Jan. 20, 2011  -- The United States launched a classified spy satellite from Vandenberg Air Force Base in California this afternoon -- and for a top-secret launch, the government was actually pretty open about it.

The satellite -- publicly designated NROL-49 -- was mounted atop a rocket 230 feet tall, known as a Delta IV Heavy. It is currently the tallest rocket in America's fleet of launchers, and when it leaves a launch pad, generating two million pounds of thrust, it is hard to miss.

It was the largest rocket ever launched from the West Coast. The Saturn V moon rockets and the space shuttles were built to generate more thrust, but they only launched from Florida.

"Launches like this have been seen all up and down the coast, as far as Los Angeles," said Mike Rein, a spokesman for the United Launch Alliance, which ran the launch for the National Reconnaissance Office. "And, of course, it leaves a vapor trail too."

Vandenberg is about 150 miles northwest of the Los Angeles basin. Officers at the base closed nearby roads and a park, did tests to make sure the vibrations from the launch would not break windows, and announced the planned launch time well in advance so that people would not think they were caught in an earthquake.

What was the rocket carrying? The launch team would not say, beyond a coy phrase on the ULA website that "this launch supplies the military's national defense mission."

Independent analysts, though, said the payload is a billion-dollar high-resolution spy satellite. Ted Molczan, an amateur sky watcher from Toronto, said it was an "imagery intelligence satellite" capable of spotting objects from orbit that are as small as four inches in diameter.

He said it is probably an updated version of a satellite called KH-11, first launched in 1976.

"KH-11s provide high resolution imagery, useful for strategic and tactical purposes," Molczan wrote in an e-mail to ABC News. "They have the highest resolution of any such spacecraft in orbit, and are among the NRO's most important. Examples of hot spots that they monitor are North Korea (its nuclear and ICBM programs) and Iran and its nuclear program."

(Click here for larger-format video of the launch.)

"I would be hard-pressed to disagree with him," said John Pike, who runs the website GlobalSecurity.org and often writes about space and national security.

NASA and the Air Force have been launching Delta rockets since 1960, but have been steadily upgrading them over the years. The Delta IV Heavy is actually three booster stages strapped together, generating enough thrust to put payloads of 24 tons in low Earth orbit.

Four of them have been launched since 2004, all from Cape Canaveral Air Force Station in Florida. This is the first from California. It is using a pad called Space Launch Complex 6 at Vandenberg -- "Slick Six" to those who know it. SLC-6 was meant to be used for military launches of space shuttles, but was heavily modified to fit the Delta IV Heavy.

Spy Satellite Launch from Vandenberg Air Force Base, California

Aside from the soon-to-be-retired space shuttles, the Delta IV Heavy is the most powerful rocket the United States now has. The shuttles, with their strap-on boosters and orange external fuel tanks, are capable of carrying more than 50,000 pounds into orbit, but they are far more complicated, and the Air Force decided to stop using them after the Challenger disaster 25 years ago.

"This launch marks a significant milestone in our nation's space capability" said Lt. Col.Brady Hauboldt, the launch director, in a press statement. "This extends our ability to cost effectively deliver payloads of all sizes and compliments."

Why make the launch public, complete with video and high-quality photographs? Molczan said there was little point in keeping it secret.

"My colleagues and I routinely track nearly every one of the more than 300 U.S. military satellites in secret orbits," said Molczan. "Most are long dead relics of the cold war, but their orbits remain secret. If we hobbyists can find them, then any nation can do likewise."