Impact consequences

In the Solar System, there is strong evidence that asteroids hit our and other planets from time to time. According to the Lunar and Planetary Laboratory at the University of Arizona in the history of the Earth there have been more than 3 million impact craters larger than 1 km in diameter produced from impacting objects, the largest craters stretching more than 1000 km in diameter!!

Today, due to many geological processes on Earth there are about 160 surviving impact craters found on Earth so far. These are shown in the picture below.

Earth is not the only place where this phenomenon takes place. In fact, all solid bodies such as the rocky planets, their moons and asteroids are covered with craters ranging from a few meters to huge structures.

The locations of terrestrial impact craters (from Lunar and Planetary Laboratory, University of Arizona)

The first crater ever recognized on Earth as being generated by a NEO impact was Arizona's mile-wide Meteor Crater, also called Barringer Crater. A second famous impact site, located off the coast of the Yucatan Peninsula in Mexico, is the Chicxulub crater, buried under ocean sediments and believed to be a record of the event that led to the extinction of the dinosaurs 65 million years ago. Fortunately for us, these big asteroid impacts are extremely rare.

But how dangerous can an impact be?

Artist's impression of asteroid impact with Earth

The answer to this question depends on the impactor’s characteristics. Firstly, on the kinetic energy of the colliding body, so the object mass and orbit are key parameters in assessing the risk to Earth. Most of the objects that fall on Earth are not bigger than a meter and are therefore destroyed before they reach the ground. In fact, on our planet about 100 tons of interplanetary material rift down to Earth daily. Luckily, most of these objects are too small to reach the ground, and don't represent a danger, since our atmosphere acts as a real shield, burning out all small debris. Burning up, the debris can leave beautiful trails of light known as meteors or "shooting stars”.

The possibility of fragmentation or explosion of the asteroid before reaching the ground, and the consequences of an impact can be dramatically different depending on the object’s internal structure and composition. Therefore, good knowledge about the physical properties of a NEO is necessary to calculate not just its impact energy but also the emerging consequences of a collision with our planet.

For our heavily populated world and highly networked society the impact of an asteroid or comet on the Earth today could produce a natural catastrophe far more damaging to civilization than any in recorded history.

…but just how often?

When the impactor is big enough, it can survive to this fall and reach the ground with some very evident consequences, such as the formation of impact craters or other possible destructive after effects.

The hazard represented by NEO impacts increases with the size of the projectile: the greatest risk is associated with objects larger than 1-2 kilometers, which are capable of causing major regional or global disasters, injecting large quantities of dust into the stratosphere and perturbing the Earth's climate on a global scale.

Luckily, these very dangerous asteroids are extremely rare: they are thought to impact Earth only a few times every 1,000,000 years, on average.

The estimated frequency and effects of impactors as a function of size are shown in the Table below.

NEO impact: Frequency and consequences (from NEOMAP report)

Impactor size (m)

Mean impact interval (years)

Energy released (megatons TNT)*

Crater diameter (km)

Possible effects/comparable event

30

200

2

-

Fireball, shock-wave, minor damage

50

2500

10

under 1

Tunguska explosion or small crater

100

5000

80

2

Largest H-bomb detonation

200

47,000

600

4

Destruction on national scale.

500

200,000

10,000

10

Destruction on European scale.

1000

600,000

80,000

20

Many millions dead, global effects.

5000

20 million

10 million

100

Billions dead, global climate change.

10,000

100 million

80 million

200

Extinction of human civilization.

*Note: The energy release estimates assume a density of 3500 kg m-3 (stony body) and an impact velocity of 20 km s-1.

For more information about the energy of an impact and how it is calculated see the SpaceGuard Impact Characteristics pages.

While good progress is being made in the discovery of NEOs with diameters of 1 km or larger, the population of NEOs in the hundreds of metres size category is still largely undiscovered; such objects impact far more frequently and can cause damage on a national scale or worse, leading to millions of deaths.

Difficulties in predicting impacts and consequences

The largest uncertainty in risk analysis arises from our incomplete knowledge of NEOs, more precisely on their existence and orbits (to assess the risk of collision with the Earth) and on their mass and physical characteristics (to assess the consequences of an impact with the Earth).

The difficulty in estimating the consequences of an impact of a sizable, say 1 km diameter asteroid, is the scaling over many orders of magnitudes from practicable laboratory or even field experiments (like nuclear explosion tests) to events that prevail in a real impact situation. In the latter case, one does not only have to simulate the excavation of the crater formation, but also the effects on the Earth's atmosphere or the response of the Earth’s crust for the prediction of earthquakes.

Another important aspect to consider is the simulation of an impact into the sea, since 3/4 of the Earth surface is covered by oceans. Since the majority of the population on the planet lives near coastal areas, the prediction of the resulting tsunami wave and its effects on the surrounding shores are of paramount importance in assessing the global risk from NEOs. Current models suggest that the threat of giant tsunamis from ocean impacts with small to medium sized asteroids (a few 100 to a few 1000 m) actually represent the greatest threat to human life. In this context, knowledge of the internal structure and strength of the NEO is of crucial importance as it affects the likelihood of the asteroid breaking up in the atmosphere. The tsunamis arising from a single giant impact or several smaller ones can be expected to have dramatically different characteristics (in terms of height and radius of the tsunami wave and hence depth of inland penetration).

For more information about known impacts and their locations follow this link to the UK NEO Information Centre website.

Last update: 23 May 2012

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