The largest rocket ever to blast off from the West Coast launched into space from Vandenberg Air Force Base in California on Thursday afternoon.

The most powerful launch rocket in the U.S., the Delta IV Heavy, took off a little after 1 p.m. Pacific Time carrying a National Reconnaissance Office (NRO) spy satellite called NROL-49.

"This launch marks a significant milestone in our nation's space capability," launch director Lt. Col. Brady Hauboldt said in a press release. "We've really restored a national capability for heavy lift on the Western Range... This extends our ability to cost-effectively deliver payloads of all sizes and compliments."

According to the United Launch Alliance (ULA), the organization that is running the launch for NRO, "this launch supports the military's national defense mission." However, that's about all of the information that has been publicly disclosed about the payload.

An ABC News report said that it's likely a billion-dollar high-resolution satellite capable of spotting things "as small as four inches in diameter" from space.

The "imagery intelligence satellite" is probably a modern version of KH-11, a spy satellite that was first launched in 1976, ABC said.

"KH-11s provide high resolution imagery, useful for strategic and tactical purposes," respected satellite tracker Ted Molczan told ABC. "They have the highest resolution of any such space craft in orbit, and are among the NOR's most important. Examples of hot spots that they monitor are North Korea (its nuclear an ICBM programs) and Iran and its nuclear program."

It's the fifth trip for ULA's Delta IV rocket, an 230-foot tall unmanned booster with two million pounds of thrust. It last launched in November, carrying what is believed to be the largest spy satellite in the world.

Vandenberg has spent three years and $100 million in infrastructure renovations to prepare for Thursday's launch.