Shoemaker-Levy 9 had collided with Earth, it would have resurfaced and sterilized the planet, and extinguished all life forever.
There are scientists and astronomers who work to safeguard Earth and its inhabitants from such calamities; NASA sponsors the Near Earth Object program, which strives to study and catalog objects that cross Earth’s orbital path around the sun. According to NASA JPL’s website, 9721 near Earth objects have been discovered as of Feb. 16, 2013, and of those discovered, 1379 are classified as PHAs (potentially hazardous asteroids). Even with programs like Near Earth Object and the Spaceguard Survey, there are fewer than 100 staff members working to keep watch over the vast skies. The concern is that with a sparse staff combing through large amounts of data and the inability to survey blind spots—areas of daylight sky and high southern latitudes—governments are leaving it up to chance that scientists will discover a potentially dangerous object before it is too late.
Part of the problem is with humankind’s perception of safety since events like the Shoemaker-Levy 9 impact with Jupiter happened on a far away world, and the KT asteroid strike that drove the dinosaurs to extinction occurred nearly 65 million years ago—these kinds of doomsday scenarios are thought not to happen here on Earth in the present time. The chances of an impact with an object more than 2 kilometers wide in the near future are remote—these types of impacts occur on Earth between one and two times every 1 million years—but, the object does not need to be the size of a mountain to bring destruction capable of collapsing human civilization, killing millions of people. The Barrington Crater in Winslow, AZ was created about 49,000 years ago by a nickel and iron asteroid some 50 meters wide and weighed nearly 300,000 metric tons, and the resulting crater measures 4,000 feet wide and 570 feet deep. An impact of that magnitude near or in a major city today could kill tens of millions of people, devastate structures, and render farmland unusable. The national economy would collapse, millions more would starve, and entire regions could be thrown into anarchy.
Humans are the first species to have the intellect to be aware of these dangerous objects from space and the technological capability to do something to divert them away from the Earth. In the best-case scenario, an object would pass by the Earth many times during its orbit and give humans a chance to see it coming decades in advance. One of the plans to avert a collision is to launch a large probe near the asteroid; the probe’s mass and gravitational influence could pull the asteroid away from a collision trajectory. Another plan involves detonating a small nuclear device near the asteroid to push it off its intersecting orbital path with Earth. Each of these action plans requires one factor for success—a decades-long, advanced warning. The worst-case scenario would be that humans would have no warning; the first indication of the impending doom would be the blinding flash and the subsequent earthquakes—then, all hope for humanity would be lost in an instant.