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Earth Impact by an Asteroid: Prospects and Effects

In the past two decades, the topic of an asteroid impacting Earth has become very popular for various reasons, including:

  • The theory that an asteroid impact led to the demise of the dinosaurs
  • Scientific recognition of additional asteroid impact sites and other evidence of Earth impacts in ancient times
  • The movies Deep Impact and Armageddon, plus some other dramatic videos
  • Probes to asteroids and comets by NASA and other space agencies, which return photos for the general public (and data)
  • Promotional campaigns for asteroid detection programs, including to the US Congress which funds them
  • With better telescope coverage, the discovery of asteroids which make close approaches of the Earth, and news thereof
  • Occasional meteor sightings, which get better coverage on the internet

Impact of Earth by an asteroid would be sensational and possibly cataclysmic. Many people think this is the worst thing which could possibly happen to civilization. However, they're most probably wrong.

A far more probable, as well as much more devastating, threat is by simply the advance of human military or commercial biotechnology regarding superviruses and some other agents which could wipe out millions or billions of people, and possibly make humans extinct, as covered on another website of mine, (where I hardly even mention asteroid impact, as I don't consider them an extinction risk, just a cataclysm).

The solution to both problems -- human extinction risks as well as asteroid impact threats -- is to develop the resources of outer space for human colonization as well as planetary defense, which means utilizing the asteroids for their resources.

Awareness of the risks of advancing biotechnology and nanotechnology are at about the same stage now as the threat of asteroids was in the 1980s, but the risks of biotechnology are sure to eventually be recognized and pass the perceived threat of asteroid impact within the next decade.

In history up to the 1970s, there was little interest in asteroids, including near Earth asteroids. They were considered low class astronomical objects. Indeed, the small comet which destroyed hundreds of square kilometers in remote Tunguska, Siberia, in 1908 was an event little known to the general public.

In the 1970s, things started to change. A small but increasing number of astronomers interested in asteroids began to realize the abundance of asteroids which passed close to Earth, by instituting processes to catalog asteroids accidentally seen on telescopic plates and previously not recorded (in most cases) but seen as a nuisance, as discussed in the PERMANENT section on discovering and cataloging asteroids.

Theoretical models, assisted by computer calculations, revealed that the gravity of the planets caused a sizeable number of asteroids from the Main Belt between Mars and Jupiter to cascade down into lower orbits approaching or crossing Earth's. Further, a significant fraction of comets passing through the inner solar system would be diverted into orbits near Earth due to gravitational encounters with the inner planets.

As a result of these discoveries, the estimated numbers of near-Earth objects (NEOs) dramatically expanded by about 1000 times! Scientists started to take note and interest.

Ever improving U.S. Defense Dept. sensor technology looking for the satellites of adversaries recorded a surprisingly high frequency of asteroid viewings as well as meteor fireballs hitting Earth's upper atmosphere, the latter greatly augmented by sound sensors listening for the booms of nuclear tests. Part of this process was discriminating between satellites and distant asteroids seen, and nuclear tests vs. meteor fireballs heard.

New telescope technology (CCDs) emerging around 1990 increased the discovery rate of all asteroids and confirmed the above theory on the abundance of asteroids (based on solid statistical sampling rates). In fact, the latest estimates project that there are about 300,000 near-Earth asteroids over 100 meters in diameter. Approximately 1000 near Earth asteroids of size 1 kilometer in diameter have already been found, though the total is not expected to be more than 20% over this amount.

Smaller asteroids are much more difficult to detect. The cutoff size of what could cause major damage to Earth, such as a tsunami or an airburst, is difficult to state because it depends upon what the asteroid is made of, e.g., a metal asteroid vs. a soft one. Some sources put it at about 150 meters. A 10 meter asteroid can produce an explosion with approximately the same power as the nuclear bomb at Hiroshima, but that would occur very high in the atmosphere where it would be harmless.

If a hard asteroid of size 200 meters hit the ocean (which covers 70% of the Earth), the tsunami (i.e., giant wave) it would create would inflict catastrophic destruction of coastal cities and substantial worldwide human casualties along coastlines. If an asteroid of size 1 kilometer hit Earth, it would cause a dust cloud which would block out sunlight for at least a year and lead to a deep worldwide winter, exhausting food supplies. The latter is what caused the dinosaur extinction, as well as other major extinctions of smaller creatures in geologic time scales. The 200 meter asteroid hits, which are far more common than the 1 km+ hits, wouldn't show up much in geologic histories on a global scale.

In the general press, too much emphasis is put on the big, 1 km size asteroids like the one that killed off the dinosaurs, which are unlikely to hit Earth for hundreds of thousands of years, if not millions of years. Too little coverage is given on the small asteroids which could cause terrible local destruction (e.g., to nearby coastal cities) but little worldwide impact, and which probably hit once per few hundred years. Our best telescopes can hardly see the 100 meter asteroids because they're so small, and many are dark.

The Tunguska, Siberia asteroid of 1908

An asteroid hit Tunguska, Siberia on June 30, 1908. It was a tiny asteroid, only about 30 to 60 meters across, i.e., difficult and unlikely to be detected by even the most modern ground-based telescope in existence today, given their necessarily selective partial coverage of the sky, and between 10,000 and 100,000 tons in mass. The Tunguska event was caused by a volatile rich asteroid which exploded due to heating during reentry.

Fortunately, the asteroid was just grazing the Earth and did not come straight down, causing a long streak in the sky seen over many territories, and was packed with volatiles rather than nickel-iron metal. It exploded in the air about 5 kilometers (3 miles, or 15,000 feet) above remote Tunguska. However, the energy released was equivalent to a nuclear bomb. In fact, the explosion was greater than the Hiroshima or Nagasaki nuclear bombs.

The forest was flattened, out to about 30 kilometers (18 miles) from the center. Below the explosion, trees were incinerated, though some remained standing -- just like at Hiroshima and Nagasaki. Trees were scorched on one side out to 14 kilometers (9 miles) from ground zero. About 200 kilometers (120 miles) away, carpenters were thrown off of a building from the shock wave, and shelves were emptied. At 100 kilometers, an eyewitness reported "the whole northern part of the sky appeared to be covered with fire ... I felt great heat as if my shirt had caught fire ... there was a .. mighty crash ... I was thrown onto the ground about [7 meters] from the porch ... A hot wind, as from a cannon ... Many panes in the windows [were] blown out, and the iron hasp in the door of the barn [was] broken."

The closest surviving observers on record were some reindeer herders asleep in their tents about 80 kilometers (50 miles) from ground zero. They were blown with their tents into the air, several of them losing consciousness momentarily. They reported thick smoke and fog from the burning trees. About 1,500 reindeer were killed in the area.

At 500 kilometers (300 miles) observers reported "deafening bangs" and a fiery cloud on the horizon.

Large seismic vibrations were recorded 1000 km (600 miles) away, and an English weathermen 3600 km (2200 miles) away noted unusual air pressure waves.

However, that was a remote part of the Earth and the year was 1908. The stories kept coming in from that desolate cold place. Due to the state of Russian science at the time, the remoteness of the area, and the harshness of the temperature during most of the year, it took 19 years until a group of scientists went on an expedition to study that remote site in 1927. What they found prompted additional expeditions, the best of which were conducted in the 1950s (when a nearby airport was built), led by those scientists of the previous expedition who had survived the second world war.

What they found was a flattened forest with young saplings growing up between the fallen trees, and a layer of carbonaceous dust, round glass melts, free metal granules and elements not normally found in the crust of the Earth, adding evidence of a carbonaceous chondrite asteroid hit. (Alternatively, some think it was a comet captured by the inner solar system whose outer volatiles had been burnt off eons ago, leaving just inner volatiles.)

It's theorized that by grazing the Earth's atmosphere, the volatiles under the surface of the asteroid heated up and eventually caused the asteroid to explode. An asteroid would enter Earth's atmosphere much faster than a re-entering spacecraft and would have practically no heat protection. The asteroid probably had plenty of hydrogen and carbon in its interior, which was basically ignited all at once. It's thought that the asteroid pretty much vaporized entirely into dust and gas in the air due to the high-heat explosion. No large chunks were found.

"[I]f the same object had exploded over New York City, the scorched area would have reached nearly to Newark, New Jersey. Trees would have been felled beyond Newark... The man knocked off his porch could have been in suburban Philadelphia. 'Deafening bangs' might have been heard in Pittsburgh, Washington, D.C, and Montreal."

If instead it had been a nickel-iron asteroid and a little bit larger, and hit the ocean (which covers 70% of Earth's surface), it could cause a tsunami wave giant enough to smash into numerous modern 20th century coastal cities with no warning.

There aren't nearly as many remote areas in the world like Tunguska, Siberia, any more due to population growth and industrialization.

Bolides: Meteor explosions and shockwaves in Earth's atmosphere

Many meteors can be detected and analyzed by sensitive sound equipment, due to their hypervelocity shock waves as well as any explosion. For example, national defense departments monitor sound waves in order to detect things like nuclear test explosions and any incoming missiles.

While a meteor also causes a bright flash of light locally, optical measurements from the ground or satellites may be missed or incomplete, so in many cases it is better analyzed by the recorded acoustic signature. Bolides are detected by satellites, and the visual and acoustic data can be put together for analysis.

Unfortunately, defense departments don't always wish to reveal the capabilities of their technology. However, non-defense entities do release information, such as the European Network (EN), the Canadian Network (CN) and the Prairie Network (PN). Typical sizes of bolides have been estimated to range from 1.2 meters to 15 meters, with corresponding masses of 3.5 tons to 80 tons.

Meteors usually explode in the upper atmosphere, and have severe shock waves due to their hypervelocity. Besides bolides, there isn't much else which can come close to the acoustic profile of a nuclear bomb explosion, except a large exploding volcano. It was important to develop this technology to analyze bolides not only for intelligence purposes but also to prevent false alarms caused by bolides.

Most bolides have an explosive force in the kilotons range, but are high enough in the atmosphere to not cause damage on Earth, just creating a very bright flash and a loud bang.

Data collected by one site, the Air Force Technical Applications Center at the Patrick Air Force Base in Florida, between 1960 and 1972 was made public. It characterized 20 explosions on 10 different dates (some dates had multiple hits). Most of the detections were within only about 5000 km (3000 miles) range. However, two bolides delivered the energy of over one megaton of TNT, which is the same as a large nuclear warhead, and over 50 times the power of the nuclear bomb dropped on Hiroshima. (ReVelle et al., 1996)

Effects of impacts on Earth - different sizes, frequencies of impact

The press and Hollywood often focus on the impact of a large asteroid, say 1 km diameter. Those kinds of catastrophic hits have dramatic impacts for all life on the planet, but are extremely rare and quite unlikely to occur in the next few thousand years. Of much greater concern should be the Tunguska-size asteroids.

The population of asteroids of size 1000 meters (1 km) or larger which cross or closely approach Earth's orbit is thought to be about 1,200. We know of many of these, which is why they make the press. They are big, so we have seen some of them with telescopes. We know the orbits of many of them, and that they won't hit us in the forseeable future, at least not for many hundreds of years. The only potential 1 km impactors would have to come from the approximately 20% still undiscovered (as of 2012) 1 km class asteroids, or else a new comet coming down.

The Meteor Crater in Arizona, measuring about a kilometer in diameter, was caused by a nickel-iron rock only about 30 meters across, which isn't all that much larger than for the largest of the bolides we've seen over the past few decades. That's a very small asteroid which we couldn't see from telescopes on Earth's surface until it's right above Earth -- when it's much too late to do anything but duck for cover. The reason why the asteroid penetrated the atmosphere and hit Arizona is because it was a nickel-iron asteroid, not a volatile rich asteroid, and it came almost straight down rather than near a grazing angle.

This small size of asteroid is very difficult to detect.

At the other end of the spectrum are the 1 km asteroids. That kind of impact would wipe out life within proximity of the impact site. However, more serious is how it would affect the whole world in indirect ways. The dust and/or vapor cloud created by an impact to either the land or the ocean could be big enough to create a "nuclear winter" like mini-ice age, and disrupt climatological wind patterns, adversely affecting major food-growing regions of the world, thus straining world food supplies, prices, governments and civilization. However, such an impact is quite unlikely over the next thousand years, at least.

The most damaging kind of impact would be an asteroid that hits the ocean, not the land. An asteroid hitting land causes mainly localized damage. An asteroid hitting the ocean can cause a tsunami (i.e., huge wave) that would inflict catastropic damage to coastal cities and assets to great distances. The Earth is covered 70% by oceans, so an ocean impact is more likely.

Earth's atmosphere gives protection against the vast majority of small asteroids which hit. Asteroids hit the atmosphere at typical speeds in excess of 10 km/sec -- an average of about 20 km/sec for asteroids whose entire orbits reside within the inner solar system, with exact relative speed depending upon their angle of approach, and with speeds over 50 km/sec common for small cometary objects making a pass from the outer solar system. At this speed, they usually break up due to severe shock pressures, and burn up due to friction with the atmosphere. Think about it -- 10 kilometers per second (6 miles per second) is awfully fast -- about 36,000 kilometers (22,000 miles) per hour.

For asteroids coming in at 20 km/sec, it's generally thought that to penetrate the atmosphere and cause major damage by tsunami, an iron asteroid must be around 40 to 60 meters in diameter, and a stony asteroid 200 meters in diameter (Hills, 1994, paper ref.). However, a stony asteroid 60 meters in diameter can cause significant damage by airbursts (Hills and Goda, 1993, paper ref.).

The exact damage inflicted by an asteroid or comet depends upon a number of factors -- size, speed, composition of object, and whether it hits land or ocean.

For a land impact, it can be said in general that an object of roughly 75 meters diameter can destroy a city, a 160 meter object can destroy a large urban area, a 350 meter object can destroy a small state, and a 700 meter object can destroy a small country.

For an ocean impact, the destruction is much greater -- smaller objects can cause far more widespread damage. The effects of an ocean impact are felt much further away than the effects of an airburst due to the more effective propagation of water waves, and the fact that human populations and assets are largely concentrated in coastal cities which historically became established due to water transport (i.e., shipping and trade) and businesses near ports.

For example, the earthquake-induced tsunami in Chile in 1960 produced waves in Hawaii 10,600 km away of height up to over 10 meters (30 feet), and up to 5 meters (15 feet) in Japan 17,000 km away with an average of 2 meters, causing heavy damages and loss of lives.

What happens with a tsunami is that when a deep water wave of, say, a third of a meter hits a continental shelf its speed decreases but its height conversely rises. For example, the tsunami from the 1960 Chile earthquake created a deep water wave of only 20 cm (8 inches) above sea level, but when it hit the shore it had risen to a height an average of ten times its ocean size -- over 2 meters (6 feet), and in some places much higher. However, the size varies depending upon the coastal features, and was higher in many places. Understand, this is not just a narrow surfable wave that dies down when it approaches the shore, but is a wide body of water that grows into a wall that smashes into the land. (When the wave hits the shallow coast it slows down, and the water of the deep ocean wave behind it piles up on top to form a wall of water.)

The effects of an airburst are far more localized because the intensity of the phenomenon decreases with the inverse square of the distance in a three-dimensional way, whereas the height of a water wave decreases only with the inverse of the distance, i.e., to the first power, due to its circular, two-dimensional nature.

The damage caused by a tsunami is due not just by a heavy wall of water hitting things, but much more due to the solid debris carried by up the powerful, churning deep water wave as it hits the continental shelf -- the solid debris rams and batters anything in its way.

The 1998 earthquake-induced tsunami in Papua New Guinea that wiped out coastal villages and killed uncounted thousands of people was only a few meters high. If an asteroid hit the ocean, we could see a tsunami wave 100 times higher.

It's not easy to determine the frequency of tsunamis in the world historically. Unusual debris has been found in high places in many parts of the world which could be the result of a tsunami, though it's not easy to determine what happened for sure and when, by the ordinary nature of the material. There has been little effort to date to systematically assess the frequency and nature of tsunamis well before the 20th century. Recorded history by civilizations along the Atlantic Ocean has not noted major tsunamis, though there wouldn’t be many people around to report it. There’s not much recorded history from many coastal regions in the world, and many long coastlines were devoid of cities.

Searches for small tsunami in the geological record have mostly been started only in the 1990s. Of particular interest are tsunami along the Atlantic coast, where earthquake-induced tsunami are rare, so that any detected tsunami would probably be due to an asteroid. The results of these ongoing efforts will shed some light on the frequency of asteroid hits into the oceans.

A mainstream scientific analysis currently estimates that an asteroid-induced tsunami exceeding 100 meters in height along the entire coast probably occurs once every few thousand years, which slightly exceeds written history in most of these ocean coastal regions. We've been living on the edge for a long time now. Such a 100 meter tsunami would cause unprecedented damage to now-developed low lying areas all along the U.S. east coast, and may totally submerge vast areas in Europe such as in Holland and Denmark. A 100 meter tsunami would travel inland about 22 km (14 miles) and a 200 meter tsunami would travel inland about 55 km (34 miles) (Hills, 1994, paper ref.).

In any case, it is clear that the cost of dealing with damage due to a hit by a sizeable asteroid causing even just a small tsunami like the 1960 one could be far higher than it would cost to embark on a crash program of developing space on a large enough scale using asteroidal materials which would in turn give us the infrastructure necessary to detect and prevent impending Earth impacts -- a Rapid Deployment Force of rockets and a few people ready on standby in space, to nudge the incoming object so that it misses Earth.

Contrary to movies and popular belief, we probably wouldn't want to blow it up as that would cause a lot of pieces being thrown in unpredictable directions. Nudging its trajectory a little is probably the most reliable way to make it miss Earth, and would be easier and cheaper, if we got to the asteroid long enough in advance.

Yet, we hardly even know what exists in our neighborhood, and a dedicated asteroid sentry system is needed. Of course, such a sentry system would also discover economically attractive asteroids.

Many close encounters with Earth-approaching asteroids are found out AFTER the near-miss has already passed.

The Spaceguard Foundation

The Spaceguard Foundation is an international body officially set up by a convention in Rome and with a large number of participating government officials and professionals from around the world. Their website gives updates to the master worldwide list of asteroids which could potentially strike Earth, though it is technically oriented rather than for the general public.

Besides asteroids in orbits near Earth's orbit, there are also small comets that pass through the inner solar system, making one in-out double pass of Earth orbit every few centuries. New ones aren't going to show up in the Spaceguard Foundation's catalog because their orbit doesn't reside in the inner solar system and they aren't detected or known until they come down from the far reaches of the solar system, and we generally wouldn't know about them until shortly before their arrival. (This is what the movie Deep Impact had hitting the Earth.)

Looking at the entire situation, one could conclude that an early warning system is of limited use unless it comes with an interceptor system. The latter would require a government body (or else a high gamble by a private initiative with prices negotiated shortly before impending disaster!). Despite all the threats, the government bodies are not funding any asteroid intercept systems, and are providing only small amounts of funding for asteroid searches and cataloging potentially threatening asteroids, plus a little bit of money for paper studies into methods to deal with a threat.

Effects on satellites and space stations in low Earth orbit

When comet Shoemaker-Levy impacted Jupiter in July 1994, the event was watched by the Hubble Space Telescope. When scientists saw the big splash of Jupiter's atmosphere rise up like a huge atmospheric wave, many could not help but wonder what would happen to satellites and space stations in low Earth orbit if a large asteroid or comet hit Earth's atmosphere, or even a large bolide.

With the increased data and analyses of asteroids, comets and bolides, it has been estimated that once per century an asteroid, comet or bolide will hit Earth's atmosphere and cause a plume to rise about 1000 km up over an area thousands of kilometers in diameter (Boslough et al., 1996, paper ref.). Countless satellites currently operate well below 1000 kilometers up, as well as the International Space Station.

The effects on these satellites and space stations would be catastrophic if they hit the plume. They would be travelling at 7 km/sec and could sustain physical damage. Unless they have substantial thrust capability, they would probably be slowed down enough to fall down into the Earth's atmosphere and burn up ... and possibly crash down onto Earth, nobody knows where.

Meteor showers

On occasion, there are meteor showers, e.g., when Earth passes through the tail of a comet. Many comets have tails of significant debris stretching hundreds of millions of miles behind the comet. The chances of a satellite being impacted by a meteor can increase by a factor up to about 10,000 times during the heaviest meteor shower, the Leonids. The probabilities are vague because we just don't have enough data on the population density of sand and small pebbles, which are sufficient to destroy a satellite at their impact speeds.

For example, a meteor from the Perseid Meteor Stream is thought to have killed the OLYMPUS telecommunications platform in 1995.

The largest meteor shower is the Leonids. Of all the meteor showers chronicled over the last 1000 years, almost half were unknowingly reporting the same cause - the Leonids, which are in turn debris from the comet 55P/Temple-Tuttle, which returns to the vicinity of Earth's orbit every 33 years. However, the severity of the meteor shower is not always the same and is unpredictable.

When Earth passed through the tail of Comet Tempel-Tuttle in 1966, it was near the dawn of the space age when there wasn't much manmade hardware up in space. However, photographic and radar data is available from this hit. In fact, one measurement of meteors on November 17, 1966, reported 150,000 per hour (that's forty per second) over a two hour period, which is by far the heaviest bombardment ever recorded.

Comet Tempel-Tuttle crossed Earth's orbit in February 1998 and the Earth passed through its tail in November. It was expected to be a fantastic meteor shower but instead it was remarkably very light, producing a maximum of only about 1,000 meteors per hour during a brief peak period of a couple of hours. Satellites were lucky in 1998, as we were spared with a surprisingly very light meteor shower.

The occurance of a meteor shower may justify vacating humans from space stations which do not have a decent level of shielding, which includes all those which exist today.

The only protection against such storms is good thick multilayer shielding, e.g., from asteroidal materials.

Planetary defense methods

There have been many scientific analyses on alternative ways to deal with a large object on a collision course with Earth.

The methods can be split into two categories -- destruction and deflection. (A third option is obvious -- turning it into useful products.)

Destruction means assuredly breaking up the object into small enough pieces so that none can penetrate the Earth's atmosphere. For example, if done by nuclear detonation, the dispersion of the fragments would mean that most pieces would miss the Earth, but some pieces could still hit Earth. The further away the detonation, the more dispersed the pieces by the time they arrive in Earth's vicinity. As you can see, blowing up the object is actually a combination of destruction and deflection -- the dispersion is a sort of deflection. The problem with destruction is the uncertainty of explosions -- success is risky.

Deflection means nudging the body so that it misses Earth. The further away the object is from Earth, the less we need to nudge it because the change in its trajectory adds up over time.

For example, for an asteroid on a trajectory to hit the Earth in the middle (as seen from its approach), we would need to deflect it a minimum of about 8000 km or 5000 miles (since Earth has a radius of 6400 km or 4000 miles) in the direction perpendicular to its trajectory. If we were to land on the asteroid roughly 6 months (4300 hours) before it would impact, then we would need to nudge it by accelerating it roughly 2 km/hour (or a little over 1 mile/hour) in a quick thrust, or about 4 km/hr (2.5 miles/hr) for a slow 6 month thrust. We'd probably want to accelerate it even more just for the sake of safety, and would certainly want to rendevous with it further in advance if possible. While a few km/hr speed seems small, keep in mind that we are moving mountains, not little cars.

The main problem is that we would probably have less than 6 months notice between detection time and impact, especially if the object is a comet coming in from the outer solar system at 50 km/sec. While we have some recently scaled up search programs, they give us very little coverage of the entire sky, and they don't detect small objects until they are close. By small, we're still talking about city-smashing tsunami sized objects.

If we detect an object on an impact trajectory, then we will need to make a decision on a method of planetary defense. The method chosen will depend upon the size of the object, how soon we can rendezvous with it, what the object consists of, the rotation rate of the object, the object's geometry, and any fractures in the object. There would be considerable uncertainty regarding the composition of the object without a thorough on-site visit. For analysis purposes at this point in time, models have considered objects consisting primarily of ice, friable material, gravel, hard rock and pure metal.

Most proposed methods have been rejected due to risk and economic and/or technical feasibility in the near future. The remaining candidate methods seriously considered to date include:

  • Blowing it up by nuclear bomb -- This option is generally unfavored because it seems unlikely that it would completely break up most objects into small enough pieces, or assuredly move all pieces into a non-impact trajectory. It's still considered because it is economical and technically feasible -- it might work, and it might be all we can do if given very short notice.
  • Nudging it by nuclear bomb -- This option explodes a nuclear bomb above the surface of a volatile rich asteroid or comet to cause intense heat at the surface in order to create gas jets which would thrust it away from Earth. Another nuclear nudge option is to chip off a piece by a subsurface explosion along an existing natural fracture -- split it into two but so that both dangerous pieces miss Earth in a straddling way (as in the movie Armageddon). The drawback to both options is the risk that it would work. However, it very well might work, and it might be the most reasonable option if given very short notice.
  • Nudging it by kinetic impact -- This option simply has a sizeable object strike the asteroid or comet at high speed in order to nudge it, possibly with an explosion upon impact to enhance the effect. This could work with small objects. The risk is that it will fragment the target and leave a sizeable chunk on a collision course with Earth.
  • thrusting the object -- This option is attractive for very small objects whereby it would be feasible to launch up the required fuel propellant with a very high performance engine, or for small to medium sized objects known to be rich in water which we could extract and use as fuel propellant in a thermal rocket. Nuclear rockets (which use a small nuclear reactor to heat any kind of propellant) would be preferred for their simplicity and high performance. The advantage of thrusting is that the object won't be fragmented and we have more control. The disadvantage is that it won't handle very large objects in a short time frame.
  • pulling by gravity -- by hovering a massive enough object next to it. This has hit the popular news circuits, as a "tractor beam".

  • ... and many, many other techniques proposed ...

If an object were approaching Earth and we were given sufficient time, we could send out multiple missions using different techniques so that if the first mission failed, a second mission could give it a shot. If an earlier mission fragments the asteroid, then a later mission may need to deal with a fragment on a collision course with Earth. If it's a large object then it could possibly fragment into multiple threats.

In all cases, the more advanced notice we have, the greater our chances for success. Time is a critical element which can make all the difference in the world.

Telescope programs to detect near Earth objects could also use telescopes in orbit (above Earth's atmosphere) to see in difficult places, such as near the horizon, and actually in all directions without regard to day or night side. However, that's not in the government budgets.

External links:

The Spaceguard Foundation

NASA Asteroid and Comet Impact Hazards

Easy to read and follow. Some parts are a bit outdated, but there are good links in the "NEO Catalog" section.

Tunguska event

B612 Foundation

B612 plans to launch a space telescope named Sentinel in December 2016 into a Venusian orbit to detect asteroids which could threaten Earth. A nonprofit founded in 2002 by astronauts and scientists, which depends on donations, its initial work was to develop asteroid deflection techniques. A goal is to someday alter the orbit of an asteroid in a controlled way, as an exercise which could be applied sometime to protect Earth.

B612 foundation on Wikipedia

Wikipedia on the B612 Foundation > Asteroids, Near Earth Objects (NEO) > Impact of Earth by a NEO

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  • The SiteMap page.

The pulldown menu and the SiteMap are the same tree of pages and links. The pulldown menu offers + and - for expand and collapse sections/subsections/sub-subsections... of the tree, sometimes multiple levels, whereas the SiteMap has everything expanded with no + or - expand and collapse options so the SiteMap is much longer, compared to the pulldown menu if not fully expanded. You may just choose which of the two formats you prefer at a particular time.

The SiteMap colors links red which you have already visited, vs. normal blue for still unseen. It is convenient to browse the SiteMap in one tab or window, and opening pages in other tabs/windows (Ctrl-click or right-click), such as browsing the whole SiteMap to skip pages you've already seen and to choose to open pages you haven't read yet.

The pulldown menu doesn't change the color of seen pages, unfortunately, unlike the SiteMap. However, using the pulldown menu, you can quickly browse the list of sections and other pages without leaving the page you're on. The SiteMap is a separate page of its own.