On the second day of a conference on cosmic threats to our planet, the proceedings were interrupted by an urgent message from Paul Chodas, the manager of NASA's Center for Near-Earth Object Studies.
New calculations suggested there was a 10 percent chance that an asteroid named 2019 PDC would strike Earth in eight years, unleashing enough energy to level a whole city.
Scientists didn't know where it might hit, though New York, Denver and a wide swath of west and central Africa were all in the path of potential destruction. Chodas called the situation "uncertain" and "unprecedented."
So, he asked his audience, what did they want to do about it?
Before you start stocking up on canned goods know that this was a fictional exercise. The asteroid 2019 PDC does not exist. No city on Earth is thought to be imperiled by a catastrophic impact. Indeed, analyses of more than 20,000 known near-Earth objects suggest the chance of any hitting us in the next century is less than 1 in 10,000.
But the scenario that played out this week at the International Academy of Astronautics' Planetary Defense Conference in College Park illuminated the very real (if very slim) possibility that such an asteroid might be discovered one day - and revealed just how hard it would be for humanity to mount a response.
Congress first mandated that NASA track near-Earth objects, or NEOs - space rocks that circle around the sun and come within 30 million miles of Earth's orbit - in 1998, after people briefly panicked over a newly discovered asteroid that would pass by our planet.
Two decades later, scientists say they have identified 90 percent of all NEOs 3,300 feet or larger - big enough to precipitate a global catastrophe. Research suggests these kinds of impacts happen once every 700,000 years.
The population of smaller objects such as the fictional 2019 PDC, which was estimated to be about 600 feet wide, is not so well defined - even though these objects could still demolish cities, states and even continents. This is the seventh tabletop exercise NASA has participated in to help game out what scientists and emergency managers would need to consider if one of these asteroids was headed our way.
But the agency's top concern this week seemed to be making sure everyone understood this was a drill. "If you tweet about this," communications officer JoAnna Wendell urged the audience, "please use the hashtag 'exercise only.' We don't want to get into a 'War of the Worlds' scenario."
Her words were comforting to keep in mind as NASA programmer Lorien Wheeler took the stage to explain just how catastrophic an asteroid such as 2019 PDC could be.
She had modeled millions of potential impact scenarios for the object, each featuring slightly different parameters. The asteroid might land in the Atlantic Ocean, triggering a catastrophic tsunami; or it could crash into New York, inflicting "unsurvivable" carnage on millions of people; or it could break up over a largely uninhabited area, causing relatively little damage. And it was still more likely than not that the rock would miss Earth altogether.
Scientists would need more than a year of observations before they could say exactly where the asteroid was headed. But it takes several years to build and launch a mission in response, and any deflection effort would have to happen before 2025 to be effective.
If humanity was going to try to stop the asteroid, we had to start considering our options now.
In reality, the discovery of any rock with a 10 percent chance of hitting Earth would trigger an automatic response from United Nations' Space Mission Planning Advisory Group -- an international coalition of space agencies whose sole job is to coordinate the world's response to impending asteroid disasters.
Preventing an impact is possible - theoretically. Humans need only change the asteroid's velocity by a few centimeters per second; over the course of several orbits around the sun, that change adds up to push the rock fully in front of or behind the Earth. But the proposed methods for deflection are expensive and untested.
One, the "kinetic impactor" technique, involves crashing a spacecraft into the asteroid at high velocity to slow it down. (NASA last year gave the go-ahead to start design and assembly of a kinetic impactor test mission called DART, which will smash a spacecraft into a binary near-Earth asteroid called Didymos.)
Alternatively, scientists could detonate a nuclear bomb beside the asteroid, vaporizing part of its surface and causing the rock to recoil. This method is equally effective at increasing or decreasing an asteroid's speed.
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