North Korea’s rocket launch Sunday has raised concerns among Japanese officials and experts that its grasp of missile technology is advancing at a worrying clip.

The latest rocket, which Seoul said flew about 5,500 km from the Dongchang-ri launchpad in North Korea before crashing into the Pacific Ocean, is believed to be significantly larger than the one test-fired in December 2012 and would require a more powerful propulsion system.

The test appeared to be largely successful, demonstrating Pyongyang’s considerable progress in its quest to master missile technology, experts interviewed by The Japan Times said.

“(The test-firing) could further advance the development of ballistic missiles,” Defense Minister Gen Nakatani told a session of the Lower House on Monday.

From the launch, the North may have learned more about multi-stage rocket technology, such as how to separate propulsion units and control the projectile’s bearing, Nakatani said.

Last year, South Korea’s Dong-a Ilbo newspaper reported that the launch tower in Dongchang-ri had been modified to be more than 10 meters higher than it was in December 2012, when a variant of the Taepodong-2 ballistic missile was launched for the last time.

Such a tower could accommodate a larger rocket at the site, which has been regularly monitored by U.S. spy satellites. Indeed, experts said Sunday’s rocket was likely bigger than those of previous tests.

Akira Sawaoka, president of Daido University in Nagoya and an expert on rocket technology, pointed out that the latest rocket’s first stage separated several minutes earlier than that of the December 2012 launch.

This means the engine power was bolstered and the rocket flew faster than the previous one, Sawaoka told The Japan Times. It also means the rocket is able to carry a heavier payload.

“This is technological progress. Eventually a rocket would be able to carry something like a nuclear warhead” if the North succeeds in further improving the technology, Sawaoka said.

According to the South Korean Defense Ministry, the first stage exploded into more than 270 pieces after separating from the rocket at around 9:37 a.m. Sunday over the Yellow Sea west of the Korean Peninsula.

But the North may have intentionally destroyed the section because Seoul retrieved the 2012 rocket’s first stage from the sea for analysis, said Hideshi Takesada, a professor and noted Korean affairs expert at Takushoku University’s graduate school in Tokyo.

“You can’t say (the test-firing) was a failure just because the first stage exploded,” Takesada said.

The test-firing was “largely successful” because the rocket was reportedly able to send an object into Earth orbit, he said.

The developments came as Pyongyang works to eventually develop an intercontinental ballistic missile that can strike Washington or New York on the U.S. East Coast, Takesada said.

According to the Defense Ministry’s 2015 white paper, a successfully developed variant of the Taepodong-2 ballistic missile could fly more than 10,000 km with a warhead weighing less than a ton. Such a range would put most of Western Europe, Asia and the Western U.S. within striking distance.

Using what it learned in the test launches, the North’s long-range ballistic “missiles could have ranges that potentially reach the central, western and other areas of the U.S. mainland,” the Defense Ministry concluded in the white paper.

Currently, the Taepodong-2 is believed to still be in the experimental stages. Experts say the North faces a number of technological hurdles before it is able to develop a functioning ICBM as well as master the miniaturization process needed to mount a warhead on the missile.

Takesada noted that one key hurdle is to develop heat-resistant materials that allow warheads to endure the intense heat generated upon re-entry from space.

It also faces an uphill battle in making a missile that can be launched at the drop of a hat.

The Taepodong-2 uses liquid — not solid — fuels, which make it almost impossible for Pyongyang to have the missile on stand-by for immediate launch.

According to Japanese government sources, a liquid-fuel rocket such as the Taepodong-2 would need to be launched within a few days — possibly a week at the most — once the fuel is injected because its strong acidic properties would badly damage the fuel tank.

Experts appear to be split over whether Pyongyang has already succeeded in developing a miniaturized nuclear warhead.

In May last year, Pyongyang claimed it has succeeded in creating a nuclear warhead small enough to fit on the tip of a ballistic missile.

In response, U.S. National Security Council spokesman Patrick Ventrell disputed Pyongyang’s claim.

“Our assessment of North Korea’s nuclear capabilities has not changed,” he said in a statement at the time, according to CNN. “We do not think that they have that capacity,” he was quoted as saying.

However, the Defense Ministry, in its 2015 white paper, refused to rule out the possibility that the North had already mastered that critical technology.

“In general, miniaturizing a nuclear weapon small enough to be mounted on a ballistic missile requires a considerably high degree of technological capacity,” the paper said.

“However, considering that the United States, the Soviet Union, the United Kingdom, France and China succeeded in acquiring such technology by as early as the 1960s … the possibility that North Korea has achieved the miniaturization of nuclear weapons and has acquired nuclear warheads cannot be ruled out,” the paper read.

However advanced the North’s nuclear and missile technologies may actually be, Sunday’s launch has posed a serious security challenge for Japan and its key allies, the U.S. and South Korea.

Masao Okonogi, a professor and Korean affairs expert at Kyushu University, said that the Taepodong-2 is being developed not to target Japan or South Korea, but to put the U.S. mainland in its cross hairs, given its long range.

Pyongyang has already developed other types of shorter-range ballistic missiles, he said, including the Rodong variations and Scud-type missiles, which pose a direct threat to Japan.

According to Okonogi, Pyongyang believes that if it acquires the capability to stage a direct nuclear attack on the U.S. mainland, Washington would scrap its treaty commitments and no longer protect Tokyo or Seoul.

The thinking in Pyongyang, Okonogi said, is that such a scenario would not only help the North defend itself but also, in the long run, aid Pyongyang in its quest to re-unify the two Koreas on its own terms.

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