A reader points me to this old NRO piece by Dennis Powell I’d forgotten about.

You are the president of the United States, and you receive the phone call.
“Mr. President,” the person on the other end says, “there is a one-in-three chance that an asteroid more than 500 feet in diameter will strike somewhere in the northern hemisphere six days from now. We cannot be more precise, though we’ll have better information in the next few days.”
You are the president of the United States. What do you do?
Your science adviser tells you that if the rock hits the planet there is a better than even chance that it will come down in the ocean, but that this is not good news — it could create waves which might destroy coastal cities. It would, of course, be a hazard to ships at sea at the time. It would be a disaster the likes of which we have never experienced both in terms of property loss and, absent an unprecedented evacuation, loss of life as well. The long-term effects on the climate would not be easy to predict, but they would likely be greater than those from a major volcanic eruption.
“Can anything be done?” you ask.
“Other than evacuating coastal cities, no. We have no means of altering the course of the asteroid. We have no way of knowing where it will it or, at this point, if it will hit. By the time we know the extent of the threat, it will be too late even to conduct an orderly evacuation.”
The secretary of Homeland Security and the director of the Federal Emergency Management Agency have more bad news: There is no provision, none at all, for such an evacuation, orderly or otherwise. We’re talking moving everyone, from both coasts, far inland. This includes the entire state of Florida and the entire northeastern megalopolis, as well as Los Angeles, San Francisco, Seattle. It can be done in six days, if we begin right now.
“Don’t forget,” someone chimes in, “you might be ordering people to move from their perfectly safe homes into the area where the thing will hit. And it probably won’t hit at all.”
The implications begin to sink in. No matter what happens, the odds are that your political career has just ended. No matter what you do, you will be second-guessed pretty much forever. Maybe the realization is liberating: whatever decision you make will be easier if it is free from political considerations.
What do you do?
It turns out that something not very much unlike this almost happened two months ago. In January there was evidence that for several hours suggested that the Earth had a frighteningly good chance of being hit by a space rock 100 feet in diameter. (For comparison, the meteor that created Meteor Crater in Arizona was about 150-feet across and hit with a force equivalent to a 20-megaton bomb.) Scientists debated whether or not NASA and the White House should be called. As it turned out, astronomers determined within hours that the asteroid did not pose a threat to Earth. No one had to make hard decisions.
This time.