NEW YORK — A huge unmanned rocket carrying a U.S. spy satellite roared into space Nov. 21 to deliver into orbit what a top U.S. intelligence official has publicly touted as “the largest satellite in the world.”

The giant booster — a Delta 4 Heavy rocket — blasted off at 5:58 p.m. EST from the Cape Canaveral Air Force Base in Florida, carrying a classified payload for the U.S. National Reconnaissance Office (NRO). “This mission helps to ensure that vital NRO resources will continue to bolster our national defense,” said Air Force Brig. Gen. Ed Wilson, commander of the 45th Space Wing, after the successful launch.

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 purpose of the new spy satellite is secret, but one NRO official has hinted at the huge dimensions of the spacecraft, which amateur satellite trackers believe is designed to intercept signals intelligence for the National Security Agency.

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 6,910-kilogram satellite equipped with a huge 18-meter antenna.

Ted Molczan, an amateur satellite tracker in Toronto, said Nov. 23 he believes the payload was the fifth in a series of so-called Mentor signals-intelligence satellites the United States began launching in 1995. Molczan said NROL-32’s launch trajectory — due east from Cape Canaveral — tells him the payload was bound for geosynchronous orbit. A rocket bound for the kind of highly elliptical orbit used to maximize a spy satellite’s dwell time over the northern hemisphere would have headed northeast up the North American coast, he said.

Molczan also said he has no reason to doubt Carlson’s claim that the payload aboard NROL-32 is among the largest ever launched. “If it isn’t the largest, it must be in the top two or three. It is undeniably a huge object,” he said.

Molczan bases his claim in part on the amateur satellite-tracking community’s observations of previous NRO satellites believed to be part of the same series. These satellites, Molczan said, are significantly brighter than other geosynchronous satellites — in astronomy terms, they are 4 or 5 magnitudes brighter than the typical telecom satellite orbiting 36,000 kilometers above the equator and can be seen with a large pair of binoculars.

The Nov. 21 launch marked the fourth launch of a Delta 4 Heavy rocket and the second satellite launch in as many months for the NRO. An Atlas 5 rocket launched the NROL-41 reconnaissance satellite Sept. 20.

In his September address, Carlson said that the plan for NRO satellite missions “is the most aggressive launch campaign that the National Reconnaissance Office has had in 20 years, almost a quarter of a century.”

Carlson went on to say that new satellites are vital for NRO’s mission and to replace older satellites before they fail.

“The other thing I can tell you is these are very important, because they all go to update a constellation which is aging rapidly,” Carlson said in his address, according to an NRO transcript. “We bought most of our satellites for three, five, or eight years, and we’re keeping them on orbit for 10, 12, and up to 20 years.”

“Now when I buy something people complain about how expensive it is, but nobody ever complains when it’s time to die and it keeps right on ticking,” Carlson added. “Some of these guys are like the Energizer bunny and they have really done marvelous work.”

The Delta 4 Heavy rocket is the United States’ biggest unmanned rocket currently in service and has 2 million pounds of thrust, making it the most powerful liquid-fueled booster available today. A Delta 4 Heavy rocket stands 72 meters tall and is actually made up of three common booster cores arranged side-by-side to give it a three-column appearance.

The rocket is built and launched by the United Launch Alliance, a partnership between Lockheed Martin and Boeing. It made its first flight in 2004 and is capable of launching payloads of up to 24 tons into low Earth orbit and 11 tons toward the geosynchronous orbits used by communications satellites.