* The truth about impacts of asteroids and comets

A scientific, very respectful and well-thought reply to the popular question "Do you believe in UFOs?"  This book evolved as a reply to one of the most frequent questions that I used to hear from the public when I was working in an astronomical observatory: "Do you believe in UFOs?". That seems an odd question to ask to scientists, but after researching conscientiously for about a full year, I discovered, to my surprise, that mainstream Science has a few things to say about the topic.  This book is not about conspiracy theory, "NASA is hiding the truth", or much less, that flying saucers have already landed on the lawn of the White House. Rather, it is a book about what is the most rational reply that a scientist, or in my case, a science writer, can offer when people insist on asking that question.  As one advances through the chapters, explores the following rationale: Is there life in the Universe? The answer is yes: us. Are there civilizations capable of spaceflight? The answer is again yes: us. Can we expand those two questions? Can we answer also: "them" and "them"?  All illustrations are also available at naturapop.com

Illustration: The Torino Scale: assessing asteroid/comet impact predictions. Public description for the Torino Scale, revised from Binzel (2000) to better describe the attention or response that is merited for each category. Credit: Courtesy RP Binzel / MIT, Copyright © 1999, 2004 Richard P. Binzel, Massachusetts Institute of Technology. Reproduced with permission of Richard Binzel.
In recent years, with increasing frequency, it is appearing in the media, specially in movies, the hypothetical case of an asteroid or comet headed toward Earth with catastrophic consequences. Movies like "Meteor", starring Sean Connery and Nathalie Wood (written by Edward H. North and directed by Ronald Neame, American International Pictures, 1979), "Deep Impact", directed by Mimi Leder and starred by Elijah Wood and Morgan Freeman (Paramount Pictures, 1998) and "Armageddon", starred by Bruce Willis, Ben Affleck and Liv Tyler (with Michael Bay on the role of director, Touchstone Pictures, 1998), are examples of hollywoodian proposals that were very successful among the public. In all of them, Humankind tries desperately to avert the impending catastrophe with varying success. How fantastic are these films, and what dose of reality do they contain? What the public really knows, and how it might be better informed? Should we take this issue seriously or are just movies? Should we be worried? How worried? What steps should we or could we take?

The first indications of the fact that asteroids and comets fell on Earth came to light in the twentieth century. In the United States, in Arizona, there is a crater 1500 m in diameter by 200 m deep called the Barringer Crater, but it was always known as Meteor Crater. The traditional explanation of geologists for this formation was volcanism. But in 1960, Astrogeology pioneer Eugene Shoemaker proved that the crater is very similar to craters from nuclear explosions. Something caused a huge explosion 50 000 years ago and opened this gigantic hole in the ground in seconds. Detailed analysis showed the existence of minerals characteristic of iron meteorites. The most convincing explanation is that an asteroid of about 50 meters fell at tens of thousands of kilometers per hour exploding with the power of nuclear weapons.

After this conclusion, the attention of many scientists turned to the Tunguska explosion of 1908. A number of witnesses reported that in that year a mysterious meteor fell over Siberia, exploding with huge force and heat, burning 2150 square kilometers of forest. Barometers in London, thousands of km away, managed to record the shockwave. The sky was filled with a strange powder that generated an aurora-like effect. This phenomenon was observed in most of Europe. Due to the isolation of the region and political problems, an expedition made it to the place only two decades later. No crater was found, despite the efforts. Interestingly, about 50 km before reaching the hypocenter, more and more burned and fallen trees began to be found, all pointing away from the hypocenter. At the hypocenter, the trees were standing, but burned and with no branches. Numerous investigations revealed that this is the result of a huge airburst. In recent times (Longo and colleagues, 1994) traces of minerals associated with stony meteorites were labouriosly found, though there is much to be done. Today there is a general consensus that Tunguska was the most recent cosmic impact with Earth.

In 1980, Alvarez and colleagues announced the discovery of a deposit layer, very thin, rich in iridium, just in the border that divides the Cretaceous sediments from those of the Tertiary geological era. The metal Iridium is rare in Earth but abundant in some types of meteorites. Iridium deposits are present in the same geological layer, 65-million years old, worldwide, suggesting that a large amount of iridium came to Earth at the same time and was evenly distributed across the planet. The conclusion reached is that a large asteroid rich in iridium hit Earth 65-million years ago. It is precisely at the time when one of the largest mass extinctions on Earth occurred, when 75% of all species disappeared, including the dinosaurs. An explosion such as the one proposed by the fall of a medium-sized asteroid would easily fill the atmosphere with dust, soot and smoke in sufficient quantity to alter the Earth's climate and disrupt photosynthesis. The result would be a serious global ecological catastrophe. In 1991, Hildebrand and colleagues were able to locate in the Yucatan peninsula the rim of a colossal crater 175 km in diameter that according to the evidence was opened 65-million years ago. It would be the culprit for the extinction, among many others, of Earth's dominant beings at that time, the dinosaurs. Currently, there are identified over 150 impact craters of considerable size in the 5 continents, and which still resist erosion and tectonic movements.

In 1993, the same Eugene Shoemaker, his wife Carolyn and writer and amateur astronomer David Levy, during a work of cataloging near-Earth objects, detected a comet whose orbit had been altered by the planet Jupiter. In fact, the calculations showed that this comet, christened from then on as D/1993 F2 (Shoemaker-Levy 9), would crash into Jupiter in July 1994. One by one the pieces of the comet were crashing against the upper atmosphere of Jupiter in a terrible spectacle not seen since the invention of the telescope in 1609, and unimagined just a few years earlier. All the great observatories of the world could see first hand that the idea of ​​a collision of a comet or asteroid with Earth was no joke anymore.

Some governments of the World, specially the spacefaring nations, devoted funds and other resources to study more deeply the issue. One of the first consequences was a renewed interest in robotic missions to asteroids. The first of these missions done under civilian control was the NEAR-Shoemaker probe, launched in 1996 and which after various setbacks successfully achieved orbit and then landed on the asteroid (433) Eros, the second-largest of the group closest to the Earth.


Asteroids are rocky or metallic bodies several-kilometers wide that orbit the Sun like real minor planets. Because of their low gravity are irregular in shape, although the larger ones, as (1) Ceres (almost 1000 km) are found to be more rounded. However, of the nearly 20 000 cataloged, few reach a hundred km; the vast majority are about 1 km. Those under 50 m do not usually open craters at falling to Earth and are known as meteoroids.

The group of asteroids that are usually close to Earth (called Near Earth Asteroids = NEA) are divided into three types: those with orbits smaller than that of Mars but do not cross Earth's orbit (called Apollos, for its more representative asteroid), those with orbits larger than Earth's orbit but do cross it (Amors) and those with orbits smaller than Earth's but do cross the orbit of our planet (Atens).

In addition to asteroids, comets can also cross the orbit of our planet. Comets are a special type of asteroid. The difference is in their much more stretched orbit and in their composition: comets have plenty of ice, primarily CO2 and H2O, which are melted by the heat from the Sun when they reach the innermost part of the Solar System, where Earth is. Vapors form the characteristic cloud that surrounds them and extends by the Sun's radiation forming the giant tail, which can reach millions of kilometers long. Comets can be long-revolution period (more than 200 years, like C/1995 O1 [Hale-Bopp] at 4000 years) or short period (less than 200 years, such as 1P/Halley at 76 years). Some comets have orbits so small that they are always in the inner Solar System (up to Mars), like 2P/Encke at 3,3 years. Due to their volatile nature, comets are being spent. The waste they release remain in the comet's orbit as swarms of small particles. When the Earth enters these clouds, the collision of small particles (cm or mm) with the atmosphere is so violent that they burn out, yielding what we know as meteors. Usually the particles are completely burned at about 80-km altitude, without any damage to the surface.


Sometimes a particular space residue is too large (20 m) and does not burn out completely. There remain some part that reaches the ground as a stone that fell from heaven, as happened for example in Peekskill, United States of America, some years ago. A bolide was observed and even filmed when falling and burning, but it was so big that the last bit reached the ground and pierced the trunk of a parked car. In this case, the stone found in the ground is known by the name of meteorite.

Still larger pieces (over 50 m) reach a point in which they can open craters, and by definition are called asteroids. An asteroid a few tens of meters that falls on the Earth explodes with a power equivalent to nuclear explosions. If this asteroid falls over a city, we would have a local destruction that could kill millions of people. I does not matter if the explosion occurs when touching the ground or even before (due to the intense heating), in the air. The Tunguska event of 1908 is in this category.

If the asteroid has a size of hundreds of meters its destructive power would be equivalent to a nuclear arsenal, capable of causing regional devastation (an entire country or continent). Contrary to what one might imagine, should it fall into the ocean the consequences would be worse still: the explosion would generate a type of great sea waves (tsunamis) that can travel thousands of km to the coasts. About 80% of the world's population is concentrated in coastal communities, so the destruction and the consequences in terms of human lives would be immense, perhaps hundreds of millions of victims.

Up the scale in size, an asteroid or comet several kilometers in diameter falls with such force that it would penetrate the Earth's crust (through the thickness of the ocean) and lift a cloud of material up to outer space that would spread quickly throughout Earth's atmosphere. This category includes the debris of comet D/1993 F2 (Shoemaker-Levy 9), which left dark spots of the size of the Earth in the upper atmosphere of Jupiter. They took months to dissipate.

On Earth this material in suspension would reduce the sunlight reaching the surface preventing photosynthesis for months.

The climate change could last a couple of years, with consequences similar to those of a nuclear winter, studied during the decade of the 80's (Turco and collaborators, 1983; Ehrlich and collaborators, 1983). It would be a climate catastrophe of global consequences. A week after the impact, the amount of light and temperature would decrease dramatically. In lakes and rivers a layer of ice of considerable thickness would form. Forests would turn dry and facilitate the spread of fires. In the oceans, phytoplankton would die and the food chain would be interrupted. The thermal difference between the continents that rapidly cool and the sea that still retains heat would generate intense storms on the coast, with implications for coastal communities and any fishing fleet who ventures into the sea in search of food. Temperatures below freezing would destroy all agricultural crops in the hemisphere that is in spring or summer. Extreme temperatures would destroy tropical rainforests: in places like Central America and South America populations would have to wander in search of food. Civilization as we know it would be destroyed. Humankind would be forced to return to the Stone Age but in a much more hostile environment. The very survival of the human species would be endangered.

The scenario may be even worse if an asteroid or comet with a size of tens of kilometers should fall. The impact would be so energetic that the infrared radiation released would be enough to ignite whole forests thousands of km away. The incandescent shockwave would carbonize much of the land surface within a few minutes. Such a terrible impact would result in truly massive extinctions almost instantly. Fortunately, today bodies of this size are rare and relatively easy to spot, so we could say that right now there are none of this kind in direction to Earth.

At the end of the scale are the asteroids or comets of hundreds of kilometers. A collision with Earth would release so much energy that it would boil all the oceans and heat up the Earth's surface and atmosphere so much that would cause the complete sterilization of the planet, with the consequent elimination of all forms of life. The Earth would become a dead planet. Collisions like this might have happened in Earth's past, specially during the formation of the Solar System when planetoids were much more abundant. It is possible that the Earth has been sterilized again and again in the first thousands of million of years of its existence, and then revived again because the physical-chemical conditions in this early period were still conducive to the emergence of life. Today, these physical-chemical conditions no longer exist so it is regarded that Earth could not regenerate again. But also there are no bodies of that size left over in our region of the Solar System. There are only 33 asteroids larger than 200 km in the Main Belt (between Mars and Jupiter), and all are in stable orbits. Therefore, this is a kind of scenario that has no chance of re-occur.


One of the greatest scientific advances obtained with the exploration of the Moon is exactly the study of its craters. Virtually all were caused by impacts, and as the Moon has no wind nor rain nor modern tectonic movements, the craters we see through a small telescope are a record of collisions occurring in this part of the Solar System during the last 3800 million years. Since the Moon and Earth are next to each other in space, we can easily extrapolate what we see on the Moon to what happened on Earth. In fact, this is what is done for other Solar System bodies, such as the surface of Mars and those of Jupiter's satellites. The higher the density of craters on the terrain, the older the surface. Measurements of the absolute age of terrains done using the lunar rocks brought back by astronauts from different regions of the Moon gives us a very clear idea of when collisions were more frequent.

In fact, most of the major impacts occurred in very early times, when theory tells us that asteroids (from which planets originated) were much more numerous. More recently the number of impacts has decreased as well as the size of impactors. Using a statistical analysis of these scars on the lunar surface, according to their different sizes and ages, and supplementing this data with the populations of small bodies currently observed and their respective orbital dynamics, with analysis of craters on Earth and data from spy satellites that monitor potential nuclear explosions on Earth we can draw the following conclusions (originally published by Clark R. Chapman and David Morrison in 1994 and refined by Morrison, Alan Harris, Geoff Sommer, Chapman and Andrea Carusi in 2003):

Bodies of a few meters but that totally disintegrate strike the Earth with a frequency of months or years. Bodies of dozens of meters that can leave meteorites strike the Earth with a frequency of years to decades. Large bodies of 50 m or more, which can cause considerable damage on the surface, such as a "local" destruction (Tunguska-type) fall at a rate of centuries to millennia. Bodies of hundreds of meters, capable of causing "regional" devastation, fall on Earth with a frequency of thousands to hundreds of thousands of years. Bodies of the order of kilometers across strike the Earth at a rate that currently varies from hundreds of thousands of years to tens of millions of years. These impacts are capable of causing climate changes of "global" consequences. And as for bodies of tens of kilometers wide, which can cause mass extinctions, the last record we have is precisely the impact of 65-million years ago, on the K/T border. This is probably the order of years to be expected between collisions of this type. 


Cross-checking this data of frequencies with the number of casualties that can be expected in each of the scenarios, it is possible to calculate the probability of death per year per individual. Thus we can compare the risk from such disasters with other types of natural disasters. The following table (based on data published by David Morrison / NASA Ames Research Center) shows this comparison:


Bangladesh (mainly flooding): 0,005 %

China (mainly floods & earthquakes): 0,002 5 % 

Turkey / Iran / Turkestan (primarily earthquakes): 0,002 % 

Japan (primarily earthquakes): 0,001 5 %

Central America & the Caribbean (storms, earthquakes, volcanoes): 0,001 % 

Asteroid / Comet ("global" type): 0,000 1 % 

Europe (various disasters): less than 0,000 1 %

Asteroid / comet ("regional" type): 0,000 01 % 

United States / Canada (various disasters): less than 0,000 01 % 

Asteroid / Comet ("local" type): 0,000 001 %

It is possible to extrapolate these probabilities for a significant period of the lifespan expectancy of a human being, for example a century, giving us a probability of cause of death in a developed country, e.g., the United States of America. The table below shows these statistics (compiled by Clark Chapman and David Morrison, "Nature" magazine, volume 367, page 39, 1994. See also Harris, 2010, below):


Motor vehicle accident: 1 % = 1 in 100

Homicide: ~ 0,3 % = 1 in 300 

Fire or smoke: ~0,1 % = 1 in 800 

Firearm accident: 0,04 % = 1 in 2 500 

Asteroid / comet ("global" type): ~ 0,03 % = 1 in 3 000 

Electrocution: 0,02 % = 1 in 5 000 

Asteroid / comet ("regional" type): 0,005 % = 1 in 20 000 

Airplane crash: 0,005 % = 1 in 20 000

Flood: ~ 0,003 % = 1 in 30 000 

Tornado: ~ 0,002 % = 1 in 60 000 

Venomous bite or sting: 0,001 % = 1 in 100 000 

Asteroid / comet ("local" type): 0,000 4 % = 1 in 250 000 

Food poisoning by botulism: ~ 0,000 03 % = 1 in 3 000 000

It follows that the collision of an asteroid or comet may be classified as a disaster comparable, in terms of individual risk, to various types of natural disasters or violent deaths occupying a lot of space in the media. Clearly the impacts of asteroids or comets are much less common than different types of tragedies to which we are more accustomed. But a single asteroid or comet explosion has the ability to annihilate, at once, as much or more people as a huge number of other natural disasters combined and repeated throughout History. That is why the consequences an impact can have on the total population is equivalent over time. This is a finding that only a few years ago was unexpected. The meaning of these statistics is that if we, both as individuals and as a society, are concerned about electrocutions, airplane crashes, floods, tornadoes, stings of venomous animals, and food contamination, we also should be concerned, in equal or even greater extent, about impacts of asteroids and comets against the Earth. If this has not happened until now is simply because our historical memory is too short to tell us that these things do happen. Now our Science has managed to overcome this defect. The difficulty is to pass this information to the public, which is at the end the main interested party, in a way that the they could, in a conscious manner, decide the actions to be taken in relation to this issue.


The scientific opinion is that the first thing to do is to know if really, at this time, there is or there is not an asteroid or comet that poses a danger requiring immediate reactions. The challenge is great, because firstly, the availability of telescopes for the task is quite small. Generally patrols are being devoted to only a few telescopes in the category of 1 m of aperture or less. This means setting priorities. Currently the projects of mapping populations of small bodies are distributed throughout the three regions in which these have already been detected: the Kuiper Belt (mostly cometary nuclei in the region of Pluto's orbit), with some hundreds of objects already cataloged, the Main Asteroid Belt (between Jupiter and Mars), with almost 20 000 objects already cataloged (the largest known population), and the region near the Earth (between Mars and Mercury), with about 2000 objects cataloged. Because the orbital dynamics of the external belts pose no threat in the short or medium term, the priority is in the region between Mars and Mercury. The objects in this region are known as Near Earth Objects (NEO). The exception to this priority would be the region of the Oort Cloud (cometary nuclei that would be far beyond Pluto), from where long-period comets (potentially dangerous) come, but our technology is still insufficient to even think about mapping this region.

Because statistically the objects that can cause the most damage are the largest one (1km), and precisely because these are the most easily detected by telescopes of the category of 1m or less, at first the proposal is to map the population of this category of objects and their orbits. The current goal is to catalog as quickly as possible what statistically is estimated would be 90% of them, maybe about 1300 objects. Smaller objects, which can cause "local" destruction or "regional" devastation are too small (tens and hundreds of meter) to be able to think realistically in cataloging them at this time.

These projects had a substantial contribution of money and resources after the collision of D/1993 F2 (Shoemaker-Levy 9) into Jupiter. Thanks mainly to technological advances that allowed a high degree of automation, the cataloging process took a leap forward. However, the projects are still quite reduced. Currently, the most successful professional projects are LINEAR, by the MIT and the U.S. Air Force, NEAT, by NASA/ JPL, project LONEOS, by Lowell Observatory, the Spaceguard Foundation project in Italy and the Catalina Sky Survey by the University of Arizona aided by the Siding Spring Observatory, Australia. There are some other professional groups but are of much less expression. Clearly the number of telescopes is wholly inadequate to the task, specially in the Southern Hemisphere, less accessible to the projects mentioned above. It is estimated that the total number of people seriously studying the subject is about 60, in the entire world, much less than the public might believe. The main problem occurs in the monitoring of the newly discovered objects. The larger telescopes are dedicated to detection and to a first identification, but the workload is such that they have no time to study with precision the orbits of each newly-discovered object. This they leave to other observers, usually amateur astronomers. There is simply no money or equipment to perform this important task of monitoring. The responsibility that lies on good-willing astronomers is enormous, as it depends heavily on the latter the advancement of the objects catalog with the necessary reliability. The first indication that an object is coming towards the Earth may well come from an image obtained through an amateur telescope. The sad reality is far from what the public might be thinking about how we stand guard to protect Humankind.


At decade and a half after the systematic efforts have began, we now see that great progress have been made in developing the catalog of near-Earth objects. In fact, the data indicate that we have already cataloged most of these objects: it has become common in observatories that the same object is detected repeatedly, without the appearance of new ones. That is, new objects are not being discovered at a pace like that of the beginning of the program, despite the efforts. This is a sign that there are not many unknown objects out there.
With this, the statistical proportion of unknown objects that might pose a threat also decreased. We already know where most of the near-Earth asteroids and comets of short period are, and we know that this almost absolute majority of objects are indeed harmless in the relatively short and medium-term.
Thus, with these new data acquired in the last decade and a half, we are now able to adjust our risk tables. Here are some cases (from Alan Harris, "Cosmic disasters, real and imagined", a presentation for the "Night Sky Network". Astronomical Society of the Pacific and Jet Propulsion Laboratory, 18 November 2010):


Motor vehicle accident:  ~ 1,1 % = 1 in 90
Homicide: ~ 0,5 % = 1 in 185

Fire or smoke: ~ 0,1 % = 1 in 1 100

Firearm accident: 0,04 % = 1 in 2 500

Electrocution: 0,02 % = 1 in 5 000

Flood: ~ 0,004 % = 1 in 27 000
Airplane crash: ~ 0,003 % = 1 in 30 000

Tornado: ~ 0,002 % = 1 in 46 000
Venomous bite or sting: 0,001 % = 1 in 100 000

Asteroid / comet ("global"type): ~ 0,000 07 % = 1 in 1 500 000

Asteroid / comet ("local"type): ~ 0,000 04 % = 1 in 2 300 000
Food poisoning by botulism: ~ 0,000 03 % = 1 in 3 000 000

Asteroid / comet ("regional"type): 0,000 005 % = 1 in 20 000 000

The result is that in relation to what we had in the mid 90's, the risk of encountering a disaster is significantly lower than estimated. Before having certainty of where most of the near-Earth asteroids and comets of short period are, the probability was higher because the margin of error was higher. Now, with the lower margin of error, the estimated risk is also lower. 


Another difficulty is how to communicate to the public any relevant discovery in a responsible manner. The big problem is the margins of error that involves taking the position and calculating the future path of any object in space, specially if it is small, is far away and will still travel hundreds of millions of kilometers before crossing the Earth. For example, asteroid 1950 DA (1 km, capable of causing a "global" climate catastrophe) will cross the orbit of the Earth in the year 2880, but it is impossible to know whether or not it will collide with our planet. The margins of error in the calculations are still too large to say whether this will happen or not. This margin of error, known as error ellipse (the ellipse in a plane inclined to the path in which the object will pass with 99% certainty) can only be reduced after repeated observations at different points and times in the orbit of the object, usually for several orbits. Another complication is that the orbits are continuously altered by the gravity of other celestial bodies, including our planet. It is considered that efforts should focus on the calculations of collisions that would occur in the short and medium term, up to 100 years. To help communicate this uncertainty (or certainty, as appropriate) to the general public Richard Binzel created the Torino Scale, adopted by the International Astronomical Union in 1999 and amended in 2005. The Torino Scale takes into consideration basically two parameters (the probability of collision and its possible consequences) to inform the public about what level of concern (or not) they should have in relation to each new asteroid or comet discovered by scientists. The Torino Scale should be mentioned each time a communication to the public about the discovery of NEOs is made.


As for the possible steps we can take once we confirm an object is on a collision course with our planet, much has been said and discussed about the feasibility, possibility and desirability of these. Obviously, each case would be a different case, but we can imagine different types of scenarios, as in the table below (adapted from the original by John Urias, Iole DeAngelis, Donald Ahern, Jack Caszatt, George Fenimore III and Michael Wadzinski, 1996):


OBJECT: Asteroid
CHARACTERISTIC: Well-known orbit

OBJECT: New asteroid, short-period comet
CHARACTERISTIC: Orbit uncertain

OBJECT: Long-period comet; new small asteroid
CHARACTERISTIC: Immediate threat
TYPE OF REACTION: All-out effort

OBJECT: Long-period comet; "difficult" asteroid

OBJECT: Non detected
CHARACTERISTIC: Detected only at impact
TYPE OF REACTION: Post Impact / post disaster

Protecting the Earth would take place in three phases: the first phase would be a phase of detecting threatening objects, the second phase would be the study of the case to determine the mitigation measure, and the third phase would be implementation of the mitigation measure.

Due to the limited technology and resources available, at present time the only realistic goal is the cataloging of 90 % of objects capable of "global" consequences (of the order of kilometers) mentioned above. At a later stage, a similar cataloging of objects with possible "regional" consequences (of the order of hundreds of meters) might be thought and finally we would pass to the cataloging of objects with possible "local" consequences  (of the order of tens of meters). To develop a catalog of 90% of the latter two types of objects we would need technologies and resources that we do not possess today, so these goals are not practical at this time.

Having identified the threatening object, it will be very important to know its physical characteristics: size, mass, density, rotation period, consistency, type of material, etc.. From there the best techniques of neutralization would be chosen. Ideally, an on-site investigation would be carried out, by orbiting or landing on the asteroid or comet one or more spacecraft. A crucial factor would be time. To prepare an unmanned space mission, even supposing economic and technical resources at will, as of today it would take a minimum of 18 months between approval and launch. And to get to the object, it could take another several years. An alternative would be having reconnaissance spacecraft on a standby state, for example in space, that could be used for other studies while they are not called to this mission. Spacecraft could be kept in the Earth-Moon system, or in the inner Solar System, or directly in the main belts of small bodies, to try to give an early reaction, even preventively. The CONTOUR spacecraft was the first one to be launched with this in mind, in this case having to remain on standby in the inner Solar System as its secondary mission. Unfortunately, the mission ill succeeded because of a launch vehicle failure.

Once the physical characteristics of the enemy are known counterattack begins. Again, time available is crucial. If we have little time all we can do is an attempt of destruction by using the stockpiles of intercontinental nuclear missiles. The goal would be to practically, almost pulverize the asteroid or comet (reduce it to fragments of less than 20 m). Even using all the known nuclear arsenal (about 5000 missiles) this result cannot be guaranteed. If we fail to achieve this, the fragments could still cause serious damage on Earth. Therefore, this solution would most likely be insufficient. There is a consensus that the best alternative would then be deflecting the object without destroying it. The concern here would be just the opposite: not to fragment it. Knowledge of the structural properties of the asteroid and the resistance of its materials would become vital. That is another argument to finance different types of missions to asteroids and comets before a threatening one appears. Depending on the object, a nuclear explosion at a sufficient distance from the Earth, that is, early enough, could achieve the slight change in trajectory that would be enough to eliminate the hazard. Maybe less violent deflection mechanisms would be needed, to avoid fragmentation. Kinetic projectiles (heavy loads that would serve as cosmic billiard balls), rocket motors attached to the body of the invader (a Delta chemical rocket motor would do if it is attached to an asteroid and ignited some 20 years before it reaches the Earth) have been proposed. Entering the field of future technologies, the United States Air Force (Air Force 2025 study) has considered lasers, microwaves and concentrating mirrors to heat a certain point of the object and try to generate a column of gases that would act as a jet for deflection. A mining operation that throws the extracted mass into space could have a similar effect. Finally, a proposal that is considered more in line with current technology would simply place giant ultra-thin sails that would receive the impact of photons from the Sun to drive the asteroid or comet in the desired direction. The research on solar sails for space travel is already quite advanced. The problem might be the scale of the deploying frame structure required to drive a body as massive as an asteroid or comet. All these alternatives have one thing in common: the closer to Earth, the greater the course-change needed shall be, and therefore, the greater the momentum or thrust. Ideally, therefore, we should have an advance notice of several decades.


Besides the technical and economic difficulties of the creation of these defense systems, there are other problems, political: Who will pay for the development of these systems? If the invading object is capable of "global" destruction the problem is not very serious: it is of interest to all countries on Earth. But what about an asteroid or comet capable of a small "local" destruction that threatens a Third-World country? This nation would be dependent on the willingness of the more advanced nations. Does the United States of America, for example, be inclined to use much or even all of its strategic missile arsenal to defend Tanzania? There is another problem, this time related to new technologies: because these systems, some directly and some indirectly, would have an enormous potential for destruction that must be used in an absolutely responsible manner, who should control these systems, which ultimately and after all can act as military systems? Who guarantees that the first nation to develop these will not use them to threaten other countries? Is it worth taking this short-term risk to try to avoid a risk that could be very long-term? Would not the proposed remedy be worse than the disease? Obviously, we speculate about these because we do not know if we are talking about an immediate threat or something that can wait. This is another reason to have catalogs of Near Earth Objects as complete and reliable as possible.

Anyway, this is an issue to be discussed at the international level and in which cooperation is vital. Because at this time the uncertainty is the same for all and all potentially have the same probability of suffering the consequences, we must consider the risk of asteroid and comet impacts as a risk to the planet Earth. As this is the home of everybody, every nation should be equally interested in reducing the uncertainties and in studying preventive plans. When and where will the next impact be? The truth is we do not know. It may be tomorrow, in a year or within centuries. The truth is we do not know. And this is the first major problem to be solved.

Amateur astronomers could provide valuable help in the program of detection, cataloging and monitoring coordinated by the Minor Planet Center (MPC) of the International Astronomical Union, based at the Harvard-Smithsonian Center for Astrophysics, in United States of America. The view of skies that in many cases are not readily accessible to professionals is a strong point that would support the decision to use facilities of amateur astronomers to provide data. The MPC publishes the technical requirements and suggestions for such collaboration. The observatories of amateur astronomers who volunteer must earn an international code of participant observatory.

Of all known natural disasters, the impact of asteroids and comets is the only phenomenon capable of destroying at once the entire Humankind. And it is the only natural disaster that we may be able to anticipate and neutralize completely. The dinosaurs did not have that capability. If we allow to them happen again what happened to them, this time to us, by our inaction, it would be a terrible negligence.

A. L. 

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Based on a lecture given at USP, on 27 July 2002. An abridged version was originally published in ABC Color, on 5 March 2006. Illustration: The Torino Scale: assessing asteroid/comet impact predictions. Public description for the Torino Scale, revised from Binzel (2000) to better describe the attention or response that is merited for each category. Credit: Courtesy RP Binzel / MIT, Copyright © 1999, 2004 Richard P. Binzel, Massachusetts Institute of Technology. Reproduced with permission of Richard Binzel.

A scientific, very respectful and well-thought reply to the popular question "Do you believe in UFOs?"  This book evolved as a reply to one of the most frequent questions that I used to hear from the public when I was working in an astronomical observatory: "Do you believe in UFOs?". That seems an odd question to ask to scientists, but after researching conscientiously for about a full year, I discovered, to my surprise, that mainstream Science has a few things to say about the topic.  This book is not about conspiracy theory, "NASA is hiding the truth", or much less, that flying saucers have already landed on the lawn of the White House. Rather, it is a book about what is the most rational reply that a scientist, or in my case, a science writer, can offer when people insist on asking that question.  Of course, "Do you believe in UFOs?" is, understandable, one of the most popular questions that common people ask (even if silently, to themselves) when they raise their eyes and look at the stars. So it has to be treated respectfully, and why not, given a well-thought reply.