The Danger from Asteroid Impact
Understanding the impact risk
Statistically speaking you're more likely to be killed by an asteroid impact than in a plane crash - but this statistic can be misleading. Every year there are millions of plane flights, and every year about a hundred people die in plane crashes. The odds of you getting on an airplane in any year are high, but the odds of being on a deadly flight are extremely low.
The statistics of asteroid impacts is a bit different. There's a wide range of impactor size, from gravel that burns up high in the atmosphere to the occasional mountainous rock that smashes down, and there's a wide range in how often they happen - many times every day for gravel, only once in millions of years for the big rocks. It turns out that if you add up the effects of the little rocks and compare to the big rocks, the big rocks are deadlier on average. Their devastation outweighs their infrequency.
What that means for asteroid hazard is that although the average rate at which people may die from impact is relatively high (about 1000 per year), that average comes about mostly because a billion people get killed every million years, rather than 1000 people dying per year. This means that we have no everyday experience with death from asteroid impact, although the section on historical impacts may surprise you. The bottom line is that the chances of you dying by asteroid impact are extremely low, perhaps 10 times greater than dying by shark attack, but lower than dying in an earthquake.
Why worry about asteroids if the impact risk is small?
The risk of your house burning down is very small and yet you still insure it against fire, right. In this case you realize that the risk is small but the investment to protect your house (and your neighbors) is also small. Simply stated, if your house does burn down it is a real hassle.
We think a similar argument can be made to protect against asteroid impacts. Sure, the impact risk is small, but if there is an asteroid out there with the Earth's name on it then we should do something about it. Our cost-savings by not looking for the next major impactor will be cold comfort if it turns out that most people on the planet are killed next year because we didn't bother to look up and see what's approaching us. Remember that the dinosaurs and about 80% of all life on the planet was wiped out due to an asteroid impact 65 million years ago. But dinosaurs had brains the size of walnuts and no opposable thumbs to hit the space bar on a keyboard. Humans have big brains and now have the capability of saving the planet from destruction and we think we should make the effort.
While it is impossible to know ahead of time if you are in extreme risk of dying from things like shark attack or hurricane or earthquake or traffic accident it is now possible to know ahead of time whether you are at risk from asteroid impact. We had an excuse not to do anything about it up until about 10 years ago because the sky is a very big place, and it was just not technically feasible to perform such a wide, deep, and fast search. Nowadays, camera and computer technology have made it possible to scan the entire sky every night to discover incoming asteroids before they hit. In nerd-speak, Moore's law met the Universe and Moore's law won.
The U.S. Congress responds to the asteroid impact threat.
In recognition of the asteroid impact threat, the US Congress in 2005 directed NASA to evaluate the danger and report on methods to survey for dangerous asteroids and to divert those that are on course for Earth (the George E. Brown, Jr. Near-Earth Object Survey Act). In response, NASA issued a 2007 report entitled "Near-Earth Object Survey and Deflection Analysis of Alternatives" that describes what they thought could and should be done. They said that it ought to be possible by 2020 to find at least 90% of asteroids with diameter 140m (500 ft) or larger (100 megaton or more worth of impact energy) that come close to the Earth. This could be accomplished, they said, by immediate, dedicated ground-based surveys or by a satellite that looks for the thermal glow of asteroids. The cost of doing this they thought would be of order a billion dollars.
Although still very undeveloped, NASA discussed ideas of how to deflect an oncoming asteroid. The methods depend strongly on finding the deadly asteroids long before the predicted impact time because the feeble push we could exert on an asteroid takes a long time to move it, and because it takes time to mount a mission to intercept the asteroid. Therefore, if you want to find an Earth-impacting asteroid early enough to divert it, you can expect that it will be at a fairly random place in the solar system and you have to be prepared to detect it at quite a large distance (typically about as far from the Earth as the Earth is from the Sun). This fact, and because a 500 foot diameter asteroid is really very dim and small compared to the vastness of the Solar System, are what makes it so difficult and expensive to find out whether we are in immediate danger from asteroid impact.
The National Research Council Study
NASA asked the National Research Council (NRC) to conduct a detailed study to expand on the problems of detecting near-Earth objects (NEO) and mitigating the hazard. In 2010 the NRC issued their report entitled "Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies".
There is a wealth of of information in the NRC report, and we reproduce some of their figures here. This graph shows the current best estimates of how many asteroids are out there, waiting to hit the Earth. The bottom scale shows the size of the objects, the left scale how many are that size or larger, the top scale the energy that would be released on impact, and the right scale the frequency with which impacts might occur. One big arrow shows the "K-T Impactor" that killed the dinosaurs with an explosion of a million Mton and is expected to happen every 100 million years. The second arrow shows the "Tunguska" explosion that was a few Mton and is expected to occur every few hundred years. Technically, this shows the number of NEOs, defined as objects whose orbits descend to 1.3 "astronomical unit" (AU, defined as the distance of the Earth from the Sun). These are potentially dangerous objects because even if they miss us now, their orbits slowly change because of interactions with planets such as Jupiter, pressure from the solar wind, and even collisions. About 20% of NEOs are "potentially hazardous objects" (PHOs), meaning they pass within 0.05 AU of the Earth's orbit (600 Earth diameters, close enough that within 100 years their orbits randomly can shift by 0.05 AU).
So what's the effect of such an explosion? We are protected by a thick atmosphere that is the equivalent of 10m (30 ft) layer of water, so impactors that are small enough will explode high above the ground and not pose a serious threat to people. These are the meteors that we see every night. Impacts of asteroids larger than 10m (30 ft, 10kton explosion) are much less frequent, arriving once every few years, and can sometimes punch down to the ground. It is extremely unlikely that someone is close enough to the explosion to be harmed, however. Still larger asteroids are much more dangerous, and the graph shows how the explosion from the impact of an asteroid of 100m (300 ft) diameter and larger is expected to kill a hundred thousand or million people, averaging over all possible impact locations. These deaths can be caused by direct effects or by tsunami (tidal wave) for ocean strikes. It is thought that an impact by an asteroid of 1-2km (1 mile) diameter will also cause secondary, global damage by the effect that it has on the Earth's climate. The vast amount of debris thrown up from the explosion and the subsequent fires will create an "asteroid winter" that might persist for several years, long enough for crop failures to lead to mass starvation. These secondary effects are a hundred times more deadly than the prompt loss of life from the impact.
Now that we have an estimate of how often the Earth is struck by an asteroid as well as an idea of how many people are killed per impact, we can estimate how many people are killed per year on average. The graph to the left shows the number of fatalities per year as a function of the size of impactor. As mentioned at the beginning, the worst effects occur from global catastrophe, amounting to a thousand deaths per year. We see that the most deadly asteroids have a diameter of a few kilometers, these explosions occur about once per million years, and the "asteroid winter" is expected to cause a billion deaths. Impacts do occur at this rate, averaged over the eons, but what about next year or next century? The question is whether we're lucky or not.
Current status of Near Earth Object surveys
Since the 1990's NASA has supported the Spaceguard Survey, an effort to find all NEOs larger than 1 km. This has been very successful and we're pretty sure that we've found about 85% of the NEOs that are 1km in diameter or larger. Fortunately none of these is expected to strike the Earth in the next century. The plot to the left shows the current expectations for fatalities per year, given this new knowledge. (Note the change in the vertical scale from the previous graph.) The chances of global catastrophe in the near future are lower than the average over the eons, because we've looked and found that most asteroids that can come close to us will not in fact strike us any time soon. There is a residual possibility that one of the 15% of undiscovered, large NEOs might strike us soon, of course. Instead of the 1000 fatalities per year that we would predict with no specific knowledge of the asteroids in the sky, the Spaceguard Survey reassures us to expect only 100 fatalities per year, because of the asteroids that have been found to be harmless.
Finding smaller impactors is more difficult than bigger impactors
It gets harder and harder to improve on previous generations of asteroid search such as Spaceguard. Smaller asteroids are dimmer -- a 140m (500 ft), 100 Mton asteroid is 100 times fainter than a 1.4 km(1 mile) monster that can cause "asteroid winter". Also, some asteroids are on "obnoxious orbits" that are very hard to spot from Earth, including orbits spend most of their time inside of Earth's orbit, objects that are phased right now to spend most of their time on the opposite side of the Sun, and long period comets that plunge down out of the darkness. Finally, orbits do evolve over time in unpredictable ways from the solar wind and deflections by close encounters with other objects. The price of security is eternal vigilance.