What is the likelihood of a "catastrophe-sized" asteroid colliding with the earth?

Question posed by Jennie.

By Fredrik at en.wikipedia [Public domain], from Wikimedia Commons

Dust and small rocks collectively measuring tens of tonnes hit the Earth's atmosphere every day, but we don't notice this as it burns up well before it gets anywhere near us. Objects between 5 and 10 metres in diameter enter the Earth's atmosphere approximately once per year, and usually explode in the upper atmosphere with a force in the order of the Hiroshima atomic bomb, with most of their mass being vaporised in the process.


Larger objects
Objects with diameters in the order of 50 metres strike the Earth approximately once every thousand years.

On June 30th 1908 a powerful explosion (the most widely accepted estimates are between 10-15 megatons*) about 5 - 10 kilometres above the Earth's surface flattened approximately 2,150 square kilometres of forest (an estimated 80 million trees) near the Tunguska river in Russia.

The explosion is now widely accepted to have been caused by the air burst of a meteoroid or comet fragment of a few tens of metres in diameter. Had the same event occurred over, for example, London, much of the area within the M25 motorway would have seen significant destruction.

The Tunguska event is widely believed to be the largest Earth impact in recent history, although it must be noted that similar incidents occurring over remote ocean locations would most likely go unnoticed. More recent, smaller, documented events include a 10 metre object exploding in the air above the Mediterranean Sea on June 6th 2002, and a small asteroid that exploded over a sparsely populated area of Sudan on October 6th 2008. This was the first Earth impact to be accurately predicted, with the object being discovered and the collision calculated the previous day.


Even larger objects
Objects with a diameter in the order of one kilometre are thought to collide with the Earth once or twice every million years. Objects in the region of 5 km in diameter are expected once in every ten million years, and larger impacts by objects of several kilometres diameter are thought to occur less often than once every hundred million years.


Likelihood of a collision
As you can see, the likelihood of a collision of an object with Earth changes with its size- small objects hit our atmosphere every day, and larger objects less often. It is futile to attempt to put a number to the probability, but it's relatively easy to make some simple observations:

• Objects large enough to do significant damage to large metropolitan areas are very rare, and most such objects will, statistically, hit unpopulated areas of land or ocean.
• Objects large enough to cause geological changes, or changes in climate, are even more rare, but one such event stands a good chance of affecting the entire planet regardless of where it hits.
• The chances of a significant impact occurring in any given year are very remote. This does not mean that they can't happen, though, and given enough time, it can be assumed that something large enough to do considerable damage will come along eventually. Whether that's in 5, 5,000 or 5,000,000 years is impossible to tell.

Some Earth impact craters

There is evidence of large prehistoric Earth impacts dotted around the planet. Here are some of the most famous.

The Barringer Crater, Northern Arizona Desert

Also known as Meteor Crater, is just over a kilometre in diameter, and  around 170 metres deep. It is around 50,000 years old and was produced by a nickel-iron meteorite about 50 metres across. The owners of the crater claim it to be the "best proven, best preserved meteorite crater on Earth, and it even has its own website. One if its most interesting features is its slightly square shape. This is believed to have been caused by cracks that existed in the surface of the Earth in the area prior to the impact.

The Manicouagan Crater, Quebec, Canada

This crater is a multiple-ring crater 100 kilometres in diameter, with its most striking feature being the annular lake that has formed in it's 70 kilometre diameter ring. It was formed around 214 million years ago by the impact of a 5 kilometre diameter asteroid. It has been suggested that the Manicouagan Crater is part of a series of craters formed by a (hypothetical) multiple-impact event. It forms a chain with four other craters (Red Wing Crater, North Dakota; Saint Martin Crater, Manitoba; C, France; and Obolon' Crater, Ukraine)** which suggests the possibility of the breakup and impact of a large body, similar to that observed on Jupiter by comet Shoemaker-Levy 9 in 1994.

The Chicxulub Crater, Yucatán Peninsula, Mexico

This is one of the largest impact craters evident on Earth, at more than 180 kilometres in diameter. This means that the object that caused the crater was around 10 kilometres in diameter and released energy equivalent to 100 million megatons of TNT on impact. The crater is not visible on the surface but is very important as it was formed around 65 million years ago and is widely believed to have been instrumental in the extinction of the dinosaurs. The impact would have caused tsunamis over a thousand metres high, as well as global earthquakes and volcanic eruptions. Longer-term effects would have included a dramatic cooling of the Earth's surface caused by particles of dust and ash blocking out much of the light from the Sun. Some scientists have suggested that this was just one event of a number of impacts that occurred in a short time frame, possibly a result of the breakup and subsequent impact of a larger body. This appears, however, to be the subject of much debate in the field.

The map at the top of this page displays the locations of all of Earth's known impact sites. The circles are not to scale, but sized proportionally to the diameter of the impact crater, and the colour indicates approximate age of each crater (in four groups). Click the image to be taken to the originators website for more information.




Have a question about this topic? Comment below! Got an astronomy related question of your own? Ask it here.

*This is about 1000 times as powerful as the Hiroshima bomb
** These craters are all hidden below the surface of the Earth, which is why I haven't included pictures!