America's biggest unmanned rocket, the Delta 4 Heavy, is shown here roaring into space Sunday. It's carrying a secret spy satellite for the United States into orbit that one official touts as "the world's largest."

The giant boosters attached to the Delta IV Heavy each provide 744,737 pounds of thrust. Adding the two boosters with the main thruster, you get a total thrust of over two million pounds of thrust. You can see all two million of those thrust pounds are being put to good use as the rocket blasts off at 5:58 p.m. ET from Cape Canaveral Air Force Station in Florida. The rocket's carrying a classified payload for the U.S. National Reconnaissance Office.

The satellite, called NROL-32, launched after a series of delays from technical glitches. The most recent glitch, a pair of faulty temperature sensors, thwarted a Nov. 19 launch attempt.

The exact purpose of the new spy satellite NROL-32 is secret, but one NRO official has hinted at the huge size of the reconnaissance spacecraft.

In a Sept. 13 address at the Air Force Association's Air and Space Conference, NRO director Bruce Carlson, a retired Air Force general, told an audience that this Delta 4 Heavy rocket would launch "with the largest satellite in the world on it."

For comparison, in July 2009 a satellite called TerreStar-1 - touted as the world's largest commercial satellite ever built - launched into space atop an Ariane 5 rocket. TerreStar-1 is a 15,233-pound (6,910-kilogram) satellite equipped with a huge 60-foot (18-meter) antenna.

The Delta 4 Heavy rocket is the United States' biggest unmanned rocket currently in service and has 2 millions pounds of thrust, making it the most powerful liquid-fueled booster available today. A Delta 4 Heavy rocket stands 235 feet (72 meters) tall and is actually made up of three boosters, each called a Common Booster Core, arranged in a line to give it a three-column appearance.

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