New Study Shows How Even Small Asteroids Can Make a Big Impact
published during a full moon.

A giant asteroid careening toward Earth threatens to wipe out human civilization as we know it, and it’s up to scientists with nukes or Bruce Willis to save the day and prevent us from going the way of the dinosaurs. We see this in the movies, but such asteroids are extremely rare. More than 90% of them have been tracked with telescopes, and they’re not on a path to Earth, at least in our lifetimes.

But impacts from asteroids between the size of a house and a city block are much more common and could pose a greater threat, according to a new study published this month in Icarus. An asteroid with a diameter of just 65 meters could do a lot of damage, from driving tsunamis to decimating a highly populated region. Moreover, asteroids between 100 and 130 meters in diameter pose an even bigger threat due to their thermal radiation, according to Donovan Mathias of NASA Ames Research Center in Mountain View, California, lead author of the study.

Small Asteroids

Propagation of the blast wave from an air-bursting asteroid to the ground. The pressure from the shock wave and the hurricane-force winds behind it break windows, knock down walls, and send debris flying. At 2 pounds per square inch (psi), most windows break. At 10 psi, most buildings collapse. Thermal radiation from the explosion can also ignite fires. Credit: Michael Aftosmis, NASA/Ames

Mathias and his team used computer simulations of hypothetical asteroids to model impact scenarios of varying sizes, density, entry angle, impact speed, and impact location. The team simulated an asteroid’s trajectory towards the Earth’s surface and then calculated the spread of the impact’s energy to determine local hazards, such as blast effects and the potential to create a tsunami. The team also mapped out the asteroid’s damage zone and estimated the number of people that would be affected by the impact.

Simulations, which can be run millions of times, have the advantage of providing statistics, allowing scientists to assess the probability and risk of these unlikely yet potentially dangerous events. Mathias and his team estimated that asteroids with a diameter of 70 meters or smaller have just a one in a million chance of affecting 10,000 people. Furthermore, collisions from 70-meter asteroids are estimated to occur less than once every two thousand years and have yet to happen during human history. With most of the simulated impacts, the asteroid splashes harmlessly in the ocean or lands in unpopulated areas.

“The average damage due to asteroids is very low,” says Mathias. To that point, the 2013 Chelyabinsk impact in southwest Russia was newsworthy because it was so rare. In that case, the wind blasts created by the asteroid burning up in the atmosphere were strong enough to shatter windows and cause injuries from the glass debris. The same thing happened in 1908 from the Tunguska impact event, when a larger asteroid crashed into a sparsely populated part of Siberia.

Small Asteroids

The meteor over Chelyabinsk. Credit: Nikita Plekhanov

Based on his simulations and statistical analysis, Mathias argues that 65-meter asteroids or larger should be considered the minimum size that constitutes a threat to the population because of the damage and casualties they can inflict, despite being rare. Mark Boslough, a physicist at Sandia National Laboratories in Albuquerque, New Mexico, agrees with Mathias’s methodology but cautions that it underestimates the damage from asteroids less than 50 meters in size. When even relatively small asteroids explode in the atmosphere, they can create airbursts that affect a large area.

“Just because it breaks up in the air doesn’t mean it’s not dangerous,” Boslough says.

Small Asteroids

Chelyabinsk meteor event consequences in Drama Theatre. Credit: Nikita Plekhanov

International space agencies such as NASA and the European Space Agency (ESA) are already working together to figure out how to deflect an asteroid long before it becomes a threat. For example, NASA scientists are planning the Double Asteroid Redirection Test, known as DART, which if approved, would send a spacecraft to intercept Didymos, a pair of near-Earth asteroids.

The spacecraft would crash itself at a speed of 13,000 miles per hour into the smaller of the two asteroids, which is 150 meters across and at least a million times heavier than the craft itself—kind of like crashing a BB pellet into a boulder. ESA has proposed deploying an additional spacecraft to observe and monitor the collision.

Small Asteroids

ESA’s AIM spacecraft watches DART impact Didymos. Credit: ESA / Science Office.

“It’s more about time until impact than distance from Earth when it comes to deflection,” says Megan Bruck Syal, a researcher at Lawrence Livermore National Laboratory in California. “If you have ten years of warning time,” she says, “then it’s only something like one centimeter per second of a nudge that it needs” to send it on a safer trajectory.

But if all other attempts fail, one might need nukes after all, like in the movies. Even if it’s less than a year before impact, a nuclear explosion would rip an asteroid into a lot of pieces that are well dispersed and have those fragments miss the Earth, Bruck Syal says. But this would be the last resort.