Airburst

Primary reference(s)

Lexico Dictionary, no date. Airburst. Accessed 17 December 2019.

Additional scientific description

Meteoroids are objects in space that range in size from dust grains to small asteroids. Think of them as ‘space rocks’: when meteoroids enter Earth’s atmosphere (or that of another planet, like Mars) at high speed and burn up, the fireballs or ‘shooting stars’ are called meteors. When a meteoroid survives a trip through the atmosphere and hits the ground, it is called a meteorite (NASA, no date).

Research has revealed why meteors explode before impacting the Earth. During model simulations of meteors entering Earth’s atmosphere, air that was pushed into the meteoroid was allowed to percolate inside, which lowered the strength of the meteoroid significantly. In essence, air was able to reach the insides of the meteoroid and cause it to explode from inside out (Tabetah and Melosh, 2017; Williams, 2017).

The hazardous effects are estimated using both semi-analytical models, realised now as simple calculators, and numerical simulations of airbursts of large meteoroids and asteroids. The numerical simulations are based on the equations of hydrodynamics and radiation transfer. Dangerous consequences include shock waves with high wind speeds, fluxes of thermal radiation capable of igniting fires and causing skin burns, seismic effects of airbursts, and ionosphere disturbances. The dangerous effects of a shock wave and thermal radiation on people are considered in the context of the Chelyabinsk meteorite of 2013 and the Tunguska airburst of 1908 (Ryabova et al., 2019).

On 15 February 2013, a meteor exploded in the sky over Chelyabinsk, southern Russia. Although no people or buildings were hit by the resulting meteorite, the shockwave from the exploding object injured about 1500 people and caused damage to 7200 buildings in the region. The fireball was caught on video, mainly by dash cameras throughout the region, and posted on the internet by news organisations and individuals. Although the Chelyabinsk meteorite probably weighed about 12,000 to 13,000 tonnes and measured 17 to 20 metres in diameter before exploding, scientists were quick to point out that it was very small compared to other objects that could potentially hit the Earth. The explosion released energy estimated at about 500 kilotons of TNT (about 20 to 30 times more energy than the Hiroshima atomic bomb). The event brought to the world’s attention the very real hazards associated with the impact of objects from outer space (Nelson, 2018).

In the early morning of 30 June 1908, a powerful explosion over the basin of the Podkamennaya Tunguska River (Central Siberia), devastated 2150 ± 50 km2 of Siberian taiga. Eighty million trees were thought to have been flattened, and a great number of trees and bushes were burnt in a large part of the explosion area. Eyewitnesses described the flight of a “fire ball, bright as the sun”. Seismic and pressure waves were recorded in many observatories across the world. Bright nights were seen over much of Eurasia. These different phenomena, initially considered non-correlated, were subsequently linked together as different aspects of the ‘Tunguska event’.

Metrics and numeric limits

Not applicable.

Examples of drivers, outcomes and risk management

The International Asteroid Warning Network (IAWN) was established in 2013 as a result of the UN-endorsed recommendations for an international response to a potential near-Earth object (NEO) impact threat, to create an international group of organisations involved in detecting, tracking, and characterising NEOs. IAWN is tasked with developing a strategy using well-defined communication plans and protocols to assist governments in analysing asteroid impact consequences and in planning mitigation responses. Currently, IAWN includes members from Europe, Asia, South America and North America (IAWN, 2020).

IAWN has proposed the following definition: An asteroid, meteoroid, or a comet as it passes near Earth, enters the Earth’s atmosphere, and/or strikes the Earth, or provokes changes in inter-planetary conditions that affect the Earth’s magnetosphere, ionosphere, and thermosphere.

Criteria and thresholds related to this definition are as follows: the probability that an NEO will impact Earth (either 1% warning and 10% for terrestrial preparedness planning); the probable size, or at least its luminosity in the night sky (greater than 10 meters or at least absolute magnitude 28); and how far in the future the NEO will impact Earth (20 years). The European Space Agency’s (ESA) Space Situational Awareness (SSA) programme partners with many countries, organisations and individuals. In particular, it has strong links with the United Nations Committee on the Peaceful Uses of Outer Space (UNCOPUOS), which facilitates the Space Mission Planning Advisory Group (SMPAG) and IAWN. SMPAG and IAWN were both established in response to the need for an international response to the threat of NEO impacts. SMPAG coordinates the technological know-how of agencies, including ESA, recommending specific responses to asteroid threats, including basic research and development, impact mitigation measures and deflection missions. Depending on the capabilities and specific technologies available to each agency, options are made to ensure the best use is made of skills of each organisation. ESA’s primary projects are the SSA programme – especially the dedicated NEO segment, mapping of threat scenarios to mission types, as well as AIM – ESA’s Asteroid Impact Mission (ESA, 2018).

IAWN and SMPAG have agreed on the following criteria and thresholds for impact-response actions:

1. IAWN shall warn of predicted impacts exceeding a probability of 1% for all objects characterised to be greater than 10 metres in size, or roughly equivalent to absolute magnitude of 28 if only brightness data can be collected. Rationale: Impact probabilities greater than 1% are rare. Most objects greater than 10 meters in size will have effects (air blast and pieces) that could reach the Earth’s surface. IAWN is compelled to warn populations if bodies will have effects that reach the ground. Setting threshold at 1% is a compromise between not being overly alarmist and not warning too late for necessary action to be initiated. It is a probability figure that individuals and governments can comprehend. An alert such as this demonstrates that IAWN is functioning. Further, it ensures the flow of communications from IAWN to the public and the United Nations.
2. Terrestrial preparedness planning is recommended to begin when warned of a possible impact: predicted to be within 20 years; when probability of impact is assessed to be greater than 10%; and when the object is characterised to be greater than 20 meters in size, or roughly equivalent to absolute magnitude of 27 if only brightness data can be collected. Rationale: Terrestrial preparedness and increased potential for impact will also involve determination of a ‘risk corridor’ from objects with 10% impact probabilities and impacts in less than 20 years. This provides populations and population centres on the Earth with the information to begin preparations for emergency preparedness if needed. The surprising effects of the Chelyabinsk event in 2013 from an object ~18 meters in size, in turn led to the establishment of a lower limit (20 meters) in these threshold criteria.
3. SMPAG should start mission option(s) planning when warned of a possible impact: predicted to be within 50 years; when probability is assessed to be greater than 1%; and when the object is characterised to be greater than 50 meters in size, or roughly equivalent to absolute magnitude of 26 if only brightness data can be collected. Rationale: Several decades warning, if available, enables sufficient lead time to mount characterisation missions. If a 1% probability on a 100-meter object is assessed, SMPAG will be informed immediately following verification of the precise orbit. Part of such a characterisation mission would likely deploy a transponder with the object.