Predicting Damage from an Asteroid Strike on Earth

Overview

As of 2015, approximately 10,000 near-Earth asteroids larger than 30 meters in diameter have been discovered—and the actual total number is estimated to be more than 1 million. Of those discovered, 1,600 are classified as hazardous. An asteroid larger than 30 meters is expected to strike the Earth every 100 years, on average. NASA is investigating the potential consequences of an asteroid or comet strike to determine conditions in which an asteroid on a collision course could be simply dealt with by evacuation, or when it would require in-space mitigation, such as altering its trajectory to miss Earth or breaking it into small pieces. In this effort, we aim to quantify how asteroid properties such as size, density, speed, strength, and trajectory will determine mitigation strategies.

Project Details

There is very little data (thankfully) on hazardous asteroid strikes on Earth. Before 2013, when an asteroid exploded over the Russian city of Chelyabinsk and caused significant damage, asteroids of its size (approximately 20 meters across) were not considered to be hazardous. Although multiple meteor showers occur each year and many impact craters have been identified on Earth, the only well-documented significant impact is the Chelyabinsk event. Consequently, most asteroid damage models use simple equations describing the break-up of the asteroid in the atmosphere and analogies to nuclear explosions, which were well-studied in the 1950s. In lieu of sufficient observational data, we are running computer simulations to determine the fragmentation and blast propagation that occurs when asteroids impact the atmosphere or the ground. Both air and ground bursts emit pressure blast waves that damage the surrounding area, and heat from the explosion can ignite fires.

Results and Impact

Simulations of the Chelyabinsk event using a simple strength model of the asteroid's rock mass show that the rock fractures when it hits the stratosphere. In the simulations, rapid fragmentation and deceleration in the stratosphere produce energy deposition curves very similar to those shown in dashboard camera videos of the event. Propagating the resulting blast wave to the ground using a computational fluid dynamics code produces damage very similar to observations of broken window patterns. The next step is to improve the simulations to include thermal effects and simulate different sizes and speeds in order to accommodate the range of possible impact scenarios. Results will be used to improve the analytical models and provide insight to help develop mitigation recommendations.

Why HPC Matters

Simulating an asteroid impact requires detailed modeling of the asteroid itself, including its composition, the behavior of the rock mass under the stresses of atmospheric entry and ground impact, and how the friction and heat erode and ablate its surface. Accurate simulations of asteroid behavior require time steps of microseconds, while the entry trajectory occurs over an altitude range of at least tens of kilometers and damaging blast waves can extend hundreds of kilometers over several minutes. Capturing this wide range of time and distance scales requires high-end computing resources. A small 2D simulation that captures just the energy deposition rate of an asteroid impact can take as few as 100,000 grid cells and as little as 2,000 processor-hours on NASA's Pleiades supercomputer; propagating the blast wave to the ground takes 100 million grid cells and 10,000 processor-hours, at ~20 gigaflops per processor.

What's Next

Our simulations will be used as part of an asteroid threat assessment project to characterize asteroid properties, model the impacts, determine the sensitivity to parameters, and provide information that decision makers can use for future characterization and observation efforts, and for mitigation strategies.

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