What is the risk?
NEOs, asteroids and the occasional comet, are objects that travel close enough to Earth’s orbit to warrant constant attention. Since the 1990s, astronomers have discovered over 39,000 of them, including more than 11,000 that are larger than 140 metres across. The largest recorded impact, the Tunguska event in 1908, is believed to have been an impactor in the lower end of this size range. It exploded in the atmosphere above the sparsely inhabited East Siberian taiga flattening trees over an area of 2,000 km2, roughly the size of greater London.
An asteroid the size of a football field, approximately 100 metres, can carry the energy of several thousand Hiroshima bombs. The chance of one striking Earth in any given year is extremely small, but not zero. Smaller asteroid impacts are expected to have an average frequency of roughly one per 1,000 years.
There are an estimated 300,000 small asteroids, making these the more likely, near-term threat. When it comes to larger asteroids, larger than 1 kilometre in diameter, scientists estimate that they have discovered and identified more than 96 per cent of the existing large asteroids, 878 individuals to date[1].
An asteroid the size of a football field, approximately 100 metres, can carry the energy of several thousand Hiroshima bombs.
What is at stake?
Various sizes of asteroids would have various degrees of impacts, if they were to hit earth.
◄ Asteroids >1 km could alter global cli-mate and threaten civilisation itself.
◄ Asteroids 140 m–1 km would devastate a region or continent, killing millions.
◄ Asteroids 50–140 m could destroy a city if it struck land.
◄ Asteroids 20–50 m usually explode in the atmosphere but can shatter windows and injure people. For example, the Chelyab-insk meteor that struck the Ural region of Russia in 2013.
What are key factors affecting risk levels?
Three things decide how dangerous an asteroid is:
- Impact probability — how likely it is to cross Earth’s path.
- Size and makeup — rock, metal or rubble determines how it behaves in the atmosphere.
- Where it hits — ocean, desert or city.
When an asteroid is identified, astronomers work on defining the orbit, size and composition. This helps determine the impact corridor, the areas on Earth where 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.
What is being done in global governance to mitigate this risk and where are there gaps?
The United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) and the United Nations Office for Outer Space Affairs (UNOO-SA) — the UN entity with a dedicated mandate to advance the peaceful uses of outer space and democratise access to space and its bene-fits — coordinate international cooperation in space safety and sustainability. To strengthen global preparedness for NEO threats, COPUOS established two specialised agencies: International Asteroid Warning Network (IAWN)
and Space Mission Planning Advisory Group (SMPAG). Together, these entities enable the detection, tracking and impact risk assessment of NEOs and support planetary defence measures, such as asteroid deflection. Collectively, these mechanisms make an asteroid impact the only natural disaster humanity has the capability to prevent — if action is taken in time.
◄ IAWN shares global observation data and issues early warnings. The IAWN links together the institutions that are already performing many vital functions, including: discovering, monitoring and physically characterising the potentially hazardous NEO population. One of its purposes is to maintain an internationally recognised clearing house for the receipt, acknowledgement and processing of all NEO observations.
◄ SMPAG is composed of member states with space agencies or intergovernmental entities that coordinate and fund space activities and are capable of contributing to or carrying out a space-based NEO mitigation campaign. SMPAG has 20 members and six observers, as of October 2025, with UNOOSA acting as its secretariat[2].
UNOOSA plays a pivotal role in fostering global cooperation and transparency in space activities, recognising the critical importance of timely and accurate information-sharing in the event of a potential asteroid threat. Should IAWN detect a credible impact threat, it is mandated to provide and disseminate relevant information through UNOOSA to all UN member states[1]. This mechanism ensures that every country — including developing nations with limited technical capacity to track or assess NEO hazards — has timely access to critical data and warnings. Such coordination directly advances the UN General Assembly’s annual resolution on international cooperation in the peaceful uses of outer space[2].
In the event of a credible impact threat, warnings are issued by IAWN if the object is assessed to be larger than 10 metres. If the object is larger than 50 metres and the impact probability is larger than 1 per cent within the next 50 years, 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.
Other risk overviews
Catastrophic Climate Change – overview
What is the risk? Climate change is the long-term alteration of the Earth’s climate system, driven primarily by the accumulation…
Ecological Collapse – overview
What is the risk? These systems can tolerate some impact from human use and recover after a period of time…
Weapons of Mass Destruction – overview
What is the risk? Hostile geopolitical relations and the deteriorated strategic context are eroding the structures that govern the world’s…
Artificial Intelligence – overview
How is AI integrated in military decision-making? Several countries now incorporate AI into their military operations through decision support systems…
