Impact Hazards

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

What is an Explosion?

In simplest terms, an explosion is the sudden release of a large amount of energy in a small space. The source of energy can be mechanical, like the bursting of a compressed air tank; chemical, like high explosives; or nuclear. Non-mechanical explosions almost always result from the instantaneous release of a large amount of heat.

Explosions in Space

In space, an explosion would cause damage mostly from radiation (heat, light, perhaps nuclear radiation) and flying debris from the exploding object or whatever else is in contact with the explosion. In free space, debris and radiation would travel indefinitely but the danger would diminish quickly with distance. Expanding vapor from chemical or nuclear blasts would rapidly expand to insignificant density. Radiation would decrease in intensity as the inverse square of distance, and the risk of being hit by flying debris would also decrease with distance. On an airless body like the moon, most debris would fall back to the surface but perhaps a little would escape into space.

So explosives wouldn't be terribly effective weapons in space battles. Only if the explosive was in direct contact with the target or very close to it would it be effective. Space military planners think mostly in terms of destroying hostile spacecraft by direct high-speed impact (aided maybe by explosives), by directed energy weapons like lasers or particle beams, or by using nuclear radiation to fry electronics.

Explosions on Earth

What makes explosions so much more effective on earth is the atmosphere or other surrounding medium (water, say, if you're talking about a depth charge). Even a mechanical explosion like a bursting air tank propels the surrounding atmosphere away at high speed, perhaps with enough force to do damage. Chemical and nuclear explosions do something else as well: they heat a large volume of atmosphere, causing it to expand violently. If the heating is strong enough, the heated atmosphere expands faster than the speed of sound, bulldozing the air ahead of it in a shock wave. Eventually the fireball of hot gas stops expanding, but the compressed air it pushed ahead keeps moving, and may be capable of causing injury and damage kilometers from the blast. A nuclear weapon in space would produce a few hundred kilograms of vapor that would have insignificant effect a kilometer away; the same weapon on earth would propel thousands of tons of air at high speeds.

The superheated fireball of air also has another effect: it emits huge amounts of heat. A nuclear blast in space would emit a near-instantaneous burst of heat, light, and radiation. The same nuclear blast on earth would create a fireball of incandescent air that would emit dangerous amounts of heat for many seconds.

One last effect of the fireball is important: being hot, the gases of the fireball expand, become less dense than the surrounding atmosphere, and rise at high velocities. The rising fireball sucks up dust and debris in its wake. Any large explosion due to heat will generate a mushroom cloud.

Effects of Large Explosions

Two units of energy are commonly used to describe large explosions. A kiloton is the energy equivalent of 1000 tons of high explosives, about 4.2 x 10¹² joules. A megaton is the energy equivalent of a million tons of high explosives, about 4.2 x 10¹⁵ joules. A kiloton has been formally defined as a trillion calories (small c - a standard food calorie is 1000 of those). Since these units are rarely printed in standard tables of units, it's worth remembering them. The largest chemical blasts ever detonated overlap the energy range of small nuclear weapons, and explosions are all about the point release of large amounts of energy, so the mechanism of the explosion is immaterial.

Fun Factoid: 1000 tons of TNT = a million kilograms = a billion grams. A kiloton of energy is a trillion calories, so a gram of TNT contains 1000 calories (small c) or one Calorie (big C). A big-C Calorie is a food calorie. So a gram of TNT has a Calorie of energy. Snap, Crackle and POP! Fat, on the other hand, contains 9 Calories per gram. So whatever other faults high explosives have, they're not fattening. Although you do run the risk of shooting your mouth off.

It's all in how the energy is released. If a candy bar or carrot gave up all its chemical energy in microseconds, they would be high explosives, too.

The scaling law for explosion effects is approximately proportional to volume; that is, a one kiloton blast affects a volume 1000 times that of a one-ton blast. The radius of blast effects is proportional to the cube root of the volume, so a one-kiloton blast affects a radius only ten times that of a one-ton blast. This scaling law explains why nobody builds 100-megaton nukes. Apart from it being impossible to contain a nuclear explosion long enough to get that much energy, the effects of a 100 megaton nuke could be achieved much more cheaply with a number of smaller weapons.

Relatively small fireballs like those due to exploding fuel tanks rise and suck smoke and debris behind them. As they rise, air flowing past the fireball is sucked into a vortex under the edge of the fireball, creating a toroidal circulation. The toroidal circulation helps define the cap of the mushroom. Much larger explosions, like those due to nuclear weapons or large impacts, also have the toroidal circulation. In addition, they are hot enough to combine atmospheric nitrogen and oxygen. Videos of nuclear blasts commonly show dark brown nitric oxide forming around the still incandescent fireball. Such large blasts also create fireballs that initially expand faster than sound. The atmospheric shock wave propelled by the expanding fireball consists of an initial abrupt compression followed by rarefaction, and the rarefaction is often great enough to cause water vapor to condense, creating a condensation hood around the fireball.

The Physics of Impact

The fundamental law behind impact is the formula for kinetic energy: K = 1/2 mv². Consider a 10-meter chunk of rock moving at 30 km/sec, the earth's orbital velocity. The density of the rock will be about 3000 kg/m³ (the SI units for density are kg/m³. To convert from the more familiar g/cm³ multiply by 1000.) So m, the mass of the rock, is 3000 kg/m³x (10m)³, or 3 x10⁶ kg. Velocity, v, is 3 x 10⁴ m/sec. So K = 1/2 x 3 x10⁶ kg x (3 x 10⁴ m/sec)² = 13.5 x 10¹⁴ joules, or about 300 kilotons. The impact of a 10 meter object at typical inner solar system velocities releases as much energy as 15 Hiroshima atomic bombs.

When that object hits a planet, say the earth, all that energy converts into other forms, like light, sound, seismic waves, and heat. Especially heat. It takes a few million joules to vaporize a kilogram of silicate rock, so the impact releases enough energy to vaporize roughly 100 million kilograms of rock, or 30 times the mass of the impacting object. So we need hardly be surprised that the impacting meteor is destroyed - imagine piling 15 Hiroshima atomic bombs around a 10-meter cube of rock and setting them off.

Atmospheric entry

First the object has to penetrate the atmosphere. Once an object is large enough, it makes little difference whether a planet has an atmosphere or not. A Kevlar vest will stop shrapnel, slow down a bullet, and have no effect at all on an artillery shell. But even for large objects, the atmosphere can have significant effects.

When the Apollo astronauts returned from the Moon, they entered the atmosphere at a grazing angle at a mere 11 km/sec, and nevertheless experienced up to 12 g's of deceleration. A meteor entering more steeply at higher speeds would feel far greater forces.

Let's imagine our 10-meter asteroid object coming straight down at 30 km/second, and let's assume it loses only 10% of its initial velocity passing through the atmosphere. It will take about 3 seconds to pass through the atmosphere, and in the process its change in velocity (delta V) will be -3000 m/sec. Its deceleration will be delta v/t = -3000 m/sec/3 sec = -1000 m/sec². Since one g is about 10 m/sec², the object experiences 100 g's of deceleration.

How much is 100 g's? It will take you 1.4 seconds to fall from a 10-meter (33 foot) building, and when you hit the ground you will be going 14 m/sec. If you land flat on your face, you will decelerate -14 m/sec in the thickness of your body, say 20 cm (0.2 m), and at 14 m/sec it will take you .014 seconds to cover that distance. So you will decelerate at (-14 m/sec)/(.014 sec) = 1000 m/sec² and experience 100 g's of deceleration. So our incoming meteor experiences forces roughly equivalent to swan diving 33 feet onto a sidewalk.

If the stresses are severe enough, the object might break up catastrophically. It was once thought the 1908 Tunguska object was a piece of a comet, but we now doubt that a small chunk of comet, with about the texture of crusty snow, could make it to within a few kilometers of the surface before breaking up. Most planetary scientists now believe the Tunguska object was a stony meteorite, able to withstand higher stresses. But if the object does break up catastrophically, it now has vastly increased surface area in contact with the atmosphere and will vaporize in the atmosphere. The result will be to release the object's kinetic energy entirely in the atmosphere, with almost all the physical effects (even a mushroom cloud) of a large nuclear weapon. The only thing missing will be radioactivity.

If the object does make it to the ground more or less intact, it will have released about 19% of its kinetic energy passing through the atmosphere. (It lost 10% of its velocity, but energy is proportional to the square of velocity, hence the 19%). Since its original kinetic energy was 13.5 x 10¹⁴ joules, it released 2.7 x 10¹⁴ joules in the form of heat in 3 seconds, or a power output of 9 x 10¹³ watts.

Let's figure out what the effects would be at a distance of 10 km from the object's path. The radiated heat is spread out over a cylinder 90 km high (the atmospheric path length) by 10 km in radius, with a total area of 5.65 x 10⁹ m². The energy flux at a distance of 10 km is thus 9 x 10¹³ watts/5.65 x 10⁹ m² = 16,000 watts/m². The intensity of sunlight is about 1370 W / m. This thing will be about a dozen times brighter than the sun. And this is a small impact - a really big one will cause burns at large distances just from its atmospheric passage. Another way to think of it is that the released heat, 2.7 x 10¹⁴ joules, is equivalent to about 56 kilotons, or roughly the equivalent of setting off four Hiroshima atomic bombs during the three seconds of atmospheric entry.

The Chelyabinsk Meteor of 2013

On February 15, 2013, a huge meteor broke up over Chelyabinsk, Russia. The sonic boom shattered windows and even collapsed a roof. Using simple methods, we can estimate the energy of the meteor.

The meteor was low in the sky as seen from Chelyabinsk, so the slant distance was probably 100 km or 100,000 meters. The meteor was much brighter than the Sun for a couple of seconds, and the energy flux from the sun is 1370 watts per square meter. So let's estimate the peak output at 10,000 W/m², or about seven times brighter than the Sun, for two seconds. That means each square meter received about 20,000 jopules of energy (a watt is a joule pre second). Since the meteor was about 100 km away, that energy was spread out over a sphere 100 km in radius, so the total energy was 20,000 J/m² * 4 * pi * (100,000m)² , or 2 x 10⁴ J/m² * 4 * pi * 10¹⁰ m² , or 25 x 10¹⁴ joules. A kiloton is 4.2 x 10¹² joules, so the energy was about 600 kilotons. This is a bit bigger than most published estimates (300-500 kt) but not bad at all considering the rough and ready approximations used. The meteor was the largest since the Tunguska event of 1908, bigger than the Sikhote-Alin meteor of 1947. All three occurred in Russia. Why does Russia get all the cool meteor impacts?

Cratering

When the object hits, it plows about its own diameter into the earth before stopping. The rear of the object keeps moving after the front stops, and the object flattens and spreads. Compressed target rock is pushed out from the impact site and flies outward, aided by vaporized impactor and target materials, creating a transient crater. Compressed rock flows along the floor of the transient crater, up the sides, and out, some of it reaching the velocity of the impacting object. Thus large impacts are capable of blasting material into space. After the initial shock wave passes, the compressed floor of the transient crater rebounds, the walls collapse, and the final crater is about twice the size of the transient crater, and typically 20-30 times the diameter of the impacting object.

Since impacts deliver their energy directly in mechanical form, and involve high velocities, they are probably more efficient than nuclear explosions at flinging ejecta. However, atmospheric blast waves will probably cause complete destruction beyond the range of most ejecta anyway.

Types of Impact

Microscopic objects: These are tiny enough that even impacts with individual atoms can be significant. They are slowed to a halt in the upper atmosphere and settle to the surface as dust.
Millimeter to 10 cm objects: These are heated by friction as they enter the atmosphere, become incandescent, and vaporize. We see them as meteors or "shooting stars." The bigger objects in this class become brighter than Venus.
Meter-size objects: These often fragment in the atmosphere, and fragments sometimes survive to reach the surface as meteorites. Objects that fragment low in the atmosphere often emit sonic booms. Falling fragments have hit houses and cars but whether any have killed someone - yet - is controversial. As the earth's population density grows, it will happen.
House-sized objects: These are packing energies of small nuclear weapons. Defense satellite data shows that objects on the low end of this scale impact the earth several times a year. Icy and fragile stony objects fragment in the upper atmosphere and create brilliant meteors brighter than the sun. Objects that fragment lower in the atmosphere may drop large meteorites on the surface, some of which may hit hard enough to dig craters. Fragile objects that disintegrate close to the surface, like the Tunguska event of 1908, will produce blast effects like the air burst of a nuclear weapon. Objects that hit intact will excavate craters the size of a football field. These impacts happen on century time scales.
100-meter objects: These have energies comparable to the largest nuclear weapons. Blast effects and flash burns will extend to distances of kilometers, and a surface impact will create a crater a couple of kilometers or so across. These impacts happen on thousand-year time scales.
Kilometer objects: These have energies comparable to thousands of large nuclear weapons. Blast effects will extend to distances of tens or hundreds of kilometers. Flash burns will extend to the limits of visibility. These objects are big enough that an atmosphere has little shielding effect, and the object will hit the surface and create a crater 20-30 kilometers or so across. Ejecta will devastate the landscape far beyond the crater. These impacts happen on million-year time scales.
10-Kilometer objects: These have energies far greater than the total nuclear arsenal of the earth. Blast effects will extend to distances of hundreds or thousands of kilometers. Flash burns will extend to the limits of visibility. These objects will hit the surface and create a crater 200-300 kilometers or so across. Ejecta will devastate the landscape on continental scales and hot ejecta will fall globally. Dust raised by the impact will have global climatic effects. These impacts happen on 10-100-million-year time scales.
100-kilometer objects: These have not struck the earth in the last 4 billion years. An impact by an object of this size would vaporize enough rock to cover the earth, sterilize the surface and boil away most of the oceans. Some deep ocean organisms might survive.
1000-kilometer objects: Collisions on this scale have not happened since the last stages of solar system accretion. An impact on this scale would melt a significant fraction of the earth and envelop the earth with vaporized rock for centuries or longer. The earth would be entirely sterilized and would probably lose most of its atmosphere.

Likely Scenarios

As fascinating as it is to consider the results of mega-impacts, they are extremely unlikely to happen in the foreseeable future. The list below deals with the scenarios that are reasonably likely to happen in the next few centuries. The probability of the first two is at least 10 per cent. The probability of the next two may be about one per cent. The last event is of very low probability but worth considering as an exercise in disaster planning.

Unpredicted Atmospheric Impact Causing Local Damage

This would be a repeat of the 1908 Tunguska Event (or the 2013 Chelyabinsk event). The physical effects would be similar to those of an air burst of a large nuclear weapon, except there would be no radioactive fallout. Given that the major powers all have surveillance for nuclear blasts, the likelihood of such an event causing a global nuclear war is probably low. However, an impact in a war zone or region of high tension might provoke responses that could be difficult to contain. Regardless of where the impact happens, a very real problem might be convincing local military, Homeland Security and law enforcement personnel that the event is not nuclear (indeed, there are persistent internet rumors that Chelyabinsk was a weapon of some kind). The populace might also be worried about fallout and might not believe assurances that there is no radiation hazard.

Unpredicted Surface Impact Causing Local Damage

The Meteor Crater event that occurred in Arizona about 20,000 years ago would be an example. The physical effects would be like a surface or shallow underground nuclear blast. The social problems would be similar to an atmospheric impact.

Anticipated Impact Causing Local Damage

100 years ago we could not have predicted any small impacts. Anything big enough to see in a telescope would result in a regional or global scale impact. Now our detection technology could probably provide at least some advance warning of at least house-sized objects, and probably years of advance warning for 100-meter and larger objects.

In this scenario we have advance warning of an incoming object in the 100-meter size range. The appropriate response would be to estimate the radius of blast and flash effects and evacuate that area plus a safety margin. How effective the response will be depends on the advance warning and the target area. A month's warning of an impact in Mongolia is a lot better than a day's warning of an event in Chicago.

Anticipated Impact Causing Regional Damage

In this scenario we have advance warning of an incoming object in the kilometer size range. The evacuation area could be too large to empty in time and involve too many people to feed and shelter. The economic effects of the event would be global. It's also possible that the impact could cause unacceptable cultural losses - picture an impact on Paris, Rome, Athens or Mecca. Offshore impacts could cause tsunamis. The stakes in such an event are high enough to warrant considering means of diverting or destroying the incoming object.

Anticipated Impact Causing Global Effects

In this scenario we have advance warning of an incoming object in the several-kilometer size range. Ejecta and dust could have global environmental effects. Oceanic impacts could cause tsunamis capable of causing global coastal damage. Destruction of infrastructure and communications would cause worldwide economic and political upheavals. Warning times of decades or centuries are desirable and require global cooperation to deal with the threat. We simply cannot allow an impact of this sort to happen.

Ranking the Hazard: The Torino Scale

The Torino Scale was largely the brainchild of Richard Binzel of MIT after a couple of highly publicized announcements of near-earth asteroid discoveries resulted in media sensationalism. The scale is named for the Italian city because astronomers at a conference there refined the scale.

No Hazard (White Zone)	0	The likelihood of a collision is zero, or is so low as to be effectively zero. Also applies to small objects such as meteors and bodies that burn up in the atmosphere as well as infrequent meteorite falls that rarely cause damage.
Normal (Green Zone)	1	A routine discovery in which a pass near the Earth is predicted that poses no unusual level of danger. Current calculations show the chance of collision is extremely unlikely with no cause for public attention or public concern. New telescopic observations very likely will lead to re-assignment to Level 0.
Meriting Attention by Astronomers (Yellow Zone)	2	A discovery, which may become routine with expanded searches, of an object making a somewhat close but not highly unusual pass near the Earth. While meriting attention by astronomers, there is no cause for public attention or public concern as an actual collision is very unlikely. New telescopic observations very likely will lead to re-assignment to Level 0.
	3	A close encounter, meriting attention by astronomers. Current calculations give a 1% or greater chance of collision capable of localized destruction. Most likely, new telescopic observations will lead to re-assignment to Level 0. Attention by public and by public officials is merited if the encounter is less than a decade away.
	4	A close encounter, meriting attention by astronomers. Current calculations give a 1% or greater chance of collision capable of regional devastation. Most likely, new telescopic observations will lead to re-assignment to Level 0. Attention by public and by public officials is merited if the encounter is less than a decade away.
Threatening (Orange Zone)	5	A close encounter posing a serious, but still uncertain threat of regional devastation. Critical attention by astronomers is needed to determine conclusively whether or not a collision will occur. If the encounter is less than a decade away, governmental contingency planning may be warranted.
	6	A close encounter by a large object posing a serious but still uncertain threat of a global catastrophe. Critical attention by astronomers is needed to determine conclusively whether or not a collision will occur. If the encounter is less than three decades away, governmental contingency planning may be warranted.
	7	A very close encounter by a large object, which if occurring this century, poses an unprecedented but still uncertain threat of a global catastrophe. For such a threat in this century, international contingency planning is warranted, especially to determine urgently and conclusively whether or not a collision will occur.
Certain Collisions (Red Zone)	8	A collision is certain, capable of causing localized destruction for an impact over land or possibly a tsunami if close offshore. Such events occur on average between once per 50 years and once per several 1000 years.
	9	A collision is certain, capable of causing unprecedented regional devastation for a land impact or the threat of a major tsunami for an ocean impact. Such events occur on average between once per 10,000 years and once per 100,000 years.
	10	A collision is certain, capable of causing global climatic catastrophe that may threaten the future of civilization as we know it, whether impacting land or ocean. Such events occur on average once per 100,000 years, or less often.

Source: http://neo.jpl.nasa.gov/torino_scale.html

Above is a graphic of the Torino Scale. Note that the scale is not really a continuum. Since the scale combines two quite different things, probability of impact and severity of damage, it juxtaposes very different values. For example, if a 5-km object has a 50% chance of impact, it rates a 7 on the scale. But if a 1000 km change in its predicted orbit guarantees it will miss the earth's atmosphere, it drops to 2, 1, or 0 on the scale (I seriously doubt something that big coming that close would ever be rated zero regardless of the math.) A 50-meter object whose impact changes from possible to certain jumps from 3 to 8 on the scale.

What "Probability of Impact" Means

Imagine you measure your property line and get measurements of 100.02, 99.97, 100.01, 99.98 100.00 and 100.02 feet. They average exactly to 100 feet so you conclude your lot is 100 feet long. The variation comes from slight differences in how tight you pull the tape, where you put the start of the tape, how exactly you follow the property line, and how you line your eye up with the tape. Two measurements are smaller, three are larger, and one is equal to the average.

On the other hand, maybe just by luck, all your measurements are too small, or too large. Maybe your lot is 100.03 feet long, or 99.96, and you somehow failed to catch it despite measuring six times.

A given measurement has a 50% chance of being smaller than the true value, so the chance of all six measurements being too small is 1/2 x 1/2 six times, or 1/64.
For five measurements being too small, any of six measurements could be the one larger one, and there are six possible ways the other five measurements could be too small. Each outcome has a probability of 1/64, so the total odds are 6/64.
For four measurements to be too small, two have to be larger. Any of six could be the first, and any of the remaining five could be the second, but they could occur in either order, so there are (6 x 5)/2 = 15 ways to get four measurements too small, thus the odds are 15/64.
For a three-three split, any of the six values could be too large, then any of the five remaining, then any of the four, but there are six different possible orders they could come in, so we have (6x5x4)/(3x2x1) = 20 possible outcomes, so the odds of a three-three split are 20/64. This is the most likely outcome but note that the probability is still less than half.
For two, one, or zero measurements being too small, the reasoning is the same as four, five, or six (the chance of only one measurement being too small is exactly the same as only one being too large.) So for zero through six measurements being too small, the odds are 1/64, 6/64, 15/64, 20/64, 15/64, 6/64 and 1/64, respectively. Note that the probabilities add up to 64/64 = 1; one of the outcomes has to happen.

Whenever we measure any quantity, there is always some probability that we are far off the mark. The most likely estimate for the true value is the average, but there is always a tiny probability that all our measurements are off in the same direction and the true value is larger or smaller than any of our measurements.

When we speak of probability of impact, many people think it means that objects change their orbits or wander unpredictably. Actually, it means that there is unavoidable observational error in all our measurements. Measure the position by eye through a telescope, and one observer will place the crosshairs more to the right of the object than another observer. Measure the positions on a photographic plate and observers will still differ. Measure the positions digitally, and pixel responses on imaging chips will affect the results.

So when an object is first discovered (above), there are only a few measurements of its position and they don't span a very long distance. We calculate an orbit for the object, and very likely none of the observations lie exactly along the estimated orbit (recall only one of the hypothetical measurements above was equal to the average). There is a small probability that all the measurements were off in the same direction and that the true orbit is significantly different from the predicted orbit. In the figure above, the left-hand column of figures is the probability that the true orbit is within the colored zone and the right hand column is the probability that the true orbit is outside it.

If those outer, low-probability zones include the earth (above), the press gets very excited. Note, however, there's a probability well over 99% that the path misses the earth completely, and even if the true orbit is outside the blue zone, it still has only a small chance of hitting the earth. We are a small target.

The cure for this problem is more data. A quick cure is to find pre-discovery observations, observations caught on earlier surveys but not noticed (there is a huge amount of data in the world's astronomical images going back over 100 years - nobody has come close to extracting it all). It's frequently possible to find out the object was photographed months or years earlier, and that goes a long way toward eliminating uncertainty. If all else fails, we wait and gather more data and refine our estimated orbit.

The Torino Scale was actually devised after a couple of asteroid discoveries initially indicated a low probability of impact with the earth. To any astronomer, these figures meant the orbit needed to be refined, and until then there was no point in speculating, but to the media they meant that we might get hit. In every case, a few days' more observation eliminated any chance of impact. The nature of probability means that the Torino Scale juxtaposes very low values with very high ones. An event in the middle of the scale (possible impact) will either quickly be shown to present no danger and drop to the bottom of the scale, or be shown to be a certain impact and move to the high end.

As of March, 2006, there are a number of quite close passes predicted for objects but no predicted impacts in the next few centuries.

The Palermo Scale

The Palermo Scale is a somewhat different and more quantitative scale than the Torino Scale, which is meant mostly for public communication. The Palermo Scale was originated by Steven R. Chesley, Paul W. Chodas, Andrea Milani, Giovanni B. Valsecchi and Donald K. Yeomans. It is based on the fact that there is a background level of impacts of all sizes, frequent tiny ones and rare big ones. A predicted event that is the same likelihood as a random background event is rated zero on the scale. For example, if we detect a one meter object that will impact in a year or two, the odds are strong that some random object of that size will also hit during that same interval, so the predicted impact would carry a probability of zero. A step up indicates a ten times greater probability, down represents a probability one tenth as great. A one kilometer object that won't hit for ten million years might also be rated zero - the odds are equally good that some random impact of that size will also happen in ten million years.

If something is ten times more likely than a random impact, it gets a Palermo rating of 1, 100 times gives it a rating of two, and so on. Something only a tenth as likely as a random impact gets a rating of -1, and so on. The scale quantifies probability of impact, time span, and severity in a single scale.

For big objects, even negative ratings are worth paying attention to. There may be a bigger chance of a random event but the probability of the known object hitting is not negligible. This really doesn't seem like a very useful scale. Different numbers mean different things depending on the size of the impactor.

Averting the Threat

If it's an impact in the multi-megaton range or less, and in a sparsely populated area, we should probably just evacuate the people at risk and let the event happen. Marine impacts, unless they are close to inhabited coasts, pose risks only to ships in the immediate impact area.

On the other hand, if the impact endangers a population too large to evacuate easily, or threatens to involve a large area, or endangers irreplaceable cultural or natural treasures, we may want to try eliminating the danger. A few decades ago, the problem was simple: launch a bunch of nukes and pulverize the puppy. Now we know it's not so simple. Fragmenting a large solid object may result in multiple impacts instead of merely one. Many asteroids are probably loose agglomerations of fragmentary material weakly held together by gravity. Setting off a nuclear weapon on or in such an object may be as effective as punching a pillow. While the blast will be very effective in breaking up the object at close range, the mechanical energy from the blast will be rapidly dissipated passing through loose material.

Deflection, not destruction, is probably the more likely approach. If the impact is imminent and catastrophic, attempted destruction may be the only course, but if we have years of warning or more, deflection is the way to go. For one thing, deflection is a lot more predictable. We have no way of knowing how an object will break up or what the pieces will do, but we can apply thrust in controlled amounts and directions with predictable effects. Also, we only have to deflect the object by at most one earth radius (6400 km or 4000 miles), a tiny amount over the hundreds of millions of miles the object will travel before impact.

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Created 3 April 2006, Last Update 11 January 2020