Asteroid Apocalypse

Back in 1908 - just yesterday, geologically speaking - a huge blast occurred over a swampy area near the Lower Stony Tunguska River. In case you forgot from high school geography class, this is a minor river in Siberia, about 40 miles from Vanavara (pop. 3090). The explosive force of the Tunguska Event has been variously calculated but agreed to be in the ballpark of an H-bomb, perhaps around 10 megatons (a megaton is the energy released by exploding a million tons of TNT). It was not a H-bomb however, but evidently an asteroid tens of meters (perhaps as low as 20) in diameter, that crashed into the atmosphere at high speed and exploded. 

I say "crashed into the atmosphere" because asteroids that approach Earth most commonly do so in the neighborhood of 20,000 miles an hour. Suppose you have a 10-mile commute that takes 20 minutes when traffic is reasonable. At 20,000 mph it would take just under 2 seconds, which sounds nice until you consider the side effects. Ever stick a hand out of the car window to feel the air resistance at highway speeds? It pushes surprisingly hard against your hand considering it's just air (try it and see). Physics tells us that drag increases proportionally to the speed squared, and 20,000 mph is a little over 300 times faster than 65 mph. That means the drag experienced at asteroidal velocities is about 300x300 or 90,000 times the drag your hand feels at 65 mph. Let's put that 90,000x into perspective. If your hand experiences 1 lb. of drag when you stick it out the window at 65 mph, it would experience 90,000 lb. of drag were you to try commuting by asteroid. Your hand would be instantly destroyed. That is why drag also destroys most asteroids in the air before they hit the ground, quickly converting their energy of motion mostly into heat. In other words they explode. The Tunguska event could easily have destroyed a city, but luckily it exploded over a remote, forested area and killed mostly trees, though several hundred square miles worth.

Powerful though it was, the Tunguska event was just a fire cracker compared to what can happen. Bigger asteroids can resist complete atmospheric breakup long enough to blow up on contact with the ground. The result is an impact crater, which can blast enough pulverized rock into the atmosphere to change the climate dramatically, perhaps for months, over the entire world. One such impact killed the dinosaurs 65 million years ago, when asteroid Dinolith slammed into the ground near Chicxulub town on the Yucatan peninsula, in Mexico. It has been argued, however, that some dinosaurs survived and live on today. We call them birds (some of these dinosaurs we call "hummingbirds").

A much more recent and, thankfully, smaller one landed only 49,000 years old. About 3/4 mile in diameter, its crater is billed as "the world's best preserved meteorite impact site just minutes from Interstate 40." In Arizona, it is owned by the Barringer Crater Co., perhaps the world's only legitimate crater company. There are some other crater companies that will sell you craters and other land on the moon and other extraterrestrial getaways. However under established international law they do not actually own what they are selling, and so can hardly be considered legitimate. Other companies sell merely certificates of ownership to extraterrestrial craters and such, not the craters themselves, leaving it to the possessors of the certificates to try to enforce their claims of ownership. Those companies may be legitimate, but are really certificate companies, not crater companies. For those who feel that priceless and unique geological treasures should not be privately owned, even the Barringer Crater Co. is not legitimate, leaving the world without even one legitimate crater company. In any event, this crater is also unique in being the world's most plainly named crater: its legal name is "Meteor Crater." It is now marketed as a tourist attraction, not having the billion dollars worth of buried iron that old man Barringer hoped for when he acquired it in 1903.

The largest impact crater on Earth known with certainty is the Vredefort crater in South Africa at about 170 miles across. It was crated when asteroid Archaeoaster ("ark-ee-oh-aster"), 3 to 6 miles in diameter, slammed into Earth just over 2 billion years ago, long before the first dinosaur stepped out of its eggshell. The largest in the U.S. is Chesapeake Bay on the coasts of Maryland and Virginia. Impact craters on Earth tend to weather to the point that it is not obvious to the naive observer that they are craters. On the other hand, the moon is pockmarked with numerous and well-preserved impact craters easily visible through telescopes, because erosion from atmospheric winds and surface liquids does not happen there.

It makes sense that larger bodies tend to experience more impacts. Just the fact that they take up more space and hence intercept more potential asteroid orbits helps explain this. Additionally, larger bodies have more gravity, which tends to suck in nearby asteroids, the poor little suckers.

The largest body in our neighborhood is the sun. Thus one might expect comparatively frequent impacts. In fact on June 1 and 2, 1998, two comets did crash ito the sun, followed shortly by a huge eruption, a "coronal mass ejection" much bigger than the entire Earth, but thought to be unrelated to the comets.

The second largest body in our solar system is Jupiter. In July, 1994 comet Shoemaker-Levy 9 plunged into the planet in over 20 pieces covering a period of several days. Comets are a lot like asteroids, but are icy while asteroids are not. The impacts left dark markings easily visible through telescopes, and emitted bursts of microwaves, x-rays, far ultraviolet, infrared, and radio waves.

At this point it is worth defining a few closely related terms (see http://www.jpl.nasa.gov/multimedia/neo/spaceRocks.html for supplementary multimedia on this). Asteroids (from aster + oid, literally, star-like) are rocky or iron planetoids, especially those whose orbits are inside Jupiter's orbit. Meteoroids (from the Greek meteoron, airborne thing; originally from meta, beyond; and eora, hovering) are smaller than asteroids, up to the size of a boulder but often smaller than a grain of sand. Particularly small meteoroids are called micrometeoroids or cosmic dust particles. If meteoroids are things high in the sky, meteors are meteoroids that have entered the atmosphere and are in the process of burning up - shooting stars! Left over pieces from meteoroids that fall to the ground are called meteorites (-ite means rock or mineral). Micrometeoroids don't burn up on entry, scientists have found high quality sources of micrometeorites in polar ice and snow, and you can actually collect micrometeorites yourself from roof runoff and other sources using a magnet, paper, and microscope.  

Comets are like asteroids except they formed farther away from the sun where it is colder, and thus contain easily vaporized substances like ice. They can leave a trail when they get near the sun and the sun heats them up and starts burning off the ice and such. Finally, a bolide, to astronomers, is a particularly bright meteor - a fireball. To geologists it is something else, an asteroid or comet that crashed and left a crater.

What can we say about impact events that have not happened, but perhaps will? A tunguska-scale event could knock over another 80 million or so trees - or destroy a city. Estimates of the frequency of such events range from an average of every couple hundred years to every few thousand. The fact that one happened barely 100 years ago argues for the low end of the range. Chicken Little may have had a point.

The bolide that created Meteor Crater in Arizona was much more destructive than Tunguska, carving out a large crater, vaporizing itself on impact and then literally raining iron on the surrounding area. Luckily impacts of that magnitude are considerably rarer. But they happen.

The mother of all impacts, however, leaves no crater today. When Earth was young, a gargantuan collision with a huge asteroid is thought to have occurred. It smashed into our planet at high speed, tearing out a huge mass of rock, and with tremendous force hurling it into orbit. From this coalesced the moon! Does the Earth bear any scars of this cosmic childhood apocalypse? Perhaps unusual minerals formed during those moments, or currents of molten rock formed at that time and still present deep below the surface? Could such currents help explain modern facts like the "hot spot" under the Earth's crust at Yellowstone National Park, which erupts every several hundred thousand years or so as a supervolcano, devastating large swaths of the United States? We still do not know.

We don't expect a moon-creating event in the future in our now-mature solar system. But NASA's near-Earth object (NEO) program is steadily finding out more about what future events are possible. For example the NEO found that on Friday April 13, 2029, asteroid Apophis will zoom by closer than 28,000 miles from Earth. Were it to crash it would release 510 megatons of energy. This is just over 10x the power of the largest thermonuclear device ever tested, the 1961 Russian weapon test known as Tsar Bomba. It is also about 50x more powerful than the Tunguska asteroid event. If it landed in a large urban area it would, therefore, no longer be a large urban area.

Now 28,000 miles away is not exactly across town. A random asteroid passing somewhere within that distance probably won't impact - but it might. What is the chance that it would in fact crash? This question is like determining the chance of a dart landing in the bulls eye of the dart board. Here is the analysis. The Earth is about 7,918 miles in diameter, so let's approximate to 8,000 miles for simplicity. The diameter of the big circle defined by 28,000 miles from the Earth's surface is 28,000 miles from the edge of the circle to the Earth's surface, plus another 4,000 miles to get to the center of the Earth, plus another 4,000 miles to get to the other side of the Earth, plus another 28,000 miles to get all the way across to the other side of the big circle. That's 64,000 miles. That is 8x the diameter of the Earth. An 8x8 square is 8 times as wide as a 1x1 square but has 8x8=64 times the area, because the area of a square, circle, or any other shape is proportional to the square of the side or diameter (or any other length measurement). Thus the Earth takes up 1/64 of the area of the big circle, so an asteroid passing randomly through the big circle has one chance in 64 of striking. So will Apophis strike? Detailed followup astronomical measurements have ruled out initial fears, showing that it will almost certainly pass just about 22,000 miles from Earth. We'll dodge a bullet this time. But for every 64 asteroid approaches of 28,000 miles or closer we can expect one impact on average. So we'll luck out most times. But we won't luck out every time. Asteroid impact can happen and, some day, it will.

Based on careful calculations by NASA astronomers, Apophis is now known to present less than 1 chance in a million of impact in its close approaches of 2036, 2068, 2076, and 2103. Discovered June 19, 2004, Apophis's calculated risk of impact during its 2029 close approach peaked at 2.7% (greater than 1 in 40) on Dec. 27, then dropped rapidly based on new calculations and is now thankfully 0. Usually unlikely risks do become more unlikely with time as better information becomes available, but it's in the nature of the mathematics that you can't count on it. Think about it this way: if a future risk is 10% now, eventually it will either become 0% (nothing happened) or 100% (it did happen). It has a 10% chance of going up to 100%, and a 90% chance of going down to 0. This concept applies to any risk from asteroids to rain tomorrow. But back to Apophis. It reached the highest Torino Impact Hazard Scale rating ever recorded, 4, meaning:
"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." (http://neo.jpl.nasa.gov/torino_scale.html)

As just explained, the last sentence is both redundant and, arguably, misleadingly reassuring. Nevertheless the Torino Impact Hazard Scale is the only major asteroid risk metric intended for public information, though the Palermo Technical Impact Hazard Scale is used more by astronomers. First described in 1995, the Torino scale was named after the city of Turin, Italy during a conference there in 1999 (Turin being English for Torino). As of this writing, all known asteroids have a Torino rating of 0 ("no hazard") except for asteroid 2007 VK184, which is a 1 ("chance of collision is extremely unlikely with no cause for public...concern"). In contrast, the highest possible rating is 10: "A collision is certain, capable of causing global climatic catastrophe that may threaten the future of civilization as we know it...".

More important than a counterfactual impact probability analysis specifically about Apophis, is a better understanding of the probability of an impact by any asteroid. After all, if a big one lands in your backyard you don't care which one it is. There are roughly 900 large (1 kilometer or more in diameter) near Earth objects known. Ninety-two were discovered in the year 2000 but there has been a marked trend of decreasing new discoveries in the years since then. In other words, we've found most of them already and keep getting closer to a full inventory. A cautionary note: there are a lot more asteroids than that that are smaller than 1 km in diameter but still big enough to be able to cause considerable damage. Recall that both Apophis and the Tunguska object were quite a bit smaller than 1 km. The bottom line is that it is still possible, though unlikely, that something big could land in your backyard tomorrow, ruining your whole day and lots more besides.

The good news is that probably no dangerous near-Earth astronomical body will crash any time soon. This is based on specific orbital predictions about the bodies of dangerous size that have been identified. More generally, at least Tunguska-sized impacts are thought to average on the order of once in 1,000 years. Larger impacts occur less frequently while smaller ones are more frequent. For example, asteroids of at least the size responsible for the demise of the dinosaurs are thought to occur only once every 200 million years or so. On the other hand, impacts with an energy of about 2 pounds of TNT explosive (that's 1/1000 of a kiloton, equivalent to about 4/5 of a gallon of gasoline) occur at the rate of roughly 3 a day. Those smaller ones (and indeed impacts up to about 1 megaton, or million tons of TNT) generally cause no significant damage on the ground because they explode or burn up high in the sky. Shooting stars are in this category.

There is, however, an exotic complication that makes frequency predictions murkier for impacts of civilization-altering (or destroying) scale: "Nemesis."

Nemesis. The Solar System is a big place. The Earth is 93 million miles from the center, where lives the Sun, "Sol" in some languages, hence "Solar System." If scaled down until that 93 million miles was instead 93 yards (a little below the length of a football field), the Earth would be between 1/4 and 1/3 inch in diameter, a small marble. The Sun would be 31 inches in diameter, a little less than a yard, or about 4.7x the width of an official NFL football. Neptune, the farthest planet, would be under an inch wide and over a mile and half (or 28 football fields) away. But part of the Solar System is a lot further away even than Neptune. Called the Oort cloud, it is composed of lots and lots of objects, some of deadly size. Beyond even the Oort cloud, hovering silently over a light year away, drifting lazily around the sun in a long, slow path, some scientists hypothesis is a very small, very dim star: Nemesis! With an orbital period (that is, year) of about 27 million Earth years, it is suggested that Nemesis passes near the Oort cloud, disturbing it and jarring loose large objects, every 27 million years. These fall inwards toward the Sun (and Earth). A small fraction of them hit the Earth, causing havoc. This would explain the apparent tendency for mass extinction events with a 27 million year period. We're about 11 million years into the cycle now, giving us 16 million years before the danger peaks. This reasonably good news, though far from a safety guarantee.

While the Nemesis explanation is coming under increasing criticism by astronomers, the 27 million year periodicity for asteroid apocalypses appears to remain. Thus although the next impact could be around dinner time, I have categorized asteroid apocalypse in the 10-100 million year generation of time in recognition of this 27 million year cycle.

Recommendations

An asteroid impact ended the dinosaurs. Another one could end us. Something, as the saying goes, should be done! But what? Undeniably, as the Association of Space Explorers, the "international professional organization of astronauts and cosmonauts" puts it, ". . .protracted debate . . .can lead to inaction; evacuation of the impact site may then be our only option." Evacuation is however a legitimate plan, one which could result in economic damage but zero loss of life. Yet there are competing strategies as well. Their economic costs and technical feasibilities should be debated first, assessed next, plans laid, and any necessary preparations made.

The asteroid problem needs to be understood. That means discovering those with potentially dangerous impacts, and tracking them, so that impacts can be predicted many years in advance. That will give the required lead time for planning effective action. Such discovery projects are called "Spaceguard" surveys, after sci-fi great Arthur C. Clarke's Project SPACEGUARD in his 1973 novel Rendezvous with Rama. Nineteen years later in 1992, NASA, the US space agency, released its "Spaceguard Survey: Report." Still later, section 321 of the NASA Authorization Act of 2005 set the goal of discovering and characterizing 90% of NEOs (near Earth objects) at least 140 meters across by 2020. This objective seems a likely to be achieved a few years late. No matter - it is far more important to achieve it, than to achieve it precisely on time. 

Knowing the dangerous asteroids is only chapter 1 of the story. Mitigating their threat is chapter 2. Some methods for neutralizing the danger a asteroid poses only work temporarily (e.g. centuries, millennia or more). These involve pushing it out of the way, no piece of cake considering that a rocky asteroid 100 feet in diameter could easily be around 600,000 tons (metric: 30 meters and 560,000 metric tons), millions of miles away, and flying at 20,000 mph (or about 6 miles/sec, reducing your commute time to under 2 seconds for a 10-mile trip if you don't stick your hand out the window). You can't just contract the local towing company to get it out of the way.

A variety of pushing strategies have been devised, some more starry-eyed than others.

·                     Land on the asteroid and install multiple mirrors positioned to simultaneously focus sunlight onto a specific area. Enough mirrors doing this would be able to heat up a spot enough to boil off material. The boiled off vapors would fly into space, pushing the asteroid little by little in the opposite direction, the same principle by which a rocket engine spews exhaust in one direction to push the ship the opposite way. (That's in accordance with Isaac Newton's 3rd Law: a force in one direction has an equal counter-force in the opposite direction.) 

·                     As before, boil off material, but this time using a powerful laser (indeed the laser could be solar powered). The laser could not be on Earth because the beam would spread out too much over the vast distance, so space travel is still required.

·                     Hover a spacecraft near the surface of the asteroid rotating beneath it. Gravity will try to pull the spacecraft toward the asteroid, but at the same time pull the asteroid toward the craft (Newton's 3rd law again). It is like when you step on the scale, thinking the Earth is pushing you down onto it but probably not realizing that you are pulling the Earth upward the same amount. 

·                     Land spacecraft on the asteroid, then use an engine on the craft to push the asteroid. Sharp fellow, Newton.

·                     Absorbing and reflecting light create small amounts of force. For example, when the sun is directly overhead, it pushes on a square mile of the Earth's surface with a weight of several pounds. Light pushes a perfectly reflective surface twice as hard as a pure black (absorptive) surface. Also all bodies emit thermal radiation, more at high temperatures and less at low. This produces a small amount of thrust - the Yarkovsky effect, named after Russian railroad employee Ivan Yarkovsky, 1844-1902 (asteroid 35334 Yarkovsky is also named in his honor). For these reasons, painting part or all of an asteroid surface black or white/silvery can gradually affect its orbit enough to get it out of harm's way.

Although a weak force for a long enough time can do the job, a strong force for a short time can too. Explosions are a good way to produce strong, short forces. 

·                     Detonate conventional explosives near, on, or under the surface of an asteroid.

·                     Nuclear explosives are stronger, hence more effective, and are feasible with current technology. They can also break up and destroy an asteroid instead of just pushing it. We should try it.

·                     Smash a spaceship into the asteroid to give it a push. For relative speeds measurable in miles/sec., the collision will be an intense explosion.

If mitigation fails, at least we should know when and where the impact will be as far ahead as possible. With a lead time of 100 years or more, an impact zone could be gradually evacuated at a deliberate pace. Even a major city could be moved or dispersed within 100 years without undue hardship. If Seoul had started moving 50 years ago to get out of range of North Korean artillery, they could be half done. With an impact warning only days or weeks in advance, emergency evauation would be needed. Some cities have such plans already. Houston's hurricane evacuation plan, executed but badly for hurricane Rita in 2005, involved converting inbound lanes on major highways into outbound lanes, thereby doubling the amount of outbound roadway. All areas should be required to have plans and those plans should be tested to the degree feasible.

Since most of the Earth is covered with deep water, most asteroid impacts will in deep water. These cause tsunamis. The disastrous earthquake-linked 2004 Indian Ocean tsunami caused over 200,000 deaths. Legends of ancient, otherwise unrecorded tsunamis saved many Andaman Islanders from that disaster, because they recognized the signs of an incipient tsunami and ran for cover. Flood stories of many cultures, including our own (think Noah's Ark), testify to the importance of maintaining both a worldwide tsunami warning system and emergency evacuation plans for all coastal communities, large and small. 

Asteroid impact as an act of God gives pause, but what about as an act of mankind? If we learn how to push asteroids out of the way, it will become possible to push them into the way. An act of war! This could cause an impact mere decades away. As weapons, asteroids actually have some desirable qualities. Since the impact must be planned and engineered years in advance, evacuation could prevent deaths. Warfare without loss of life is better than the other kind, so with some trepidation I recommend going forward with research on asteroid warfare.

Finally, a couple of recommendations that are less earthshaking (literally). One is to check out some shooting stars. They are space rocks, too small to cause damage, that burn up high in the atmosphere, putting on brief but awe-inspiring shows. Shooting stars can and do happen on any night, but there are more during meteor showers. The Perseid shower peaks around Aug. 12-13, typically at a rate of dozens per hour. Meteor "storms" are meteor showers with particularly high rates, hundreds or at times even thousands per hour. The so-called "king of meteor showers" is the Leonid shower, peaking yearly on a night on or near Nov. 17-18, with peak rates that vary greatly over a 33-year cycle (because comet Tempel-Tuttle is the source of the Leonids and has an orbital period is 33 years). One can get an idea of how many shooting stars will be visible on what nights of what years and from what meteor showers at http://leonid.arc.nasa.gov/estimator.html. Cloudy, too much light pollution, or you don't feel like going outside? Then view videos of shooting stars available online at sites like youtube. 

Another thing you can do is hunt for micrometeorites. As discussed earlier, anyone can do it.

A final suggestion: check out an old impact site. This can be part of a fun vacation! For example, Meteor Crater in Arizona is commercially developed into a tourist attraction and is easily accessible from Interstate Highway 40. If you have more time and money, you can arrange a Tunguska site tour, see e.g. http://www.sibtourguide.com/tunguska.html. Such activities can help impress visitors with the awe-inspiring power of the universe.

"It was not a H-bomb however, but evidently an asteroid tens of meters (perhaps as low as 20) in diameter...": M.B.E. Boslough and D.A. Crawford, Low-altitude airbursts and the impact threat, International Journal of Impact Engineering, Vol. 35, Issue 12, Dec. 2008, pp. 1441-1448.

"I say 'crashed into the atmosphere' because asteroids that approach Earth most commonly do so at around 20,000 miles/hr." Derived from data at http://neo.jpl.nasa.gov/ca.

"In fact, on June 1 and 2, 1998, two comets did crash into the sun...": Twin comets race to death by fire, NASA Goddard Space Flight Center, June 3, 1998, http://umbra.nascom.nasa.gov/comets/comet_release.html.

"The impacts left dark markings easily visible through telescopes, and emitted bursts of microwaves, 'unexpectedly bright' x-rays, far ultraviolet, infrared, and radio waves."Microwaves: H. O. Vats, M. R. Deshpansde, O. P. N. Calla, N. M. Vadher, B. M. Darji, V. Sukumaran, Microwave Bursts from Jupiter due to K, N, P2 and S fragments, Earth, Moon, and Planets, vol. 73, pp. 125-132, 1996, http://www.springerlink.com/content/m14h632291434u05/fulltext.pdf. X-rays: C. J. Hamilton, Hubble observations shed new light on Jupiter collision, Views of the Solar System, http://www.solarviews.com/eng/levyhst.htm.Far ultraviolet: G. E. Ballester and 22 others, Far‐UV emissions from the SL9 impacts with Jupiter, Geophysical Research Letters, vol. 22, no. 17, p. 2425-2428, 1995. Infrared: J. Rosenqvist and 11 others, Four micron infrared observations of the comet Shoemaker‐Levy 9 collision with Jupiter at the Zelenchuk Observatory: spectral evidence for a stratospheric haze and determination of its physical properties, Geophysical Research Letters, vol. 22, no. 12, pp. 1585-1588, 1995. R. W. Carlson and 9 others, Galileo infrared observations of the Shoemaker‐Levy 9 G impact fireball: a preliminary report,Geophysical Research Letters, vol. 22, no. 12, pp. 1557-1560, 1995. Radio waves: S. J. Bolton, R. S. Foster, and W. B. Waltman, Observations of Jupiter's synchrotron radiation at 18 cm during the comet Shoemaker‐Levy/9 impacts, Geophysical research letters, vol. 22, no. 13, pp. 1801-1804, 1995.

"...you can actually collect micrometeorites yourself from roof runoff and other sources using a magnet, paper, and microscope." Just hunt around on the Web for pages and videos about how to do it!

"Were it to crash it would release 510 megatons of energy." 99942 Apophis (2004 MN4) Earth Impact Risk Summary, http://neo.jpl.nasa.gov/risk/a99942.html.

"Detailed astronomical measurements have ruled out the initial fears, showing that it will actually pass just about 22,000 miles from Earth." Ibid.

"Based on careful calculations...Apophis is now known to present under 1 chance in a million of impact in its close approaches of 2029, 2036, 2068, 2076, and 2103." 99942 Apophis (2004 MN4) Earth Impact Risk Summary, NASA Near Earth Object Program, http://neo.jpl.nasa.gov/risk/a99942.html.

"Apophis's calculated impact risk peaked at 2.7%...on Dec. 27, then dropped rapidly...": J. D. Giorgini, L. A. M. Benner, S. J. Ostro, M. C. Nolan, and M. W. Busch, Predicting the Earth encounters of (99942) Apophis, Icarus, 2008, vol. 193, pp. 1-19, http://neo.jpl.nasa.gov/apophis/Apophis_PUBLISHED_PAPER.pdf.

"It is the only asteroid to ever reach a Torino Impact Hazard Scale of 4...": D. Yeomans, S. Chesley and P. Chodas, Near-Earth asteroid 2004 MN4 reaches highest score to date on hazard scale, NASA Near Earth Object Program, Dec. 23, 2004, http://neo.jpl.nasa.gov/news/news146.html.

"...asteroid 2007 VK184, which is a 1 ('chance of collision is extremely unlikely with no cause for public...concern')". 2007 VK184 Earth Impact Risk Summary, NASA Near Earth Objects Program,http://neo.jpl.nasa.gov/risk/2007vk184.html.

"There are roughly 900 large (1 kilometer or more in diameter) near Earth objects known." NEO Discovery Statistics, NASA Near Earth Object Program, http://neo.jpl.nasa.gov/stats/.

"Ninety-two were discovered in the year 2000 but there has been a marked trend of decreasing new discoveries in the years since then." NEO Discovery Statistics, NASA Near Earth Object Program, http://neo.jpl.nasa.gov/stats/.

 "...Tunguska-size impacts are thought to average on the order of once in 1,000 years." P. Brown, R. E. Spalding, R. O. ReVelle, E. Tagliaferri and S. P. Worden, The flux of small near-Earth objects colliding with the Earth, Nature, Nov. 21, 2002, vo. 420, pp. 294-296.

"For example, asteroids of at least the size responsible for the demise of the dinosaurs are thought to occur only once every 200 million years or so." C. R. Chapman, Meteoroids, meteors, and the near-Earth object impact hazard, Earth Moon Planet, 2008, vol. 102, pp. 417-424.

"On the other hand, impacts with an energy of about 2 pounds of TNT explosive...occur at the rate of roughly 3 a day."  Ibid.

"While the Nemesis explanation is coming under increasing criticism by astronomers...": A. L. Melott and R. K. Bambach, Nemesis reconsidered, Monthly Notices of the Royal Astronomical Society: Letters, 2010, vol. 407, issue 1, pp. L99-L102. Preprint at http://arxiv.org/ftp/arxiv/papers/1007/1007.0437.pdf.

Association of Space Explorers, the "international professional organization of astronauts and cosmonauts": R. L. Schweickart, T. D. Jones, F. von der Dunk, and S. Camacho-Lara, Asteroid threats: a call for global response, 2008, Association of Space Explorers, http://www.space-explorers.org/ATACGR.pdf.

". . .protracted debate . . .can lead to inaction; evacuation of the impact site may then be our only option." Association of Space Explorers, www.space-explorers.org (document no longer online).

"Spaceguard Survey: Report." D. Morrison, Spaceguard Survey: Report of the NASA international near-Earth object detection workshop, Jet Propulsion Laboratory, Cal. Inst. of Tech., Pasadena, CA, 1992, http://impact.arc.nasa.gov/downloads/spacesurvey.pdf (linked from http://impact.arc.nasa.gov/gov_nasastudies.cfm).

"This objective seems a likely to be achieved a few years late." Near-Earth object survey and deflection analysis of alternatives (report to congress), National Aeronautics and Space Administration (NASA), 2007,http://neo.jpl.nasa.gov/neo/report2007.html.

"a rocky asteroid 100 feet in diameter could easily be around 600,000 tons (metric: 30 meters and 560,000 metric tons)." C. Q. Choi, Small asteroids pose big new threat, part (1b) of Tunguska revision, and a possible NEA impact on Mars, Dec. 21, 2007, News Archive, NASA, http://impact.arc.nasa.gov/news_detail.cfm?ID=179.

"A variety of pushing strategies have been devised, some more starry-eyed than others." Near-Earth object survey and deflection analysis of alternatives (report to congress), National Aeronautics and Space Administration (NASA), 2007, http://neo.jpl.nasa.gov/neo/report2007.html.

"Explosions are a good way to produce strong, short forces." ibid.