Part 1 * Part 2 * Part 3 * Part 4 * Part 5
Don Yeomans: To sum up part of the discussion from last week, near-Earth comets and asteroids are important in understanding the origin of our solar system. The planets, including our own Earth, were formed from collections of these near-Earth objects (NEOs). Life itself may have been enabled by the water and carbon-based materials brought to the early Earth as a result of comet and asteroid impacts. NEOs are among the easiest objects within the solar system to visit with spacecraft, so in the future NEOs may provide the raw materials (e.g., water, minerals, metals) for human survival and for building structures in space.
Of course, most of the newspaper accounts reporting upon near-Earth comets and asteroids have nothing to do with their scientific importance. As last week's debate made clear, a hot topic is the effect of past NEO impacts on Earth and the possibility of future impacts. In fact, most of the press reports about NEOs have to do with those that could impact Earth and possibly cause a significant number of deaths. With regard to the possible threats that NEOs pose, scientists can point to no person in recorded history who has been killed by an asteroid or comet. On the other hand, we can all cite examples of people who have been killed by auto accidents, floods, hurricanes, and airplane crashes. So why all the fuss about comet and asteroid impacts with Earth?
Benny Peiser: I am not so sure about the accurateness of the claim that "no one in recorded history has been killed by an asteroid or comet." As a matter of fact, there are historical records that give accounts of suspected meteorite impacts leading to fatalities. For example, a number of historical reports exist about an alleged disaster in 1490 AD, said to have occurred in the Chinese city of Qingyang (Shaanxi province). According to these reports, over 10,000 people were killed when "stones fell from the sky like rain." Perhaps, as John Lewis has suggested in his book "Rain of Iron and Ice" (1996), it would be wiser to say, "No one in recorded history has ever been killed by a meteorite in the presence of a meteorite scientist and a medical doctor".
Clark Chapman: Still, the average American's chances of dying as a result of an asteroid impact is about the same as an average American's chances of dying in a tornado. The distinction is that many tornadoes occur each year, and some of them kill up to dozens of people, whereas big asteroid impacts occur very rarely, but might kill millions or even billions of people.
Several U.S. citizens die every day, one at a time, by accidental electrocution. Roughly the same number die in the very few jet airliner crashes each year, where hundreds die at one time. Airline crashes are relatively rare events: there were only two days during 2001 on which people died in airliner crashes in the United States; one was Sept. 11th. Asteroid impacts are just a more extreme case of a rare but extraordinarily deadly event.
Since the chances of death and destruction by cosmic impact are on the same order for Americans as death by airliner crash, flood, tornado, and other hazards society takes seriously, it is reasonable that the impact hazard be taken seriously. In fact, an asteroid impact is a much more serious hazard, statistically speaking, than many other hazards we have experienced in the last few decades, including death by terrorism, by nuclear power plant accident, by shark attacks, etc. And cosmic impacts - if large enough - are nearly unique (along with nuclear war and perhaps some 'Andromeda Strain' pandemic) of having the possibility of sending civilization back into a Dark Age or even exterminating our species -- although the chances of such a cataclysmic impact are extremely tiny.
Historically, society has not spent equally to mitigate various hazards, in terms of dollars per life saved or damage averted. The comparisons I have cited do not necessarily require that society must spend the same amount on NEO searches and mitigation measures as we do, for instance, on tornado research, but policy-makers ought to at least seriously examine the impact hazard.
Joe Veverka: Today asteroids and comets are largely irrelevant to life on Earth on any reasonable human time scale. So why all the fuss? Probably the most important reason is that most humans worry a lot and are not equipped to evaluate certain risks properly. If there is vigorous publicity concerning ANY potential danger to status quo and well being, some people will worry and some will even become terrified - no matter how trivial the alleged danger really is when viewed in comparison with the other dangers that surround us. The fact that some individuals feel threatened by asteroids and comets does not imply anything about the reality or severity of the threat. Exactly the same statement can be made about dozens of other "dangers'' peddled by well-established advocacy groups to scare the public and thereby gain support and funds. Thus, the reasons for the fuss are very clear and fundamentally very human. They have very little to do with what is really out there in space, or with what really are the dangers to life, society, and environment posed by comets and asteroids.
|Pie chart representing US deaths over the last century. Click image for larger view.
Credit: John Pike
Clark Chapman: Joe Veverka's answer implies that there are other catastrophes that have been more deadly, at least in the last century. That is illustrated in this chart, produced for an asteroid workshop by John Pike. The statistical hazard from asteroids and comets would be a slice of pie in this chart that is roughly the same size as the one for volcanoes.
Joe Veverka: Regarding the advocacy groups I mentioned, there are at least three different, well-established advocacy groups whose future and welfare depends on focusing the public's attention on the "threat from space." (The term "advocacy group" is a polite one. A much more accurate but cruder term can be found in Jean Giraudoux's play 'The Mad Women of Chaillot.') The first advocacy group is the media. If all else fails, stories about comets and asteroids destroying New York or Tokyo sell newspapers and magazines and make for popular TV fodder.
Second, there is a strong advocacy group among astronomers. They want more resources devoted to studying comets and asteroids. Publicizing the "threat from space" has certainly proven an effective means for generating government support for the study of NEOs.
Finally, there is an evolving engineering/industrial/military advocacy group that promulgates the "threat from space" because members of this group want public support to build and provide the defenses that will shield us from this "peril."
|Fragments of Comet P/Shoemaker-Levy 9 colliding with Jupiter (July 16-24, 1994).
Benny Peiser: I'm afraid I regard as misleading allegations that the NEO community (or the missile defense community for that matter) deliberately exaggerate the impact risk for selfish reasons. In reality, most NEO researchers - in particular those in the U.S. - have underrated the potential hazards from space. NASA only reluctantly began to address the issue following the considerable 'wake-up call' caused by the impact of comet Shoemaker-Levy 9 on Jupiter in 1994. Without these harmless reminders, I doubt whether NASA would have established even a rudimentary program to inventory the number of large asteroids out there.
Alan Harris: In response to Joe Veverka's point on advocacy groups, it is often the case that those who are most qualified to offer advice also have special interests in the matters of their expertise. You go to a car salesman to buy a car, but if you are not equipped to evaluate what he tells you, you risk being "taken for a ride." The same is true even for consulting a doctor about an ailment. By Joe's argument, you should never buy a car or see a doctor because any advice you receive may be tainted by special interests!
Benny Peiser: Professional astronomers and planetary scientists study comets and asteroids for a better insight into the evolution of our solar system. In contrast to this outlook, the vast majority of people around the world are not bothered with the specific aspects of NEO research. What they want to know, first and foremost, is quite simple: Do these objects pose any threat to me, my family, or to the stability of our societies?
The information provided by scientists has not been very reassuring. What do we expect the interested public to think about claims that the K/T impact was "unique" and that it was the only impact that caused a mass extinction? How credible are such sweeping statements, given how incomplete our current understanding remains regarding the mechanisms, effects, and rate of hypervelocity impacts?
|The painting titled "K/T Hit" by artist Donald E. Davis. This impact occured 65 million years ago, ending the reign of the dinosaurs.
Image Credit: Don Davis
Peter Ward: David Kring has suggested to me that another K/T type impact could be devastating for the animal and plant world. New calculations show that our planet would go into another "Snowball Earth event" like the one that occurred 600 million years ago, when the oceans froze over. While bacteria might readily survive such calamitous impacts, our new understanding from the record of the Earth's mass extinctions clearly shows that plants and animals are very susceptible to extinction in the wake of an impact.
Anything that would increase the rate of large asteroid or comet impacts on the Earth - or on any inhabited planet in this or any other galaxy - would be detrimental to the evolution and survival of higher life forms. But what determines impact rates? That depends on how many comets and asteroids exist in a particular planetary system. It also depends on how often those objects are perturbed from safe orbits that parallel the Earth's orbit to new, Earth-crossing orbits that might, sooner or later, result in a catastrophic K/T or Permian-type mass extinction.
Our understanding of those events now leads us to believe that any comet or asteroid greater than 20 kilometers in diameter that strikes the Earth will result in the complete annihilation of complex life - animals and higher plants. How many times in our galaxy alone has life finally evolved to the equivalent of our planets and animals on some far distant planet, only to be utterly destroyed by an impact? Surely this has been a commonplace event in the vast cosmos.
Yet it appears that Jupiter, with its stable circular orbit far from the sun, assures the Earth a low number of impacts resulting in mass extinctions. Jupiter sweeps up and scatters away most of the dangerous Earth-orbit-crossing comets and asteroids. In 1995, astronomer George Wetherill calculated that without Jupiter, the impact rate on Earth by comets and asteroids would be 10,000 times higher. Under such bombardment it is hard to conceive of how complex life would survive. Jupiter acts as our cosmic guardian, making our neighborhood vastly safer through a gravitational Planetary Protection plan.
Benny Peiser: Given the exceptionally low odds of large impacts, astronomers are rightly staggered by the public's concern with and the media's obsession about the impact hazard. I think one reason for this bewilderment has to do with a genuine lack of sociological understanding. After all, the irrational unease about the impact risk needs to be seen in context of a global public that, to an overwhelming extent, still adhere to religious beliefs that predict cosmic catastrophes of apocalyptic proportions. For hundreds of millions of devout Christians and Muslims, comets and asteroids are extremely relevant to life on Earth - not for any scientific reasons, but as traditional portents of doom.
In the past, one way to deal with such celestial dread was to deny that there is any cosmic threat whatsoever. For most of the last 2,000 years, astronomers have been extremely reluctant to acknowledge that there are any risks from space. This approach, however, is no longer available to modern science and any attempt to discount, minimize, or ridicule the impact hazard will backfire. That's why I am uncomfortable with ongoing efforts to underestimate or belittle the concerns people have about the impact hazard.
Part III of the Great Impact Debate (02-24) series is entitled "The Large and the Small", and features a discussion of whether we should invest the resources to minimize the effects of smaller impacts.
Related Web Pages
Great Impact: Part I
Impact Hazards Website
NASA/JPL Near Earth Object Program