Totally no need to worry or anything though.
The year is 2020 and Los Angeles is about to be wiped off the face of the planet. You've known this day was coming since the fall of 2016, when the asteroid that is on a collision course with Earth was first discovered. At the time, the global consortium of scientists who've tasked themselves with saving Earth from giant asteroids estimated that there was a 2 percent chance that the 800 foot-wide space rock would collide with Earth in late September of 2020. Nobody was particularly worried about it—Earth has had much closer brushes with annihilation before.
Yet as astronomers continued to track the asteroid after it was initially discovered, the probability of impact climbed to 65 percent by January of 2017. After a four month hiatus on observation while the asteroid was obscured by the sun, the astronomers resumed tracking the asteroid in May 2017 only to find that the probability of impact had jumped to 100 percent. It was now a question of where the asteroid would hit, not when or if it would hit. By November 2017, astronomers were able to conclude with some certainty that the asteroid's trajectory would place the impact site near the coast of southern California.
This was the scenario laid out during a joint NASA-FEMA emergency planning meeting in late October. For now it remains a hypothetical, but one day it will be a reality.
"It's not a matter of if—but when—we will deal with such a situation," said Thomas Zurbuchen, Associate Administrator for NASA's Science Mission Directorate in Washington. "But unlike any other time in our history, we now have the ability to respond to an impact threat through continued observations, predictions, response planning and mitigation."
The agencies have run two previous impact emergency simulations before, but this scenario was more bleak than its predecessors because the asteroid was discovered too late to launch a mitigation mission. NASA and other national space agencies have poured a lot of money into figuring out how to deflect a killer asteroid bound for Earth, with techniques ranging from nuking the thing into thousands of smaller pieces to using massive sun-powered lasers to slightly change its orbit.
Unfortunately none of these plans would be ready to go by 2020 if the earthbound asteroid was discovered today and that's still a pretty generous advance warning given how common it is for large asteroids to be discovered only days ahead of their near Earth flybys. Since actually changing the course of the asteroid wasn't an option in this scenario, NASA and FEMA employees were instead tasked with figuring out how to deal with evacuating the greater Los Angeles area (home to some 18 million people), managing hysteria, and dealing with the post-impact fallout.
"It is critical to exercise these kinds of low-probability but high-consequence disaster scenarios," FEMA Administrator Craig Fugate said. "By working through our emergency response plans now, we will be better prepared if and when we need to respond to such an event."
As for what would happen post-impact, Paul Chodas, the manager of NASA's Center for NEO Studies and the person responsible for designing the scenario, said it depends on whether the asteroid hits the ocean or the California coast.
"If the asteroid struck off the coast, with these size parameters there'd be a small tsunami depending how far off the coast it impacts," Chodas said. "It would not be like in the movies, it would be small: probably 2-3 foot tsunami by the time it reached the coast. There would be inundation which would cause infrastructure damage. The air burst is the main hazard if it hit anywhere near land, which in this case would be equivalent to a 50 megaton explosion which would cause a large shock wave. It would be devastating for a region with a 40-50 kilometer radius around the impact point."
Chodas emphasized there's no need to panic: "Bear in mind the scenario we put together is extremely unlikely to happen, but it could."
While it's good to know that NASA and FEMA are planning ahead for these types of cosmic calamities, the reality is that the plan to save Earth from the next giant asteroid is still woefully disorganized and underfunded. This is probably why Lindley Johnson, who until recently was the only NASA employee working at the agency's Planetary Defense office, was happy to see the issue getting some attention last month.
"We receive valuable feedback from emergency managers at these exercises about what information is critical for their decision making, and we take that into account when we exercise how we would provide information to FEMA about a predicted impact," said Johnson. "These exercises are invaluable for those of us in the asteroid science community responsible for engaging with FEMA on this natural hazard."
UPDATE: This story has been updated with comment from Paul Chodas, the manager of NASA's Center for NEO Studies.