Asteroids and Comets


[The] story of how Phaëton, child of the sun, harnessed his father’s chariot, but was unable to guide it along his father’s course and so burnt up things on the earth and was himself destroyed by a thunderbolt, is a mythical version of the truth that there is at long intervals a variation in the course of the heavenly bodies and a consequent widespread destruction by fire of things on earth.

How Big Is The Risk?

Here is a rundown of the different sizes, their probabilities, the chances of us seeing them coming, and the theoretical approximates of the damage they could inflict. Because we know so little about the makeup of comets, I’ve lumped them in with asteroids for convenience.

1 kilometer diameter or larger
The aim of NASA and their various branches and allies is to discover at least 99% of these monsters.
A 10km asteroid strike would create waves in the Earth’s crust higher than houses, and a blast of 500ºC air travelling at 2500 kph. Any creature within 12 million sq km would be wiped out.(1) That’s roughly the size of the USA, Europe or Australia

500 metre
According to Duncan Steel (2), we are unlikely to discover more than half of the asteroids and comets in our solar system with a 500 meter diameter. Just one of these would create a crater 10km wide, and destroy all life within 1,000sq km.(3)

100 metre
These are so small, in terms of our ability to discover them, that only a few percent are likely to be spotted, says Steel. If a 100m asteroid struck Earth at 19km/sec the resulting crater would be 2km across, and it would destroy all life within 200sq km. These hit Earth with an average frequency of one every 22,000 years.(4) Or according to Gerrit L. Verschuur, as often as every 1,000 years (5).

50 metre – Tunguska size
Objects with a diameter of 50-60 meters pass closer to Earth than the Moon about once per week.(6) Aside from the famous Tunguska incident, and serving as a reminder that Tunguska was not a “one-off”, a smaller asteroid exploded mid-air over eastern Siberia in 1947, leaving “122 craters up to 26m wide and 5m deep.”(7) It weighed about 70 tonnes.

They Strike Earth All The Time!

Meteors do make it to Earth – here are some examples that serve as a reminder:

1 metre – Barwell, UK – 1965
Roughly half of Coventry, a city of 300,000 that it passed over, say they heard it. Some of the high frequencies given off meant horses heard it before it became visible. Those that could see it through the evening clouds estimated the tail to be 20 degrees long. It broke up into many pieces, and although some struck buildings, nobody was hurt.

Astrophysicists from the Herzberg Institute in Ottawa, Canada, have estimated that an average of 16 buildings are damaged by meteorites each year, with a human being hit every nine years, sometimes fatally.(8)

Recorded deaths by meteorites and asteroids:

  • 588 AD, 10 people, China
  • 1490, supposedly 10,000 people, China
  • 1511, Franciscan monk, Cremona, Italy
  • 1650, Another monk (!), Milan, Italy
  • 1647-54,  2 sailors at sea
  • 1790, A farmer and cattle, France
  • 1825 , A man, India
  • 1827, A man, India
  • 1874, Child, China
  • 1879, Man in bed, Indiana, USA
  • 1879, Farmer, France
  • 1897, Horse, West Virginia, USA
  • 1907, Entire family, China
  • 1908, 2 people reported, Tunguska
  • 1911, Dog, Egypt
  • 1929, 1 member of a bridal party, Zvezvan, Yugoslavia

Buildings are stuck, and people nearly hit, most years. A recent example (June 2009) involved a German schoolboy, Gerrit Blank, who was left with a scar on his hand when he was grazed by a meteorite that left a 30cm-wide crater in the pavement.

In 1931, three asteroid fragments struck a Brazilian jungle and 1,300 square kilometers of rainforest were destroyed by wall of fire. (9)

On February 1, 1994, near the Marshall Islands in the western Pacific, a handful of fishermen witnessed a hundred-kiloton explosion (that’s 10x Hiroshima) that momentarily flashed brighter than the sun. This asteroid has been estimated to be just 6-17 metres across, but plenty sufficient to decimate a city – so it was extremely fortunate (for humans) that it exploded above the ocean. According to Duncan Steel:

“It is therefore not surprising that the 10-meter-or-so asteroid that blew up over a largely vacant area of the western Pacific on February 1, 1994, producing an explosion equivalent to at least ten times that of the Hiroshima bomb (and possibly rather more), was not seen prior to impact. Surveillance satellites registered it as the brightest such explosion that they have picked up so far. Despite the efforts of numerous scientists in this area of study to make the military aware that such detonations do occur naturally, it appears that the U.S. President was awakened because the Pentagon thought that this incident might be a hostile nuclear explosion.” (10)

Relatively recent, large impacts

Merewether crater, west of Ungava Bay in Canada, is 200m in diameter and was formed less than 10,000 years ago. More recent is the Henbury crater cluster near Alice Springs, Australia. The twelve craters have been dated at between 2,000 and 6,000 years ago. The largest is 180m across and 15m deep. According to Aboriginal legend, the site is known as “sun walk fire devil rock”, suggesting that the event had witnesses.

Recent Near Misses

In 1937 an asteroid called Hermes, with a diameter of one kilometer, became the closest recorded passage to Earth. When it crossed our orbit it was 780,000kms away, twice the distance of the moon. In terms of time, it missed us by a mere 5 hours. It was reported 2 months later, with newspapers claiming we almost witnessed the destruction of our planet. (11)

1989 – a 300m asteroid (known as 4581 Asclepius or 1989 FC) missed us by 690,000kms and 7 hours. It was not spotted until after it had flown by. It is due to return in 2012, but is not expected to come as close.
1991 – a 100m asteroid (1991 BA) passed within 170,000kms.
1996 – a 300–500 m asteroid, (1996 JA1_, passed within 450,000 km of Earth

In the near future, the number one concern is a 320 m asteroid known as 99942 Apophis. Although when first discovered it was considered to have a 1 in 17 chance of hitting Earth, it is now understood to only come as close as 25,600 kilometres – close enough to knock out a communications satellite!

What Are The Odds?

Given recent examples, it is easy to believe that incidences of humans being killed by such impacts are rare. However, when you consider the catastrophic impacts that happen less often ( but when they do, can kill millions), the odds become a little bit more sobering. Austen Atkinson says the odds of being killed by a comet or asteroid is 1 in 24,000. (12) And then he points out that the odds of contracting mad-cow disease is 1 in 15 million, yet that scared most of the world enough to ban the importation of British beef.

1 in 24,000 is roughly the same odds for dying as a result of a plane crash. I’m not alone in worrying about this every time I board a plane…

The problem we have is easily understood – governments generally react after the fact, not before. It is only after some of us have suffered somehow that the government decides to do something about it – they are not in the business of scaring us with predictions of what might happen, unless it suits them (think pre-emptive military strikes).

The folk running the city of New Orleans were very aware of the risk of flooding they faced from hurricanes, and they chose to gamble that it wouldn’t happen on their watch, a gamble that they lost with Katrina. Appropriate spending, relative to the risk they faced, would have saved New Orleans. If it is an “Act of God”, our governments seem to universally elect to be underprepared for it. Especially when, in the case of New Orleans, to be prepared would have taken a commitment of $1billion and 20 years.

At present, NASA’s “Spaceguard Survey”, which aims to spot Near-Earth Objects greater than 1 kilometer in diameter, has a budget of $4.1 million per year from 2006 through to 2012. This is a pitiful effort, relative to the risk, and explains why most new comets are discovered by amateurs.

Effect of a Comet/Asteroid Striking Earth

The easiest to predict is the most likely – the collision occurring in the ocean. First of all, here’s some data regarding recent tsunamis caused by earthquakes and volcanoes…

1960: An earthquake in Chile with a magnitude of 9.5 (the largest magnitude ever recorded) caused the death of 6000 people worldwide. 61 of the victims were in Hawaii, as the result of a tsunami that arrived 14 hours later – when it struck it was 10-15 meters in height . The same tsunami killed 142 people in Japan, when it arrived 22 hours after the earthquake. By then its height had reduced to between 1 and 5 meters. The deep-water wave (the height of the tsunami before it reaches land) was only 20 cms.

2004: The Indian Ocean tsunami was caused by the second largest earthquake ever measured (9.1-9.3), and was much more devastating. It killed more than 225,000 people in eleven countries with waves as high as 30 meters. However, before reaching land it was mere 60 cms in height.

We don’t know when an asteroid or meteor will strike Earth next, but we know that in the past there have been many – they leave a crater. The type of tsunami they can create has not been recorded by humans (as far as we know), so all we have are best guesses coming from experts.

The threat of tsunamis caused by asteroid impacts has only recently been recognized, due to the work of Jack Hills and Patrick Goda of the Los Alamos National Laboratory in New Mexico. They have performed calculations showing that the hypothetical 500-meter asteroid mentioned earlier would produce a deep water wave 50 to 100 meters in amplitude, even at a range of 1,000 kilometers from ground zero. Since the tsunami height could be amplified by a factor of 20 or more in the run up as continental shelves are encountered, we are referring here to a tsunami several kilometers in height. Even if the impact were between New Zealand and Tahiti, the tsunami breaking on Japan would be perhaps 200 to 300 meters high, and heaven help New Zealand and Tahiti themselves!… (13)

In case you are wondering how far inland you would need to be to survive, formulas have been determined. A 200-300 meter tsunami hitting a populated coastline (buildings will slow it hinder its advance) will travel 50-100 kilometers inland, or even further if the terrain is flat. (14)

If you are thinking to yourself “fair enough, but I can’t imagine a tsunami that high ever occurring”, consider this; coral has been found in Lanai, Hawaii, 326 meters above sea level, quite possibly due to a tsunami passing through.

There have been several studies made regarding the Tunguska event. Two figure it was an asteroid, with a diameter of 60, or 90-190 meters. Another study decided it was a comet with a diameter of 1200 meters. (15) According to Duncan Steel, generally speaking, anything that can make it through the atmosphere without disintegrating, and affect more than just the spot where it crashes, would need to be 50 meters wide (for an asteroid) and 100 meters wide (for a comet). To put it another way, if it hits the ocean we either won’t notice, or there will be a substantial tsunami, and nothing in between. (16)

A 100-meter object will typically strike Earth once every 1,000 years, and if it struck land would lay waste to an area of about 10,000 square kilometers – roughly the same size as Connecticut. The deaths just from the impact would depend on the population density, but would be much less than if it struck the ocean. Shin Yabushita has calculated that the odds of most of the Pacific Rim cities being wiped out by an asteroid/comet driven tsunami, in the next century, is 1%. A sobering figure. (17)

In terms of survival, land and sea impacts are quite different. A sea impact will create a tsunami, and depending on the location, could wipe out many major cities. Once the waves subside, Earth is pretty much back to normal, but we will be missing the people and the infrastructure that were destroyed. While this would be tragic for the global economy, people living outside the path of the tsunami will still have their crops and climate, and life will go on.

A terrestrial impact might cause less immediate damage, but create long-term hardships.

Overall, Gilmour and his colleagues have identified a dozen “environmental stresses” caused by the K-T impact. The strong winds and tsunamis lasted for a matter of hours; fires lasted for months, as did the darkness and cold partly caused by the fires; the greenhouse effect began to take grip as the darkness cleared, boosted early on by the presence of water vapour in the air, and maintained by the long-term presence of carbon dioxide; poisons and mutagens remained active for years, as did the effects of acid rain; the ozone layer must have been severely disrupted by the disturbance to the atmosphere, and then there was the volcanic activity triggered by the impact. (18)

As a recent example, the Gribbins mention a “relatively modest” forest fire in California, 1987, which reduced valley temperatures by 15 degrees Celsius for an entire week.
With the ozone layer depleted by nitric oxides, crops would be burnt, and humans venturing outside unprotected would risk cancer. Few crops would survive acid rain, fires, extended periods of darkness and ozone depletion. Humans and animals would starve, and we would also miss the ability of plant life to remove carbon dioxide from our atmosphere. The amount of time it would take for Gaia to return to a steady state environment is not known.

The great K/T boundary extinction of 65 million years ago is a good example of how bad it can get, and how extinctions occur. A 15 kilometer wide asteroid crashed into North America. Debris from the impact was ejected into the atmosphere, and then fell as billions of tiny bullets. A fireball engulfed the continent, soot adding to the dust in the atmosphere. Global temperatures dropped by as much as 10 degrees Celsius. Plants died –they could not survive the triple-whammy of fire, acid rain and lack of sunlight. Large animals starved. Some smaller animals, those that didn’t mind feeding on dead tissue and rotting vegetation, managed to survive, as a species. Phytoplankton, dependant on sunlight, died. Because it was the fundamental basis of the oceanic food chain, the oceans became more about death than life. It is estimated that 75 percent of all Earth’s species became extinct following that singular asteroid impact.

Would we be warned? And how?

In March 1996, a declaration by the Council of Europe, discussing the dangers of Near-Earth Objects, and name-checking Tunguska and Shoemaker-Levy, said that the “possible consequences are so vast that every reasonable effort should be encouraged to minimise them.” (19)

A single impact by a rock the size of the Millennium Dome could devastate the surface of the globe with an explosive release of energy five times more powerful than the entire world’s nuclear arsenal. On 19 May 1996, just such an object came within 280,000 miles of Earth: six hours from collision.

Humankind could have been eradicated.

The asteroid (named JA1) sailed into our system – the largest object to approach Earth, other than the moon, since records began in 1833 – and was only four days away before two astronomers (Tim Spahr and Carl Hergenrother) in Tuscon, Arizona, detected it and alerted the US National Aeronautics and Space Administration (NASA). No one was prepared. Nothing could be done to prevent its approach. Yet no one was told: no public warning was given. The world’s powers watched the asteroid approach, impotent and unable to prevent the end of human civilisation. At the last moment, when it was only 400,000 miles, or seven hours, away from impact, its trajectory carried it away from our world. (20)

A few months later physicist Edward Teller wrote to the British Prime Minister, warning him of the serious threat posed by asteroids and comets – Teller, as a key player in the development of the hydrogen bomb, knew all about how fragile human existence is. (21)

The same year the US Department of Defense created a report that said:

Due to a lack of awareness and emphasis, the world is not socially, economically or politically prepared to deal with the vulnerability of …ECO (Earth Crossing Object) impacts and their potential consequences.

…These authors contend that the stakes are simply too high not to pursue direct and viable solutions to the ECO problem. Indeed, the survival of humanity is at stake. (22)

One of the brightest comets ever seen was Halle-Bopp. With a nucleus estimated at 40kms it is certainly large enough to wipe out all of humankind. It is rather discomforting to learn that we only noticed it in 1995, and if it happened to have been aimed straight at us, would have struck in 1997. Two years would not be enough time to plan/build/launch a defense.

While asteroids mostly follow the same plane of orbit as the planets, quite a narrow band of sky that is under regular observation – comets can come from anywhere, they can sneak up on us. Consequently comets tend to be discovered by amateur astronomers, while NASA lacks the funds (and perhaps willingness) to carry out full-sky observations.

It is not for me to speculate on our future ability to change the path of an asteroid or comet, should one have our planet in its sights. But based on what is currently known, it seems unlikely that we will be prepared to take on such an object if it was destined to crash into us in 2012. However, you never know what NASA might have been putting together in secret.

Therefore my presumption is this, we cannot thwart a comet or asteroid, and if one strikes us in 2012 there will be significant loss of life, depending on the size and location of impact. The bigger it is, the more likely we are to see it coming, yet the greater the odds of it wiping out all of humanity.

1. Peter Grego, Collision Earth (Blandford, 1998), 92.

2. Duncan Steel, Rogue Asteroids and Doomsday Comets (John Wiley & Sons, Inc., 1995), 222.

3. Grego, Collision Earth, 106.

4. Ibid.

5. Vershuur, Gerrit L., Impact!: the threat of comets and asteroids, 166

6. Steel, Rogue Asteroids and Doomsday Comets, 236.

7. Grego, Collision Earth, 79.

8. Ibid., 71.

9. Austen Atkinson, Impact Earth (Virgin, 1999), 81.

10. Steel, Rogue Asteroids and Doomsday Comets, 203-204.

11. Grego, Collision Earth, 101.

12. Atkinson, Impact Earth, 8.

13. Steel, Rogue Asteroids and Doomsday Comets, 40.

14. Ibid., 41.

15. “The Tunguska event,”

16. Steel, Rogue Asteroids and Doomsday Comets, 44.

17. Vershuur, Gerrit L., Impact!: the threat of comets and asteroids, 166.

18. John & Mary Gribbin, Fire on Earth (Pocket Books, 1996), 37.

19. Atkinson, Impact Earth, 84.

20. Ibid., 3.

21. Ibid., 4.

22. Ibid., 6.