ULA successfully launched a giant Delta IV rocket this afternoon that's supposed to be carrying a secret spy satellite into space for the National Reconnaissance Office. The rocket, manufactured by the United Launch Alliance, was originally scheduled to launch on Thursday, but plans were canceled due to bad weather. The launch was then rescheduled for Saturday afternoon, when it successfully took off.

The flight is dubbed the NROL-37 mission, the purpose of which remains classified since it "supports national security." Despite the intense level of secrecy, NROL-37 does have one very cryptic mission patch associated with it. According to the NRO, the patch "depicts a knight, a symbol of courage with a chivalrous code of conduct representing bravery, training, and service to others. The knight stands in front of the U.S. flag in a defensive posture as to protect at all cost." The eagle on the knight’s chest is supposed to represent freedom, according to the NRO, while the sword is "a message of tenacious, fierce focus with the claws representing extreme reach with global coverage."

Another clue about NROL-37 is the Delta IV rocket it’s launching on. This is the heavy version of the rocket, which is the largest vehicle that ULA makes. The Delta IV Heavy consists of one main rocket core with two additional boosters attached to either side, which makes for an impressive sight during takeoff. Altogether, those boosters create a whole lot of thrust capable of getting upwards of 30,000 pounds into geosynchronous transfer orbit — a very elliptical orbit located high above Earth’s surface. The satellite needed for the NROL-37 mission is likely pretty hefty and will travel to a very high orbit if it needs the heavy version of the Delta IV to carry it into space.

First/second stage separation, Delta Cryogenic Second Stage Ignition and fairing separation! All systems nominal. #DeltaIV #NROL37 — ULA (@ulalaunch) June 11, 2016

The Delta IV Heavy may be ULA’s largest vehicle, but it also doesn’t fly too often. The last time the vehicle flew was in 2014, when it lofted an uncrewed test version of NASA’s Orion crew capsule into space. Since the Delta IV Heavy’s first flight in 2004, this version of the rocket has only flown eight times, and most of those launches have been for the NRO.

Update June 9th, 6:05PM ET: This article was updated to indicate the launch has been cancelled.

Update June 9th, 4:48PM ET: This article was updated to indicate the launch has been holding.

Update June 9th, 2:06PM ET: This article was updated to include the new target launch time.

Update June 11th, 1:15PM ET: This article was updated to include the new target launch time.

Update June 11th, 2:00PM ET: This article was updated to reflect the rocket's successful liftoff.

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