SEOUL (Reuters) - Last month, a rocket launched by North Korea soared to an altitude five times higher than the International Space Station. A Reuters analysis of publicly available data shows how that may have brought leader Kim Jong Un closer to his goal of producing a missile to hit the United States.

North Korean leader Kim Jong Un reacts during a ballistic rocket test-fire through a precision control guidance system in this undated photo released by North Korea's Korean Central News Agency (KCNA) May 30, 2017. KCNA/via REUTERS

The principle, experts say, is that the higher a rocket can travel, the further it can reach. See an interactive graphic here: (tmsnrt.rs/2t0oSv7).

“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.

“They fly much higher into space that way, but come back down relatively close to their launch points.”

North Korea is not unusual in sending rockets to high altitudes to test their range, but does so more than other countries because it is surrounded by other states, he said.

Kim Jong Un watched the May 14 launch of the Hwasong-12 rocket in the early hours of a Sunday morning on North Korea’s east coast. The missile splashed down just 787 km (490 miles) from the launch site, but reached an altitude of 2,111.5 km, according to North Korean state media.

The South Korean and Japanese governments measured similar distances at the time.

Had the Hwasong-12 been fired at a more shallow angle, it would have flown more than 4,800 km, according to David Wright, co-director of the Global Security Program at the Union of Concerned Scientists, which works to reduce the risk of nuclear use.

That would fall short of the mainland United States, but Wright’s calculations put parts of Alaska within range.

North Korea has been conducting missile tests at an unprecedented pace under Kim Jong Un, who has tested about 80 rockets. His father Kim Jong Il conducted 16 tests, according to monitoring by the Nuclear Threat Initiative.

North Korea has test-fired missiles at lofted trajectories in the past, but none as high as the Hwasong-12. It has used larger and more powerful rockets for space launches, but the Hwasong-12 is smaller, more mobile, and harder to detect.

The rocket has a powerful engine. Photos of the launch show the missile had four additional steering engines as well as its main engine, giving slightly more power and accuracy, according to Ralph Savelsberg, writing on the 38 North website.

FLYING HIGH

After the May 14 launch, North Korean state media said the missile had been launched at such a high angle “in consideration of the security of neighbouring countries.”

In 1998, Kim Jong Il ordered the launch of a two-stage ballistic missile, one half of which flew over Japan, prompting strong protests from Tokyo. Japan still protests at every North Korean test.

As Kim Jong Un has accelerated his missile programme and developed more mobile rockets with longer ranges, he has launched them ever higher.

This has other benefits, according to Pollack: It puts more stress on the nosecone as it re-enters the atmosphere, providing a more rigorous test than an ordinary trajectory.

For a missile to become a threat, its nosecone has to be able to carry a nuclear warhead, and that warhead has to withstand the incredible stresses of re-entering the Earth’s atmosphere from space.

Missiles which land closer to home also make it easier for North Korea to collect and analyse data to improve the missile programme, according to Pollack.

As threatening as these developments sound for the United States, it took North Korea 20 years to finish developing the rocket which eventually put a satellite in space in 2012, said German aerospace engineer Markus Schiller, who has closely followed the North’s missile development programme.

“It will be even longer until they have a real weapon deployed - if ever,” he said. A “real weapon” means an Inter-Continental Ballistic Missile that can be reliably launched under any circumstances, he added.

But if North Korea were to win support from outside enabling it to buy in rocket parts, he said, “they could have an ICBM next month.”