Experts speculate that the satellite-carrying rocket Naro 1 blew up in three stages during a second launch attempt last Thursday, not all at once 137 seconds after blastoff, as previously assumed. The cause of the explosion has yet to be determined.

Prof. Yoon Woong-sup of Yonsei University on Monday said, footage of the launch shows that a second explosion occurred about 0.5 seconds after the first one, and a third explosion some 5 to 10 seconds later. There is a likelihood that the explosions occurred in stages after fuel or oxidizing agent leaked from the first-stage engine of the rocket.

The Korea Aerospace Research Institute is going to take this assumption into consideration when it tries to figure out the cause.

Fire does not easily break out at low pressure and temperature, but it is possible that consecutive explosions occurred if fuel or oxidizing agent leaked from the first-stage engine of the rocket, Yoon said.

The ignition starts when fuel and oxidizing agent are mixed at a proper ratio. "Based on the recorded footage, the flame from the Naro looked yellow at first, then the periphery became red and mottled," Yoon said. "This is not a normal phenomenon. It appears that either fuel or oxidizing agent was improperly supplied."

But a spokesman for Energomash, the Russian developer of the Naro's engines, said technicians in Korea reported no significant engine problem. "The failure was due to a flaw in the Naro's control device," he added.

Kim Jung-hyun, vice minister of education, science and technology, denied the Russian report, saying the control device for the Naro's second-stage engine had not even been activated yet.

Meanwhile, Korean and Russian technicians exchanged data at a first session of a joint investigation team. They agreed to analyze the Naro's flight data carefully.