Asteroid impact

What is at stake?

The largest near-Earth asteroids – those with a diameter of more than 1 kilometre – have the potential to cause geologic and climate effects on a global scale, disrupting human civilisation, and perhaps even resulting in the extinction of our species. Smaller near-Earth objects (NEOs) in the 140 metre to 1 km size range could cause regional or continental devastation, potentially killing hundreds of millions of people. Impactors in the 50 to 140 metre range are a local threat if they hit in a populated region and have the potential to destroy city-sized areas. NEOs in the 20 to 50 metre range generally disintegrate in Earth’s atmosphere but can cause localised blast and impact effects.

The Chelyabinsk Event in Russia in 2013 is believed to have been caused by an airburst of an NEO with a 20 metre diameter. It caused localised damage in the city and injured nearly 1,600 mainly from debris and shattered glass from the blast.

"The largest near-Earth asteroids – those with a diameter of more than 1 kilometre – have the potential to cause geologic and climate effects on a global scale, disrupting human civilisation, and perhaps even resulting in the extinction of our species."

How much do we know?

Surveys of the NEO population since the 1990s have discovered more than 22,800 NEOs as of May 2020. A record 2,433 NEOs were discovered in 2019. In the United States, NASA’s Planetary Defense Program has a congressionally directed requirement to discover at least 90 per cent of potentially hazardous asteroids larger than 140 metres across.

As of May 2020, 9,100 NEOs larger than 140 metres have been discovered. This is believed to be approximately 38 per cent of the total population of NEOs above this size.

Smaller asteroids are also continually being discovered, with the reservoir of NEOs with diameters between 50 and 140 metres expected to be approximately 300,000. This means these are the more likely impact threat in the near term. Impactors of these sizes are expected to have an average frequency of one per ~1000 years. The Tonguska event (1908) is believed to have been an impactor in the lower end of this size range.

"The impact of an asteroid bigger than 1km in size would release enough particles in the atmosphere to dim the sun for a number of months."

What are key factors affecting risk levels?

The risk presented by an NEO is related to the probability of impact with Earth, the size and composition of the asteroid and the location of impact. The first step required in assessing the risk from larger NEOs is the completion of the census of NEOs larger than 140 metres across. Risk assessment also requires an observational assessment programme to refine knowledge of the orbit and to characterise the size and composition of the asteroid. This could include specialised ground and space-based observations, or a spacecraft reconnaissance mission to the asteroid.

Accurate orbital knowledge is required to establish the “impact corridor” – the areas on Earth where, given uncertainties in the orbital knowledge, the impact is most likely to occur. The size and composition of the asteroid are used to model impact effects and determine the potential severity of an impact.

"The goal is the global protection of human civilisation and our ecosystem."

In the event of a credible impact threat prediction, warnings will be issued by the IAWN if the object is assessed to be larger than 10 metres. If the object is larger than about 50 metres and the impact probability is larger than 1 per cent within the next 50 years, the SMPAG would start to assess in-space mitigation options and implementation plans for consideration by the Member States. The goal is the global protection of human civilisation and our ecosystem. With vigilance and sufficient warning, an asteroid impact is a devastating natural disaster that can be prevented.

Reviewed by

Gerhard Drolshagen

University of Oldenburg and the European Space Agency

Lindley Johnson

NASA Planetary Defense Officer and Program Executive of the Planetary Defense Coordination Office

Romana Kofler

United Nations Office for Outer Space Affairs

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