The frequency of impacts of objects of various sizes is known only to limited precisions. In particular, objects up to several meters in diameter explode in the atmosphere without reaching the surface. Although the energy released in these explosions may be many times greater than that released by the Hiroshima bomb, they most frequently occur over the ocean or sparsely inhabited regions of Earth and go unreported [Or unrecognized as events in the past – CT].
Hazards due to Comets and Asteroids, Current Deputy NASA Administrator Lori Garver, et. al., 1994
Efforts to persuade governments to invest significant resources in evaluation of the hazard of asteroid impacts must overcome what has been called the “giggle factor.”
See Full Excerpt below
Our understanding of the history of Earth and its inhabitants is undergoing a radical change. The gradual processes of geologic change and evolution, it is now clear, are punctuated by natural catastrophes on a colossal scale – catastrophes resulting from collisions of large asteroids and comets with Earth. It is, to use the popular term, a “paradigm shift.”
This “new catastrophism,” is not unlike the revolutions brought about by the heliocentric solar system of Copernicus, or Darwinian evolution, or the big bang. In retrospect, such revolutionary ideas always seem obvious. On reading the Origin of Species, Thomas Huxley remarked simply: “Why didn’t I think of that.” Now, looking at the Moon, we find ourselves wondering why it took so long to ask whether the process that cratered its surface is still going on. […]
The long time scale between major impacts has implications for public policy. Governments do not function on geologic time. On the North Dakota prairie near the town of Grand Forks, lie the abandoned ruins of America’s ballistic missile defense system. … Built in accordance with the ABM treaty, the Grand Forks facility was meant to defend our retaliatory capacity. It was declared operational in 1975 – and decommissioned the same year. National leaders had been persuaded by some scientists that the Grand Forks facility would meet the threat to our intercontinental ballistic missile fleet, even though other scientists warned that the system was dangerous and ineffective. It was closed because the money to operate it was needed for other projects that were deemed to be more urgent.
The lesson of Grand Forks is as old as human history: societies will not sustain indefinitely a defense against an infrequent and unpredictable threat. Governments often respond quickly to a crisis, but are less well suited to remaining prepared for extended periods. Even on the brief sacle of human lifetimes, resources are eventually diverted to more immediate problems, or defenses are allowed to decay into a state of unreadiness. According to news accounts, in the great flood of 1993, the U.S. Corp of Engineers prepared to close the massive iron gates in the vast complex of levees on the Mississippi and its tributaries only to discover that some of the gates had been removed and sold for scrap. Periodic inspections had been suspended to save money. Indeed, civilization will do well to survive long enough to be threatened by a major asteroid impact; our own destructive impulses of the unanticipated consequences of our technologies seem likely to do us in first. It is unrealistic to expect governments to sustain a commitment to protection against a rare occurrence when they are constantly under pressure to respond to some perceived immediate crisis.
Particularly now , with nuclear weapons being dismantled by the major powers, any talk of a nuclear defense against such an unlikely hazard as cosmic collisions will be seen as an effort by the weapons community to sustain itself. The risk of diversion of any mitigation system to military uses must be regarded as a more immediate hazard. […]
Given the frequency of past collisions, major impact is unlikely to occur in the next century. […]
Discussion of mitigation may serve one public purpose. It is important that devastation not be accepted as inevitable, otherwise society might prefer not to know when it is coming. An asteroid interception workshop hosted by NASA in 1992 concluded that available technology can deal effectively with a threatening asteroid, given warning time on the order of several years. That conclusion validates the view that current efforts should concentrate on detection and orbit determination.
The challenge of science is to identify objects that threaten Earth and work out the timetables for their arrival. Here the challenge is straightforward and technical. […]
The emphasis has properly been on impacts that would be expected to have global consequences. Even for objects too small to produce more than local effects, however, it has been pointed out that an impact might be misidentified as a nuclear explosion. Misidentification would be most likely among nations that have recently joined the ranks of “nuclear powers” and would therefore be expected to have less sophisticated means of verification.
It is more than a hypothetical concern. We recall that the 1978 South Indian Ocean anomaly, detected by a Vela satellite, was suspected at the time of being a South African-Israeli nuclear test. In spite of the failure to find any confirming evidence from intelligence sources or atmospheric monitoring, it created international tensions that lasted for years. At the time, there were suggestions that it might have been an artifact produced by micrometeorite impact on the Vela satellite itself, but little serious consideration seems to have been given to the idea that the satellite had observed the fireball from an asteroid impact in the atmosphere. A 1990 satellite observation of an apparent asteroid impact fireball over the Western Pacific has been described by Reynolds (1993). The danger of misidentification, which grows as weapons proliferate among less sophisticated nations, is meliorated in part by publicizing the possibility. The only sure means of avoiding an unfortunate response, however, would be for everyone to know the impact is coming. Which again places the emphasis on detection.
Efforts to persuade governments to invest significant resources in evaluation of the hazard of asteroid impacts must overcome what has been called the “giggle factor.” Clearly, elected officials in Washington are not being inundated with mail from constituents complaining that a member of their family has just been killed or their property destroyed by a marauding asteroid. […]
Congressional involvement has been confined to the Committee on Science, Space and Technology of the U.S. House of Representatives, whose current chair, George Brown of California, has maintained an interest in the asteroid issue for several years. The committee directed NASA to conduct two international workshops on the asteroid threat. […]
In March of 1993, the Space Subcommittee held a formal hearing to examine the results of the two workshops. Some members remain skeptical that the threat is real. But even among those who recognize that it is only a question of when a major impact will occur, there was no sense of urgency. […]
The frequency of impacts of objects of various sizes is known only to limited precisions. In particular, objects up to several meters in diameter explode in the atmosphere without reaching the surface. Although the energy released in these explosions may be many times greater than that released by the Hiroshima bomb, they most frequently occur over the ocean or sparsely inhabited regions of Earth and go unreported. […]
Congress is unlikely to take any action in the absence of public pressure. Once the public understands that Earth and the life on it have been shaped by cosmic collisions (and the process is continuing), they will be more likely to support the science needed to evaluate the threat. The scientific community must, therefore, concentrate on public education. […]
All of this creates a dilemma. While it is important to inform the public, it is dangerous to encourage fear mongering. … Scientists would do well, for example, to avoid such terms as “near miss.” The public understands “near-miss” as the draft of wind from a truck that passes as you step off the curb – not a truck that went by six hours earlier. […]
Even in such staid newspapers as the New York Times and Washington Post, articles may include a well-reasoned discussion of relative risk, but the headline writers find “doomsday rock,” “space bullets” and “killer comet” irresistible. These headlines exploit the excessive fear engendered by events people feel powerless to control. The image of an indifferent mountain of stone and metal guided by the immutable laws of physics toward an inevitable rendezvous with Earth, is the stuff of nightmares. Remarkably, however, Nature has apparently provided a non-threatening demonstration. The impact of comet Shoemaker-Levy 9 on the back side of Jupiter in July of 1994 provides an historic opportunity to educate the public without terrorizing anyone.
Shoemaker-Levy 9, in its last pass by Jupiter, broke into a string of 21 major pieces. The energy released by the impacts of the full string will be equivalent to about a billion megatons of TNT. Although the pieces will impact on the side of Jupiter away from Earth, millions of amateur astronomers will be watching to see the flashes reflected from Jupiter’s moons. A few hours later, the rotation of Jupiter will bring the impact region into view. There is great disagreement about what will be seen, but no one suggests that it will not be spectacular.
The asteroid-comet community needs only to insure that everything is fully and accurately explained; the message will take care of itself: (1) the energy deposited by the cosmic impacts is enormous (2) this is a process that is still going on.