What is at stake?

The largest near-Earth asteroids (> 1 km diameter) have the potential to cause geologic and climate effects on a global scale, disrupting human civilization, and perhaps even resulting in extinction of the species. Smaller NEOs in the 140m to 1 km size range could cause regional up to continental devastation, potentially killing hundreds of millions. Impactors in the 50 to 140-meter diameter 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 m diameter range are generally disintegrated in Earth’s atmosphere but can cause localized blast and impact effects.

"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 on-going since the 1990s have discovered more than 26,000 NEOs of all sizes as of June 2021. A new record 2959 NEOs of all sizes were discovered in 2020. The current surveys have discovered more than 96% of the population of NEOs larger than 1 km in diameter – 889 individual asteroids. In the United States, NASA’s Planetary Defense Program has a Congressionally directed objective to discover at least 90% of potentially hazardous asteroids 140 meters and larger in size. As of 1 June 2021, 9663 NEOs larger than 140 m have been discovered. This is estimated to be approximately 39% of the total population of NEOs this size or larger.

Smaller asteroids are also continually being discovered, with the reservoir of NEOs with diameters between 50 and 140 meters expected to be approximately 300,000, making these 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 Tunguska event (1908) is believed to have been an impactor in the lower end of this size range. The total number of NEOs larger than 10 m could be as high as 50-100 Million.

What are key factors affecting risk levels?

The assessment of the risk presented by an NEO is related to the probability of impact with Earth, the size and composition of the asteroid, and where on Earth the impact occurs. Beyond discovery of NEOs, the risk assessment for a NEO with the potential to impact Earth requires an observational assessment program to refine knowledge of the orbit and to characterize the size and composition of the asteroid. This could include specialized 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 impact location and potential severity of damage will determine the risk level, and the required governmental response, either in terms of disaster preparedness or potential asteroid deflection attempts.

The recent sample return missions to the asteroids Ruygu (Hayabusa2) and Bennu (OSIRES-REx) contribute considerable to our knowledge of these NEOs. The main objectives of these missions were scientific, but the characterisation of natural parameters of these objects is also important for planning of potential future Planetary Defence missions.

"With vigilance and sufficient warning, an asteroid impact is a devastating natural disaster that can be prevented."

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 meters in size. If the object is larger than about 50 m and the impact probability is larger than 1% within the next 50 years, the SMPAG would start to assess in-space mitigation options and implementation plans for consideration by the Member States. With vigilance and sufficient warning, an asteroid impact is a devastating natural disaster that can be prevented.

At the end of 2021 the DART impactor will be launched by NASA. DART will impact Dimorphos, the smaller 160 m companion of the 780 m large Didymos, in early fall 2022. It will test the capabilities to deflect an asteroid by a high velocity impact of the spacecraft. A few years later ESA will launch the HERA spacecraft to study the impact effects in detail. If successful these missions will demonstrate that an impact can be avoided by active measures if the object is discovered soon enough.

SMPAG plans to perform exercises to test their function and coordination for realistic impact threats. These exercises should also clarify the working procedure, form of recommendations and flow of information. They should also identify missing technologies and other potential deficiencies in the field of space-based NEO mitigation.

Governance of asteroid impact

The issue of near-Earth objects (NEOs) has long been on the agenda of the Committee on the Peaceful Uses of Outer Space (COPUOS), the primary United Nations body...

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