A Delta 4 rocket blasted off Saturday night, carrying a national security satellite into space.

The 23-story rocket, which replaced the Titan fleet two years ago, blasted slowly at 8:50 p.m., taking nearly 15 seconds to clear the launch pad.

The 1.6 million pound rocket, which can carry the military's largest payloads, uses three common booster cores that are strapped together with a trio of hydrogen-fueled motors and then added to an upper stage rocket.

It carried the 23rd and final satellite of the Air Force's Defense Support Program. The satellite was expected to set into orbit 6 1/2 hours after liftoff, more than 22,000 miles above the Earth.

The reconnaissance satellites use infrared sensors that detect heat from missile and booster plumes against the background of the Earth.