Fears that such a test may be imminent were heightened last month, when North Korea marked an important anniversary with a long-range rocket launch. Its two previous tests came soon after such launches. Satellite imagery also suggested stepped-up activity at the North’s Punggye-ri nuclear testing site.

Little progress at the site has been reported since, which could mean the activity was a ruse or the device is simply not ready yet. It also could mean that the new regime headed by Kim Jong Un, who assumed power after the death of his father in December, is having second thoughts about whether to risk international sanctions by forging ahead.

Sooner or later, however, a test is highly likely.

“The North Koreans clearly value the demonstration effect of nuclear and missile tests, even if the test is only partially successful,” said Jeffrey Lewis, of the Center for Nonproliferation Studies at the Monterey Institute of International Studies. “North Korea gets a tremendous amount of leverage from our fear that these weapons might work someday.”

But he noted that Pyongyang failed miserably in its attempt to launch an ICBM-style rocket last month, then capped off a lavish military parade with the unveiling of a half-dozen ominous-looking new missiles that analysts now believe were low-quality mockups of a design that could never fly.

“They are trying to run before they can walk, with the predictable outcome of tripping,” Lewis said.

A test could have two practical goals.

North Korea may be developing devices that use highly enriched uranium, instead of the harder-to-obtain plutonium it has relied on in the past. If so, it needs to try one out and see if it works. Either way, the North has to shrink its warheads down to fit them on a missile — so it needs to test that capability as well.

“There can be a huge difference between a nuclear explosive ‘device’ and a weapon,” said Ivan Oelrich, a nuclear weapons consultant and former head of strategic weapons at the Federation of American Scientists. “We have no idea how large North Korea’s bombs are, or even whether they have anything that would be described as a ‘bomb.’”

North Korea’s devices are likely along the lines of the first plutonium bomb the U.S. built — Fat Man, which was dropped on Nagasaki in 1945. That bomb was 3 meters (10 feet) long and weighed more than 4.5 tons.

Such a bomb could be loaded on a ship or an airplane, but without significant “miniaturization,” which requires difficult technological redesigning, it would be useless as a missile payload.

“A weapon has to be light and compact, a more or less self-contained package,” said Oelrich. “To fit on a missile, they would have to be less than a few hundred kilograms (about 600 pounds) and smaller than a cubic meter or two.”

Though estimates vary, outside experts say the North has enough plutonium for about four to eight “simple” bombs, more if it can employ uranium. But, so far, North Korea’s attempts to demonstrate it has mastered the technology — with tests in 2006 and 2009 — have not been entirely successful.

The first produced a yield of less than 1 kiloton of TNT, and the second was equivalent to only 4 kilotons, both quite small by nuclear standards, though some experts believe North Korea in the second test may have been trying out the smallest device it could put on a missile.

Success or failure, the tests provide an opportunity for North Korea’s nuclear scientists to learn valuable lessons. That’s why the international community has imposed harsh sanctions after each of its previous underground blasts. But turning those lessons into a viable weapon is no easy task.

“Testing a device underground is relatively easy, as one can initiate the test once everything is in order and verified to be ready,” said Michael Elleman, of the International Institute for Strategic Studies. “A military or strategic nuclear weapon must be able to detonate on demand, with little forewarning.”

Then there is the other problem — how to deliver it to a target.

South Korea and Japan — and the more than 70,000 U.S. troops based in those countries — are already within range of the North’s Nodong weapon, which was test fired in 1993 and can travel up to 1,300 kilometers (800 miles) with a 1,200-kilogram (2,600-pound) payload.

If tipped with nuclear weapons, they would put millions of lives at risk.

But all of North Korea’s long-range rocket launches have ended in failure, meaning it is 0-4 since 1998. That has led some experts to doubt whether North Korea, lacking in resources and expertise and hamstrung by stringent international trade sanctions, will ever succeed in fashioning an ICBM of its own.

The “Unha-3″ rocket has three stages, or parts. If all goes well, the first will burn out and fall in waters off the west coast of South Korea in about 2 minutes. The second will separate 4 to 6 minutes after the launch, then splash down in the Pacific off the northern Philippines. The third will burn up in the atmosphere after boosting the satellite into orbit.

All this will take 10 minutes. The satellite will then enter a near-polar, sun-synchronous orbit, meaning it will pass over the same point on the Earth at about the same time each day. The 500-kilometer (300-mile) altitude announced by the North is considered a low Earth orbit, which is where most artificial satellites go.

TRAJECTORY

Professional and amateur satellite watchers question North Korea’s calculations. They infer from data provided by Pyongyang that the probe will not be able to reach its orbit without conducting difficult navigational maneuvers very soon after liftoff. One expert says the rocket will skirt China’s east coast near Shanghai, fly over Taiwan and shed its second stage into the sea, 50 kilometers (30 miles) off the northern Philippines.

Japan and South Korea say they will shoot down the rocket in the unlikely event it veers off course and heads toward their territory.

THE SATELLITE

The minibar refrigerator-sized satellite is covered with solar panels and golden foil to protect its instruments. One meter (three feet) tall and weighing 100 kilograms (220 pounds), the “Bright Shining Star 3″ is designed to monitor weather, natural disasters and agriculture patterns. Experts say the satellite and its predecessors, Bright Shining Stars 1 and 2, were probably developed with China’s help. It is unlikely the latest version could effectively multitask, since it is so small and North Korea has no demonstrated track record with simpler missions.

ODES FROM SPACE

Pyongyang says the satellite will broadcast martial music praising North Korea’s founder, Kim Il Sung. It says Bright Shining Stars 1 and 2, launched in 1998 and 2009, did the same. But no broadcasts were ever detected, and neither probe is believed to have reached orbit.

If the broadcasts on the UHF and X-bands are successful, the first independent confirmation could come from western Australia, which might pick them up within 20 minutes. The west coast of South America would be next, followed by the U.S. east coast. North Korea would not likely hear them until as much as 12 hours later.

But the international condemnation is much louder over the satellite launch North Korea plans sometime between April 12 and 16. The U.S. is promising to scrap a just-signed food aid deal if the rocket is launched. Tokyo and Seoul have vowed to shoot it down if it veers off course. Russia and China, which have long-standing ties with the North, have urged Pyongyang to rethink its plans.

Though the international community is concerned about both countries’ satellite launches, which require technology also needed to launch missiles, Pyongyang’s efforts are seen as a greater threat, in part because it already is believed to be capable of producing nuclear weapons.

Experts also are more skeptical of North Korea’s claim that its launch is a scientific mission. It has nothing to show for nearly 15 years of off-and-on efforts to launch a satellite, in contrast to Iran, which has successfully launched three.

While the two countries, which have a long history of cooperating to develop long-range missiles, appear to be pulling pages out of the same playbook, experts say Pyongyang’s launch plans are particularly bold.

“They are deliberately pushing the envelope,” said James Moltz, a professor at the U.S. Naval Postgraduate School in Monterey, California.

Moltz said the launches underscore how both countries, isolated from the international community, “are struggling to develop military capabilities that no one else wants them to get.”

For Iran, long-range missiles are seen as a key to keeping Israel at bay. North Korea wants a credible threat to counter the United States. Nothing could accomplish that better than raising the fear that it will put a nuclear weapon on the tip of an intercontinental ballistic missile, though North Korea is not yet believed to be capable of making weapons small enough.

Developing missiles is a tricky game for both countries. North Korea is under heavy U.N. sanctions to prevent it from conducting any ballistic missile tests. Iran is under sanctions preventing other nations from selling it missile-related technology.

Even so, conducting satellite launches helps both countries achieve military goals, though they have had different levels of success.

North Korea first tried to launch a satellite back in 1998. It tried again in 2006 and 2009, but international observers say neither succeeded.

Iran launched its first satellite aboard an Iranian-built rocket in 2009 and its second in June last year. In February, it successfully launched the Navid satellite into orbit with a missile launch-vehicle called the Safir.

But playing the satellite card has two big pluses for North Korea and Iran.

The launches augment data for military programs and at the same time boost national pride, just as the U.S., Soviet and, more recently, Chinese space programs have done for their countries.

Iran and North Korea claim their satellite launches are for strictly peaceful purposes, and say they have a sovereign right to space programs.

Recent satellite work by South Korea could complicate efforts to get North Korea to stop, said physicist David Wright of the Union of Concerned Scientists.

“South Korea has developed a space launcher — interestingly, using Russian technology — and has been trying to place satellites in orbit,” Wright said. “South Korea is currently negotiating with the U.S. to allow it to deploy missiles with longer range — the current range limit is 300-600 kilometers (185-370 miles). So North Korea is likely feeling a double standard, and probably does not want to be shown up by South Korea.”

Cooperation between Iran and North Korea on missile technology goes back at least two decades.

Iran is believed to have begun cooperating with North Korea on the development of a medium-range missile in the early 1990s. The North Koreans also provided Soviet-designed Scud short-range ballistic missiles and the industrial and technical infrastructure for Iran to produce the missiles domestically.

“Iran is now considered more advanced than the North Koreans in missile technology — particularly in the category of solid-fueled missiles,” said Greg Thielmann, a former senior U.S. intelligence official who is now at the Washington, D.C.-based Arms Control Association. “I think Iran no longer needs North Korean help to continue making advances in improving its ballistic missiles.”

Thielmann said Iranian launches suggest it is making significant progress.

“The second stage of the Safir-2 space launch vehicle exhibited technical features — gimbaled engines and higher-energy fuel — not previously seen in either Iran or North Korea,” he said. He also said Iran is developing a two-stage, solid-fuel, medium-range ballistic missile that is “superior to any the North Koreans possess.”

Iran’s claim to having ambitions beyond missile testing for its satellite program is also seen as more credible than North Korea’s. It has followed a rather predictable pattern of developing its space program with considerable foreign cooperation, which North Korea has shown little interest in.

“In Iran, you can observe a rational, logical, slow but steady program that is aimed at space launches, with expected failures and impressive successes,” said Markus Schiller, an analyst with Schmucker Technologie in Germany and a leading expert on North Korean missiles. “In North Korea, you only see singular events now and then, with no clear line or rationale visible.”

Schiller said military analysts will be watching closely to see how much the Unha-3 rocket North Korea plans to launch resembles the Iranian Safir. The upper stages of the rocket North Korea used in its 2009 launch bore several similarities to the Safir, which suggests ongoing collaboration between the two programs.

“It is known that North Korean rocket experts were in Iran in the 1990s,” he said. “Personally, I would expect a continuing cooperation between these countries on the missile-rocket sector, but it remains unclear to what degree they still cooperate.”

Military planners want to know how much progress North Korea has made since its last attempt to launch a satellite three years ago. Arms negotiators will be looking for signs of how much the rocket, a modified ballistic missile launcher, depends on foreign technology.

“There are a number of things they will be watching for,” said Narushige Michishita, a North Korea expert with Japan’s National Graduate Institute for Policy Studies. “If North Korea does get a satellite into orbit, that means it could deliver an object anywhere on the globe, and that has intercontinental implications.”

One thing analysts could quickly put to the test is North Korea’s insistence that the satellite launch is a peaceful mission. Experts can easily estimate from photographs the rocket stages’ mass ratio — a measure of their efficiency — and that will give a quick indication of whether the rocket is designed primarily to be a space vehicle launcher or long-range missile.

They also will be watching where the rocket goes.

North Korea says it will fire the satellite into a polar orbit. The “splash zones” for the booster stages suggest it will travel south over the East China Sea and the Pacific, rather than the easterly path it chose for a launch in 2009 that sent the rocket directly over Japan’s main island.

That could indicate North Korea is being more cautious about its neighbors’ reactions — though it has alarmed others such as the Philippines which could be in the rocket’s path. But the launch could also have military implications.

In North Korea were to attack the United States, Michishita said, it would likely launch to the north. It can’t feasibly conduct such a test, because that would anger Russia and China, which would be under the flight path. Launching to the south can provide similar data.

Actually reaching the splash zones is another hurdle. In its 2009 launch, the stages barely made their zones, suggesting they had lower thrust than expected.

Analysts stress that success by no means suggests North Korea could pull off an attack on the U.S.

North Korea has a long way to go in testing the technologies required for re-entry — a key to missile delivery that is not tested in satellites. And while it is believed to be capable of producing nuclear weapons — and almost certainly wants to put them on a military-use missile — it is not yet able to make them small enough to load into a warhead. Doing so will likely require another nuclear test, which North Korea hasn’t done since 2009.

The launcher itself is another issue — and it has a history of failure.

The Unha-3 rocket that will be used in the launch is believed to be a modified version of North Korea’s long-range Taepodong-2 ballistic missile, which mixes domestic, Soviet-era and possibly Iranian designs.

North Korea launched its first Taepodong-2 in 2006 and it exploded just 40 seconds after liftoff. A followup attempt in 2009 got off the launch pad and successfully completed a tricky pitching maneuver, but analysts believe its third stage failed to separate properly, sending it and the satellite it carried into the Pacific.

Even so, physicists David Wright and Theodore Postol of the Union of Concerned Scientists say the 2009 launch displayed major strides over the Taepodong-1. If modified as a ballistic missile, they say, it would potentially give the North the capability to reach the continental United States with a payload of one ton.

In an analysis of the 2009 launch, Wright and Postol suggested North Korea relies heavily on a stockpile of foreign components, likely from Russia. If data from the upcoming launch confirms that, it may mean Pyongyang’s missile program is severely limited by the isolated country’s ability to procure new parts from abroad.

That could figure into future arms talks. If North Korea is running out of the parts it needs, it isn’t likely to conduct frequent missile tests, and may be more willing to agree to test moratoriums. More emphasis on blocking its imports would also make sense if the North cannot manufacture what it needs.

What analysts find out will figure into regional security planning for years to come — as North Korea’s first attempted satellite launch did in 1998.

Japan and the United States responded to that launch by pouring billions of dollars into the world’s most advanced ballistic missile shield. That shield includes a network of sea-based SM-3 interceptor missiles and land-based PAC-3 Patriot missiles.

Japan is now mobilizing PAC-3 units in Okinawa, which is near the path of the upcoming launch and where more than half of the 50,000 U.S. troops in Japan are deployed. It’s also mobilizing PAC-3 units in Tokyo, which is much farther from the rocket’s expected path. South Korea is taking similar steps — which it didn’t do in 2009.

The U.S. will be watching with equipment that was unavailable in 2009: a Sea-Based X-Band radar system, aboard a Navy ship that left Pearl Harbor late last month.

U.S. officials claim the SBX system is so powerful it can track a baseball-sized object flying through space 2,500 miles (4,000 kilometers) away. Further, if U.S. military satellites detect a flash of heat from a missile launch in North Korea, within a minute computers can plot a rough trajectory and share that information with Japan.

Tokyo and Seoul warn they will use their interceptors on anything that threatens their territory, though that is highly unlikely. No country has ever shot at another country’s satellite launch, and, barring any major surprises, the North Korean rocket will be traveling mostly over water, not populated areas.

“Whether it comes close to our southwestern islands or not, this will have significant implications for our missile defenses, and how they should be adjusted in the future,” said Hiroyasu Akutsu, a senior fellow and Korea expert with the National Institute for Defense Studies, a think tank run by Japan’s Defense Ministry.