Leaping off the launch pad Friday in its most powerful arrangement to boost its heaviest payload into orbit, a United Launch Alliance Atlas 5 rocket successfully deployed the second in a series of sophisticated spacecraft to grow the U.S. Navy's new mobile communications network that will span the globe.

Read our launch story.

SPACECRAFT SEPARATION! The Centaur upper stage has deployed the U.S. Navy's second Mobile User Objective System satellite, dubbed MUOS 2, 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 RL10 engine.

We will pause our live updates at this time. Check back around 12 noon EDT for confirmation of the third Centaur burn and deploy of the MUOS 2 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 57 minutes into flight. Deployment of MUOS 2 from the rocket to complete the launch is expected 2 hours, 53 minutes after launch.

T+plus 26 minutes, 25 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, 30 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, 15 seconds. Continued good engine operation is being delivered by the RL10, no problems reported.

T+plus 21 minutes, 10 seconds. The engine is burning well. This is a planned six-minute firing by the Centaur's single Aerojet Rocketdyne RL10 engine.

T+plus 20 minutes, 28 seconds. Ignition! The Centaur's single RL10 engine has re-ignited to climb into a highly elliptical intermediate orbit on the way to the final perch later tonight.

T+plus 20 minutes, 20 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 RL10 restarts for a final time to accelerate MUOS into the geosynchronous transfer orbit and separating the payload. Here's the planned track map.

T+plus 17 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 16 minutes, 30 seconds. That first burn by Centaur inserted the rocket into the precise target orbit as planned.

T+plus 15 minutes. Centaur's onboard systems are stable in this coast period continues.

T+plus 12 minutes, 26 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 RL10 engine re-ignites for the next burn.

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

T+plus 11 minutes. The RL10 continues to perform well, burning liquid hydrogen and liquid oxygen propellants.

T+plus 10 minutes, 20 seconds. The rocket is 1,774 miles downrange and traveling at 16,254 mph.

T+plus 9 minutes, 30 seconds. All systems reported stable as the Centaur fires to reach an initial Earth orbit.

T+plus 8 minutes, 30 seconds. A review of first stage shows the performance was right on the mark.

T+plus 7 minutes, 55 seconds. The rocket is 147 miles in altitude, some 1,146 miles downrange and traveling at 14,883 mph.

T+plus 7 minutes, 30 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, 15 seconds. RL10 performing well.

T+plus 5 minutes, 20 seconds. The vehicle is traveling at 13,787 mph.

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

T+plus 4 minutes, 45 seconds. Centaur has ignited! The RL10 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, 34 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, 26 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, 20 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, 10 seconds. The RD-180 main engine continues to fire normally, burning a mixture of highly refined kerosene and liquid oxygen.

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

T+plus 2 minutes, 30 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, 10 seconds. Vehicle is right on course.

T+plus 1 minute, 55 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 second 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 Atlas 5 launching the U.S. Navy's second MUOS satellite to grow a global constellation for mobile communications. And the tower is clear!

T-minus 20 seconds. "Go Atlas" and "Go Centaur" 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 second Mobile User Objective System communications satellite to continue orbital construction of a five-spacecraft network to span the globe.

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.

Standing by to pick up the count.

The MUOS spacecraft is running on internal power and configured for launch.

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 "go" for a liftoff today at 9:00 a.m. EDT.

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.

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

• The 621st launch for Atlas program since 1957
• The 331st Atlas to occur from Cape Canaveral
• The 210th mission for the Centaur upper stage
• The 187th use of Centaur by an Atlas rocket
• The 437th production RL10 engine to be launched
• The 45th flight of the RD-180 main engine
• The 39th launch of an Atlas 5 since 2002
• The 17th Atlas 5 launch with solid boosters
• The 61st Evolved Expendable Launch Vehicle flight
• The 14th Atlas 5 dedicated to the Defense Department
• The 33rd Atlas 5 to occur from the Cape
• The 27th daytime Atlas 5 from the Cape
• The 72nd United Launch Alliance flight overall
• The 6th ULA launch of the year
• The 31st Atlas 5 under United Launch Alliance
• The 13th 500-series flight of the Atlas 5
• The 4th Atlas 5 to fly in the 551 configuration
• The second MUOS satellite
• The second Atlas launch for MUOS
• The 5th Atlas launch of 2013

NEW TIME! A new liftoff time is being targeted for 9:00 a.m. EDT.

The 44-minute launch window is now open.

The winds aloft are not part of the official weather forecast. The winds are monitored by weather balloons dispatched throughout the countdown to measure speed and direction. That data is then computed to determine if the conditions are safe for the rocket to fly through, based on structural capability and steering controls.

There is no immediate estimate on how long this extra hold might last for upper level wind conditions. Today's launch opportunity extends to 9:32 a.m. EDT.

RED for upper level winds! The launch director has instructed team to remain in this hold at T-minus 4 minutes.

The first stage liquid oxygen tank and Centaur's liquid oxygen and liquid hydrogen tanks are reported at flight level.

T-minus 4 minutes and holding. The countdown has entered the planned 10-minute hold to give the launch team a chance to review all systems before pressing ahead with liftoff.

The MUOS 2 satellite is switching to internal power for launch.

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

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Today marks the 39th flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 38 missions since debuting in August 2002, the tally shows 13 flights dedicated to the Defense Department, 9 commercial missions with communications spacecraft, 9 for NASA and 7 with spy satellites for the National Reconnaissance Office.

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

The final planned weather briefing to launch officials just occurred and the odds of acceptable conditions today has improved to 80 percent. Although there are some clouds over the Cape this morning, the current conditions do not appear threatening to any of the weather rules for remainder of the count or the launch window, the launch weather team says.

Just 30 minutes until liftoff time. Today's launch is the second 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 2 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 early next year.

Centaur liquid hydrogen has reached flight level.

Now 45 minutes from liftoff. Today begins a string of five national security launches that the Air Force will perform with United Launch Alliance in a three-month span through October, using both Atlas and Delta 4 rockets from the Cape and Vandenberg Air Force in California. The future flights will deploy two Air Force communications spacecraft, a massive spy satellite for the National Reconnaissance Office and a new GPS bird.

The surge begins with the Navy's Mobile User Objective System satellite No. 2, taking the next step to construct a worldwide communications system using 3G-cellular technology for ships, submarines, aircraft, land vehicles and terminals in the hands of troops.

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

Fast-filling of the first stage liquid oxygen tank has been completed. Topping mode is now underway.

Centaur's liquid hydrogen tank is three-quarters full.

The official launch weather forecast has improved to a 70 percent chance of acceptable conditions for liftoff this morning during the 8:48 to 9:32 a.m. EDT window. All cloud conditions are within limits for liftoff at the current time.

Centaur's liquid hydrogen tank is 30 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Aerojet Rocketdyne-made RL10 engine.

Now 60 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen is progressing smoothly as the countdown continues on schedule for a liftoff at 8:48 a.m. EDT. Weather remains GO.

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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.

First stage liquid oxygen tank is 50 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 prior to today.

The Centaur engine chilldown sequence is being initiated.

The first stage liquid oxygen tank has reached the 30 percent mark.

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

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

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.

Centaur liquid oxygen is 75 percent loaded.

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.

Half of the Centaur upper stage's liquid oxygen tank has been filled thus far.

Passing the 20 percent level on the Centaur upper stage's liquid oxygen tank.

The current weather is GO for launch. The previous concerns about thick clouds have passed.

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 RL10 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 2 mobile communications satellite.

Clocks have one more built-in hold planned at T-minus 4 minutes. That pause will last 10 minutes during which time the final "go" for launch will be given. All remains targeted for liftoff at 8:48 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 GO 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 8:48 a.m. EDT launch of the Atlas 5 rocket.

This initial pause was designed to give the team some margin in the countdown timeline to deal with technical issues or any work that could fall behind schedule before fueling starts. But all is going smoothly today, with officials not reporting any troubles in the count.

The final hold will 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 "no go" at the moment due to thick clouds over Cape Canaveral. The cumulus cloud rule may go "red" shortly as well, but meteorologists predict both issues should clear around 7 a.m. today, with it being a dynamic "real-time" call during the launch window this morning.

Guidance system testing has been accomplished as the countdown goes smoothly today at Cape Canaveral.

Atlas first stage propulsion, hydraulic and liquid oxygen system preps have been accomplished. And launch pad cameras have been verified configured for monitoring fueling operations.

The hazard area roadblocks around the launch site's safety perimeter have been established. Also, the launch team is configuring the pad's water deluge system.

Internal battery checks are getting started as the counts the final four hours to launch.

A check of the weather shows current conditions at Cape Canaveral have gone "red" due to thick clouds over the launch site. Meteorologists continuously update the status of the various weather criteria, and expect this cloud condition to clear in about an hour. The outlook for launch time remains 40 percent favorable.

The C-band and S-band systems are being tested at this point in the countdown. They are used for vehicle tracking and telemetry relay, respectively. Also, final preps to the Centaur liquid hydrogen system are reported complete as the countdown continues for the 8:48 a.m. EDT launch this morning.

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 8:48 a.m. EDT liftoff time stand at 40 percent favorable. The main concern that meteorologists are watching is the thickness of clouds that the rocket would fly through and a slight concern about electrically-charged anvil clouds drifting over the Cape.

The outlook for the launch window now predicts scattered clouds at 2,500 feet, broken clouds at 15,000 feet and overcast conditions at 25,000 feet, good visibility, southerly winds of 10 gusting to 14 knots, a temperature of 78 degrees F and humidity level of 95 percent.

If the launch should slip to Saturday for some reason, there is a 70 percent chance of acceptable wether.

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 begins now for this morning's launch of the Atlas 5 rocket to deploy the Navy's MUOS 2 communications satellite for mobile military users.

Clocks are picking up the seven-hour sequence of work that will prepare the booser, payload and ground systems for blastoff at 8:48 a.m. EDT (1248 GMT).

Soon 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 C-band system used to track the rocket as it flies downrange, plus a test of the S-band telemetry relay system. The Complex 41 site will be cleared of all personnel at 5:53 a.m.

A planned half-hour hold begins at 6:08 a.m. when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 6:35 a.m. to verify all is in readiness to start fueling the rocket for launch.

Supercold liquid oxygen begins flowing into the Centaur upper stage around 6:55 a.m., followed by the first stage filling around 7:08 a.m. 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 starting at 8:34 a.m. That 10-minute 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 from 8:48 to 9:32 a.m. EDT (1248-1332 GMT).

They are the U.S. military's heaviest spacecraft looking for rides to orbit -- the Navy's next-generation satellites providing communications to forces on the move -- and the United Launch Alliance's mightiest version of the Atlas 5 will do the heavy lifting Friday morning to deploy the second bird in the series.

Read our launch preview story.

The Atlas 5 rocket stands atop its launch pad at Cape Canaveral for blastoff Friday morning to deploy a massive communications satellite for the U.S. Navy, becoming the second in a series that will ring the planet and create a new cellular-like mobile network for calls and data transmissions between U.S. military forces.

The United Launch Alliance booster was rolled out to the Complex 41 pad shortly after 11 a.m. EDT today aboard a mobile platform, emerging from the assembly building where the rocket's two stages, five strap-on boosters and the payload were integrated since late May.

Countdown clocks begin ticking Friday at 1:48 a.m. EDT, leading to activation of the rocket, final testing and system preps. Fueling operations start at 6:55 a.m.

The day's available launch opportunity begins at 8:48 a.m. and closes at 9:32 a.m. EDT to send the rocket on an easterly heading to geosynchronous transfer orbit with the Mobile User Objective System satellite No. 2.

Weather forecasters will be keeping a close eye on coastal showers and any isolated thunderstorms that could violate cumulus and anvil cloud rules and delay the liftoff.

"The weather is one of dozens of considerations for being fully ready to go, we'll see how that plays out Friday morning," said Navy Capt. Paul Ghyzel, manager of the Satellite Communications Program Office.

Watch this page for live updates throughout the countdown and flight, plus live streaming video.

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The United Launch Alliance Atlas 5 rocket's rollout to the pad is underway!

This slow half-hour drive from the 30-story Vertical Integration Facility to Cape Canaveral's Complex 41 pad uses a pair of specially-made "trackmobiles" to carry the rocket's 1.4-million pound mobile launching platform along rail tracks for the 1,800-foot trip.

The 206-foot-tall satellite booster is moving to Cape Canaveral's Complex 41 for launch of MUOS 2, the next spacecraft in the Navy's next-generation communications network.

The two-stage rocket, its five strap-on boosters and the payload were put together inside the assembly building over the past month-and-a-half in preparation for this rollout event. The Atlas 5 is designed to spend minimal time at the launch pad, which does not include a service gantry like other sites.

"As with any large and complex project, it takes thousands of talented men and women working diligently, and I thank them for their hard work and dedication. We are honored to serve alongside such a strong government and industry team to ensure we deliver the capability to significantly improve communications for our U.S. forces," said Jim Sponnick, ULA's vice president of Atlas and Delta Programs.

"Thank you to all of our mission partners who have worked with us tirelessly to make this launch a success. We look forward to a great launch on Friday."

It is rollout day for the next Atlas 5 rocket at Cape Canaveral. A massive Navy communications satellite is loaded aboard as the payload flying on mission headed for an eventual spot in geosynchronous orbit to provide voice and data transmission services to U.S. troops on the move.

The booster is scheduled to depart its assembly building around 11 a.m. and we will update this page when the move gets started.

The launch weather forecast continues to predict a 40 percent chance of acceptable conditions, with isolated showers and thunderstorms the main concerns on Friday morning.

"An upper level trough is near the Florida peninsula and the Gulf of Mexico today and slowly migrates west through launch day enhancing morning coastal showers and isolated thunderstorms. In this deep easterly flow regime, overnight and morning coastal showers are likely. By early afternoon, showers and thunderstorms will begin to favor the interior of the peninsula," Air Force meteorologists report.

The outlook includes scattered and broken decks of clouds, isolated showers and thunderstorms, good visibility, southerly winds of 10 to 14 knots, a temperature of 79 degrees F and relative humidity of 95 percent.

If the launch slips to Saturday for some reason, the forecast improves to 70 percent favorable.

The Launch Readiness Review was held this morning and gave concurrence to proceed with the planned Friday morning liftoff of the Atlas 5 rocket carrying the Navy's second mobile communications satellite in the MUOS series.

The United Launch Alliance rocket will be rolled out from its assembly building to the pad at Cape Canaveral's Complex 41 starting at 10 a.m. EDT on Wednesday.

Thursday will be a sleep-shifting day for the launch team in preparation for Friday morning's countdown. Clocks start ticking before 2 a.m. EDT.

Liftoff on Friday will be possible during a 44-minute window that opens at 8:48 a.m. EDT and extends to 9:32 a.m. EDT.

Weather forecasters continue to predict a 40 percent chance of acceptable weather during the launch window. Coastal showers and a possible isolated thunderstorm around the Cape are the main concerns.

The outlook improves to 70 percent favorable on Saturday morning, as the atmospheric steering pattern shifts. Saturday's window opens a few minutes earlier.

The Atlas-Centaur will fly a three-hour mission to heave the 15,000-pound MUOS 2 satellite into a geosynchronous transfer orbit for the rocket program's fifth mission of the year.

"This is a very exciting time for the MUOS program. On Friday, we will launch our second MUOS satellite where it will join an operational MUOS 1 as we grow a constellation," said Iris Bombelyn, vice president of Narrowband Communications, Lockheed Martin, maker of the MUOS satellites.

The MUOS 1 was launched atop an Atlas 5 on February 24, 2012.

The Navy will deploy five MUOS spacecraft at a pace of one a year to cover the entire globe, creating a new mobile communications network using 3G cellular technology for voice and data transmissions to warfighters on the move.

The preliminary weather forecast for Friday morning's Atlas 5 rocket launch carrying a Navy communications satellite calls for a 40 percent chance of acceptable conditions, with coastal showers or an isolated thunderstorm the main threats against flying at 8:48 a.m. EDT as scheduled.

"The Atlantic high pressure ridge axis is to the north with deep on-shore flow through the week. An upper level trough approaches and is near the Florida peninsula and the Gulf of Mexico by mid-week and persisting into the weekend enhancing morning coastal showers and isolated thunderstorms. In this deep easterly flow regime, overnight and morning coastal showers are likely," Air Force meteorologists reported this morning.

"On launch day, the easterly to southeasterly flow pattern persists in the mid and upper levels with an enhanced coastal shower and isolated thunderstorm threat associated with the upper level disturbance and surface trough off the East coast. Low level winds are southerly and relatively light with gusts in the low teens during the count and window."

The launch time outlook includes scattered low and high clouds, good visibility, southerly winds of 10 to 14 knots, a temperature around 78 degrees F and humidity of 95 percent.

If the launch is delayed for some reason, the odds increase to 70 percent on Saturday morning.

"The pattern begins to transition through the day on Friday with steering level winds becoming southerly to south-southwesterly and afternoon and evening thunderstorms beginning to favor the East coast," forecasters said. "In the event of a 24-hour delay, the flow pattern continues to slowly transition with low level and steering level winds becoming southerly to southwesterly. If this transition occurs, coastal showers on Saturday would tend to remain to the east with a reduced threat of nearing the coast."

The second satellite in the orbital assembly of the U.S. Navy's new mobile communications network was being loaded atop its United Launch Alliance Atlas 5 rocket booster today at Cape Canaveral.

Liftoff is scheduled for July 19 during a morning window of 8:48 to 9:32 a.m. EDT (1248-1332 GMT) from Complex 41.

At nearly 15,000 pounds, the Mobile User Objective System satellite is a hefty cargo requiring the most powerful version of the Atlas 5 to carry the payload into a highly elliptical geosynchronous transfer orbit.

Read our full story.

The most powerful version of the United Launch Alliance Atlas 5 rocket has been assembled at Cape Canaveral to deploy the second satellite in the Navy's new mobile communications system.

The massive rocket, equipped with five strap-on solid-fuel boosters, will lift the 15,000-pound Mobile User Objective System satellite into orbit July 19.

It will be the year's fifth Atlas, the third for the Pentagon and the first using the expansive five-meter-diameter nose cone and solid motors.

This rocket is known as the 551 configuration of the multi-varient Atlas 5 family, which is tailored with different sized nose cones and strap-on solids to match the cargo's mass, size and targeted destination.

Read our full story.