The size of a small city, the target asteroid is imposing. The cracks and craters on its surface reflect years of wear in the extreme and dangerous environment of deep space.
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| This mosaic image of the asteroid Bennu is composed of 12 images collected on Dec. 2, 2018 by the OSIRIS-REx spacecraft. They were taken at a range of 15 miles. Image Credit: NASA/Goddard/University of Arizona |
Scientists predict that the force of the impact will completely destroy the asteroid, turning a grave threat into a cloud of dust. But they underestimate its strength.
On impact, a gaping hole forms at the collision point. Stress waves ripple through it, creating tiny cracks in the rocky material. As the cracks grow and merge, chunks break off and shoot out into the darkness. Other pieces flow like sand along the surface. The damage is severe, but the target isn’t destroyed. The threat remains.
Luckily, it’s only a simulation.
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| Panoramic image of Meteor Crater in Arizona, an impact crater about 1,200 m wide caused by a meteorite. Image Credit: Cburnett (CC BY-SA 3.0). |
After running what may be the most comprehensive simulation of an asteroid impact to-date, researchers from Johns Hopkins University (JHU) and the University of Maryland (UMD) have concluded that asteroids are probably stronger than scientists have been assuming. Their work was published in a recent issue of the journal Icarus, and could have implications for how we interact with asteroids in the future.
Humanity hasn’t had to reckon with a major asteroid impact yet, but if a giant space rock comes barreling toward the Earth, we had better know what we’re dealing with. That’s one important reason to study asteroid impact scenarios, but there are others.
For example, it might be possible to mine asteroids for water, precious metals, or other minerals that we can use here on Earth or to build infrastructure elsewhere in the solar system. The potential for mining in the distant future is one motivation for the Japanese Space Exploration Agency’s ongoing Hayabusa 2 mission to explore the asteroid Ryugu, which celebrated a successful touchdown just a few weeks ago.
NASA is in the middle of a similar asteroid exploration mission—the spacecraft OSIRIS-REx is currently mapping the surface of the asteroid Bennu so that scientists can determine the best place for OSIRIS-REx to land and collect samples. Bennu formed along with our solar system, so the samples could help us better understand asteroids and our own history.
“We look at asteroids as potential stepping stones into deep space travel,” says Charles El Mir from JHU, the lead author on this new research. “These objects are sources of metals and minerals, and they could provide the raw materials for refueling spaceships without having to return to Earth. Understanding their evolution and internal structure could provide us with an understanding of what to expect if robots or humans were to land on an asteroid and attempt to mine its resources,” he explains.
From self-preservation to self-realization, we can learn a lot from studying asteroids. But they are not easy subjects. Asteroids exist in a harsh environment. They have been so marred, shaped, and fragmented by high-speed collisions with other asteroids that it’s difficult to determine what their internal structure is today. Given how hard it is to study them in the lab and in the field, one of the best ways to study asteroids is by impact simulations.
To simulate an asteroid impact, scientists input what they know about asteroids and their environment into mathematical models that describe how things change as a result of an impact. This means that the accuracy of a simulation depends on the accuracy of the inputs and the models.
In this new research, El Mir worked with K.T. Ramesh from JHU and Derek Richardson from UMD to create a comprehensive two-phase simulation of high-speed asteroid impacts.
During phase I, the simulation explores the stress-induced damage experienced by the asteroid during and immediately after the impact. The force of the impact crushes the asteroid’s rocky surface, leaving behind a giant crater. Stress waves emanate from the site and cause all kinds of damage. Cracks form and spread, fragments break off and fly away, and damaged material that doesn’t break free from the asteroid’s irregular gravitational field flows along the surface.
To accurately capture all of this activity, the team based phase I of their simulations on a model previously developed by Ramesh and one of his former graduate students, Andrew Tonge (now at the Army Research Lab). The “Tonge-Ramesh material model” incorporates subtle but key aspects of how a rocky object responds to impacts that haven’t been included in other asteroid impact simulations.
After phase I is complete, the results are funneled over to phase II. Phase II takes a longer view, looking at the minutes and hours following an impact. Remember those fragments that flew out into space? After being ejected, their motion is primarily governed by gravity—the attraction they feel to all of the other asteroid fragments. Depending on their distribution, size, and velocity, sub groups of fragments might coalesce into one or more gravitationally bound piles of asteroid rubble, or even be pulled back to a remaining asteroid core (if there is one). Phase II is based on a model of gravity developed in part by Richardson.
This isn’t the first asteroid impact simulation, but it is the first one to incorporate the full set of physical processes included in the Tonge–Ramesh material model and to be structured in this way. To study the influence of these changes, the researchers ran a scenario that was simulated by a different group of researchers in 2013—a head-on collision at 5 km/s between an asteroid 25 km in diameter and one 1.2 km in diameter.
In the 2013 simulation, the larger asteroid was completely shattered by the impact. But not this time. The new simulation yielded a strong but damaged core that recaptured most of the ejected fragments with its gravitational pull, returning to about 85% of its original mass after just three hours. This suggests that very large asteroids are probably more shatter-resistant than we thought, and may be composed of a strong internal core surrounded by a layer of loose fragments.
This is especially interesting given that activity is well underway for NASA’s DART mission, the Double Asteroid Redirection Test mission, which involves crashing a spacecraft into an asteroid at 6 km/s and measuring the impact on its motion. The encounter is planned for 2022 and will provide precious experimental data for further refining our understanding of asteroids. In the meantime, it’s probably best to give asteroids the benefit of the doubt—as Lao Tzu says in the Tao Te Ching, “There is no greater danger than underestimating your opponent.”