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## Asteroid Day: How do you prepare for an asteroid impact?

      By Astrogeology Science Center      June 28, 2022    

The United Nations observes Asteroid Day every year on June 30 to raise awareness of the risks of asteroid impacts, coordinating events worldwide to share knowledge. Flagstaff’s Lowell Observatory and Meteor Crater will be hosting events, and although USGS Astrogeology isn’t hosting an event, our scientists work tirelessly to understand hazards and mitigation for asteroid impacts.

### Quick Facts:

Asteroid impacts on Earth happen all the time, but most of the time we never know about them. When an asteroid travels from the outer solar system or near Earth’s orbit and falls to Earth and impacts, it becomes a meteorite.
                      - Smallest meteors burn up in the upper atmosphere before ever reaching Earth’s surface to become meteorites.
                      - Slightly larger meteors may make it through the atmosphere but often crash at locations that cause minimal or no immediate damage. Earth’s oceans cover ~70% of Earth’s surface, and many meteorites wind up there or land harmlessly in open areas (often to be picked up by meteorite collectors or enthusiasts), but some this size can cause minimal damage.
                      - Larger meteors can cause local effects and damage over small areas or regions.

**The Chelyabinsk meteor** was ~20 m (66 ft) in diameter and although the impact produced minimal damage to the local area, the entry of the meteor created an air burst in the atmosphere. This air burst created a shock wave that resulted in about 1,500 injuries to people and a large amount of damage to buildings and houses. Larger meteors can cause damage over regional areas that have effects outside of the range of the fallout from the impact itself.

**The Tunguska event** is known as the largest impact event in recorded history and happened in 1908 in east-central Russia. The meteor was potentially ~65 m in diameter and flattened a forested area of 2,150 square kilometers around it in the air blast. The air burst produced elevated levels of ice in the upper atmosphere, causing the sky to glow over Europe and Asia for a few nights. This was also observed by telescopes in the US, as global dust increased making it more difficult to see the stars through the telescopes.

**The largest impacts** we have evidence of, like the Chicxulub (pronounced, IPA: [tʃikʃuˈlub] or sometimes pronounced like “Chicks-u-lube”) crater in Mexico formed from an asteroid impact, created global effects that lasted for millennia. The impact ignited wildfires nearly globally, generated megatsunamis, generated earthquakes as far away as Texas and Florida, and vaporized sulfur rocks creating devastating climate effects. So many combined effects led to myriad species stresses and extinctions, and marks a specific point in geologic time called the K-T Boundary.

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#### So, how do you prepare for the effects of an asteroid impact?

USGS Astrogeology Scientists study asteroids in our solar system to understand their potential to come to Earth and the potential risks if asteroids from a wide variety of class sizes and compositions were to impact. Dr. Timothy Titus is working with NASA and Planetary Defense partners in academia to understand the effects of an asteroid impact. This can help us be better prepared to assist in the future for impacts that may harm our air, water, or soils. Dr. Titus and other authors with the USGS and other partners wrote a document called “Planetary Defense Preparedness: Identifying the Potential for Post-asteroid Impact Time Delayed and Geographically Displaced Hazards” to describe possible cascading hazards and mitigations of asteroid impacts. Dr. Titus and colleagues at the USGS and elsewhere also submitted an abstract called “Asteroid Impacts – Downwind and Downstream Effects” to the Planetary Defense Conference, describing how to understand how widespread possible cascading hazards might occur using more common natural hazards ( volcanoes, wild fires, etc.) as analogs.

 

#### Immediate effects of asteroid impacts

In an asteroid strike, there are effects that happen within the first few seconds and minutes of the impact. These immediate effects often reduce with distance from the impact site and can be devastating to the surrounding area. According to the Planetary Defense Preparedness paper, the immediate effects include “shock waves that can knock down forests and buildings, thermal radiation that can set fire to the surrounding environment, and tsunami waves for impacts in the ocean.” These immediate effects are different, depending on the size of the impactor, or the composition of the meteor that impacted. Small meteors have smaller effects over a shorter distance, and larger meteors have a larger effect over a large distance. Dr. Titus and his team are trying to answer what is the largest meteor that society can safely tolerate allowing to impact in a sparsely populated area?

 

#### Long-term effects of asteroid impacts

Shock waves and the aftereffects of asteroid strikes damage ground cover and soils in the short term, which is an immediate effect, but long after these effects, the damage can change how the ecosystem evolves. Bare soil allows more rainwater runoff, changing the way that erosion works in the ecosystem. Dust from the impact or air burst can make it into the atmosphere, with climate effects. This was observed with the Tunguska explosion in Siberia in 1908, where lofted debris created bright nights for a several days over entire Europe from Siberia to the Atlantic Ocean.

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#### “Downstream” and “Downwind” effects of asteroid impacts

Asteroid impacts create debris that falls back to the surface and can potentially affect food crops, water reservoirs that serve as drinking water supplies, and air quality. Shock waves and thermal waves of asteroid strikes damage ground cover, which leaves bare soil that can cause rainwater to flood low-lying areas. This is similar to what happens after wildfires, and studying how wildfires affect rainwater runoff flooding downstream can help understand the effects of asteroid strikes in areas where there is a potential to have these conditions. This is especially critical as asteroid strike debris will likely wind up in the rainwater runoff and could make it into drinking water reservoirs, so this is critical research for utility infrastructure.

According to Dr.Titus and team, “Although no recent documented asteroid impacts or air bursts have affected agricultural food crops,” it is important to understand the potential risks from this scenario to mitigate its effects. Volcanic eruptions release substantial amounts of ash into the atmosphere and can serve as an analog to this effect. Material from volcanic eruptions can make it into the upper atmosphere, circulate around the globe, and have significant effects on the climate for a long time. Asteroid impact strikes can distribute material over crops and plants, and it may take a long time for plants to recover, depending on the amount of debris the plants are covered with.

Dr. Titus and his team ran through simulations of a potential asteroid strike in the San Juan Mountains in southwestern Colorado. The impactors ranged in size from 42 m to 600 m, to investigate what size impactors do downwind/downstream effects become important and how to evaluate post-recovery efforts based on these effects. The figure below shows the results from one of the scenarios: a 600-m impactor and wind patterns from a day in March 1997. The lines indicate how thick the debris would be in each circle. For example, the innermost circle would have debris deposited on the ground that is between 30  and 100 mm thick (~ 1 in – ~4 in) and the deposited debris would get thinner the farther from the asteroid impact that you get.

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*Figure from the Planetary Defense Conference Abstract showing the potential dispersal of asteroid outfall from an impact in Colorado.*

With all this information, rest assured that the experts all over the world are working on how to identify asteroids that could be hazards, mitigate potential impacts before they happen, determine the risks of asteroid strikes in different places and at different scales, and understand how to mitigate asteroid strikes if they do happen. USGS Astrogeology, NASA, FEMA, the U.S. Space Command, and academic and industry partners are all working diligently to protect our precious Earth.

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### *Happy Asteroid Day!*

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