After five nuclear tests in a decade, North Korea has already shown that it poses a nuclear threat to South Korea and Japan, roughly 80,000 American soldiers stationed in those countries, and to China, its nominal ally. But although Kim Jong-un has dramatically increased missile testing since he took power in 2011, North Korea has yet to test an intercontinental ballistic missile (ICBM) that could cross nearly 5,500 miles of the Pacific.

North Korea would need to overcome two feats of engineering to threaten the American mainland: a working ICBM system and a warhead for one of those missiles. Unlike shorter-range missiles, long-range missiles have multiple engines and flight stages, meaning North Korean engineers have to make rockets – and bombs – that can survive the violent vibrations of launch, the wrenching g-forces of flight, and the temperature changes of takeoff and re-entry from space.

“Producing a warhead that can handle all that is a challenge,” said Joseph Bermudez, an analyst for 38 North, a thinktank affiliated with the Johns Hopkins School of Advanced International Studies. Although Kim has said he wants to test an ICBM later this year, Bermudez doubted the test would be a success.