What is the real danger of the Earth being hit by an asteroid?

Summary: To what extent can we think we're quite safe here in this corner of the universe, when thousands of objects, sometimes with unpredictable orbits, or simply not yet cataloged, drifting through space, and obeying blindly to the inescapable laws of celestial mechanics? Responses in the interview with Dr. Jorge Carneiro, researcher at the Astronomical Observatory of the University of Porto.

Cosmos - What is the real risk of being hit by a smaller celestial body, such as an asteroid?
Dr. Jorge Carneiro - Man, of course, always took risks and will continue to run. The space around Earth, more precisely, the space around the Sun, is swarmed by asteroids many sizes. And you can not put aside the possibility of such a focus on Earth asteroids. It is believed, for example, that the 65 million years, a relatively large asteroid has collided with Earth, although no formal proof of exactly one collision exists. But this may not be the only record. When historians analyze the Bronze Age find that there was from 2500 BC to 800 BC breaks in the civilizations that existed in the Bronze Age, and also that these interruptions could be explained by natural disasters caused by asteroid strikes. However man is accustomed to living in a risky environment. The inhabitants of Lisbon, for example, know they live in a state of constant risk, the city of Lisbon is terribly bad from the standpoint of seismic risk. Many millions of people living on the San Andreas fault, or in their vicinity, they know they have scheduled an earthquake in the very short term; the inhabitants of Mexico City, living in the shadow of the volcano, you know that if there is a gambling have no chance; the inhabitants of the southern United States, living in the area through which the tornadoes, are constantly buffeted by hurricanes. There are areas of land that are structurally inhospitable and yet are densely populated.

Cosmos - In terms of the probability of an impact of these happen, can we quantify it?
Dr. Jorge Carneiro - "It is difficult to quantify these impacts. Estimates are really about these risks and it is estimated, for example, that a small asteroid (10 to 30 meters in diameter) should have an impact with Earth in 100 years. This has a local impact, with fewer risks. But if we pass to an average asteroid (200 meters to 2 kilometers in diameter), then it would have a regional impact, it would be a threat to civilization and it is estimated that asteroids of this size could fall to Earth with a frequency ranging between 10000 1 million years. This interval between impacts is a risk that insurance companies are not accustomed to consider. It is a remote threat, but because it is remote we can not ignore it. The fact that there are now 1 million years of not having any impact, isn't not to say that there might be no tomorrow.

Cosmos - What effect would such an impact, in terms as realistic as possible?
Dr. Jorge Carneiro - Two Russian specialists in explosives, calculated that an asteroid about 200 meters who fell in the middle of the Atlantic Ocean, where the ocean is deep, raise a wall of water in 40 seconds with 35 kilometers high; the wall that was propagated and literally flooded the entire U.S. coast and throughout the Portuguese coast, and, regardless of their films does not illustrate the Portuguese case, I am convinced that reached the Serra da Estrela. Anyway, it's a sham, is a hypothetical situation, but is not at all a situation that one can say that is not an option. We're talking about, literally what happens immediately after the impact. But there are other consequences, including two that would be: the weather certainly altered substantially, and there would be a very difficult winter, and since we are generally accustomed to a situation of stability, since communication would be seriously affected, all large companies, banks, distribution companies, would be paralyzed. And note that: contrary to what happened 100 years ago, when people had local livelihoods, currently we do not. The goods supply are currently distributed by large distribution companies, which were paralyzed. Here there is a cascade effect, which is difficult to predict. (...) It has been thought to live without cash card? »

Cosmos - How long these effects to disappear?
Dr. Jorge Carneiro - It is very difficult to quantify it, because if that happened to the dinosaurs, was certainly one thing that lasted for many months. Depending on conditions of impact, and also depends on the asteroid. It is very difficult to create scenarios for every taste.

Cosmos - There has been talk for some time, the asteroid 1997 XF11 would collide, or pass too close to Earth in October 2028. Preliminary calculations indicated that. Shortly thereafter, new calculations showed that it would probably be about 950,000 kilometers. To what extent is that the projections made by astronomers are rigorous and consensual? How to measure the margin of possible error?
Dr. Jorge Carneiro - The asteroid 1997 XF11, is a small asteroid passing near Earth's orbit, is one of the objects of risk, classified as having some risk. He appeared in 1997, as its name implies, the first observations and, indeed, led to the stroke of an orbit that would almost colliding with Earth. There Astronomy in a centralized information on these objects, which has been operating for many years, which is the Central Bureau of Astronomical Telegrams, which is in the Smithsonian Institute, and the first information was, as usual, forwarded to this office, who calculated the orbit and has, like you said, an orbit of nearly colliding with Earth in 2028.A function of this office is to centralize information and then disseminate it. And that's what he did. He spread the observation, and other institutes were also warned that imminent.The calculation of the orbit of a celestial object, it is a trivial mathematical problem today, what can be done with great accuracy as a sum, since the initial elements are also strict. What happens is that the calculation has been done initially relied on elements relatively undemanding. The next day was possible to have more elements, more observations of that asteroid, and it was possible to recalculate an orbit that showed that there was really no danger of collision. Would pass by, no doubt, but there had been passages closer, but he certainly went further than the Moon, and thus that risk, the next day, was eliminated because what happened (at first) was made a calculation with imprecise data "." Astronomers do not know how to react to such a situation of real danger. So, since 1997, several people working in this area, decided to draw lines to prevent the repetition of this mistake in future, draw lines and strict performance to future situations similar to this one. This work, recently came to culminate in a meeting in Turin, where he was formally introduced a range of risk classification of asteroids. That is, here in the future, rather than an astronomer jump to the phone and say "Look out! Lower the head, which an asteroid is coming !!!", he says" The asteroid X has degree 4. (. ..) The Turin conference hangs unfounded speculation.

Cosmos - What is being done to prevent future incidents of collisions with celestial bodies?
Dr. Jorge Carneiro - Of the 2,000 known asteroids, only about 200 are listed. NASA has an office in charge, exactly, to do an exhaustive survey by the year 2010, the situation of the remaining elements. And to objectively measure what risks it may face in the coming future.

Additional information:

Closest Approaches to the Earth by Minor Planets
Closest Approaches to the Earth by Comets
Forthcoming Close Approaches To The Earth

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