Expert Voices

Stopping an Earth-Bound Asteroid in its Tracks (Op-Ed)

An artist's illustration of an asteroid headed for Earth.
An artist's illustration of an asteroid headed for Earth. (Image credit: European Space Agency)

Cathy Plesko, Ph.D., is a research scientist at Los Alamos National Laboratory in New Mexico. She contributed this article to's Expert Voices: Op-Ed & Insights.

In a couple of weeks, at the Planetary Defense Conference just outside of Washington, D. C., I'll be taking my turn at one of the highest-stakes role-playing games on the planet: an emergency response drill where astronomers, emergency management experts, planetary scientists (like me), meteoriticists, rocket scientists and other experts work together to practice our response if a large asteroid or comet were heading toward us. Like a fire drill, we practice our roles and test new technology and scientific data to see how it changes what we think the best response would be.  

Even though asteroids and comets large enough to be dangerous don't hit the Earth very often, they do hit at random, so there is a chance that an extinction-level event (like what happened to the dinosaurs) could happen to us. It would be like winning the worst lottery prize ever. So planning for a way to protect ourselves is important.

Related: Potentially Dangerous Asteroids (Images)

Fortunately, we're getting to the point where we can see these potentially hazardous objects coming and maybe even do something about it. Astronomers using Earth- and space-based telescopes have discovered almost 20,000 near-Earth objects so far, and the pace of discovery is speeding up. Like a new pair of glasses, future telescopes such as the upcoming NeoCam space telescope have the potential to show us hazards that we couldn't see before.

Once the telescopes reveal what an object is and where it's going, we can simulate its orbit for hundreds of years into the future. If that orbit crosses our planet's path at any point, the United Nations' International Asteroid Warning Network will notify member governments and serve as a clearinghouse for information about it.   

If the object has a chance of hitting Earth — say, one in a few thousand in the next 20 years — NASA's Space Missions Planning Group would coordinate among governments that are working on a defense plan so that everyone is on the same page when any action is required. A threatening object probably won't be kept a secret, because all the information about it will be published and the object will be visible to telescopes in many countries. 

Most objects are announced with estimated probabilities of impact similar to betting odds in Vegas. As more observations come in, the odds typically go to zero as we figure out that their orbit and Earth's don't actually cross. If, instead, data tells us that the object is more likely to hit than we thought, the Space Missions Planning Group would coordinate any space missions to go look at it up close or to push it off course or destroy it. 

Using science to answer the question, 'What if?'

As a research scientist at Los Alamos National Laboratory, I study what happens to the atmosphere and crust of a planet when an asteroid or comet hits and possible ways to stop that from happening. I use the supercomputers at Los Alamos, some of the fastest in the world, to run high-fidelity simulations to accurately model the physics of an impact. These simulations are constantly updated with cutting-edge data from NASA missions and Earth-bound laboratory experiments.  

I work on a team of scientists from national laboratories and NASA centers studying particular what-if scenarios, and report the results to the Planetary Defense Coordination Office. 

Our first what-if case study focused on asteroid Bennu, the target of the NASA OSIRIS-REx mission. Bennu is about as wide as the Empire State Building is tall, and weighs as much as 800 aircraft carriers. It approaches Earth once every six years, so astronomers can study it well and even use the Arecibo and Goldstone radio telescopes to make a 3D model of its shape. Fortunately, Bennu has only a one in 2,700 chance of hitting Earth, about 100 years from now. 

Related: OSIRIS-REx: NASA's Asteroid Sample-Return Mission in Pictures

We used computer models to study two ways of pushing Bennu off course so it wouldn't hit us: smashing it with a cannonball-like kinetic impactor or roasting one side by detonating a nuclear explosive device from a couple football fields away. We fed the best estimates of Bennu's shape, composition, mass and strength into our computer models and predicted what would happen in each scenario. Then we designed a spacecraft that could do the job. 

We learned that moving Bennu would be a big challenge if it was made of the type of rock that NASA meteoriticists hypothesize. We would need to launch that spacecraft 10 to 25 years before the predicted impact in order to push it off course. And we would need not just one, but a full fleet of kinetic cannonball impactors — more than we could currently launch in time. 

We published our predictions in a scientific journal last year, before OSIRIS-REx got to Bennu. For the Planetary Defense conference exercise, the OSIRIS-REx team is providing us with everything they are now learning by orbiting the asteroid. Soon, they'll use a robotic arm on the spacecraft to grab a sample and send it back to Earth for analysis. We'll feed that data into our models and rerun them to see what difference it makes. 

While we wait for OSIRIS-REx to send that space-rock sample back to Earth, we're studying another asteroid, Didymos B, or Didymoon, which is the smaller member of a binary asteroid system. NASA is designing the Double Asteroid Redirection Test, or DART mission, to test what really happens when we hit an asteroid with a kinetic cannonball impactor. They're targeting Didymoon to alter its orbit around another asteroid in the system, Didymos A, without changing either asteroid's orbit around the sun. That experiment will allow them to test the kinetic impactor deflection process without accidentally knocking Didymoon onto a collision course with Earth. 

I'm glad we're doing this research now, while we can take the time to carefully study the problem and triple-check our models without the pressure of a specific, potentially hazardous object coming at us. If we prepare well, then for the first time our species could prevent a major natural disaster. We can't yet push a hurricane off course, cork a volcano or lock an earthquake-prone fault, but in a few years, we could be ready to stop a comet in its tracks. 

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