In a breath-taking display at daybreak today, an Atlas rocket powered an enormous U.S. Navy satellite into space to close the last remaining gap and give global reach to the military’s new mobile communications system.

Read our launch story.

“The ULA team is proud to support the U.S. Navy and the U.S. Air Force by delivering this critical communications asset to orbit today,” said Jim Sponnick, ULA vice president, Atlas and Delta Programs.

“Today’s successful launch will enable the MUOS constellation to reach global coverage. The Lockheed Martin built MUOS 4 satellite will deliver voice, data, and video communications capability, similar to a cellular network, to our troops all over the globe.”

“The most dangerous part of a satellite’s life is launch and getting into orbit. I really want to thank our entire team whose hard work prepared MUOS 4 for this mission-critical event and the Atlas team who ultimately carried us safely to our transfer orbit,” said Iris Bombelyn, vice president of Narrowband Communications at Lockheed Martin.

“We look forward to completing our on-orbit health checks and delivering this important asset to the U.S. Navy and these new capabilities to our mobile forces.”

SPACECRAFT SEPARATION! The Centaur upper stage has deployed the U.S. Navy's fourth Mobile User Objective System satellite, dubbed MUOS 4, for a 15-year mission to provide mobile communications to U.S. military forces on the move.

Just a few minutes before separation, the Centaur completed its third burn of the day to raise the orbit's low point and reduce orbital inclination for the MUOS spacecraft. It was a planned minute-long firing by the Centaur's single Aerojet Rocketdyne RL10C engine.

MECO 3. Centaur has completed its third burn of the day to raise the orbit's low point and reduce orbital inclination for the MUOS spacecraft. It was a planned minute-long firing by the Centaur's single Aerojet Rocketdyne RL10C engine.

A video replay of tonight's launch is posted here.

We will pause our live updates at this time. Check back around 9:15 a.m. EDT for confirmation of the third Centaur burn and deploy of the MUOS 4 satellite to complete today's mission.

T+plus 35 minutes. A typical Atlas 5 ascent to geosynchronous transfer orbit, the standard dropoff point for communications satellites, uses just two firings by the Centaur to achieve the highly elliptical, egg-shaped orbit to deploy the payloads. From there, the satellites use their own engines in the subsequent days to fly into the operational locations 22,300 miles above the equator.

But the sizable MUOS, at nearly 15,000 pounds, drove planners to create a three-burn launch profile for the hefty payload to use all of the available performance from the Atlas-Centaur that in turns saves the satellite's precious onboard fuel supply for maneuvering over its 15-year life.

The three burns provides 1,000 pounds greater lift capability than a conventional 2-burn geosynchronous transfer orbit.

T+plus 30 minutes. The second burn by Centaur inserted the rocket into another precise orbit as targeted.

T+plus 28 minutes. Centaur now begins a quiet two-and-a-half-hour coast through space, moving higher and away from the planet before the third and final burn occurs nearly 2 hours and 49 minutes into flight. Deployment of MUOS 3 from the rocket to complete the launch is expected 2 hours, 54 minutes after launch.

T+plus 26 minutes, 23 seconds. MECO 2. Main engine cutoff confirmed. Centaur has completed its second burn of the day, this one taking the next step upwards to substantially increase the altitude from the previous parking to nearly geosynchronous height.

T+plus 25 minutes, 45 seconds. Engine performance remains normal.

T+plus 24 minutes, 30 seconds. About two minutes are left in the burn to reach the next orbit. Everything continues to look nominal.

T+plus 23 minutes. Continued good engine operation is being delivered by the RL10C, no problems reported.

T+plus 22 minutes. The engine is burning well. This is a planned six-minute firing by the Centaur's single Aerojet Rocketdyne RL10C engine.

T+plus 20 minutes, 37 seconds. Ignition! The Centaur's single RL10C engine has re-ignited to climb into a highly elliptical intermediate orbit on the way to the final perch later this morning.

T+plus 20 minutes, 26 seconds. Liquid hydrogen and liquid oxygen system prestarts are underway.

T+plus 20 minutes. Centaur getting into its pressurization sequence.

T+plus 19 minutes. The flight path is taking the vehicle over the equatorial Atlantic Ocean, soon to near the western coast of Africa where the next Centaur burn will occur. It will soar away from the Indian Ocean over the course of the next couple hours before the RL10C restarts for a final time to accelerate MUOS into the geosynchronous transfer orbit and separating the payload.

T+plus 18 minutes. Centaur is half-way through this coast period. It is completing a slow roll to keep thermal heating even across the rocket's surfaces.

T+plus 15 minutes. That first burn by Centaur inserted the rocket into the precise target orbit as planned.

T+plus 12 minutes, 31 seconds. MECO 1. Centaur's main engine has shut down following its first burn today, achieving a preliminary orbit around Earth. The rocket will coast in this orbit for about 8 minutes before the RL10C engine re-ignites for the next burn.

T+plus 12 minutes, 2 seconds. Centaur is now orbital.

T+plus 11 minutes, 30 seconds. About one minute are left in this burn of Centaur.

T+plus 10 minutes. All systems reported stable as the Centaur fires to reach an initial Earth orbit.

T+plus 9 minutes. The RL10C continues to perform well, burning liquid hydrogen and liquid oxygen propellants.

T+plus 8 minutes. Vehicle traveling at 14,869 mph.

T+plus 7 minutes, 15 seconds. The rocket has performed a planned roll to improve antenna links with NASA's orbiting Tracking and Data Relay Satellite System.

T+plus 6 minutes. RL10C performing well.

T+plus 5 minutes. Centaur engine readings look good as this burn gets underway.

T+plus 4 minutes, 46 seconds. Centaur has ignited! The RL10C engine is up and running at full thrust for its first of three firings today to reach the intended geosynchronous transfer orbit.

T+plus 4 minutes, 35 seconds. The Atlas 5's Common Core Booster first stage has been jettisoned, and the Centaur upper stage's liquid hydrogen and liquid oxygen systems are being readied for engine start.

T+plus 4 minutes, 28 seconds. BECO. Booster Engine Cutoff is confirmed as the RD-180 powerplant on the first stage completes its burn. Standing by to fire the retro thrusters and separate the spent stage.

T+plus 3 minutes, 40 seconds. The two-halves of the Atlas 5 rocket nose cone encapsulating the MUOS spacecraft have separated, exposed the satellite to space. Also jettisoned was the Forward Load Reactor, a two-piece deck that rings the Centaur stage to support the bulbous fairing during launch.

T+plus 3 minutes, 30 seconds. The RD-180 main engine continues to fire normally, burning a mixture of highly refined kerosene and liquid oxygen.

T+plus 3 minutes. The vehicle weighs just a quarter of what it did at liftoff.

T+plus 2 minutes, 50 seconds. Reaction control system has been activated.

T+plus 2 minutes, 40 seconds. Everything still looking good on the first stage as the rocket powers downrange on the thrust being produced by the main engine.

T+plus 2 minutes, 30 seconds. Vehicle is right on course.

T+plus 1 minute, 53 seconds. All five of the Aerojet Rocketdyne-made solid rocket motors have successfully separated from the Atlas 5, having completed their job of adding a powerful kick at liftoff.

T+plus 93 seconds. Solid rocket booster burnout has occurred. But the spent motors will remain attached to the first stage for a few seconds, until the Atlas 5 reaches a point where the airborne dynamic pressure reduces to an allowable level for a safe jettison.

T+plus 60 seconds into this complex three-hour mission to deploy the Navy's fourth Mobile User Objective System communications satellite in a geosynchronous transfer orbit.

T+plus 50 seconds. A period of maximum dynamic pressure is being experienced by the rocket.

T+plus 45 seconds. Mach 1.

T+plus 40 seconds. The main engine is throttling down to ease the stresses on the vehicle in the lower atmosphere.

T+plus 30 seconds. The vehicle is heading due eastward to reach a 28-degree inclination preliminary parking orbit 12 minutes from now.

T+plus 15 seconds. Pitch, yaw and roll maneuvers are underway as the Atlas 5 thunders away from Cape Canaveral with its RD-180 engine and five solid rocket boosters burning.

LIFTOFF! Liftoff of the Atlas 5 rocket with the Navy's fourth MUOS satellite, expanding coverage to encompass the globe with mobile military communications.

T-minus 20 seconds. "Go Atlas", "Go Centaur" and "Go MUOS" was just called by launch team during a final status check.

T-minus 40 seconds. Centaur's liquid oxygen and liquid hydrogen tanks are stable at flight pressures.

T-minus 55 seconds. Range is green.

T-minus 1 minute. Now 60 seconds away from launching the fourth Mobile User Objective System communications satellite.

T-minus 90 seconds. The rocket's safety system has been armed.

T-minus 1 minute, 45 seconds. Liquid oxygen and liquid hydrogen propellant topping to the Centaur upper stage is being secured.

T-minus 1 minute, 55 seconds. The launch sequencer has been commanded to start.

T-minus 2 minutes. The Atlas first stage and Centaur upper stage are now switching from ground power to internal batteries.

T-minus 2 minutes, 30 seconds. The first stage RP-1 kerosene fuel tank and the liquid oxygen have stepped up to proper flight pressure levels.

T-minus 3 minutes. The Atlas first stage liquid oxygen replenishment is being secured so the tank can be pressurized for launch.

T-minus 3 minutes, 50 seconds. The ground pyrotechnics are enabled.

T-minus 4 minutes and counting. Clocks have resumed for the final minutes of today's countdown to launch the Atlas 5 rocket with a new mobile communications satellite for the U.S. Navy from Cape Canaveral, Florida.

The MUOS 4 satellite is on internal power.

Standing by to pick up the count.

ULA and the Air Force have given their respective final approvals to resume the countdown.

Polling of the team by Atlas launch conductor just occurred. All systems are reported GO for flight.

Standing by for the final readiness check to be conducted. The launch team will be polled for a "go" or "no go" to proceed with the count.

The Atlas 5 rocket stands 20 stories tall and weighs 1.3 million pounds at launch.

Here's a look at some stats about today's mission. This will be:

• The 638th launch for Atlas program since 1957
• The 344th Atlas launch from Cape Canaveral
• The 227th mission of a Centaur upper stage
• The 204th use of Centaur by an Atlas rocket
• The 46th Atlas 5 launch from Cape Canaveral
• The 460th production RL10 engine to be launched
• The 5th RL10C-1 engine launched
• The 62nd flight of the RD-180 main engine
• The 56th launch of an Atlas 5 since 2002
• The 22nd Air Force mission for an Atlas 5
• The 86th Evolved Expendable Launch Vehicle flight
• The 99th United Launch Alliance flight overall
• The 48th Atlas 5 under United Launch Alliance
• The 20th 500-series flight of the Atlas 5
• The 6th Atlas 5 to fly in the 551 configuration
• The 4th MUOS satellite to launch
• The 5th Atlas 5 launch of 2015

The vehicle's three cryogenic tanks are at flight level.

T-minus 4 minutes and holding. The countdown has entered the planned 8-minute hold to give the launch team a chance to review all systems before pressing ahead with liftoff.

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

The MUOS 4 satellite is switching to internal power for launch.

Today marks the 56th flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 55 missions since debuting in August 2002, the Atlas 5 has flown 21 flights dedicated to the Defense Department, 12 for NASA, 11 with spy satellites for the National Reconnaissance Office. and 11 commercial missions with communications and Earth-observing spacecraft.

The fuel-fill sequence for the first stage main engine is starting.

Weather remains GO for liftoff based on the current conditions and expected to stay favorable for an 6:18 a.m. EDT launch today.

Just 30 minutes until liftoff time. Today's launch is the fourth for the Navy's new Mobile User Objective System satellite constellation to provide mobile communications to U.S. military forces.

In its launch configuration atop the Atlas rocket with the two power-generating solar arrays stowed on the sides and the umbrella reflectors folded up, MUOS stands 22 feet tall, 12 feet wide and 6 feet deep. The craft was built on Lockheed Martin's A2100 satellite design used by dozens of previous communications spacecraft.

Following separation from the launch vehicle nearly three hours after liftoff today, ground controllers will begin maneuvering MUOS 4 into geosynchronous orbit, deploying its power-generating solar arrays to span more than 90 feet, unfurling a pair of umbrella-like antenna reflectors on boom assemblies and fully checking out the onboard communications equipment.

Eight orbital raising burns are expected over the next 8-9 days, followed by the antenna reflector deploys, then a test program that Lockheed Martin will perform. The military will take control of the satellite in a few months to conduct its own acceptance evaluation before the craft enters service.

Now 45 minutes from liftoff. The vehicle is fully fueled.

The liquid hydrogen tank in the Centaur upper stage just reached the 96 percent level. Topping is now beginning.

Centaur's liquid hydrogen tank is 50 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Aerojet Rocketdyne-made RL10C engine.

Now 55 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen is progressing as the countdown continues on schedule for a liftoff at 6:18 a.m. EDT.

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NEW LAUNCH TIME. A new Atlas 5 liftoff time has been confirmed: 6:18 a.m. EDT (1018 GMT). The final built-in hold at T-minus 4 minutes will be shortened from 15 minutes to 8 minutes in duration.

Chilldown of the liquid hydrogen system has been accomplished. The launch team has received the "go" to begin filling the Centaur upper stage with the supercold fuel.

The Centaur engine chilldown sequence is being initiated.

First stage liquid oxygen tank is 90 percent full thus far. Chilled to Minus-298 degrees F, the liquid oxygen will be used with RP-1 kerosene by the RD-180 main engine on the first stage during the initial four minutes of flight today. The 25,000 gallons of RP-1 were loaded into the rocket after rollout on Monday.

To recap, the environment control system's gaseous nitrogen supply issue has been resolved, allowing the countdown operations to resume for launch this morning. Liftoff will be delayed a few minutes into the window, however.

T-miunus 1 hour, 22 minutes and counting.

The chilldown conditioning of liquid hydrogen propellant lines at Complex 41 is starting to prepare the plumbing for transferring the Minus-423 degree F fuel into the rocket. The Centaur holds about 12,300 gallons of the cryogenic propellant.

The launch team has been directed to pick up the countdown clock.

The countdown remains holding.

The first stage liquid oxygen tank has reached the 50 percent mark.

Centaur liquid oxygen tank has reached flight level.

The first stage liquid oxygen loading is transitioning from slow-fill to fast-fill mode.

And the Centaur liquid oxygen tank reached the 95 percent level. The topping off process is starting now.

Centaur liquid oxygen is 90 percent loaded.

The countdown has gone into an unplanned hold due to a gaseous nitrogen problem.

Now half-full on the Centaur upper stage's liquid oxygen tank.

The conditioning of the systems for the first stage liquid oxygen tank have been completed. And a "go" has been given to begin pumping supercold liquid oxygen into the Atlas 5's first stage.

The Common Core Booster stage's liquid oxygen tank is the largest tank to be filled today. It holds 48,750 gallons of cryogenic oxidizer for the RD-180 main engine.

Passing the 10 percent level on the Centaur upper stage's liquid oxygen tank.

Filling of the Centaur upper stage with about 4,100 gallons of liquid oxygen has begun at Cape Canaveral's Complex 41 following the thermal conditioning of the transfer pipes.

The liquid oxygen -- chilled to Minus-298 degrees F -- will be consumed during the launch by the Centaur's single RL10C engine along with liquid hydrogen to be pumped into the stage a little later in the countdown. The Centaur will perform three firings to propel the MUOS satellite into its intended orbit today.

The Centaur liquid oxygen pad storage area has been prepped. The next step is conditioning the transfer lines, which is now beginning to prepare the plumbing for flowing the cryogenic oxidizer.

T-minus 120 minutes and counting! The launch countdown has resumed for today's flight of the Atlas 5 rocket on a mission to deploy the U.S. Navy's MUOS 4 mobile communications satellite.

Clocks have one more built-in hold planned at T-minus 4 minutes. During that pause, the final "go" for launch will be given. All remains targeted for liftoff at 5:59 a.m. EDT from Cape Canaveral's Complex 41.

In the next couple of minutes, chilldown thermal conditioning of the mobile launch platform upon which the rocket stands will begin. This is meant to ease the shock on equipment when supercold cryogenic propellants start flowing into the rocket.

All console operators have reported READY status during the pre-fueling readiness poll. The ULA launch director also voiced his approval for moving forward with the countdown as scheduled today.

Loading of cryogenic liquid oxygen and liquid hydrogen into the Atlas 5 rocket will be getting underway a short time from now.

The ULA launch conductor at the Atlas Spaceflight Operations Center is briefing his team on procedures before entering into the final two hours of the countdown.

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T-minus 2 hours and holding. The countdown has just entered the first of two planned holds over the course of the day that will lead to the 5:59 a.m. EDT launch of the Atlas rocket. The holds give the team some margin in the countdown timeline to deal with technical issues or any work that is running behind. The final hold is scheduled to occur at T-minus 4 minutes.

The final hands-on work has wrapped up at the launch pad and technicians have departed the complex. Safety officials just confirmed that the surrounding danger area has been cleared of all workers for the remainder of the countdown.

Weather conditions remain "GO" at the present time.

The early portion of the countdown has been going well. There are no issues being reported in the count and activities are on schedule.

In today's first weather briefing to mission managers, all current conditions are observed GO for launch of the Atlas 5 rocket and odds for the launch window stand at 80 percent favorable. The main concerns that meteorologists will be watching are cumulus and anvil clouds.

The outlook predicts scattered low- and high-level clouds, coastal showers, good visibility, southerly winds of 10 to 14 knots and a temperature in the low 80s F.

The Atlas-Centaur rocket has been powered up at Complex 41 and guidance system testing is getting started for today's launch, as the countdown progresses as planned.

The countdown is beginning for launch of the Atlas 5 rocket to deploy the Navy's MUOS 4 communications satellite for mobile military users.

Clocks are picking up the seven-hour sequence of work that will prepare the booster, payload and ground systems for blastoff at 5:59 a.m. EDT (0959 GMT).

The launch team will begin powering up the rocket to commence standard pre-flight tests. Over the subsequent few hours, final preps for the Centaur's liquid oxygen and liquid hydrogen systems will be performed, along with a test of the rocket's guidance system and the first stage propulsion and hydraulic preps, internal battery checks and testing of the GPS metric tracking system used to follow the rocket as it flies downrange, plus a test of the S-band telemetry relay system.

A planned hold begins when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled to verify all is in readiness to start fueling the rocket for launch.

Supercold liquid oxygen begins flowing into the Centaur upper stage, followed by the first stage filling. Liquid hydrogen fuel loading for Centaur will be completed a short time later.

A final hold is scheduled at the T-minus 4 minute mark. That pause will give everyone a chance to finish any late work and assess the status of the rocket, payload, Range and weather before proceeding into the last moments of the countdown.

The launch window extends 44 minutes to 6:43 a.m. EDT (1043 GMT).

The seven-hour countdown begins tonight for the United Launch Alliance Atlas 5 rocket to boost into orbit the Navy's fourth MUOS satellite that will expand coverage to encompass the globe with mobile communications.

Liftoff is targeted for early Wednesday at 5:59 a.m. EDT (0959 GMT), the opening of a 44-minute window that extends to 6:43 a.m. EDT (1043 GMT).

The rocket was positioned atop Complex 41's launch pad on Monday morning, following rollout from the nearby vehicle assembly building.

Weather forecasters predict an 80 percent chance of acceptable conditions on Wednesday morning.

"During the countdown, favorable conditions are expected with isolated showers over the Atlantic Ocean. The primary concern for launch is cumulus clouds," forecasters say.

The launch time outlook calls for scattered low- and high-level clouds, coastal showers, good visibility, southerly winds of 10 to 12 knots and a temperature of 77 degrees F.

Watch this page for live coverage throughout the overnight countdown and early morning flight of the Atlas-Centaur rocket.

Be sure to sign up for our Twitter feed to get occasional countdown 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.)

And if you are need tips on picking a good viewing spot, check out this authoritative guide on where to go.

Delivering two-and-a-half million pounds of ground-shaking thrust at takeoff, more than any rocket flying from Cape Canaveral these days, the mighty vehicle was rolled from its assembly building to the launch pad Monday for Wednesday’s early morning blastoff.

This is the ascent timeline to be followed by the United Launch Alliance Atlas 5 rocket in launching the Mobile User Objective System satellite No. 4 on Wednesday, Sept. 2 at 5:59 a.m. EDT.

Plans to launch a Navy communications satellite aboard an Atlas 5 rocket early Monday have been interrupted by Tropical Storm Erika, an unpredictable cyclone with its sights on Florida.

Read our full story.

PREVIEW: Likened to raising a cellular tower 22,300 miles high, a United Launch Alliance Atlas 5 rocket will put the Navy's fourth mobile communications satellite into space Monday in a deployment that expands the rugged smartphone system to global coverage.

Read our full story.