In mid-May, a rocket launched by North Korea soared to an altitude five times higher than the International Space Station, and an analysis of publicly available data by Reuters shows how that may have brought leader Kim Jong Un closer to his goal of producing a missile capable of hitting the United States.

The principle, experts say, is that the higher a rocket can travel, the further it can reach.

"To avoid firing long-range missiles into or over Japan, the North Koreans have been launching them nearly straight up instead," said Joshua Pollack, editor of the Washington-based Nonproliferation Review.