By Jun Ji-hye

North Korea has apparently proven its ability to send a ballistic missile far enough to reach the U.S. mainland with its latest intercontinental ballistic missile (ICBM) test, Wednesday. But it still remains uncertain whether the North has secured "re-entry technology," according to defense analysts.

Re-entry technology is needed to protect a nuclear warhead so it can withstand the extreme heat and other challenges when re-entering the Earth's atmosphere from space. As the covered warhead re-enters the atmosphere at Mach 24, it needs to be capable of withstanding temperatures of around 7,000 degrees Celsius to 8,000 degrees Celsius. Securing such technology has been cited as the toughest challenge for the North in developing an operational ICBM.

During Wednesday's test, the Kim Jong-un regime fired the new Hwasong-15 ICBM, which flew 950 kilometers for 53 minutes and reached an altitude of 4,475 kilometers, according to the North's state-run Korea Central News Agency.

Experts said the altitudes of the North's ICBM tests have become higher as the Hwasong-14 ICBM, launched July 4, reached an altitude of 2,802 kilometers, while the same missile fired July 28 soared to an altitude of 3,724 kilometers. The flying time has become longer too, from 39 minutes to 47 minutes to 53 minutes.

Citing these numbers, experts said the latest test appeared to be its longest yet.

Kim Dong-yub, a professor at the Institute for Far Eastern Studies at Kyungnam University, said the fact the North has used lofted trajectories in its ICBM tests should be considered.

N. Korea's new ICBM can hit any target in US 2017-11-29 21:34  |  North Korea
"If the missile had been launched on a normal trajectory this time, it could have flown more than 11,000 kilometers, which is far enough to reach cities in the eastern United States such as Washington, D.C., and New York," Kim said.

David Wright, a co-director of the Union of Concerned Scientists, also assessed that the missile could have had a range of more than 13,000 kilometers, saying such a missile could reach any part of the continental United States.

However, it remains to be seen whether the North has truly secured re-entry technology.

To prove such ability, the North would have to show a warhead mounted on a ballistic missile exploding in the air after returning to the Earth's atmosphere, Kim said.

"North Korea has never shown warheads that returned to the Earth's atmosphere exploding in the air during its previous tests of the Hwasong-10, 12 and 14," Kim said, adding that the North will have to resolve such technical problem to complete its nuclear arsenal.

Wright also called into question the warhead used in Wednesday's test-firing.

"We do not know how heavy a payload this missile carried, but given the increase in range it seems likely it carried a very light mock warhead," he said. "If true, that means it would not be capable of carrying a nuclear warhead this long a distance, since such a warhead would be much heavier."

A day after the July 4 test, the North claimed it had perfected re-entry technology, saying the test finally verified "all technical features of the payload of the rocket during its atmospheric re-entry including the heat-resistant features and structural safety of the warhead shroud made of a newly developed carbon compound material, in particular."

But the Ministry of National Defense downplayed the claim at the time, saying nothing has been confirmed about the technology.