Impact History

Bombardment of the Earth by asteroids and comets has been a recurring phenomenon since shortly after the Earth was formed.  In fact, the biggest impact of all is now thought to be the episode that formed the Moon.  Current thinking has it that the early Earth was hit by another planet, about the size of Mars, about 50 million years after it had formed.  The end result of this was our dual-planet system, Earth and Moon.  The outer layers of both bodies ended up forming the Moon and the heavy core materials of both combined to form the core of our present day Earth.  Of course, lucky for us, there were some lighter materials that also formed the new surface of the Earth as well.

Since that time the Earth has been continually hit by asteroids and comets of varying sizes.  Following the period referred to as the "late heavy bombardment", about 3.8 billion years ago the impact frequency of these impacts has been quite steady.  Astronomers and astrobiologists who have researched the origins of life are somewhat divided on the details, but generally believe that the water, rock and even hydrocarbons brought to Earth by comet and asteroid impacts played a strong role in the origins and development of life on the planet.  Indeed, it now appears that once life started on the planet, it never entirely gave up!  It is clear from the geologic record that as the tree of life began to branch and diversify (between major impacts) it was pruned again and again by infrequent but persistent impacts by large cosmic visitors.  One has to realize the very long timescales being addressed here.  If the Earth is hit, on average, by a 10-15 km diameter asteroid every 100 million years, then there have been 45 periods 100 million years long where life has been free to develop unmolested!  This, of course, is not entirely true since objects smaller than 10 km also present a serious threat to life.  However, once we begin dealing with objects 1 km. in diameter and smaller, the threat to life becomes regional and finally local - and, of course, more frequent.

The seminal event in recognizing the importance of NEO impacts in the evolution of life was the proposal made by Louis and Walter Alvarez* in 1980 that the extinction of the dinosaurs 65 million years ago was caused by a cosmic impact.  After much scientific debate the clinching event in bringing general acceptance to this claim was the 1990 discovery of the Chicxulub impact crater just off Mexico's Yucatan Peninsula.  It is now generally accepted that this impact, by an asteroid of about 12 km diameter, created a global disaster which wiped out not only the dinosaurs, but approximately 75% of all species living on Earth.  While 65 million years ago seems to many people like a long time, it is interesting to realize that if one equates the origin of the Earth with midnight, January 1 a year ago, and today as midnight, December 31, then the dinosaurs met their demise on December 27!  It is, by geologic timescales, quite recent.

But how big does an impacting object have to be to be on concern?  And how frequently does an impact by a NEO of this size occur?  These questions are the basis of much of the work that has been done by the scientific community regarding this issue for the past 15 years. If it is of genuine concern, the question goes, then why haven't we seen one in our lifetime?  This is a good question, but one which is devilishly tricky to answer convincingly.  One key data point is the so-called Tunguska event of 1908, loosely speaking, in our lifetime. In this instance a stony asteroid about 50-60 meters in diameter entered the atmosphere over Siberia and exploded at about 20,000 feet over a very remote and unpopulated forest.  While this event went largely uninvestigated for decades, recent work has determined that the explosion was the equivalent of a 15 megaton bomb.  In Siberia it flattened and burned 2000 square kilometers (800 sq. miles) of forest.  Had the impact been over London or New York City, millions would have been killed.  It is now thought that a Tunguska sized event occurs about once per 1000 years.

Based on the realization that the frequency of impacts of concern occur on the timescale of human interest, and that we can detect them coming, and might even be able to prevent them from impacting, the scientific community swung into action.  The Spaceguard Survey, first mentioned in Arthur Clarke's Rendezvous with Rama, was established and championed by Clark Chapman (SwRI), Dave Morrison (NASA/ARC) and others.  Congressional hearings on the issue were held in 1993 and again in 1998.  In the 1998 hearings Dr. Carl Pilcher, Science Director for Solar System Exploration in the Office of Space Science, declared it NASA's goal to detect 90% of the NEAs greater than 1 km. in diameter within 10 years.  That goal has been the driving force behind much of the discovery of NEAs since that time. 

Most of the initial detection and discovery of these objects is performed by several large survey systems in the United States.  The confirmation of these objects and especially their long-term tracking is done by both professional and amateur astronomers around the world.  The primary clearinghouse for all discovery and tracking of these objects is the Minor Planet Center at Harvard University's Smithsonian astrophysical Observatory.  Through these efforts we are approximately 65% of the way to meeting NASA's goal.

The remaining concerns of many who are involved in this matter are twofold; 1) detecting and tracking the much larger population of NEAs that will do substantial local and regional devastation on impact, and 2) taking the initial actions necessary to establish a deflection system.  The public policy reality is that when the scientific community makes an announcement that a NEA is headed toward an impact with Earth, the public will expect that something has already been done to prepare for this eventuality.  The horror is that, at this time, nothing of this kind is being done!