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!
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