Asteroids and comets have hit the Earth throughout geological history. The smallest impacts occur on a regular basis and have even been turned into the TV show "Meteorite Men". Large impacts are much more rare but can have devastating effects on the planet Earth. In the last few decades, researchers have realized the important effects of these cosmic impacts on the biology and history of our planet. This knowledge has also penetrated popular awareness through popular articles, television shows and movies about "killer asteroids" and "killer comets." Well most of the meteorites that strike the surface of the earth are only a few centimeters to maybe a few 10s of centimeters across, these so-called "killers" are interplanetary bodies larger than a kilometer or so in diameter, which is big enough to disrupt agriculture and civilization. An impact that size could kill a quarter or more of Earth's human population. The energy released by such an impact would be equivalent to that from a million megatons of TNT. The estimated interval between impacts of kilometer-sized bodies is 300,000 years, an interval so long that there has never been one in recorded history.
Any asteroid or comet that passes close to the Earth's orbit (a Near-Earth Object, or NEO) has a fair chance of eventually hitting the Earth, usually within a few million years. Even if it's not initially on an Earth-crossing orbit, the gravitational influence of other planets can perturb its orbit until it hits Earth itself. New asteroids and comets are constantly injected into the inner Solar System from the asteroid belt and the Oort Cloud, so there's a constant supply of these objects. Although there are far fewer now than in the beginning of the Solar System, an era of heavy bombardment when these objects with much more prevalent in number.
About every century or two, we can expect an explosion on the scale of an atomic bomb or a hydrogen bomb to occur somewhere on Earth. Of course, most of these would happen over oceans and have minimal effect; it may happen over populated land only once every 500 to 1,000 years. The most recent two examples are the Russian events in 1908 at Tunguska and in 2014 at Chelyabinsk . Every hundred million years or so, we may get an impact big enough to wipe out most species and reset the evolutionary clock. In fact, such an impact happened 65 million years ago, at the end of the Mesozoic Era. It caused temporary climate changes severe enough to wipe out the dinosaurs, along with about 75% of the species that existed at that time.
Intermediate in size between these two types of impacts are the highly disrupting kilometer sized impactors. Such an object might disrupt agriculture and kill a quarter of the human population while releasing the energy equivalent of at least 20 million atomic bombs (Mind you, there aren't 20 million atomic bombs available to replicate this size of an explosion). An impact of this size occurs about once every 300,000 years, according to astronomers’ estimates. The Mesozoic-ending impact had energy closer to that of 100 billion atomic bombs! Such a catastrophe only occurs once every 100 or 200 million years.
Large impacts are not predictable. We can only estimate their occurrence statistically, in averages over enormous spans of time. We presume that impacts are random, because the orbits of interplanetary bodies are mostly chaotic. However, there is tentative evidence that large impacts may not be entirely random. Some researchers have argued that a variety of indicators of large impacts — mass extinctions, drastic changes in sea level, deposition of meteoric material — may coincide with the Solar System's movement through arms of the Milky Way every 30-50 million years. Since there is more debris in the regions of space where many stars live, our passage through these dense regions would lead to an enhanced impact rate since it becomes more likely that an object in the outer parts of the Solar System will have its orbit disrupted.
While impacts are rare, it's been a long time since the last major event occurred on the planet Earth. There is perhaps a 50% chance that another explosion of the magnitude of the Tunguska or Chelyabinsk events will make the front pages within your lifetime. The odds are high for a disaster on the scale of a state or county within a few centuries, for a regional disaster within a few thousand years, and for an impact big enough to end civilization within the next hundred thousand or million years.
How should we respond to this threat? One approach is to continue survey programs to map out the orbits of all of the Earth crossing bodies in the inner solar system. Within the next decade, especially if new search telescopes can be brought on line, we should have a complete inventory of the largest NEOs. One of the telescopes being built to fulfill this mission is the Large Synoptic Survey Telescope. With improved data on their orbits, we can determine which objects may be on collision courses with Earth. With enough warning to plan some response, it may be possible to alter the orbits of these objects, placing them on trajectories that leave the Earth safe. Unfortunately, science and politics have become mixed in this debate. Although the chances of a catastrophic hit are small, some United States Defense Department experts have suggested developing a system to deflect, or blow up any approaching body before it can hit Earth. Edward Teller, known as the father of the hydrogen bomb, and members of the Strategic Defense Initiative ("Star Wars") project have been active in suggesting an H-bomb based asteroid defense system. However, unless it was very carefully done, blowing up a small asteroid would create a cloud of fragments that would still carry the same amount of energy to earth, but would spread the impacts over a much larger area of land and sea. Critics also have suggested that such ideas are merely an attempt to maintain military funding in a post-cold-war world: China in 1996 cited the asteroid threat as a reason to continue its nuclear testing.
Instead of using nuclear weapons in an attempt to destroy a large impactor, several safer methods of avoiding destruction have been proposed. If we have enough warning, a very small change in the body's orbital velocity would make it miss the Earth entirely. Velocity changes could be instigated by small amounts of explosives on the surface of the asteroid, like thruster jets used to steer spacecraft. Scientists are also learning that the surface properties of asteroids (for example, their color or porosity) may be important in determining their orbits. So it may be possible to merely paint an asteroid and thus deflect it away from the Earth — again, if we have enough advance warning. One of the more intriguing ideas is to use gravity to nudge an asteroid very gently by simply placing a very heavy rocket nearby, or just using the gravity of the rocket to slightly deflect the asteroid, a method called a "gravity tractor." Such a steerable spacecraft could maintain a position just to the side of the asteroid calling it in the direction we wish to deviate its orbit.
Rationally, we should not take the overheated rhetoric and media sensationalism too seriously. A simple three-phase strategy can protect us from disaster. Existing and planned surveys will detect and gather data on all large Earth-crossing asteroids. The vast majority of these will not be headed directly for the Earth. For those asteroids that appear to be on course for us, we will use computer simulations to calculate the orbits more accurately. Again, in most cases they will miss the Earth. And if an impactor does appear likely to slam into the Earth, we will, thanks to proper surveying, have several years' notice to prepare, bring out the "big guns" (or at least the most obese spacecraft) and if necessary to save the planet. The probability of such an event in our lifetimes is tiny, and with adequate warning and planning, the risk can be dealt with. Don't lose to much sleep over the possibility of a catastrophic impact from space!
Diagram of the Oort Cloud and Kuiper Belt. Click here for original source URL.