A nuclear attack from space would generate an electromagnetic pulse, or EMP, that would "inflict devastating damage" on the U.S.

The House Homeland Security Committee heard expert testimony yesterday on the effects of a high-altitude nuclear detonation that could knock out the U.S. electrical grid for up to a year, resulting in the deaths of 90% of Americans.

Washington Examiner:

In calling on the Pentagon and President Trump to move quickly to protect the grid, the experts testified that an explosion of a high-altitude nuclear bomb delivered by a missile or satellite "could be to shut down the U.S. electric power grid for an indefinite period, leading to the death within a year of up to 90 percent of all Americans." Two members of the former congressional EMP commission said the threat to the U.S. has never been higher, in part because of the current high level of saber rattling by both sides and North Korea's surprising display over the past six months of its ability to deliver on its threats. "With the development of small nuclear arsenals and long-range missiles by new, radical U.S. adversaries, beginning with North Korea, the threat of a nuclear EMP attack against the U.S. becomes one of the few ways that such a country could inflict devastating damage to the United States. It is critical, therefore, that the U.S. national leadership address the EMP threat as a critical and existential issue, and give a high priority to assuring the leadership is engaged and the necessary steps are taken to protect the country from EMP," the experts told a House Homeland Security subcommittee. William R. Graham, chairman of the former EMP commission and its former chief of staff, Peter Vincent Pry, said that the U.S. has ignored the warning signs for years and that North Korea's military moves this year must be seen as a wake-up call.

An EMP is created when a nuclear bomb detonates and, for about a millionth of a second, generates temperatures as hot as the surface of the sun. That heat is instantaneously turned into high-energy particles that destroy electronics – everything from electronic ignitions in cars to your mobile devices. The electrical grid is especially vulnerable, given its interconnected nature. The destruction of part of the grid would overload the entire system, leading to cascade failures across the board.

It is not a question of whether such an attack is beyond the capability of North Korea. The question is, is this worst case scenario probable?

An EMP is a line-of-sight weapon. Its effects have been observed only on the ground. In theory, a blast – or several blasts – in outer space directly over the U.S. would result in the kind of catastrophe some experts believe would occur.

But the destructive effects of an EMP from space have never been measured. Some scientists believe that EMP damage is limited to a few square miles directly below the detonation. The farther away from the explosion, the less energy is distributed because the wave dissipates so quickly.

Also, EMP devices are specifically made for this purpose. Does North Korea have the expertise and tech ability to construct one?

The bottom line is that we simply can't take a chance. Hardening our electrical grid is a good idea even if an EMP attack is remote.