A Delta IV Heavy carrying the classified NROL-37 payload climbed to orbit from Florida this afternoon. Weather cooperated this time, and the rocket rose out of its customary hydrogen fireball in a completely successful mission, placing a reconnaissance satellite of some kind into orbit. The final orbit is undisclosed, as are the spacecraft’s type and mission. Presumably, however, it is a very large satellite, possibly in a high orbit, perhaps even geosynchronous, since there are few payloads large enough to call for this massive rocket.
The Delta IV Heavy is the most powerful rocket operating today, and the third or fourth most powerful in history (depending on how you count the Space Shuttle — Space Shuttle had considerably more thrust but had a smaller payload capacity as most of its upmass was consumed by the Orbiter itself; note that I am not counting N-1 as despite its incredible liftoff power, it never achieved a successful flight). This is not a title it will hold for long; SpaceX is moving towards the first flight of Falcon Heavy, which will combine three Falcon 9 rockets in the same manner as the three Common Booster Cores of Delta IV Heavy. As Falcon 9 outperforms Delta IV Medium (which is essentially just a naked CBC with no boosters), the total performance of the Heavy variant is expected to be greater as well.