Extraterrestrial Impact Hazard

You're probably all heard of some of these famous meteor and asteroid impacts and the craters they've left on Earth:

• The Tunguska impact in Siberia back in 1908, which flattened about 2,000 square kilometers of forest -- an airburst a few kilometers above ground

  

• Meteor Crater in Arizona, created about 50,000 years ago -- about 1.2 km across

  

• And, of course, the celebrity impact of some 65,000,000 years ago, at the Cretaceous-Tertiary boundary, which wiped out the last of the big dinosaurs and created Chicxulub Crater in Yucatán, Mexico, geological evidence of a massive tsunami running up the shallow sea embayments that at the time dominated much of the middle of the North American continent, evidence of wildfires triggered all over the world from re-entering impact ejecta comprised of molten rock. Sorry I couldn't resist this melodramatic image! For a detailed and very accessible summary of the science behind the discovery of this impact (about half a hour), check out https://www.youtube.com/watch?v=tRPu5u_Pizk, "The End of the Mesozoic," by HHMI BioInteractive. As if all that weren't bad enough, it is possible that the impact triggered such seismicity that the earth "rang like a bell" from the interactions of the body and surface waves, especially in the area antipodal to the impact. This would be enough to trigger volcanic activity in many parts of the world, but especially in the antipode, or area on the other side of the earth from the impact. The antipode at that time would have been in the then-island continent of India, in an area that for hundreds of thousands of years had experienced large flood basalt eruptions: the Deccan Traps. While the Chicxulub impact would not have triggered the initial flood basalt volcanism of the Deccan Traps, geological evidence and argumentation suggest that the impact did result in three huge accelerations in these flows: the Poladpur, Ambenali, and Mahabaleshwar flows that account for some 70% of the main Deccan activity. And the current sense of the research community is that the dinosaurs were done in by a double blow: the Chicxulub impact and the intense outgassing associated with massive volcanism.

  

There's a growing recognition that these are not just weird events that come in and make a mess of things --

• They are an integral part of the earth's history
• They constitute an important connection between Earth and the solar system of which it is a part and with which it still interacts
• and a repeated influence on the development of life forms here

Physical dynamics

Such impacts reflect the ongoing evolution of the solar system

Quickie Earth history (this is optional but it does form an interesting background to the ongoing extraterrestrial impact hazard: I've set it off in blue if you want to fast-forward):

• The Milky Way is the galaxy to which the solar system belongs
  • It is one of some 100 to 200 billion galaxies (there are some estimates based on extrapolation from the Hubble Ultra Deep Field experiment running as high as 2 trillion)
  • The Milky Way itself consists of
    • 100 - 400 billion stars
    • The interstellar medium
      • Gases
      • Dust
      • Assorted cosmic rays (electrically charged subatomic particles)
    • Dark matter (invisible but exerting gravitational influence, consistent with it being >90% of the mass in the galaxy)
  • All locked in a dance between
    • Attractive force of gravity
    • Various expansive forces
      • Pressure (motion of gasses, interference)
      • Magnetic fields
      • Rotation and its centrifugal force
  • The Milky Way is close to as old as the Universe (which is ~13.8 billion years old)
    • It has seen generations of stars come and go, and our sun is thought to be part of the third generation
    • The oldest ones were made of nothing but hydrogen and helium
    • Nuclear fusion within these ancient stars toward the end of their lives created
      • Oxygen
      • Carbon
      • Iron
    • Most of the carbon and iron stayed with these primordial stars when they died and became white dwarves and slowly cooled off -- there are probably all sorts of black lumps of cold carbon and iron out there
    • The most massive stars (> 4 x Sol's mass) die more dramatically
      • They go supernova: a humongous explosion
      • During the seconds of initial explosion, other elements in the periodic table are created
      • The larger the star, the more elements on the periodic table can be created in that final paroxysm
      • The explosion spews this newly created stuff, and the carbon and iron, into space as dust
    • The sun formed from a cloud of gases enriched by these minute remnants of ancient supernovae -- a molecular cloud -- the sun, Earth, and each of us, then, are composed of ancient stellar material released during these explosions -- we are truly stardust
    • The part of the interstellar medium that became the sun and solar system would normally not come together due to pressure -- the collisions between gas and dust molecules disrupts the tiny gravitational attraction in the molecular cloud
    • Something or other caused the local concentration to increase to such an extent that the collective gravitation would be greater than the internal pressure, turning the molecular cloud into a protostar
      • Maybe a gravitational shock wave, like the spiral arms of the Milky Way, passed through here some 4.5-4.6 billion years ago
      • Maybe another star passed nearby, creating instabilities in the molecular cloud
      • Maybe a massive star went supernova in the neighborhood
    • Once internal pressure could no longer resist the attractive force of gravity within this cloud of material, it began to contract of its own accord and rotate
    • Contraction of a rotating object increases its rate of rotation due to the conservation of angular momentum and set up a slightly offsetting centrifugal force, which shaped the cloud of material, or primordial solar nebula, into a disk shape
      • About 10 billion km in diameter (out to Neptune)
      • Nearly 200 million km thick (> 2x Earth's orbit)
      • Twice the mass now included in the solar system (half was scattered into outer space The Final Frontier in something called the T-Tauri wind, an explosive outpouring of stellar material once nuclear fusion initiates in the now drastically compressed core of the new star and blasts out its outer layers, which sweep the primordial solar nebula of any finer gasses and dust not yet accreted into a larger object, such as a planet).
    • Contraction also changed the temperatures in the nebula
      • Compression increased temperatures at the center
      • Temperatures on the perimeter stayed pretty cold
      • The reason this is important is that different elements can condense out of the gas state at different temperatures
        • Close to the protosun, only iron and nickel can condense into little dust grains
        • A bit farther out, various rocky compounds can condense into dust: iron silicates and various metal oxides
        • Still farther out, water and various other ices can form, sublimating onto the dust grains out there
        • The result, then, is a stratification of chemical composition by the temperature gradient in the primordial solar nebula
      • The reason THIS is important is this debris is the stuff out of which the planets and other objects in the solar system are made
        • The inner planets are rocky, with iron cores that roughly increase in proportion with declining distance from the sun (Mars' core is ~40% of the planet's radius; Earth's is ~55%; Venus' is ~50%; and Mercury's is ~75%)
        • The outer planets are gas giants
    • So, out in the primordial disk, these condensed grains, floating about in orbit around the protosun, sometimes bumped into one another and stuck together via gravity or certain chemical surface interactions and then settled together along the midline of the solar nebula
      • This sheet of clumps had random irregularities in distribution and density, which were then magnified by mutual gravitation
        • Clumps wandered toward areas with lots of other clumps
        • They then packed into even larger objects, about asteroid-sized, called planetesimals
      • Planetesimals close to the protosun were nearly altogether rocky; those farthest out also contained frozen methane, ammonia, water
    • This process kept repeating itself at larger and larger scales, until the planetesimals, through repeated collisions and fusion, coalesced into still larger objects, called protoplanets, which internal gravity then shaped into spheroids and ultimately internally differentiated them by density (cores, mantles, crusts in the terrestrial planets, for example).
    • This process, for Earth and for the solar system in general, has been dated through a variety of methods to around 4.54 billion years ago + ~50 million years ago.
    • Once accreted and differentiated, some of these early planets even smashed into one another!
      • Earth was struck by a Mars-sized planet called Theia very early in its history, between 20-100 million years of its formation!
        • Theia apparently formed farther out and was ice-encrusted and may have been the source of a lot of Earth's water
        • The impact shattered, pulverized, and vaporized all of Theia and probably much of Earth's mantle and crust, giving Earth a Saturn-like ring system for a while
        • This debris of mixed Earth and Theia ejecta material eventually coalesced into our rather large moon, giving our planet's axis a great deal of stability through tidal interactions (probably A Good Thing for the evolution and maintenance of life!)
      • Mars, too, may have been struck by a large impactor near its north pole, which blasted a lot of material off the planet and gave it its odd shape, with the northern third of the planet drastically lower in elevation than the southern two thirds ("the great dichotomy")
    • Configuration of our solar system, thus, took quite an adventurous path to settle down:
      • Four inner "terrestrial" planets (Mercury, Venus, Earth, and Mars)
      • Four much huger outer protoplanets (Jupiter, Saturn, Uranus, and Neptune) dominated by hydrogen, helium, and ices of water, ammonia, and methane, and these went through drastic gravitational re-arrangements among themselves until they settled into a stable pattern of mutually resonant orbits
        • If you're curious about the drastic changes among the outer solar system back in the day (with Jupiter, Saturn, then Neptune, and Uranus bunched up much closer than the group is now, Jupiter moving toward the Sun, Saturn saving its cookies, and Neptune getting flung past Uranus, you can do a search on the "Nice" model (neese, not nice) or you can get a really "nice" short synopsis at Britannica:
          https://www.britannica.com/video/186481/discussion-model-Nice-formation-planets-structure-belt
        • The re-arrangement of these giant planets perturbed a lot of small planetesimals, asteroids, and comets out in the Kuiper Belt and sent a boat-load of them hurtling into the inner solar system somewhere around 4.0-3.7 Ga (gigayears or billions of years) ago: This was the Late Heavy Bombardment
        • And it was the LHB that caused a whole new shooting gallery, with every inner solar system object getting smashed up the wazoo by impactors, leaving their craters behind as evidence
        • Looking at Mars, Mercury, and our own moon (and Mars' tiny moonlets), you can see evidence of the LHB
        • Earth and Venus, not so much: It happened just as badly to both of them but geological activity hid them from view:
          • Earth alone of these has plate tectonics and that acts to subduct ancient terrains and their cratered surfaces: There are very few surfaces left on Earth that are older than ~3.6 Ga
          • Earth also retains a lot of water and a very active hydrological cycle, which grades the surface and, again, hides surface expressions of all but a few craters
          • Venus underwent some kind of massive, planet-wide volcanic resurfacing about 700 million years ago, which, besides hiding any ancient, crater-hammered landscape under great outpourings of lava, destroyed what may have been a habitable world complete with oceans in a runaway Greenhouse Effect. To learn more about what happened to Venus, including a short, clear video, head over to Universe Today's page, "Ancient Terrain on Venus Looks Like it Was Formed Through Volcanism" by Matt Williams, at
            https://www.universetoday.com/148000/ancient-terrain-on-venus-looks-like-it-was-formed-through-volcanism/. The embedded video clip by Fraser Cain of Universe Today is entitled "Venus Could Have Supported Life for Billions of Years."
      • And then there's the Pluto-Charon rocky/icy system (now called a dwarf planet or Kuiper Belt object, demoted from full planet status) and a whole bunch of other, similar objects even farther out than Pluto/Charon

The important point of the history of the solar system is that there are still many of these smaller objects in the solar system and they continue to plow into the larger objects, such as Earth

• The most famous recent example in recent decades and, in fact, the first collision of two solar system bodies actually to be observed was the collision of Comet Shoemaker-Levy into Jupiter in July of 1994 (see http://www.jpl.nasa.gov/sl9/)
• There are two basic categories of potential impactors of relevance to the hazard under discussion here, which are collectively called Near Earth Objects or NEOs
  • Earth-crossing short-period comets (the ones that have return intervals ≤200 years) or NECs (Near Earth Comets)
  • Potentially hazardous asteroids or PHAs (aka Near Earth Asteroids or NEAs), which can be further divided into four subclasses:
    • Amors, which are minor planets with orbits between Mars' and Earth's: These could be perturbed into crossing Earth's orbit
    • Atiras, which have orbits between Earth's and Venus': These, too, could be perturbed into crossing our orbit
    • Apollos, with orbits that DO cross our orbit twice a year. These have semi-major axes (the long side of the orbital ellipse) longer than Earth's
    • Atens, with orbits that DO cross our orbit twice a year like Apollos, but with semi-major axes shorter than Earth's
• Just because they do or potentially can cross our orbit does not mean that they do so at just the moment that Earth is occupying that spot in its orbit, so you can take a deep breath!
• If such objects do enter the Earth system, they may or, more usually, may not actually hit the planet. They do get a name change, however:
  • Comets, asteroids, and small asteroids (<100 m) known as meteoroids are the objects moving through the solar system before they encounter the Earth system
  • The ones that graze Earth's atmosphere and light up with the heat of ablation are called meteors during their trip. Most of these vaporize completely in the atmosphere.
  • Those of metallic or rocky composition that are large enough to make it all the way to the surface are called meteorites when found on the surface
  • Truly large ones that do strike the surface and excavate a large crater are often shattered and pulverized, if not vaporized, by the tremendous compression and dilation of their instant deceleration. Their constituent materials and the shattered and pulverized Earth surface material at the point of impact are lauched upward and outward in an ejecta curtain and larger pieces in the curtain may fall and create secondary craters away from the primary crater.
• Sources of meteorites that are recoverable:
  • They can be primordial bits of space debris left over from the early phases of the solar system: Study of these can offer valuable clues to the nature of the primordial solar nebula and primitive solar system
  • More commonly, they are fragments of comets or asteroids that broke up in the solar system -- these are the ones that form meteor showers, e.g., Taurid meteor showers from October-January Perseids
  • Indeed, some of them are fragments of other inner solar system planets and moons that were struck with such force by some impactor that these chunks were launched at velocities that allowed them to escape their home planets' gravity and orbit the sun on their own, until they were rudely interrupted by Earth.
    • There are quite a few known Mars meteorites here, about 70 (including a fall in Los Angeles County's Mojave Desert back in 1980!).
    • More than 350 lunar meteorites have been identified, probably from 30 separate falls.
    • And there is ONE known meteorite from Mercury that landed in Morocco in 2012 (giving new meaning to the old Ford Motor Company's Mercury Meteor cars from the 1960s -- I had a 1966 Mercury Comet!).
• With their variety of origination, these projectiles range substantially in density. For the smaller ones (i.e., under a few hundred meters in diameter), what determines the fate of a meteoroid as it plunges into the earth's atmosphere are its density, uniformity, strength, velocity, and angle of incidence (low angles with the horizon mean a longer trip through the atmosphere)
  • Meteoroids are hauling at anywhere from 10-11 kilometers per second up to 72 km per second before encountering Earth's gravity, which accelerates them even faster. Such velocities translate into kinetic energy that far exceeds an equivalent mass of chemical explosives (such as TNT).
  • When they encounter the Earth's atmosphere, friction begins to slow them and that tremendous kinetic energy has to translate into something else: heat. This heat begins to vaporize, melt, and ionize the surface of the meteor (ablation) and the surrounding atmosphere is heated and ionized, too. This creates a brilliant light many times the diameter of the object and a long tail behind (the "shooting star").
  • The increasing air pressure the descending object meets on its way down creates terrific compressional forces, too, and these may exceed the strength of the object's material and it begins to fragment. This ablation and fragmentation tend to destroy most smaller objects by 50-80 km above ground. Whew!
  • The less dense and more weakly structured ones are, thus, likelier to break up and ablate and never make it to the surface
  • Denser, larger, and more coherent ones can survive the trip
• Past a certain size (about 25 m/82 feet in diameter), these physical characteristics become increasingly meaningless
  • The objects WILL survive the trip
  • Of greater importance is the energy of their impact, which is a function of their mass and velocity (force = mass times acceleration, or in this case extreme deceleration on impact)

As with many of the other hazards we've examined this semester, there is an inverse relationship between the energy yield of an impactor and the typical impact interval: the familiar frequency and magnitude concept

• Little, bitty guys (< 3 m in diameter, with under, oh, 3,000 metric tons of TNT equivalent energy) might come in an average of one per month
• Tunguska-scale guys (maybe 50 meters and maybe 30 megatons of TNT equivalent energy) might be expected, on average, about every 3 centuries or so
• The Chicxulub impactor, somewhere close to 10 km in diameter and releasing some 100 million megatons of TNT equivalent energy, is extremely rare, with an average frequency of 100 million years
• It should be understood that the specific form of the magnitude-frequency curve shown here may obscure a potential clumping in time of impacts, given that many of these objects result from the breakups of comets or asteroids
• Most of the larger Near Earth Objects that pose potential hazard are known (those larger than about 1 km in diameter), but we know about smaller and smaller proportions of the potentially hazardous objects the smaller they are: We often learn about them less than a week in advance because they are so small and so dark.

Forms of damage to be expected from a major impactor (at least 50 m for rocky objects and at least 100 m for the looser cometary objects), say, about the size of Tunguska

• If it explodes within the troposphere (say, within 20 km), the resulting airburst can be highly destructive, enough to be significantly hazardous in a wide area (up to a few hundred kilometers away from the point directly below the explosion): a major local or regional event, as Tunguska was. Less spectacular but still quite damaging was the fireball that exploded about 25 or 30 km above Chelyabinsk, Russia. Look that one up: Many people caught it on cell-phone videos and uploaded them. Here is an image from the Russiatrek blog:

  

• A larger or stronger object might make it to or near the ground and have a kinetic energy release as great as that of a nuclear bomb of the same energy potential (minus the radioactivity): again, a local or regional hazard
• A really huge one (like the Chicxulub one) that hits the ground:
  • Huge crater (Chicxulub is at least 200 km in diameter)
  • Wildfires, which may ignite thousands of miles from the impact as the heat flash races around the earth and, more importantly, as extremely hot ejecta begins to rain down across a wide radius of the planetary surface
  • Fine dust (Chicxulub is believed to have sent some 10 million billion kilograms of dust up into the stratosphere), which would darken the entire planet for as many as several months and cause thereby the virtual collapse of most terrestrial and marine ecosystems for want of photosynthesis, as Chicxulub did:
    • Even a smaller event could drop human agricultural production enough to induce massive famine even in the developed world (societies do not store even as much as one full year's food supplies)
    • An agricultural disaster could predictably result in collapse of the global economy, breakdown of social norms, and the destruction of political structures (think of what the Great Famine did in Europe in the early 14th century)
    • This dust, then, could lead to massive biocide in the worst case scenario or the deaths of more than a quarter of the human population and destruction of modern civilization in the more optimistic scenarios, depending on the size and velocity of the terrestrial impactor and where it hit
• Those major impactors that hit the oceans pose the danger of tsunami
  • The geographical scale of destruction is greater for an ocean impact than a land impact
    • Area is a direct function of impact energy and depth of ocean
    • A hazard arises insofar as that area of destruction encompasses a populated coastal or island zone
  • The height of the tsunami out at sea depends directly on:
    • The impact energy of the impactor: Composition (density, mass, strength) of the impactor Size of the impactor Velocity of the impactor
    • The depth of the ocean it falls into
    • So, in deep water (e.g., 4 km deep):
      • A 10 m radius iron impactor might raise 0.1 m wave
      • At 100 m radius, it would be about 5 m
      • At a 1000 m radius, we're looking at 200 m!!!
  • But remember that a tsunami runs up on encountering a continental shelf. Remember from the earthquake wave discussion, waves increase their amplitude or height as their velocity drops, and hitting shallow waters slows ocean waves (including tsunami) down. This issue was also brought up in the tsunami lecture.
    • The run up is greater the shallower and broader that shelf is
    • It can be over an order in magnitude -- so multiply those ocean-going wave heights by 10-20 (assuming deep water/runup wave height relations hold at these massively greater sizes)
  • Another consideration is the degree to which the wave gets inland. The maximum distance inland depends on:
    • Runup height at the shore
    • Slope of shore away from the coast
    • Roughness of terrain that the water traverses
    • Examples in fairly flat, smooth terrain:
      • A 100 m tsumani (yow!) would travel 22 km inland
      • A 200 m one could get 55 km inland
    • These could create unprecedented damage in such places as Long Island, Delaware, Florida and totally submerge entire countries, such as Holland or Denmark or Bangladesh

How many of these things are there "out there"?

• Estimated number of Near Earth Asteroids with diameters of at least 1 km is 890 (as of today). There are an additional 4,444 NEAs in the 300-1,000 m range. To learn more about, check out NASA JPL's Center for Near Earth Object Studies at https://cneos.jpl.nasa.gov/. Additionally, there are about 800 Earth-crossing short-period comets.
  • The great majority of these known objects are inner solar system asteroids, with their slower speeds and fairly predictable orbits, and active near-Earth short-period comets
  • Long period comets are not as well-known, because there are fewer observations for these things, with their long periods
    • There are two sources of primordial cometary debris that can lobb comets into classical cometary orbits, but we can't predict these orbital disturbances:
      • Oort cloud out about 20,000 AU (source of long-period comets; AU means Astronomical Unit or the Earth-Sun distance, which averages roughly 150,000,000 km or 93,000,000 mil)
      • Kuiper belt starting out roughly at Neptune's orbit (source of short-period comets)
    • These may be disturbed by gravitational interactions with passing stars, clouds, ?
  • Then, there are extinct comets, those that have outgassed their ices or fine materials (that create detectable tails): it's estimated that they might add up to another 25 percent to the number of potential impactors
• So, the probability of impacts by such objects at least 1 km in diameter in any given 10 year period is:
  • about 0.00000167 or one impact every 60,000,000 years
  • So, a very tiny probability of something with absolutely epochal effect, of existential threat to the human species and much of the biosphere. This is not the kind of situation humans are particularly good at thinking through for disaster planning and emergency response purposes!

Social dynamics

So, are such dinky probabilities worth action by government agencies -- such as development of a deflection capability?

• REALLY tiny probabilities
• REALLY extreme effects
• For the smaller, less globally-affecting objects, a government needs to adjust these tiny probabilities by the percentage of the earth occupied by that country as a target:
  • US is 1.9% of the total area and 6.4% of the land area
  • Russia is 3.3% and 11.5%, respectively
  • ESA member nations is 0.5% and 1.7%
• Only currently space-faring nations are able to even try doing anything about the impact hazard -- will they commit their own resources to defend what will likely be someone else's territory against these low probability events (smaller objects)?

Now, we're talking pretty expensive programs here

1. Specify the number of potential impactors, their sources, their trajectories, the effects of various masses (gravitational fields) on their trajectories -- really basic science of prediction
   • So far, we know of at least 26 NEA > 5 km; 425 > 1 km; and 1,149 > 140 m (JPL Small-Body Database Search Engine at https://ssd.jpl.nasa.gov/sbdb_query.cgi -- note that searching this database doesn't square with the numbers on the JPL CNEOS site).
   • So far, we know about a couple dozen active EC comets, but we have no idea how many extinct cometary lumps there are out there -- we may know of as few as 5-10% of them, given how much harder they are to detect (darkness and veloc- ity)
2. We need to know more about their compositions!
   • Impact energy = f(composition, strength, density)
   • We need more of this kind of information to know how to go about deflecting them
   • To get these data, we need a few rendezvous missions or take every opportunity to encounter these objects in the course of other missions (Cassini rant here: missed opportunity because of Congressional penny-wise budget- cutting which wouldn't allow the Cassini-Huygens mission to study asteroids en route to Saturn during its 1997-2004 cruise there) -- very expensive
3. Very expensive deflection programs:
   • Strategies available with current technology:
     • Interception with surface explosion beside potential impactor to give it some transverse velocity = deflection
       • Downside is the tight timing precision needed
       • Almost certainly must be nuclear
         • Interceptor weight is 3 orders of magnitude less, and weight affects launch logistics and cost
         • Nuclear explosion, though, gives you a little latitude on precision
     • Penetration to create a crater, the blow-off of which would impart the necessary transverse velocity
       • Timing less critical than simple aim
       • Relative velocity of missile and object should easily get missile buried appropriately, assuming nuclear explosive is protected by a billet (which adds weight, though)
       • Downsides:
         • Explosion could take place so deeply that it just creates a useless interior cavity with no effect on trajectory (so much for the whole plot of the movie, "Armageddon"!
         • Could fragment the object into lots of projectiles, many of which might hit Earth (a major reason we need to know more about these things' internal structure, composition, density, strength)
         • The shallower the penetration, the more effective the blast -- so, penetration really offers little advantage over a side blast unless your aim is to pulverize it (when deflection is really all that's needed)
     • Stand-off deflection/slow push
       • Mitigates fracturing: Instead of a crater being blown-off and creating a sidewise velocity, the entire surface on the side of the explosion loses a thin layer to create the blow-off (less internal shear stress)
       • This can be done in a variety of ways, some not particularly mature in development (rendezvous to aim laser or focussed solar energy on one side to boil off mass; position spacecraft to exert small amount of gravitational attraction as a kind of gravity tractor; using a spacecraft to connect to object and tug it enough to deflect it)
       • Downside: much less efficient, not well-developed

   • How expensive are all these? I'm going on projections dating back to 1994, in the T. Gehrels (ed.) Hazards due to Comets and Asteroids (Tucson: The University of Arizona Press), which was the definitive anthology for bringing this hazard to wider attention -- and no-one has done a comparable fiscal nuts-and-bolts analysis that I could find.
     • Detection
       • Costs of constructing and operating a system of telescopes on Earth or in orbit for detection
         • Affected by number, size, and placing of detectors
         • Affected by the size of objects looked for and the distance being searched for them
       • Examples:
         • Going out, say, 1 AU looking for objects 1 km in size adds up to about $200,000,000, as would looking just for 10 km objects but going 3 AU out
         • Going out 3 AU looking for 1 km objects gets you up to $20 billion, as would looking for 0.1 km objects only 1 AU out
     • Deflection/displacement costs -- estimated to run about $50 million per ton of interceptor final mass, and that mass is anywhere from 0.5 metric tons (to yield 1 megaton of energy output) to 25 tons (to get 100 MT)
     • So, factoring in both detection costs (which increase with increasing distance and decreasing size of objects) and deflection costs (which increase with size of object), one winds up with systems costing about:
       • $10 million for 0.1-0.3 km objects with 0.01 years warning (3-4 days)
       • $100 million could detect and deflect 1 km objects with 0.1 year warning (over a month)
       • $1 billion could detect and deflect 3 km objects with half a year's warning

   Is all this cost-effective?

   • Well, an impactor big enough to have global impacts could create inconceivable amounts of economic damage (let alone the costs in human suffering and ecological mayhem). Global economic output estimated at $86 trillion (this is a 2019 update from Visual Capitalist: https://www.visualcapitalist.com/the-86-trillion-world-economy-in-one-chart/). If disruption were global and lasted about 20 years, the loss would be nearly two quadrillion (if anyone is left to count!). Detection/deflection would be cost-effective against that
   • But:
     • You have to consider the probability of this happening: $1.72 quadrillion times 0.00000167 average/year
     • This could still be cost-effective (~$2,875,000 budget per year spent on detection and developing credible deflection options)
   • On researcher, Gregory Canavan, however, argues that detection AND deflection would be cost-effective only for objects up to ~8 km in diameter
     • Above that size, getting into Chicsulub territory, he opts only for detection
     • This would be coupled with a frantic effort to store up enough food to get the predicted potential number of survivors through a 20 year period of ecological collapse
     • Something not touched on is who the survivors would be, the elite in their inbred entirety or a more representative array of human genetic material to give the human species a wide array of genes to draw on in the very different natural selective environment after a catastrophe like this (and, in either case, who decides?)?

   Plot complication: how cost-effective would all this be when you compare dollars per life saved against expenditures mitigating other hazards? How does society balance competing hazards needs?

   • How effective is each dollar spent on these programs when the same dollar spent on overcoming, say, preventable childhood diseases?
     • 1.5 million kids die each year for lack of vaccinations for diseases which we already have but can't (or choose not to) get to those kids and which could be provided for a lot less than impactor detection and deflection
     • And now, how about the global cost of meeting the pressing need to get COVID-19 vaccines out to all human beings right now?
   • How effective is each dollar spent on this particular hazard compared to the costs of educational and legal programs to get people to wear seat belts, install smoke detectors, reduce exposure to carcinogens, and, now, wear their masks, wash their hands, and socially distance (and what about the effectiveness of that public education campaign)?

   Further plot complication: to get a society to do anything about a hazard, including pressuring their politicians to pony up a lot of their money, requires a pretty massive education and sensitization program.

   • Think how ineffective flood, earthquake, hurricane, and chaparral fire hazard education, zoning, insurance have been -- and we've seen that that has come a cropper in dealing with COVID-19!
   • Extraterrestrial impactors are not yet on the public's radar to anywhere near the degree of these other hazards (though that may have changed slightly with the "Deep Impact" and "Armageddon" movies, but whether that change lasted much past 1998 is debatable)
   • Small upticks in public interest are created by asteroids that pass within lunar orbit distance, such as 2020 VT4 that passed within 400 km (˜250 mi.) of Earth on Friday the 13th of November 2020!!! Mind you, it was only 5-10 m across but, still, this was the closest known pass on record. Like so many of these things, it wasn't discovered until on Saturday the 14th, AFTER it made its passage. Another object, 2020 QG, had previously broken the "too close for comfort" record earlier this year, passing not quite 1,900 km (3,000 mi.) from the surface of the earth on the 16th of August. By way of context, the Moon averages not quite 400,000 km (250,000 mi.) from Earth.

   So, given the nature of this rare hazard (a hazard so rare that Homo sapiens has never experienced it during our time on Earth), the political economy of societal hazard response, and the quirks of human perception, it would seem that a standing defense against asteroid and comet impacts may be impractical

   • This is a very rare hazard, with a very long recurrence interval
   • The recurrence interval is longer than human civilization, so there really is no cultural memory of the severity of the hazard (kind of like how there was no cultural memory or legends describing tsunami in the Indian Ocean basin to help people know what to do during the Boxing Day 2004 Sumatra earthquake and tsunami or how Pompeii and Herculaneum had no cultural memory of the Avellino eruption of Vesuvius)
   • As if that weren't bad enough, many astronomers and planetary scientists despair that discovery of an object headed for Earth cannot be made in a timeframe usable for mounting a deflection mission.
     • Such missions would require detection 6-18 months before impact
     • The orbits they take are subject to slight alterations because of interactions with planets and other objects in the solar system, which make Type I and Type II errors more likely (and a Type I error that cost society a huge amount of money would result in Boy Who Cried Wolf lawsuits!).
     • Again and again in recent years, fairly sizable objects have not been detected before a day or two, when they're just about to zip by Earth or between it and the moon, as we just saw on Friday the 13th!
   • Governments in general are under constant pressure to respond to immediate crises
     • These crises command lots of resources in the short-term, perhaps appropriately, in terms of dollars spent and lives saved through these other competing programs
     • Few resources are available in such a competitive environ- ment for unprecedented massive scale but very rare and infrequent hazards
     • Possibly the military may latch onto this as a self-serving way of justifying bigger budgets in the post Cold War/post Iraq War era, but even they may emphasize shorter term and serious crises, such as nuclear proliferation, chemical and biological warfare, and terrorism, as a likelier way to enhance those budgets (there's an interesting article that made just this point:

       Mellor, Felicity. 2007. Colliding worlds: Asteroid research and the legitimization of war in space. Social Studies of Science 37, 4: 499- 531. doi: 10.1177/0306312706075336. It's available from
       http://sss.sagepub.com/content/37/4/499.refs or from the CSULB Library electronic collection.

   • Citizens may get interested in this, but this interest, if present, will likely endure for just a short while compared to the recurrence interval of the hazard -- we are trying to imagine preparing for a hazard that may hit long after the United States is but a faint cultural memory
   • Basically, this is a deep-time hazard, and humans do not handle deep time too well. A fascinating book on the subject of human response to risk communication in deep time is a book by Gregory Benford, Deep Time: How Humanity Communicates across Millenia (Harper Perennial).

   For more information on this vanishingly small probability but extreme consequence hazard:

   Gehrels, Tom, ed. 1994. Hazards Due to Comets and Asteroids. Tucson and London: University of Arizona Press

   URLs:

   • https://cneos.jpl.nasa.gov/ NASA-JPL's Center for Near Earth Object Studies
   • https://newton.spacedys.com/neodys/ ESA-sponsored Near Earth Objcts Dynamic Site 2
   • https://www.asteroidanalytics.com/resources/ List of research and advocacy organizations and data sources
   • http://simulator.down2earth.eu/planet.html?lang=en-US Impact simulator
   • http://www.jpl.nasa.gov/sl9/) Comet Shoemaker-Levy hitting Jupiter in 1994