North Korea said Tuesday it had conducted the final ground test of a solid-fuel rocket engine for a long-range ballistic missile in its latest advancement toward having an arsenal that could viably threaten the continental United States, the Associated Press reports. The test Monday observed by leader Kim Jong Un was the ninth of the solid rocket motor built with carbon fiber and capable of producing 1,971 kilonewtons (443,000 pounds) of thrust, more powerful than past models, according to the North's official Korean Central News