History provides clues to next steps in North Korea's nuclear program After Tuesday's nuclear test, questions arose as to whether or not North Korea has advanced to the point where they could reach the continental U.S. with a missile. By James Oberg, Space Analyst, NBC News This week's North Korean nuclear test raises the crucial question of how close Pyongyang is to "weaponizing" its nuclear devices and rockets. It's difficult to tell, due to the country's trademark secretiveness, but the progress to date suggests that North Korea is committed to a long-term, trial-and-error campaign to put all the pieces together. The country's two rocket launches over the past year serve as an apt illustration: Last April, an NBC News team spent 10 days in North Korea to observe the preparations and launching of a "peaceful" Earth-observing satellite. The trip turned into a P.R. debacle for the North Koreans. They held back the key evidence that could have demonstrated peaceful intent, and their rocket failed after a covert blastoff. In December, North Korea finally launched a satellite into orbit, but again failed the "openness" test by concealing what the rest of the world soon found out anyway: that the satellite was dead on arrival, a drifting derelict. So what's next? If North Korea's secret plan is to develop a system capable of menacing the United States, an intercontinental launch and re-entry system is the key part of the puzzle. Last year, U.S. intelligence sources told NBC News that North Korea already may have up to a "few dozen" nuclear weapons that could be fitted on ballistic missiles. Based on historical precedents from the original U.S.-Soviet missile race, more than half a century ago, two technological challenges stand out: Providing a heat shield for a re-entry vehicle carrying a nuclear warhead: One option would be to use an "ablative" shield that absorbs the tremendous heat of atmospheric re-entry by burning away layers of material. For all we know, testing of such materials has already started. Fabricating components rugged enough to weather the stresses of a rocket ascent: It's conceivable that the satellite failure in December was due to inadequate robustness of the satellite's systems. Too rough of a ride? One of the lessons from April's visit was that the North Koreans prided themselves on working things out for themselves. They seemed almost to boast about how they didn't need to learn from any other country's experiences. After April's failure, North Korean statements reflected a more cautious approach to the challenges. News reports told how the rocket team had vowed to increase the quality and care of their own work. The Koreans accepted responsibility for the failure, rather than blaming foreign enemies for sabotage. December's launch attempt marked a success for the rocket, but an apparent failure for the satellite. Post-launch reports from U.S. intelligence sources as well as the global satellite-watching community confirmed that the spacecraft never emitted a single beep. To this day, it continues to tumble end over end in orbit. Such a failure would have a direct bearing on the construction of a reliable nuclear warhead for a long-range missile, since an intercontinental nuclear strike would use the same type of rocket that launched the satellite. True, a warhead and a satellite are different devices - but they would share critical control components, including electronics as well as rocket thrusters. The plans announced by North Korea call for launching two more Earth-observing satellites - which would provide at least two more opportunities to test the ruggedness of onboard systems. How to test a heat shield Meanwhile, the North Koreans would also need to test the heat shielding for re-entry vehicle, either for purportedly peaceful purposes or for nuclear warheads. Ground laboratories can simulate some of the heat flow, but flight tests with actual deceleration stresses are also required. Historically, even Nazi Germany's short-range V-2 missiles experienced enough heating to detonate their explosives. This required adding layers of plywood as makeshift heat shielding. By the time long-range missiles appeared in the mid-1950s, rocket designers had run into a "wall" of severe heating. It took years for the Soviets and the Americans to develop adequate heat shields and the means for testing them. For the United States, the creation of the Jupiter-C rocket, later upgraded and renamed the Juno-1, marked the key advance. That rocket successfully pushed a test warhead back into the atmosphere at speeds equivalent to long-range flights - and soon afterward, it launched the first U.S. satellite, Explorer 1. This underscores a contemporary issue: A rocket built to test warhead heat shields would be virtually identical to a rocket for launching small satellites, because the desired speeds are similar. North Korea could conduct such test flights at any time, either as overt missile tests, or disguised as a satellite mission. Theoretically, the rocket wouldn't even have to fly very far across Earth's surface to its full range. It could pop up into space and then, with its upper stages still firing, turn back down into the atmosphere to attain the speeds equivalent to a full-range firing. December's launch of North Korea's Unha-3 rocket is displayed on an array of screens at the country's satellite control center. In that profile, outside observers would see a shorter-range flight. It might be harder to determine what was actually being tested. Seeing recovery ships in an impact zone - say, 600 miles (1,000 kilometers) off the North Korean coast - might be one clue. The test payload need only relay a brief radio burst confirming its survival before splashing into the ocean and sinking out of sight. Such a payload could piggyback on a satellite launch - which raises the possibility that the North Koreans could have conducted heat-shield tests during last year's launches. Despite repeated promises, our hosts never showed us what was under the rocket's aero shroud last April. So there was no way to rule out the possibility of a hidden test. Although the North Koreans have the home-field advantage, by virtue of their status as the most closed society on the planet, the outsiders who are trying to figure them out have a greater advantage: The reality of rocket science is the ultimate judge, rewarding and punishing the same traits in all space programs. Where Pyongyang leaves blanks and shadows, experienced rocketeers can often fill in the gaps with reasonable implications and deductions from other national projects. That's why our April "road trip" to North Korea was a success for those trying to understand North Korea's rocket program - and a failure for those trying to distort it. More about North Korea's nuclear intentions: He was part of the NBC team that visited North Korea last April.