Saturday, July 1, 2017

NASA Researchers Rock the World of Asteroid Simulation

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 4 psi, most houses collapse. At 10 psi, most buildings collapse. Thermal radiation from the explosion can also ignite fires. Michael Aftosmis, NASA/Ames

On a winter’s day in February 2013, a small, 20-meter asteroid—about the size of a president’s head on Mount Rushmore—exploded over the Russian city of Chelyabinsk after entering Earth’s atmosphere without warning. The blast from the asteroid’s shock wave broke windows as far out as 58 miles, and more than 1,200 people were treated for injuries from shattered glass and building debris.

The good news is, our ability to assess the potential damage from an asteroid strike—and plan appropriate mitigation strategies—is now faster and more accurate due to state-of-the-art impact simulations and risk models run on NASA supercomputers. Researchers in the NASA Advanced Supercomputing (NAS) Division working on the Asteroid Threat Assessment Project (ATAP) are employing a unique combination of high-fidelity simulations and probabilistic risk models to provide highly accurate estimates of the damage asteroid impacts could cause.

"Asteroid impacts are one of the only natural disasters we can actually predict and then take action to protect people," said Michael Aftosmis, an aerospace engineer who leads the ATAP blast wave and ground damage modeling work at NAS.

But then why were asteroid watchers around the world taken by surprise by the Chelyabinsk event? "The meteor came from the direction of the sun, hidden from the view of even the biggest telescopes—and asteroids that small were previously thought to be of little risk," Aftosmis explained. Knowing ahead of time what sizes and types of asteroids pose a significant threat, and what damage they might cause is critical to planning asteroid surveys, mitigation strategies, or disaster response decisions.

Although the NAS team is not directly involved with response or mitigation aspects, their research in support of NASA's Planetary Defense Coordination Office (PDCO) is shared with scientists at universities, national labs, and government agencies who identify potentially life-threatening asteroids and develop response strategies. Astronomers from around the world share their telescope observations with the International Astronomical Union’s Minor Planet Center, and the Goldstone Deep Space team at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California catalogs all the observations of known near-Earth asteroids. This is the where NAS Division’s simulations come into play.

One of the main challenges in this research is that there is a limited amount of data due to the rare occurrence of large asteroid impacts, as well as uncertainties in the density and strength of different asteroid types. High-fidelity simulations, however, can model the key physics of an asteroid’s atmospheric entry, airburst, or surface impact, and provide valuable estimates of the resulting damage due to blast waves, thermal radiation, or even tsunamis.

Blast waves from asteroids that burst in the atmosphere are the predominant source of damage from the more common small-to-mid-sized asteroids—like the one that burst over Chelyabinsk. Large-scale simulations of the Chelyabinsk airburst event were run on the Pleiades supercomputer using NASA’s Cart3D software to propagate the blast from the asteroid’s entry corridor to the countryside surrounding the city. These simulations allowed the team to compare their predictions of the blast overpressures and shock arrival time at specific locations covering over 40,000 square kilometers (15,000 square miles) with actual data recorded on dashboard and building cameras—and their results matched to within a fraction of a second.

“These are some of the world’s most detailed simulations of this event,” Aftosmis said. “We were able to produce many scenarios quickly because Cart3D, normally used for aerodynamics analysis, is dozens of times faster than most hydrocodes used for 3D numerical modeling of the fluid flow that occurs when asteroids melt and vaporize as they break up in the atmosphere.”

Simulations have also been critical to assessing the potential for ocean impacts or airbursts to generate tsunamis, helping to address the question of whether land or water impacts are likely to cause more damage.

"We were very much surprised,” Aftosmis said. “Everyone thought a tsunami was going to be more of a threat. However, modern simulations consistently produce less coupling to motion in the water than the earlier, less detailed analysis."

In addition to simulations of specific impact cases, the team has developed a state-of-the-art Probabilistic Asteroid Impact Risk (PAIR) model to assess the overall risk posed by near-Earth asteroids. Running on the Pleiades supercomputer, the model performs physics-based damage analyses for millions of impact cases, capturing the full range of potential sizes, uncertain asteroid properties, impact trajectories, and location-specific human population densities across the globe.

“What’s unique about our analysis is that we can rapidly assess so many scenarios while faithfully representing the key physics,” said aerospace engineer Donovan Mathias, who leads the NAS Division’s Engineering Risk Assessment (ERA) team and ATAP risk modeling work. “This is done by leveraging the airblast, ground, and water impact simulations created by the team, as well as by taking advantage of the NAS compute resources.” Mathias is aware of only one other active group in the world doing similar work, at the University of Southampton, UK.

Once the NAS team’s modeling and simulation results are disseminated, NASA’s PDCO and other agencies can make informed plans for how best to identify and defend against dangerous asteroid strikes. The agency’s Science Definition Team used the results to assess which asteroid sizes pose a significant enough threat to warrant tracking and which search strategies are most effective for discovering them. NASA program directors then decide which future science surveys or missions may be most useful for assessing and averting potential impact threats.

Results are also used in tabletop exercises that allow NASA, the U.S. Department of Defense, and the Federal Emergency Management Agency (FEMA) to develop assessment and response plans in the event that a potential impact threat is discovered. These exercises use a realistic, hypothetical impact scenario to look at potential damage to infrastructure, warning times, evacuations, and other options for protecting lives and property.

Most recently, the NAS teams performed risk assessments and high-fidelity simulations of ground damage and tsunami generation for a hypothetical impact scenario that was conducted as part of the 2017 Planetary Defense Conference (see sidebar, "Practicing for Worst-Case Scenarios").

Aftosmis summed up the bottom-line significance of the team’s asteroid work: "If we can see it coming, we can help the agencies who predict where and when the impact will happen, and they can warn people a couple of days in advance to give them time to gather important stuff and get to safety."

Credit: NASA