NEOs impact the Earth every day. Yes, every day. 

But these are very small NEOs, and most burn up within the Earth's atmosphere and don't even make it to the surface. Every now and then a NEO makes it to the surface and is then classified as a meteorite.

There are many more smaller NEOs than larger ones (see the NEO information page) and so there are likely to be more impacts with smaller NEOs. And luckily for the Earth, it is only the larger ones that can seriously harm our civilisation.



It's all about Probabilities

It's therefore not a question of if a NEO will hit the Earth, but rather when and how big.

The question of mitigation - that is, preventing a NEO from hitting the Earth (see the Mitigation section for information on space missions to mitigate NEO impacts) - is a question of probabilities.

In the table below, the average interval between impacts for different sized NEOs is given. For example, a NEO of 100m diameter will impact the Earth on average every 10,000 years. Also shown is a comparison of the energy released from an impact to the energy released by the Hiroshima atomic bomb.

It should be noted that these estimates are very uncertain and based on statistical considerations. The potential of a 30m - 50m object to cause damage on the ground is not well understood. If such an object exploded over a city it could cause major loss of life and severe material damage.

Furthermore, the estimated statistical frequency of impacts can provide a false sense of security: on the basis of statistical studies a "Tunguska-like event" is predicted to occur once every 2000 years; the actual Tunguska event, however, occurred only 100 years ago (and the size of the object is still being debated). Statistical analyses indicate a 300 m diameter object should pass by the earth at a distance of 30,000 km once every 1000 years. However, the next occurrence of such an event, namely the close approach of the potentially hazardous asteroid Apophis, will be in 2029!

NEO diameter (m) larger than:	Average interval between impacts (years)	Energy released (megatons of TNT)	Crater diameter (km)	Possible effects/comparable event
-	-	0.015	-	Hiroshima atomic bomb detonation.
30	300	2	-	Fireball, shock-wave, minor damage.
50	2000	10	≤1	Tunguska-type explosion or small crater.
100	10,000	80	2	Largest H-bomb detonation.
200	40,000	600	4	Destruction on national scale.
500	200,000	10,000	10	Destruction on continental 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.

(Table from Alan W. Harris (US) “Estimating the NEO population and impact risk: past, present and future” presented at the 1st IAA Planetary Defense Conference, 2009)

Past Impacts
The first crater to be identified as being caused by an asteroid impact is the Barringer crater in Arizona, USA.  It is 1.2 kilometres across and caused by an impact around 49,000 years ago.

Below an image of the crater (image courtesy Alan Harris, DLR). 

Probably now the most famous asteroid impact is the one that most likely caused the extinction of the dinosaurs 65 million years ago. The so called Cretaceous-Tertiary (K-T) mass extinction was caused by a 10km - 15 km asteroid hitting the Yucatan Peninsula in Mexico. (Dinosaur image source - BBC News © 2009 BBC)

The Chicxulub crater, as it is now known, can be seen in this Shuttle Radar Topography Mission (SRTM) image (credit NASA/JPL). The crater can be seen in the top-left of the image.

      

Another asteroid impact in more recent times was the impact of an asteroid over Tunguska in Siberia, Russia. On 30 June 1908 the asteroid most likely exploded in the air before hitting the ground, causing the destruction of 2000 square kilometres of forest.

The asteroid that caused this damage was around 30m - 50m in diameter.

An image from an expedition to Tunguska by Leonid Kulik in 1927 is shown here.

Reports on the Threat from NEOs
There have been many reports on the threat from NEOs. A selection of links to reports available online is given here:

National Research Council (US) Report, 2010:
Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies

Association for Space Explorers (International), 2008:
Asteroid Threats: A Call for Global Response

International Accademy of Astronautics (IAA) (International), 2009:
Dealing with the Threat to Earth from Asteroids and Comets

Task Force on Potentially Hazardous Near Earth Objects (UK), 2000:
Report of the Task Force on potentially hazardous NEAR EARTH OBJECTS