IF NORTH KOREA’S test of a ballistic missile on February 12th was intended as a provocation, Donald Trump, unusually, failed to take the bait. For once, the president’s Twitter account stayed silent. When Mr Trump was given the news, he was entertaining Japan’s prime minister, Shinzo Abe, at Mar-a-Lago, his resort in Florida. In a joint appearance, Mr Abe described the launch as “absolutely intolerable” and demanded that North Korea should comply with a raft of UN Security Council resolutions that have so far done little to restrain its missile and nuclear programmes. Mr Trump did not refer to the missile test directly, but pledged that America would stand “100%” behind “its great ally” Japan.

The statement was far more measured than some of his previous pronouncements on North Korea. After Kim Jong Un, the country’s leader, gave a bombastic New Year address in which he boasted of being in the “final stages” of preparations to test-launch a missile with the range to threaten America, Mr Trump tweeted back: “It won’t happen!” Mr Trump’s retort suggested that any such attempt would be met with a military response.

The missile tested on Sunday may not have crossed Mr Trump’s red line (it fell into the Sea of Japan, some 500km from its launch site). But it was indicative of North Korea’s rapid progress towards developing medium-range and, eventually, intercontinental missiles capable of carrying nuclear warheads. At first, South Korean officials monitoring the launch said the missile was either a modified version of the Nodong, first deployed more than a decade ago, or the 3,000km-range Musadan, possibly equipped with a solid-fuel rocket motor. The Musadan was tested eight times last year, though only once successfully.

North Korea’s official newspaper said the missile had used solid fuel, and identified it as the Pukguksong-2. According to John Schilling, an analyst who writes for the website 38 North, it looked very similar to the submarine-launched missile North Korea successfully tested in August (known in the West as the KN-11 and by the North Koreans as the Pukguksong-1). Mr Schilling estimates that it has a range of 1,200km, based on the trajectory of the test launch—enough to reach the whole of South Korea and much of Japan.

More important than its range are its greater mobility, durability and ease of use compared with liquid-fuelled missiles, such as the Nodong. Solid-fuel missiles do not have to travel with a retinue of tankers carrying propellant, and they can be launched at five minutes’ notice, against the hour required to prepare the Nodong.

Michael Elleman, a missile expert at the International Institute for Strategic Studies (IISS), is surprised by the speed with which the North Koreans have developed a powerful solid-fuel motor. He thinks it possible that they have acquired the engines illicitly from either China or Russia. A further concern is that the Pukguksong-2 was fired from a transporter-erector-launcher vehicle with tracks like a tank, rather than wheels, giving it the ability to move beyond North Korea’s limited road network. Mr Schilling concludes that the Pukguksong-2 would be much harder to find and destroy than other North Korean missiles.

When North Korea comes to testing a missile with the range to hit America, it is likely to use liquid fuel, since that is an easier technology to master. Even so, keeping Mr Trump’s pledge to prevent such a test would be far from straightforward. One approach would be to try to destroy North Korean missile bases pre-emptively. Earlier this month the commander of American forces in South Korea, General Vincent Brooks, called for greater capability to do just that. However, as Mark Fitzpatrick, also of the IISS, points out, South Korea would bear the full brunt of the North’s retaliation. Convincing it that a shower of missiles on Seoul was a fair exchange for protecting America from a notional threat would not be easy.

Another approach would be to try to destroy the long-range missile early in its flight using interceptors fired from a naval vessel. But Mr Elleman warns that until the much faster and more capable version of America’s SM-3 interceptor becomes available, perhaps by next year, the chances of success would be low.

So too is the likelihood of diverting North Korea diplomatically. The day after the test the UN Security Council did what it usually does, deploring the launch and calling for a redoubling of efforts to enforce existing sanctions. These include measures aimed at cutting North Korea’s exports of coal and metals, which were passed in November after a nuclear test. But until China decides that the dangers of its exasperating neighbour’s nuclear programme outweigh those that might follow the collapse of his regime, Mr Kim will not be deterred. Mr Trump promised on February 13th to deal with the “big, big” problem of North Korea “very strongly”. But as ever, the options are dismal.