Asteroid or NEO impact and cause a threat to Earth everyday!
Contents at a Glance
However, these NEOs are usually very small and most of them burn up within the Earth’s atmosphere, not even making it to the surface. Every now and then a NEO makes it to the surface and it is then classified as a meteorite.
Smaller NEOs are greater in number than larger NEOs (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 probability
It’s therefore not a question of if a NEO will hit the Earth, but rather when it will hit and how big it will be. 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 probability.
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 asteroid impact to the energy released by a tsunami.
It should be noted that these estimates are very uncertain and based on statistical considerations. The potential a 30 – 50m object has 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 1000 years; the actual Tunguska event, however, occurred only 100 years ago (and the size of the object is still being debated).
Statistical analyses indicate an approach at a distance of 30,000 km from the Earth’s surface by an object with a diameter of 270 m occurs, on average, at intervals greater than ~800 years. (Giorgini et al., 2008, Icarus 193, 1-19)”.
However, the next occurrence of such an event, namely the close approach of the potentially hazardous asteroid Apophis, will be in 2029!
Estimated NEA numbers, impact intervals, impact consequences, and mitigation possibilities
(Harris, A. W. ; Boslough, M. ; Chapman, C. R. ; Drube, L. ; Michel, P. ; Harris, A. W.: "Asteroid Impacts and Modern Civilization: Can we Prevent a Catastrophe?", Asteroids IV, Univ. Arizona Press, 2015)
|NEA diameter, D [m]||Estimated total number with diameter ≥ D in the NEA population||Indicative impact interval [yr] for NEAs with diam. ≥ D||Possible consequences of impact near populated region*||Appropriate mitigation/deflection strategy assuming current technology|
|10||100 million||5||Meteorite falls; crater unlikely||Civil defense only|
|30||3 million||150||Chelyabinsk/Tunguska-type airburst; crater, depending on composition; some injuries and deaths||Civil defense only|
|50||500,000||1,000||Violent Tunguska-type airburst; crater, depending on composition; potentially many injuries and deaths||Slow push/pull (e.g. gravity tractor) or kinetic impactor, if feasible, civil defense only if not|
|100||50,000||10,000||Crater 1-2 km in diameter; local destruction; tsunami risk from near-shore impacts; many deaths likely||Civil defense; slow push/pull or kinetic impactor - slow push/pull combination|
|300||7,000||70,000||Crater several km in diameter; regional/national destruction; tsunami risk; potentially millions of deaths||Kinetic impactor - slow push/pull combination if feasible, explosive impulse if not, plus civil defense if practical|
|500||3,500||140,000||Crater some 10 km in diameter; international catastrophe; tsunami risk; potentially millions of deaths||Kinetic impactor - slow push/pull combination if feasible; several kinetic impactors may be necessary. Explosive impulse if no alternative|
|1000||1,000||500,000||Global effects; partial disruption of civilization||Series of kinetic impactors or large explosive impulse|
|10,000||3**||100 million||End of present civilization||Series of large explosive impulses; deflection may not be feasible with current technology|
* The effects listed are rough estimates based on incomplete knowledge of NEA physical characteristics and impact processes in the atmosphere and on the ground. The actual outcome of an impact could differ considerably, depending on parameters such as composition and density of the impactor, impact velocity, impact angle, and the nature of the surface impacted.
** The remaining risk is dominated by long-period comets; deflection would not be possible with any foreseeable technology.
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 was caused by an impact around 49,000 years ago.
The most famous asteroid impact to date 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.
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 as a faint circular arc across the top-left of the peninsula.
More recently, on the 30th of June 1908, there was an asteroid impact over Tunguska in Siberia, Russia. The asteroid, which was around 30 – 50m in diameter, most likely exploded in the air before hitting the ground, causing the destruction of 2000 square kilometers of forest.
Left: an image from an expedition to Tunguska. Leonid Kulik. 1927.
On February 15th 2013, a 17m-large meteorite blew up in the Russian sky, damaging thousands of buildings and injuring 1,500 people with its shock wave.
After the explosion the so called Chelyabinsk meteorite broke into approximately seven large fragments, one which made an 8m wide hole in the ice of the Chebarkul Lake. The asteroid speed was 10 – 29 kilometers per second. Scientists claim the NEO was part of a 190m-diameter celestial body.
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 provided 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