The 375th Delta rocket doubled the size of the Air Force’s “neighborhood watch” program today with the successful launch and deployment of two surveillance satellites into the vast geosynchronous belt of spacecraft around the planet.

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The next Delta 4 launch is planned for November from Cape Canaveral. The Medium+ vehicle with four strap-on solid boosters will deploy the Air Force's Wideband Global SATCOM 8 communications satellite into geosynchronous transfer orbit.

United Launch Alliance's next mission is planned for Sept. 8 from the Cape. That is when an Atlas 5 rocket will launch NASA's OSIRIS-REx probe on a 7-year, roundtrip sample return adventure to Asteroid Bennu.

MISSION SUCCESS. Launch of the Delta 4 rocket to deliver the second pair of Geosynchronous Space Situational Awareness Program satellites into orbit 22,000 miles above Earth this morning has been declared a success in the post-flight press release from ULA.

"Thank you to the ULA, Air Force and industry partners for the outstanding teamwork and flawless execution that made today’s mission a success," said Laura Maginnis, ULA vice president of Custom Services.

"This morning's AFSPC 6 launch is a prime example of why our customers continue to place their trust us to launch our nation's crucial space capabilities."

VIDEO REPLAY of today's Delta 4 rocket launch with the GSSAP satellites.

To recap, United Launch Alliance has sent a pair of Air Force satellites on a journey to geosynchronous orbit today aboard a Delta 4 rocket.

The 375th Delta lifted off at 12:52 a.m. EDT (0452 GMT) from Complex 37 at Cape Canaveral Air Force Station.

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

The rocket is launching the U.S. Air Force's second pair of Geosynchronous Space Situational Awareness Program, or GSSAP, satellites. The first pair deployed from a single Delta 4 rocket in July 2014.

GSSAP is designed to image and track man-made objects like spacecraft, rocket bodies and debris in geosynchronous orbit. GSSAP embeds itself just below and just above geosynchronous orbit to perform its reconnoitering duties.

The Delta 4 rocket has flown into a pre-arranged news blackout. The veil of secrecy surrounding the launch of these military satellites 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 4 minutes, 35 seconds. The protective payload fairing enclosing the GSSAP satellites atop the rocket has separated in two halves.

T+plus 4 minutes, 26 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.

T+plus 4 minutes, 12 seconds. The Common Booster Core first stage and the attached interstage have been separated in one piece from the Delta 4's upper stage. The upper stage engine's extendible nozzle is dropping into position.

T+plus 4 minutes, 3 seconds. MECO! Main engine cutoff confirmed as the Aerojet Rocketdyne RS-68A powerplant shuts down.

T+plus 3 minutes, 57 seconds. The main engine is throttling down to its minimum power setting in preparation for shutdown.

T+plus 3 minutes, 30 seconds. Chamber pressure holding steady and good engine control on the RS-68A.

T+plus 3 minutes. A minute left in first stage. The main engine continues to perform well, consuming its liquid hydrogen and liquid oxygen propellants.

T+plus 2 minutes, 30 seconds. The RS-68A is consuming nearly a ton of propellants per second as the powerplant pushes the Delta 4 rocket closer to the edge of space.

T+plus 2 minutes, 20 seconds. Mach 5.

T+plus 2 minutes. The vehicle weighs half of what it did at liftoff, burning propellant at a rate of nearly 2,000 pounds per second.

T+plus 1 minute, 45 seconds. Solid motor separation! The spent boosters have been shed from the first stage. Delta 4 continues powering its way toward space on the thrust generated by the RS-68A main engine.

T+plus 1 minute, 35 seconds. The twin Orbital ATK solid rocket boosters have burned out of their propellant. Standing by for jettison.

T+plus 60 seconds. Now passing through the region of maximum aerodynamic pressure as the vehicle accelerates through the lower atmosphere.

T+plus 30 seconds. The Delta 4 rocket is climbing away from Cape Canaveral with its main engine firing at full throttle and the two strap-on boosters giving a powerful extra kick.

T-minus 10, 9, 8, 7, ignition sequence start, 4, 3, 2, 1 and LIFTOFF! Liftoff of Delta 375 and the GSSAP satellites for the safety and security of geosynchronous orbit!

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

T-minus 22 seconds. The steering system for the solid rocket motor nozzle has been activated.

T-minus 30 seconds. Final status check: "Go Delta" and "Go AFSPC 6" to signify all systems with the rocket and payload are ready for launch.

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

T-minus 55 seconds. The Air Force-controlled Eastern Range is green, giving its final clear to launch.

T-minus 1 minute and counting. Computers are verifying that the RS-68A main engine is 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 tank has reached flight level and pressure.

T-minus 2 minutes and counting. Still GO for launch of the 375th Delta rocket. The spacecraft have switched to internal power.

T-minus 2 minutes, 30 seconds. The liquid oxygen tank in the Common Booster Core first stage is 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 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 rocket carrying the twin GSSAP 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 Air Force 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 9 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 12:52 a.m. liftoff.

This will be the 375th Delta rocket launch since 1960, the 33rd Delta 4 since debuting in 2002 and the 14th Medium+ (4,2) version with two solid-fuel boosters. It's also United Launch Alliance's 110th flight since 2006 and seventh this year. And it is the second launch for the Geosynchronous Space Situational Awareness Program, or GSSAP.

T-minus 4 minutes and holding. The countdown has entered the planned 10-minute hold leading toward the 12:52 a.m. EDT liftoff of the Delta 4 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.

T-minus 5 minutes. Standing by to go into the hold.

Propellant conditioning has been completed for all four cryo tanks on the vehicle.

Weather is observed and forecast GO for launch of the Delta 4 rocket from Cape Canaveral at 12:52 a.m. EDT (0552 GMT). Odds have increased to 100 percent favorable tonight.

Launch minus 35 minutes! United Launch Alliance's Delta 4 has been a workhorse for U.S. military and national security spacecraft since entering service in 2002.

The vehicle stands 206 feet tall, weighs about 720,000 pounds fully fueled and will unleash 1.1 million pounds of thrust at launch.

The rocket comes off the pad powered by an Aerojet Rocketdyne RS-68A engine on the Common Booster Core first stage and a pair of Orbital ATK GEM-60 solid rocket boosters.

The throttleable RS-68A engine burns liquid hydrogen and liquid oxygen to produce 702,000 pounds of thrust. The stage measures 155 feet in length with interstage permanently attached, 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.

Range Safety checks are now underway.

Fifty minutes till launch! Weather remains acceptable and no technical issues being worked with the rocket for the targeted liftoff at 12:52 a.m.

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Steering tests are complete. Checks of the safety system are on hold while the Range works an issue.

The flight hazard area has been established.

Now entering the final 60 minutes until the Delta 4 rocket launch from Cape Canaveral at 12:52 a.m. EDT (0452 GMT). Here's a look at some stats about the mission. This will be:

• The 375th Delta rocket launch since 1960
• The 33rd Delta 4 rocket mission since 2002
• The 14th Medium+ (4,2) configuration to fly
• The 51st main engine from RS-68 family used
• The 9th RS-68A main engine flown
• The 50th-51st GEM-60 solid rocket motors flown
• The 471st production RL10 engine to be launched
• The 36th RL10B-2 engine launched
• The 27th Delta 4 rocket launch from Cape Canaveral
• The 35th launch from Pad B at Complex 37
• The 18th use of Delta 4 by the Air Force
• The 97th Evolved Expendable Launch Vehicle flight
• The 110th United Launch Alliance mission since 2006
• The 44th ULA launch for the Air Force
• The 26th Delta 4 under the ULA banner
• The 7th ULA launch this year
• The 3rd launch of the Delta family in 2016

The flight slews and commanding tests for the vehicle steering systems are being performed. The Common Booster Core, the strap-on solid rocket motors and upper stage engine steering checks were run through a pre-launch test pattern.

All the cryo tanks are in topping now.

Countdown activities are progressing well tonight and the launch team is working no technical issues.

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

The 720,000-pound rocket stands fueled and ready for launch at 12:52 a.m. EDT tonight.

Now entering into the final 90 minutes of the countdown to launch of Delta 375 and the second launch for the Geosynchronous Space Situational Awareness Program for the U.S. military.

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

Two hours and counting! Still targeting 12:52 a.m. EDT (0452 GMT) for launch tonight.

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

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.

This is the last of the rocket's four cryogenic supplies to be filled in today's countdown to launch.

Fast-filling of the CBC liquid hydrogen tank has completed. 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.

Chilldown of the upper stage liquid oxygen system is complete for loading the rocket's tank with 4,500 gallons of supercold LOX.

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

A check of the radar shows all is quiet along the Space Coast tonight. The launch weather forecast was improved to 90 percent favorable earlier this evening.

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

Now three hours till launch. The Delta 4 rocket is being loaded with supercold liquid oxygen and liquid hydrogen as the countdown rolls on for launch at 12:52 a.m. EDT (0452 GMT).

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

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

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

And now the chilldown of the liquid oxygen system on Delta's Common Booster Core is starting. This preps the tank and pumping to guard against shock when the supercold oxidizer begins flowing into the rocket a short time from now. The first stage will be loaded with 40,000 gallons of supercold LOX.

Liquid hydrogen flow is confirmed. About 110,000 gallons of LH2 will fill the rocket's first stage.

Still targeting 12:52 a.m. EDT (0452 GMT) for launch tonight.

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

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

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.

A photo gallery of today's gantry rollback for the Delta 4 rocket launch is posted here. First motion occurred at 5:44 p.m.

T-minus 4 hours, 15 minutes and counting. The team is ready for cryogenic fueling as the countdown is underway for tonight's opportunity to launch the Delta 4 rocket with the Geosynchronous Space Situational Awareness Program satellites at 12:52 a.m. EDT.

NEW LAUNCH TIME. Liftoff time has been adjusted to 12:52 a.m. EDT (0452 GMT). This hold will last 25 minutes in total duration. The final hold at T-minus 4 minutes will last 10 minutes.

The launch team is manning stations for the start of fueling operations. Some 165,000 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 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 tonight's launch operation will begin.

At the launch pad, clearing of personnel is underway in preparation for the start of fueling operations this evening and liftoff at 12:47 a.m. EDT.

In the pre-fueling weather update, all conditions are favorable on a beautiful evening at Cape Canaveral for liftoff of the United Launch Alliance Delta 4 rocket.

The odds of acceptable weather for an on-time launch tonight have improved to 90 percent. A stray cumulus cloud violation is the only concern.

The specifics call for scattered clouds, good visibility, southeasterly winds of just 5 knots and a temperature of 80 degrees F.

Now 6 hours till launch. The countdown is proceeding well this afternoon and the launch team is not reporting any issues. A full weather briefing to mission managers will occur in about an hour.

Tonight's launch is the 375th for a Delta rocket since the flight of the experimental Echo 1 communications reflector in 1960.

The Delta family has recorded 358 successful launches in 56 years dating back to Aug. 12, 1960.

The 330-foot tall mobile service tower has been retracted at Cape Canaveral's pad 37B for tonight's launch of the Delta 4 rocket that will place twin GSSAP satellites into space for the Air Force.

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 solid rocket boosters and 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 first and second stages 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 engine 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 5 seconds.

Liftoff remains scheduled for 12:47 a.m. EDT (0447 GMT).

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Tower rollback is underway at Complex 37 to reveal the 206-foot-tall Delta 4 rocket for tonight's launch.

The weather predictions remain favorable for tonight's United Launch Alliance Delta 4 rocket launch from Cape Canaveral carrying a pair of U.S. military surveillance satellites to patrol geosynchronous orbit.

The 65-minute launch opportunity runs from 12:47 to 1:52 a.m. EDT (0447-0552 GMT).

There is an 80 percent chance of acceptable launch weather. The only concern is isolated coastal shower popping up in the wrong place at the wrong time that would violate the cumulus cloud rule.

"The forecast for the launch period remains largely unchanged from the previous forecast," Air Force launch forecasters report this morning.

"The Bermuda high pressure ridge has moved a bit south and is currently over the northern parts of the Florida peninsula. Very isolated showers have developed over the water and were moving along the periphery of the ridge. Showers and storms will, again, develop along the sea breeze near noon, but will move inland keeping the weather along the coast favorable.

"This evening and early tomorrow morning, a trough moves into the southeast U.S., suppressing the Bermuda high pressure ridge further into Florida. Winds will shift and become weak from the southeast, and isolated showers are possible causing a slight concern for a cumulus cloud Launch Commit Criteria violation."

The outlook for the launch window includes scattered low- and high-level clouds, good visibility, winds from the southeast of 5 knots, a relative humidity of 85 percent and temperature of 80 degrees F.

With the final readiness review now complete, a United Launch Alliance Delta 4 rocket has been cleared to enter the countdown Thursday evening for its late-night liftoff to deliver two Air Force satellites into space from Cape Canaveral.

Liftoff is planned for early Friday morning at 12:47 a.m. EDT (0447 GMT). The day's launch window extends 65 minutes to 1:52 a.m. EDT (0552 GMT).

If the launch slips to Saturday, the launch window moves four minutes earlier per day.

We will have complete live play-by-play coverage of the count and launch on this page, as well as a webcast of liftoff.

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

Weather forecasters give an 80 percent chance of acceptable launch conditions. Cumulus clouds from isolated coastal showers are the only concern threatening the weather rules.

The launch is the second of two spacecraft deployments for the Geosynchronous Space Situational Awareness Program, or GSSAP. The first was successfully performed by a Delta 4 rocket in July 2014.

GSSAP is used by U.S. Strategic Command as maneuverable eyes to patrol geosynchronous orbit. The spacecraft have optical payloads to image the location, orbit, size and status of space objects such as satellites, rocket bodies and debris. That surveillance data will improve the military's ability to rapidly detect and warn of impending collisions.

This will be the 33rd Delta 4 rocket launch since 2002 and the 27th Delta 4 to fly from Cape Canaveral.

The two-stage rocket was rolled from the Horizontal Integration Facility to the pad on July 5 to be hydraulically raised into the vertical position atop the launch table on July 6.

A full countdown dress rehearsal and fueling exercise occurred on July 27.

The payload, already encapsulated in the rocket's four-meter nose cone, was transported to the pad for mating with the rocket on Aug. 5.

Countdown activities will get underway in the late Thursday afternoon with retraction of the mobile service gantry around 5:30 p.m. EDT. Fueling operations will start around 8:30 p.m. EDT.

PREVIEW: The Air Force will launch a second pair of small security probes 22,300 miles above the Earth using a Delta 4 rocket on Friday to alert ground controllers of impending space collisions and monitor potential meddling with U.S. orbiting assets.

Read our preview story.

WEATHER: Air Force meteorologists are giving favorable weather odds for the Delta 4 rocket to launch a pair of space surveillance satellites early Friday morning from Cape Canaveral.

Read our full story.