The team is now preparing to conduct steering checks of the Delta 4 rocket's engines.

Checks of the rocket safety system have been completed.

Now 90 minutes away from the NROL-15 launch for the National Reconnaissance Office.

"Today, the United States is preeminent in satellite reconnaissance. The NRO enlists the expertise of highly skilled engineers from across government and industry to maintain this edge in edge in space - the ultimate high ground from which to watch, listen and learn," the NRO says.

"Always vigilant, NRO's eyes and ears give America's policy markers, intelligence analysts, warfighters and homeland security specialists the critical information they need to keep America safe, secure and free."

The latest update from the launch weather team still puts odds at 90 percent chance of acceptable conditions for liftoff today.

The official launch forecast calls for a few scattered clouds at 2,000 feet, 7 miles of visibility, a temperature in the 70 degrees F and westerly winds from 270 degrees at 5 peaking to 10 knots.

Launch time continues aim for 6:13 a.m. EDT, about 15 minutes before local sunrise. The rocket has been filled up with fuel for the mission. But as the countdown continues, all eight propellant tanks will be replenished to replace the cryogenics that naturally boil away.

Loading of the upper stage liquid oxygen tank is complete and the Delta 4-Heavy rocket now stands fully fueled for launch.

Now moving into the final two hours of the countdown.

Today's mission will add a critical new spacecraft to the nation's spy satellite fleet operated by the National Reconnaissance Office.

"They are silent sentinels. They look and listen from the cold reaches of space. They capture signals and images critical to America's intelligence community, warfighters and policymakers. They reveal threats to the homeland, provide battlefield situational awareness, support counter-narcotics, survey the damage from natural disasters and much more. They are the satellites of the National Reconnaissance Office: America's eyes and ears in space," the agency says.

"Today, the U.S. government openly acknowledges the NRO and a variety of users depend on the enormous amount of data NRO satellites collect. Quickly and reliably delivering the information to all 16 intelligence community agencies, the 5 military branches and various civil users and U.S. allies is a critical NRO mission. To do this NRO, from its headquarters in Chantilly, Virginia, builds, operates and maintains a high-speed global information system of satellites and ground-based communications."

Post-loading checks of the Common Booster Core liquid oxygen systems have been accomplished and the tanks are going into topping mode.

Fast-fill loading of the upper stage liquid hydrogen tank just wrapped up.

The chilldown of the upper stage liquid oxygen system is reported complete, clearing the way for loading the rocket's tank with 4,500 gallons. This is the last of the rocket's eight cryogenic supplies to be filled in today's countdown to launch.

Three hours and counting as the United Launch Alliance team works to get the Delta 4-Heavy rocket fully fueled with cryogenic propellants for today's launch. Liftoff remains scheduled for 6:13 a.m. EDT. Weather is fine and there are no issues being reported in the count.

The launch team is starting the chilldown thermal conditioning of the upper stage liquid oxygen system. This is the last tank left to fill today.

The loading of liquid oxygen into the Common Booster Cores has been completed. The launch team will be performing vent and relief checks following tanking and begin chilldown procedures for the upper stage liquid oxygen system.

No problems or issues are being worked in the countdown as activities proceed on pace for a liftoff at 6:13 a.m. EDT.

Fast-fill loading of the upper stage liquid hydrogen supply is starting. The tank will be loaded with 10,000 gallons of propellant for the RL10 engine.

Fast-fill loading of the liquid hydrogen tanks on the three Common Booster Cores has been completed. Some 110,000 gallons of LH2 are put into the rocket's tanks for flight. Vent and relief checks will be performed shortly.

The liquid oxygen loading is progressing as planned for the three Common Booster Cores. Each tank will be loaded with about 40,000 gallons of supercold LOX.

With the CBC liquid hydrogen tanking continuing, the launch team has been given approval to begin chilldown conditioning of the upper stage liquid hydrogen system. This is a precursor to fueling the upper stage.

Now four hours till launch. The Delta 4 rocket's three Common Booster Cores currently are being loaded with super-cold liquid oxygen and liquid hydrogen propellants.

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 three 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 boosters 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.

With the Common Booster Cores' liquid oxygen systems properly conditioned for cryogenic temperatures, the loading of Minus-298 degree LOX into the Delta 4-Heavy rocket is beginning. The liquid oxygen tanks in all three Common Booster Cores will be filled over the next hour or so. The oxidizer will be consumed by the RS-68 main engines during launch.

The liquid hydrogen loading is switching to the "fast-fill" mode.

And now the liquid oxygen chilldown is starting in advance of feeding the cryogenic oxidizer into the Delta 4-Heavy rocket.

The cold gas chilldown for the hydrogen side has been completed and the launch team is beginning the initial loading of liquid hydrogen propellant into the three Common Booster Core stages. This "slow-fill" will be sped up to "fast-fill" after a small portion of each tank is loaded.

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

The cold gas chilldown conditioning of the Common Booster Cores for liquid hydrogen fueling has begun.

The "go" has been given for fueling operations. The launch team will start thermal conditioning steps to ready equipment for pumping the super-cold liquid oxygen and liquid hydrogen propellants into the Delta 4-Heavy rocket today.

T-minus 5 hours, 15 minutes and counting! The Terminal Countdown has commenced for today's launch of the Delta 4-Heavy rocket and a satellite payload for the U.S. National Reconnaissance Office. With one final hold planned at T-minus 4 minutes, liftoff is still targeted to occur at 6:13 a.m. EDT.

The multi-step process of loading all eight cryogenic propellant tanks in the rocket is scheduled to begin in the next half-hour and continue into the overnight.

The call to "man stations for cryogenic tanking" is sounding to the launch team.

Weather continues to look just fine for the Delta 4-Heavy rocket's launch today. The outlook is calling for a few scattered clouds, light winds and temperatures in the 70s F with a 90 percent chance of meeting the launch weather rules.

T-minus 5 hours, 15 minutes and holding. Clocks have been paused for a planned one-hour hold, during which time the full launch team will be seated at their consoles, the launch pad is scheduled to be cleared of all workers and readiness polls will be conducted by mission management to ensure everyone is ready to proceed with the count.

The Terminal Countdown begins when the clocks resume ticking at 12:43 a.m. EDT, leading toward a liftoff of the Delta 4-Heavy rocket at 6:13 a.m. EDT from Cape Canaveral.

The 330-foot tall mobile service tower has been retracted from the Delta 4-Heavy rocket at Cape Canaveral's pad 37B for Friday's sunrise launch that will place a clandestine payload into orbit for the agency that runs the nation's fleet of spy satellites.

The wheeled structure was 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 stay on the pad, provided workers 360-degree access to the various areas on the vehicle and was used to attach the strap-on solid motors and 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 vehicle a little after 1 a.m.

Liftoff remains scheduled for 6:13 a.m. EDT.

Good evening from the Cape Canaveral Air Force Station where photographers are gathering right now to enter the launch pad and set up their sound-activated cameras to capture tomorrow's liftoff. We'll also grab some pre-launch shots while we're out there to post a little later tonight.

Out at Complex 37, ground technicians are making final preps to retract the dual-purpose assembly gantry and mobile shelter away from the United Launch Alliance-made rocket for the countdown to blastoff.

We'll be out of communication once we head into the complex. So we'll resume with updates and confirmation of tower rollback as soon as possible.

Ground crews are busy this evening getting launch pad 37B prepared for tomorrow's 6:13 a.m. EDT sendoff of the Delta 4-Heavy rocket carrying a critical spy satellite.

Retraction of the 330-foot tall mobile service tower from around the rocket is expected to begin around 8 p.m. EDT. The wheeled gantry structure moves along rail tracks to its launch position about the length of a football field from the rocket's mount.

The 9-million pound tower shields the Delta 4 from the weather, provides workers 360-degree access to the various areas on the vehicle and is needed to hoist the payload atop the upper stage during the launch campaign. The tower is 90 feet wide and 40 feet deep.

The Cape's Complex 37 is the same site used in the 1960s to fly unmanned Saturn 1 and 1B rockets that helped prepare for mankind's voyage to the moon. The site was rebuilt for the Delta 4 era, successfully supporting 15 liftoffs in the vehicle family so far.

Air Force meteorologists are predicting very nice conditions for a Friday liftoff of the Delta 4-Heavy rocket from Cape Canaveral. There's now a 90 percent chance of acceptable weather, with clouds posing only a slight concern.

"Rapid improvement to the local weather conditions to continue. Post-Tropical Depression Debby has moved well to the east of Florida and surface and upper level high pressure has begun to build into Florida. There are no concerns for a nominal (mobile service tower) roll on Thursday evening. There is only a very slight chance for an isolated cumulus cloud or thick cloud layer near the pad area during the launch window," the weather team says.

The forecast for Friday morning's launch window opening at 6:13 a.m. EDT includes a few clouds at 2,000 and 8,000 feet, a temperature in the 70s F and westerly winds at 5 peaking to 10 knots.

The odds of acceptable weather on Saturday and Sunday, if the launch should be delayed, are 80 percent favorable both mornings due to cumulus and thick cloud concerns.

With Debby, now a tropical depression, quickly scooting across Florida, meteorologists are becoming much more optimistic conditions will allow countdown operations to commence Thursday evening and the Delta 4-Heavy rocket to launch Friday morning at 6:13 a.m. EDT (1013 GMT).

"Tropical Depression Debby has begun a faster movement across Florida and will exit the east coast today. This faster movement away from Florida will significantly improve conditions for a Friday launch attempt. It is expected that winds will decrease enough to allow for a nominal (mobile service tower) roll on Thursday evening. Main concerns during the launch window will be for cumulus and thick layered clouds associated with any isolated showers that may be in the area. Winds are not expected to be a major concern during the window," the launch weather team reports.

The forecast for Friday has jumped to 70 percent favorable, and the backup opportunities on Saturday and Sunday mornings have a 70 percent of acceptable weather too.

Friday's launch window forecast is calling for some scattered low- and mid-level clouds, isolated showers in the area, winds from the northwest shifting around to the north-northeast at 10 peaking to 15 knots and a temperature in the low 80s.

DELAY. With a weather forecast showing a Thursday launch attempt would be futile, mission managers this afternoon decided to wait until at least Friday to try flying the mammoth Delta 4-Heavy rocket from Cape Canaveral.

The Sunshine State is anything but sunny this week due to Tropical Storm Debby dithering in the northeastern Gulf of Mexico, which has been pumping a constant stream of clouds, rain and thunderstorms over the peninsula.

Air Force meteorologists predict slightly better conditions on Friday morning for a 6:13 a.m. EDT liftoff.

Here is the ULA statement on the delay:

"Due to the forecast for Tropical Storm Debby, the launch of a Delta 4 carrying a national security payload for the National Reconnaissance Office is delayed 24 hours. The launch is now planned for Friday, June 29 at 6:13 a.m. from Space Launch Complex-37 at Cape Canaveral Air Force Station in Florida. The protective Mobile Service Tower will remain around the launch vehicle and is scheduled to be moved for launch Thursday evening."

Hoping for a little cooperation from Tropical Storm Debby later this week, the Delta 4-Heavy rocket passed its Launch Readiness Review today for blastoff Thursday morning, weather permitting, to deploy a national security payload into orbit.

Liftoff from Cape Canaveral's Complex 37 is targeted for 6:16 a.m. EDT (1016 GMT), some 12 minutes before sunrise along Florida's Space Coast.

Meteorologists, however, predict only a 20 percent chance that weather conditions during Thursday's launch window will be within allowable limits for liftoff to occur.

The forecast is calling for multiple decks of clouds, showers and thunderstorms in the area, south-southwesterly winds of 30 peaking to 40 knots and a temperature in the low 80s. The launch rules threatened include the wind criteria, cumulus and anvil clouds, plus the thickness of clouds.

The pad crew plans to begin its activities Wednesday evening and retract the 330-foot mobile service tower away from the Delta 4-Heavy vehicle, with rollback beginning around 8 p.m. EDT. The gantry was used to provide full access to the rocket since arriving at Complex 37 in early April, plus was vital in installing the encapsulated satellite cargo during pre-flight work and serving as a protective structure to shield the booster from adverse weather conditions.

The launch team will get most of Wednesday off to rest before their night owl countdown to the dawn launch. They will be reporting on station around 12 midnight EDT when a key weather briefing occurs to decide whether to commit to the fueling process. The weather officer will give managers the latest information about the kind of conditions that the rocket could face during the overnight hours while it sits exposed during the propellant loading.

The Terminal Count is scheduled to begin at 12:46 a.m. EDT, some five-and-a-half hours before the targeted liftoff time. The countdown will be spent fueling the eight cryogenic fuel tanks aboard the rocket, testing critical systems and verifying the hardware is ready to fly.

At 5:57 a.m., the countdown enters a planned 15-minute hold at the T-minus 4 minute mark. This offers managers a chance to perform final readiness polls of the entire launch team to confirm there are no issues or concerns before entering the last phase of the count.

Assuming all systems are go, clocks will resume ticking at 6:12 a.m. During those final four minutes, the rocket will switch to internal power, ordnance will be armed, all eight propellant tanks will be secured and the Range will announce a clear-to-launch.

At T-minus 14 seconds, the sparkler-like radial outward firing ignitors -- or ROFIs -- are started beneath the main engine nozzles to burn away gaseous hydrogen in the launch table. The Terminal Countdown Sequencer will grab control at T-minus 8.5 seconds to manage events in the crucial last seconds and oversee the rocket's status. The ignition sequence for the three RS-68A powerplants follows at T-minus 5.5 seconds as the main hydrogen fuel valve in each engine is opened. As fuel floods through the engines, spectacular flame erupts at the base of the rocket as free hydrogen reaches the ROFIs.

The oxygen valves in the engines are opened at T-minus 2 seconds as the RS-68As begin roaring to life. The engines must rev up to full throttle -- 108.5 percent thrust level -- and undergo a rapid computer-controlled health check to ensure all parameters are met.

If any problem is detected before T-minus 40 milliseconds, the engines will shut down and the rocket prevented from lifting off.

A successful engine startup leads to clocks striking Zero, the 12 hold-down bolts that have been restraining the rocket to Earth detonating and the 23-story, 1.6-million pound vehicle blasting off at 6:16 a.m. EDT (1016 GMT) on the NROL-15 spy satellite deployment mission.

Each booster core features a Pratt & Whitney Rocketdyne RS-68A main engine that generates 702,000 pounds of liftoff thrust while burning supercold liquid hydrogen and liquid oxygen.

This will be the debut flight of the upgraded powerplant, an evolution from the previous basic RS-68 engine that has propelled Delta 4 rockets since 2002. The internal turbomachinery has been modified to enable the engine to operate at a higher thrust level -- 108.5 instead of 102 percent -- to provide 39,000 pounds of extra performance, and improved combustion efficiency to achieve better miles-per-gallon fuel consumption.

"We are confident. We've done everything we would normally do and everything we believe needed to be done to appropriately certify (and) get this engine qualified for flight. I'm really looking forward to it," said Steve Bouley, vice president of Launch Vehicle and Hypersonic Systems for Pratt & Whitney Rocketdyne.

"There are no radical visual differences in either the look of the engine or the manner in which the rocket will fly," he added.

The RS-68A satisfies the Heavy Upgrade program to give additional lift capacity for the National Reconnaissance and its NROL-15 mission. The engine will be phased into the rest of the Delta 4 fleet starting in 2015, providing the extra power that enables all various Medium configurations to evolve into a standardize structure instead of being tailor-built for a given launch, substantially streamlining factory operations.

"The current RS-68 engines performance limitations drive each Delta 4 booster configurations to be different, and therefore they cannot be readily switched to another configuration like Atlas core booster," said Michael Gass, ULA's chief executive officer.

"The RS-68A engine upgrade additional performance enables the launch vehicle structure to now become common, while simultaneously incorporating the upgraded engine into the Delta fleet we get the advantages of maintaining a single engine production line -- all RS-68A engines, instead of producing both types together."

Bouley said his firm has residual assets to build three more RS-68s before moving exclusively to RS-68A production.

A half-minute into Thursday's flight, the center Common Booster Core's engine is throttled back to its minimum power level of 54.5 percent thrust to conserve fuel that becomes important later. The starboard and port boosters continue firing at full throttle -- 108.5 percent thrust -- through the launch's first four minutes before emptying their liquid hydrogen and liquid oxygen propellant tanks and shutting down the RS-68A engines. The engines will ease back to 54.5 percent power just before cutoff. The 15-story boosters then peel away and plummet into the Atlantic Ocean.

Once the outer boosters are shed, the center stage finally throttles back up to 108.5 percent for more than a minute of propulsion, consuming that fuel supply saved during the period of reduced thrust. Once the stage is jettisoned, the rocket's cryogenic upper stage powered by a Pratt & Whitney Rocketdyne RL10B-2 engine will continue the push to orbit.

The shiny white metallic nose cone, originally developed to shroud payloads on Titan 4 rockets, is jettisoned after the upper stage is ignited.

Soon thereafter, the launch will enter a news blackout and no further information about the secretive mission's flight events is expected.

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The weather outlook for Thursday's launch has deteriorated due to Tropical Storm Debby, dropping the odds of acceptable conditions to just 20 percent.

"Tropical Storm Debby remains in the northeast Gulf of Mexico and will move very slowly to the east over the next few days. Model guidance is in better agreement on a track across north central Florida. Strong southerly flow around Debby will keep windy conditions along with periods of showers and isolated thunderstorms in the forecast over the next 2-3 days.Main concerns will be for strong winds associated with Debby along with showers and thunderstorms," meteorologists report.

The forecast for Friday is just 30 percent favorable, then jumps to 70 percent on Saturday.

Thursday's launch of the Delta 4-Heavy rocket carrying a clandestine payload from Cape Canaveral Air Force Station is targeted to occur at 6:16 a.m. EDT (1016 GMT). The duration of the morning's launch window remains a secret.

Officials previously announced that liftoff would happen some time between 5:30 and 10:30 a.m. EDT (0930-1430 GMT).

Tropical Storm Debby, swirling in the Gulf of Mexico and blanketing Florida with clouds and rain, is dampening the odds of acceptable weather to launch the Delta 4-Heavy rocket from Cape Canaveral on Thursday.

Meteorologists put the chances of allowable liftoff conditions at just 30 percent during Thursday morning's launch opportunity.

Meanwhile, technicians at Complex 37 have successfully removed and replaced a vent relief valve on the rocket. Officials report launch preparations are back on schedule following that work.

Now, the launch date is in the hands of Debby.

The weather odds do improve to 40 percent on Friday and jump to 70 percent favorable on Saturday morning, but the forecast comes with the obvious uncertainty on the tropical storm's path and the timing of its forward movement.

Watch this page for continuing pre-flight status and live coverage throughout the launch countdown.

And 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 Delta 4-Heavy rocket, sporting more-powerful main engines, is counting down the days to blastoff this week carrying a super-secret payload for the National Reconnaissance Office.

Liftoff of the triple-barreled booster will occur some time Thursday morning during a period of 5:30 to 10:30 a.m. EDT (0930-1430 GMT), officials say.

At last check Friday, engineers were looking at a vent relief valve on the rocket that could need replacing. But officials said if such a swapout got ordered the unplanned work should fit within the existing schedule with no impact to the targeted Thursday launch date.

It will mark the debut launch of the upgraded RS-68A powerplant on the three booster cores. Built by Pratt & Whitney Rocketdyne, each hydrogen-fueled engine will deliver 702,000 pounds of liftoff thrust, an increase of 39,000 pounds over the current RS-68 engines successfully flown 29 times to date.

The Heavy will be flying for the sixth time in history and the Delta 4 program will be achieving its 20th mission to date.

About a month before liftoff, the NROL-15 mission payload was delivered to Cape Canaveral's Complex 37, hoisted into the mobile service tower and mounted atop the rocket's second stage.

Check out this photo gallery of the event.

The first launch weather forecast will be issued Monday, we'll have a pre-flight feature story on the RS-68A this week and rollback of the pad gantry occurs Wednesday evening.

Watch this page for live coverage of the countdown and streaming video of the launch.

Marking a milestone in flight at the fore of American rocketry, an Atlas 5 majestically rose away from Cape Canaveral this morning on the 50th mission for the Evolved Expendable Launch Vehicle program.

Read our full story.

The final days of preparations are underway at Cape Canaveral for next Monday morning's Atlas 5 rocket flight on the milestone-setting 50th mission for America's Evolved Expendable Launch Vehicle fleet.

See our Atlas journal for more.

The 51st EELV launch will follow fast when this Delta 4-Heavy carries out another NRO deployment mission from Cape Canaveral on June 28 during a period between 5:30 and 10:30 a.m. EDT (0930-1430 GMT).

While a United Launch Alliance Delta 4 rocket was being fueled during its countdown to blast off on the West Coast Tuesday afternoon, a mammoth Delta 4-Heavy vehicle was being rolled out to its East Coast pad.

The dual operations resulted in a successful launch from Vandenberg Air Force Base in California and delivery on the program's next booster to its Cape Canaveral pad for flight in June.

Both missions are giving space-lift power to the U.S. National Reconnaissance Office, the agency that operates the country's fleet of spy satellites. Tuesday's mission, known as NROL-25, placed a new-generation radar-imaging surveillance spacecraft into a retrograde orbit.

Read our full story.