Two million pounds of firepower successfully launched a behemoth Delta 4-Heavy rocket and a top-secret eavesdropping surveillance satellite into space Saturday, embarking on a taxing climb directly into geosynchronous orbit 22,300 miles above the Earth to deploy one of the pinnacles of U.S. intelligence-gathering.

Read our full story.

The next Delta 4 launch is planned for Aug. 4 from Cape Canaveral. The Medium+ vehicle with two strap-on solid boosters will carry another pair of Geosynchronous Space Situational Awareness Program, or GSSAP, inspector satellites into geosynchronous orbit for for U.S. Strategic Command.

United Launch Alliance's next mission is planned for June 24 from the Cape. That is when an Atlas 5 rocket will launch the Navy's MUOS 5 mobile communications satellite into a high-perigee geosynchronous transfer orbit.

MISSION SUCCESS. Launch of the Delta 4-Heavy rocket on NROL-37 has been declared a success in the post-flight press release from ULA.

"We are so honored to deliver the NROL-37 payload to orbit for the National Reconnaissance Office during today's incredible launch," said Laura Maginnis, ULA vice president of Custom Services. "This was the ninth time ULA launched the Delta 4-Heavy, the most powerful launch vehicle in existence today."

PHOTOS of today's Delta 4-Heavy rocket launch as seen from press site on Cape Canaveral Air Force Station.

VIDEO REPLAY of today's Delta 4-Heavy rocket launch with NROL-37.

To recap, United Launch Alliance has sent America's largest booster to space today carrying a clandestine payload for the U.S. National Reconnaissance Office.

The Delta 4-Heavy rocket departed Complex 37 at Cape Canaveral Air Force Station on the NROL-37 mission at 1:51 p.m. EDT (1751 GMT).

With the rest of today's mission occurring in secrecy, we will pause our live coverage at this time. Check back later tonight when the launch result should be announced.

The National Reconnaissance Office is the government agency responsible for the design and operation of the country's fleet of spy satellites.

Experts believe the payload is an eavesdropping spacecraft that will be launched into a circular geosynchronous orbit 22,300 miles high to perform intelligence-gathering on terrorist networks and adversarial nations for U.S. warfighters and policy-makers.

The Delta 4-Heavy rocket has flown into a news blackout. The veil of secrecy surrounding the launch of this classified satellite means no further information about the progress of the upper stage engine firings and release of the payload will be announced in real-time.

T+plus 6 minutes, 40 seconds. The tri-sector nose cone that enclosed the classified National Reconnaissance Office satellite during ascent through the atmosphere has jettisoned.

T+plus 6 minutes, 3 seconds. Engine start! The Aerojet Rocketdyne RL10B-2 cryogenic rocket engine is up and burning for the first firing during today's launch of the Delta 4-Heavy.

T+plus 5 minutes, 47 seconds. Pyrotechnics have detonated to jettison the spent center Common Booster Core. The rocket's upper stage and attached payload are now flying free.

T+plus 5 minutes, 40 seconds. Main engine cutoff! The center booster's RS-68A engine has finished firing and shut down.

T+plus 5 minutes, 10 seconds. The Delta 4-Heavy rocket is 59 miles in altitude and 272 miles downrange from the launch pad.

T+plus 4 minutes, 30 seconds. The center Common Booster Core's RS-68A engine has revved up to full throttle for the Delta 4-Heavy rocket's on-going journey to space. The booster is identical to the outer strap-on stages, carrying the same propellant supply and engine package, but it employed a more conservative fuel consumption strategy over the past three minutes. That has left enough cryogenic fuel to fire nearly 90 seconds longer.

T+plus 4 minutes, 10 seconds. The 15-story tall starboard and port Common Booster Cores that provided the vast majority of thrust during the first four minutes of flight have expended their fuel and peeled away from the center stage. Tiny solid-fueled motors on the discarded boosters gave helpful nudges to ensure a clean separation.

T+plus 4 minutes, 1 seconds. Engine cutoff! Standing by for booster separation.

T+plus 3 minutes, 30 seconds. Coming up in about 20 seconds, the outer Common Booster Cores will throttle down as a precursor to engine shutdown and jettison of the stages. It will take a few seconds to ease the power setting to 54.5 percent. The boosters will operate at that throttle for another five seconds before the RS-68As are shut down.

T+plus 3 minutes. The center engine remains at 54.5 percent thrust while the starboard Common Booster Core's engines are firing at 108.5 percent. The outer boosters have just over one minute remaining in powered flight.

T+plus 2 minutes, 45 seconds. The Delta 4-Heavy now weighs half of what it did at liftoff. The rocket is burning vast amounts of cryogenic propellant to accelerate away from the planet.

T+plus 2 minutes. The 8-foot diameter bell-shaped nozzles on the three main engines gimbal during flight, allowing the rocket to steer itself on the intended trajectory eastward across the Atlantic Ocean and toward space.

T+plus 1 minute, 45 seconds. The outer Common Booster Cores and their RS-68A main engines continue to consume the supply of super-cold liquid hydrogen and liquid oxygen rocket fuel while firing at full throttle. The RS-68A is considered the world's largest hydrogen-fueled rocket engine.

T+plus 1 minute, 30 seconds. The vehicle is ascending through the flight regime that provides the maximum aerodynamic pressures on the rocket. This period is called Max-Q. And the Delta 4-Heavy is breaking the sound barrier as its speed reaches Mach 1.

T+plus 60 seconds. One minute into the flight of the Delta 4-Heavy. The rocket is slowly rising away from Earth with three distinct red-hot main engine plumes trailing 20 stories long, backdropped against the sky.

T+plus 45 seconds. The center Common Booster Core's main engine is throttling back to 54.5 percent thrust as a fuel conservation effort. The starboard and port boosters continue to operate at their maximum power setting of 108.5 percent thrust.

T+plus 30 seconds. All three Aerojet Rocketdyne RS-68A main engines are firing at full throttle, gulping three tons of propellant per second to produce over two million pounds of thrust.

T+plus 15 seconds. The vehicle is beginning pitch and roll programs that will place the rocket on the proper heading to fly eastward from Cape Canaveral.

T-minus 10, 9, 8, 7, ignition sequence start, 4, 3, 2, 1 and LIFTOFF! Liftoff of the Delta 4-Heavy rocket, powering the National Reconnaissance Office to the high ground of space. And the tower is clear!

T-minus 12 seconds. Residual hydrogen burnoff ignitors have been fired beneath the main engines.

T-minus 30 seconds. Final status check: "Go Delta" and "Go NROL-37" to signify all systems are ready for launch.

T-minus 40 seconds. Upper stage liquid hydrogen tank is secure at flight level.

T-minus 55 seconds. Range is green.

T-minus 1 minute and counting. The three RS-68A main engines are ready for ignition.

T-minus 80 seconds. Upper stage liquid oxygen tank is being secured.

T-minus 1 minute, 30 seconds. The Common Booster Core liquid hydrogen tanks have reached flight level and pressure.

T-minus 2 minutes.

T-minus 2 minutes, 30 seconds. The liquid oxygen tanks in the three Common Booster Cores are confirmed at the proper level and pressure for flight.

T-minus 3 minutes and counting. Ordnance devices aboard the vehicle are being armed.

T-minus 3 minutes, 25 seconds. The systems of the Delta 4-Heavy rocket have switched from ground-fed power to internal batteries for launch.

T-minus 3 minutes, 30 seconds. Replenishment of liquid hydrogen and liquid oxygen to the Common Booster Core first stage is being secured in preparation to pressurize the tanks for launch.

T-minus 4 minutes and counting! Now into the final phase of the countdown for liftoff by the Delta 4-Heavy rocket carrying the NROL-37 spacecraft from pad 37B at Cape Canaveral Air Force Station.

Resuming the countdown in one minute.

Permission to proceed has been granted by the ULA launch director and the National Reconnaissance Office mission director.

The final readiness polls of the launch team and mission managers have been completed. No technical problems are being worked and all rocket, spacecraft and ground systems are "go" for liftoff.

Now 12 minutes to the target launch time! The launch team will be polled in the next few minutes to confirm all systems are "go" to press onward for the 1:51 p.m. liftoff.

T-minus 4 minutes and holding. The countdown has entered the planned 15-minute hold leading toward the 1:51 p.m. EDT liftoff of the Delta 4-Heavy rocket.

This pause is designed to give the launch team the opportunity to catch up on any work running behind schedule and verify all is in readiness for the final moments of the count. Also, a series of polls will be conducted during the hold to give approval to proceed with the launch.

Weather is observed and forecast GO for launch of the Delta 4-Heavy rocket from Cape Canaveral at the opening of the window, the launch weather officer just reported in the final scheduled briefing before liftoff.

Launch minus 40 minutes! United Launch Alliance's Delta 4-Heavy is America's biggest unmanned rocket currently in service, capable of lofting the largest and heftiest cargos. Its most recent mission launched NASA's Orion capsule on the Exploration Flight Test in 2014.

The Heavy is created by taking three Common Booster Cores and strapping them together to form a triple-barrel rocket, and then adding an upper stage.

The vehicle stands 235 feet tall, is 53 feet wide, weighs 1.6 million pounds fully fueled and will unleash 2.1 million pounds of thrust at launch.

The rocket comes off the pad powered by three Aerojet Rocketdyne RS-68A engines, one apiece on the Common Booster Cores. The throttleable engine burns liquid hydrogen and liquid oxygen to produce 702,000 pounds of thrust each.

The Port and Starboard liquid rocket boosters stand more than 150 feet tall and the Center Core is over 175 feet with interstage permanently attached. Each booster measures 16.7 feet in diameter and is covered in orange insulating foam.

The cryogenic upper stage also burns liquid hydrogen and liquid oxygen to power its single Aerojet Rocketdyne RL10B-2 engine for 24,750 pounds of thrust. It features a cone-shaped carbon-carbon extendible nozzle that is 7 feet in diameter.

The stage carries 60,000 pounds of propellant and can operate for nearly 19 minutes in all. It features a larger diameter liquid hydrogen tank and a lengthened liquid oxygen tank from the stage used by the lower-power Delta 4-Medium rockets.

Propellant coniditioning has been completed for all 8 cryo tanks on the vehicle.

Fifty minutes till launch! Weather remains acceptable, no technical issues being worked, all is on schedule for liftoff at 1:51 p.m.

A reminder that if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

The odds of acceptable weather at the opening of today's launch window have improved to 70 percent.

Now entering the final 60 minutes until the Delta 4-Heavy rocket launch from Cape Canaveral at 1:51 p.m. EDT (1751 GMT). Here's a look at some stats about today's mission. This will be:

• The 374th Delta rocket launch since 1960
• The 32nd Delta 4 rocket mission since 2002
• The 9th Heavy configuration to fly
• The 48-49-50th main engine from RS-68 family used
• The 6-7-8th RS-68A main engine flown
• The 468th production RL10 engine to be launched
• The 35th RL10B-2 engine launched
• The 26th Delta 4 rocket launch from Cape Canaveral
• The 7th Heavy launch from Cape Canaveral
• The 10th classified Delta 4 for the NRO
• The 6th Heavy for the NRO
• The 44th NRO launch since acknowledging flights in 1996
• The 21st NRO launch from the Cape
• The 94th Evolved Expendable Launch Vehicle flight
• The 107th United Launch Alliance mission since 2006
• The 25th Delta 4 under the ULA banner
• The 4th ULA launch this year
• The 2nd launch of the Delta family in 2016

The flight hazard area has been established.

The flight slews and commanding tests for the vehicle steering systems are being performed. The three Common Booster Cores and upper stage engine steering checks are running through a pre-launch test pattern.

Countdown activities are tracking to the scheduled timeline today and the launch team is working no technical issues.

Now entering into the final 90 minutes of the countdown to launch of Delta 374 and the NROL-37 mission for the U.S. National Reconnaissance Office. The count remains on schedule.

The vehicle is fully fueled! Loading of the upper stage liquid oxygen tank has been accomplished.

The 1.6-million-pound rocket stands fueled and ready for launch at 1:51 p.m. EDT today.

Two hours and counting! Weather remains acceptable at this time.

Loading of the upper stage liquid hydrogen tank has been accomplished as fueling proceeds today at Complex 37.

Fast-filling of the CBC liquid hydrogen tanks was completed a little while ago. After post-filling checks and valve tests, the tank will be placed in topping mode. The launch team will confirm the propellant is conditioned for flight.

In the latest update, odds of acceptable of weather for Delta 4-Heavy today remain at 60 percent favorable for the opening of the launch window. But then fall to 40 percent. Cumulus, anvil and thick clouds are the issues now.

The CBC liquid oxygen loading just finished. The tanks have been loaded with its supercold oxidizer that is chilled to Minus-298 degrees F. Topping will be completed as the count rolls on.

A check of the radar shows a line of showers south of Palm Bay that is moving eastward and a cluster west of Orlando.

Chilldown of the upper stage liquid oxygen system is complete for loading the rocket's tank with 4,500 gallons of supercold LOX. This is the last of the rocket's four cryogenic supplies to be filled in today's countdown to launch.

Now three hours till launch. The Delta 4 rocket's Common Booster Cores and upper stage are being loaded with supercold liquid oxygen and liquid hydrogen as the countdown rolls on.

Complex 37 has two giant sphere-shaped fuel tanks to store the cryogenic liquid oxygen and liquid hydrogen. The LOX tank holds 250,000 gallons and LH2 sphere about 850,000 gallons.

The cryogenics are fed from the storage tanks through pipelines to the pad. For the Common Booster Cores, the propellants are routed up to the launch table upon which the rocket sits. Tail service masts, the large box-like structures at the base of the vehicle, feed the oxygen and hydrogen to the booster via separate umbilicals.

The upper stage receives its cryos from the middle swing arm that extends from the Fixed Umbilical Tower to the front-side of the rocket.

The "go" has been given for the upper stage liquid oxygen chilldown in advance of filling that tank.

After chilldown of the upper stage liquid hydrogen system, the clear was given for loading the rocket's tank with 10,000 gallons. The launch team is actively filling the upper stage's liquid hydrogen tank with propellant for the RL10 engine.

CBC liquid oxygen tanking operation is switching from "slow-fill" to "fast-fill" mode.

Local weather remains GO at this time.

The CBC liquid oxygen chilldown is complete. "Slow-fill" mode is beginning to load a small percentage of the tanks. The process then speeds up to the "fast-fill" mode until the tanks are nearly full.

The launch team is preparing to start fueling the Delta 4-Heavy rocket's upper stage. The "go" has been given to start the chilldown conditioning of the upper stage liquid hydrogen system.

CBC liquid hydrogen tanking operation is switching from "slow-fill" to "fast-fill" mode.

And now the chilldown of the liquid oxygen systems on Delta's Common Booster Cores is starting. This preps the tanks and pumping to guard against shock when the supercold oxidizer begins flowing into the rocket a short time from now. The three boosters will be loaded with 120,000 gallons of supercold LOX.

Liquid hydrogen flow is confirmed. About 330,000 gallons of LH2 will fill the rocket's three boosters.

The cold gas chilldown conditioning of the liquid hydrogen system has been accomplished. Liquid hydrogen propellant will begin to flow into the Common Booster Cores in "slow-fill" mode. That is sped up to "fast-fill" after a small portion of the tanks are loaded.

Chilled to Minus-423 degrees Fahrenheit, the liquid hydrogen will be consumed by the three RS-68A main engines along with liquid oxygen during the first minutes of the launch.

A photo gallery of this morning's gantry rollback for today's Delta 4-Heavy rocket launch is posted here.

A "go" has been given to start the cold gas chilldown conditioning of the liquid hydrogen system. This is the precursor to filling the vehicle with propellant.

T-minus 4 hours, 15 minutes and counting. The team is ready for cryogenic fueling as the countdown is underway for today's opportunity to launch the Delta 4-Heavy rocket with the NROL-37 satellite at 1:51 p.m. EDT.

The launch team is manning stations for the start of fueling operations. Some 465,000 gallons of cryogenic liquid hydrogen and liquid oxygen will be loaded today.

T-minus 4 hours, 15 minutes and holding. The countdown has entered a 15-minute built-in hold, a pre-planned pause designed to give the team time to catch up on any work that could be running behind schedule. Once the clocks resume ticking, the main countdown for today's launch operation will begin.

In the pre-fueling weather briefing to mission managers from the Air Force launch weather officer Kathy Winters, this morning's conditions are more stable than they were on Thursday and the stubborn frontal boundary that has been lingering over Central Florida for days is just south of the Cape right now.

The odds of acceptable weather for an on-time launch today have increased to 60 percent. If liftoff is delayed later in the window this afternoon, the odds fall to 40 percent.

So the opening of the window offers the best shot to launch, with the boundary south of the launch pad. Anvil clouds from storms to the west-northwest will be watched, however.

If the liftoff slides later in the window, storms are expected to overtake the Cape area and present difficulty in finding any sort of hole in the weather.

The specific launch commit criteria in jeopardy of being violated today include weather rules governing cumulus clouds, lightning and anvil clouds.

At the launch pad, clearing of personnel is underway in preparation for the start of fueling operations this morning and liftoff at 1:51 p.m. EDT this afternoon.

Now 6 hours till launch. The countdown is tracking to the timeline this morning and the launch team is not working any issues. A full weather briefing to mission managers will occur in about an hour.

After being put back in position around the Delta 4-Heavy rocket after Thursday's weather scrub, the 330-foot tall mobile service tower has been retracted again at Cape Canaveral's pad 37B for today's launch that will place the NROL-37 payload into Earth orbit.

The wheeled structure moved along rail tracks to its launch position about the length of a football field away from the rocket. The 9-million pound tower shielded the Delta from the elements during the its stay on the pad, provided workers 360-degree access to the various areas on the vehicle and was used to attach the payload during the launch campaign. The tower is 90 feet wide and 40 feet deep.

Crews will spend the next couple of hours securing the complex for launch before leaving the danger area around the pad. All workers must be clear of the area for the start of hazardous operations in the countdown, which include fueling the Delta 4's three Common Booster Cores and the upper stage with supercold liquid oxygen and liquid hydrogen propellants.

Testing of communications links between the rocket and Air Force Eastern Range will occur after fueling is accomplished. Steering checks of the RS-68A engines and upper stage RL10B-2 powerplant are on tap in the last hour of the count.

A build-in hold is slated for T-minus 4 minutes, during which time teams will go through final polling to grant clearance to launch. The Delta 4 will transition to internal power as the count resumes, ordnance will be armed and the propellant tanks pressurized as clocks target the main engine ignition time at T-minus 7 seconds.

Liftoff remains scheduled for 1:51 p.m. EDT (1751 GMT).

If you will be away from your computer but would like to receive occasional updates, sign up for our Twitter feed to get text message updates sent to your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

And check out Spaceflight Now's Facebook page!

The launch weather officer has amended the official forecast for Saturday's launch attempt and dropped the odds of favorable conditions from 60 down to 40 percent.

The United Launch Alliance Delta 4-Heavy rocket is scheduled to fly from Cape Canaveral on Saturday to deliver a classified U.S. national security satellite into space.

Liftoff of the NROL-37 mission is targeted for 1:51 p.m. EDT (1751 GMT).

"The trough that lingered in the area all week and caused multiple weather Launch Commit Criteria violations yesterday will continue to plague the area today," the Air Force reported this morning.

"Meteorological models are now showing the boundary still lingering in the area Saturday, and an upper-level short wave will also move through during the launch window. Showers and thunderstorms are still likely along the trough. Also, anvils from inland thunderstorms will migrate toward the Space Coast.

"The primary weather concerns for launch are cumulus clouds, lightning and anvil clouds."

Conditions will improve into Sunday and Monday as an upper-level, long-wave trough to the east deepens, moving the boundary south of the area. That gives an 80 percent chance of accepable weather for Monday's backup launch opportunity.