If the skies are dark and weather are right, Earthlings should be able to see Apophis swish by with their naked eyes. The asteroid will resemble a shooting star as it streaks through space.
The geologic record reveals that Earth hasn't always been so lucky. The Chesapeake Bay was formed by a meteor that struck the East Coast 35 million years ago, scattering debris from Texas to New Jersey. Some 15 million years after that, a meteorite blasted the sand of the Libyan desert, creating a sea of foggy green glass.
And of course, there's the Chicxulub impactor 66 million years ago, which triggered the extinction of three quarters of all life on Earth, including most of the dinosaurs. That rock was thought to be dozens of miles in diameter and unleashed the energy of billions of atomic bombs.
"We know for a fact the dinosaurs did not have a space program," Bridenstine said, quoting television personality Bill Nye. "But we do, and we need to use it."
By Wednesday, the scenario fast-forwarded to December 2021. The reconnaissance spacecraft had finally given astronomers their first good look at 2019 PDC, revealing that the asteroid was actually a peanut-shaped "rubble pile" about 450 feet across and 850 feet long.
And, unless humanity took drastic action, just after 10 p.m. local time on April 29, 2027, it would enter Earth's atmosphere at a speed of 43,000 miles per hour and smash right into Denver.
The resulting blast wave would obliterate the city and some of the surrounding suburbs - setting fire to buildings, vaporizing vegetation, melting cars. People as far away as Pueblo, Colorado, and Laramie, Wyoming, would feel the Earth tremble and see their windows break. The drought-parched trees of the Rockies would be set ablaze and billions of dollars worth of infrastructure would be lost.
Worst of all, there were some 2 million people living in the "unsurvivable" zone.
Horrific though the scenario sounded, physicist Mark Boslough, a global catastrophe expert, chose to look on the bright side.
"That asteroid was headed toward Denver even before it was discovered," he said. "So it seems like very bad news, but this is actually good news, because we still have five years to deflect this thing."
Between Wednesday night and Thursday afternoon, the clock spun forward three years. The diverted scientific spacecraft arrived at the asteroid and began to observe it. Six kinetic impactors were built, but one blew apart during launch and two others failed en route to their target. Widespread controversy over the use of a nuclear device meant that the second rendezvous spacecraft launched without any cargo. But that may have been for the best: It, too, suffered a total system failure before arrival.
Now it was Sept. 3, 2024, and the world found itself facing a whole new kind of disaster: The three surviving kinetic impactors succeeded in deflecting most of the asteroid. But in the process they'd broken off a 50- to 80-meter fragment that was still headed our way.
The resulting debris cloud destroyed the observing spacecraft, and the asteroid was behind the sun, blocking our view of it. Yet early calculations suggest that the fragment would strike somewhere along a path that extended from Omaha to New York and into the Atlantic. The object was now too small to pose a significant tsunami risk, Wheeler said, but if it hit land -- particularly a densely-populated place such as New York -- as many as 11.5 million people would be affected. And it was too late to attempt another deflection.
The prospect of losing a global financial center was shocking, even as make-believe. "You may freeze the economy," one audience member worried. "You might actually be looking at total economic collapse."
Aerospace engineer Brent Barbee agreed. That's why his team was considering a "last-ditch effort" to prevent an impact. The world might "scramble," he said, to design and deploy a 300-kiloton nuclear device that would blast the asteroid into tiny pieces two to four months before it was due to hit Earth.
The proposal immediately raised concerns.
"If you couldn't launch a nuclear device before, what makes you think you could launch it now?" one researcher wanted to know. ("Great question," was Barbee's reply.)
"Can we do a small kinetic impactor that puts it safely in the ocean?" asked another. ("Too many uncertainties," Chodas answered.)
"It worries me when you say that we are going to scramble to put together a 300-kiloton nuclear device," said a third.
She had a point, Barbee acknowledged. That so much of the initial fleet had failed shows how difficult it is to quickly build a successful spacecraft. "We probably should have been planning the backup option from the beginning," he said.
Breaking the fourth wall, some of the researchers complained that the scenario was starting to seem unrealistically worst-case.
"It is not likely at all," Chodas agreed, "and we're not saying it is likely. But we will learn by studying these what-ifs."
Things were about to get even more sensational. On the last day of the exercise, with 10 days to go before impact, Chodas announced that the disruption mission never got off the ground, and the fragment - now known to be 60 meters - was on a path to obliterate much of New York City.
The skyscrapers of central Manhattan would be vaporized. Fire tornadoes would rip through the region. The "mother of all gusts" would blow from Yonkers to Staten Island and New Jersey to Queens. Ten million people would need to escape the disaster zone -- more Americans than have ever been evacuated before.
A chorus of "oohs" rippled through the ballroom. "Tough," one person muttered.
Suddenly, the tone of the conversation shifted. They had left behind the comfortable certainty of physics. Now they had to plan for the unpredictability of people.
"We're very concerned," said Carol Lewis, who was playing the role of an "affected citizen." "We've actually called ourselves 'mad as hell.' And we have questions."
What would happen to people in nursing homes? What would happen to pets? Who would prevent looting of abandoned buildings? Would they be compensated for their losses? Where would the survivors go? And would they ever be allowed to return?
"Don't think we haven't been doing anything before we're at T minus 10 days," said Leviticus Lewis, a real-life operations manager for FEMA (and Carol Lewis's husband).
"I can tell you there already is a New York evacuation plan," he assured people. "I can't tell you what it is."
Then Joseph Nuth, a NASA asteroid researcher, stood up and stated the obvious: "It's our fault."
"How are we going to compensate those people, get them their lives back?" he asked. "This is an enormous liability that we took on when we screwed this up."
That led to a question of whether the U.N.'s "Convention on International Liability for Damage Caused by Space Objects" could apply to citizens suing their own government.
"The questions now are so deep and so far-reaching," said Victoria Andrews, NASA's deputy planetary defense officer.
And, she noted, they're not quite so esoteric as they might seem. Most natural disasters cannot be predicted as far in advance as an asteroid impact, but as a consequence of climate change, many are becoming more and more likely. Deadly wildfires and disastrous hurricanes that once were rare now seem inevitable - and communities have to prepare.
"This exercise gets us thinking about how to do that," Andrews said.