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2355 GMT (6:55 p.m. EST)
Fast-filling of the first stage liquid oxygen tank has been completed. Topping mode is now underway.
2350 GMT (6:50 p.m. EST)
The liquid hydrogen tank in the Centaur upper stage just reached the 96 percent level. Topping is now beginning.
2343 GMT (6:43 p.m. EST)
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 7:43 p.m. EST.

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233 GMT (6:3 p.m. EST)
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 RL10C engine.
2332 GMT (6:32 p.m. EST)
First stage liquid oxygen tank is 60 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.
2327 GMT (6:27 p.m. EST)
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.
2319 GMT (6:19 p.m. EST)
The first stage liquid oxygen tank has reached the 30 percent mark.
2318 GMT (6:18 p.m. EST)
The Centaur engine chilldown sequence is being initiated.
2315 GMT (6:15 p.m. EST)
Centaur liquid oxygen tank has reached flight level.
2309 GMT (6:09 p.m. EST)
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.

2308 GMT (6:08 p.m. EST)
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.
2306 GMT (6:06 p.m. EST)
Centaur liquid oxygen is 90 percent loaded.
2302 GMT (6:02 p.m. EST)
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.

2247 GMT (5:47 p.m. EST)
Passing the 10 percent level on the Centaur upper stage's liquid oxygen tank.
2241 GMT (5:41 p.m. EST)
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.

2234 GMT (5:34 p.m. EST)
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.
2228 GMT (5:28 p.m. EST)
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 3 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 7:43 p.m. EST 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.

2226 GMT (5:26 p.m. EST)
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.

2224 GMT (5:24 p.m. EST)
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.
2223 GMT (5:23 p.m. EST)
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2213 GMT (5:13 p.m. EST)
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 7:43 p.m. EST 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.

2158 GMT (4:58 p.m. EST)
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.

1850 GMT (1:50 p.m. EST)
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 7:43 p.m. EST liftoff time stand at 80 percent favorable. The main concern that meteorologists are watching is the thickness of clouds that the rocket would fly through.

The outlook for the launch window now predicts scattered clouds at 3,000 and 12,000 feet and broken clouds at 22,000 feet, good visibility, southerly winds of 8 gusting to 12 knots, a temperature of 61 degrees F.

If the launch should slip to Wednesday for some reason, there is an 80 percent chance of acceptable wether.

1837 GMT (1:37 p.m. EST)
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.
1753 GMT (12:53 p.m. EST)
The countdown begins now for this evening's launch of the Atlas 5 rocket to deploy the Navy's MUOS 3 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 7:43 p.m. EST (0043 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.

A planned half-hour 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 from 7:43 to 8:27 p.m. EST (0043-0127 GMT).

An Atlas 5 rocket has been rolled out to its launch pad at Cape Canaveral for liftoff Tuesday to deploy the third satellite in the U.S. Navy's new mobile communications network.

ASee a photo gallery.

The United Launch Alliance rocket was wheeled to Cape Canaveral's Complex 41 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 over the past two months.

This slow half-hour drive from the 30-story Vertical Integration Facility to the launch pad used 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 was moved to Complex 41 for launch of MUOS No. 3, the latest satellite to join the Mobile User Objective System in geosynchronous orbit.

"It is an exciting time for the Navy, for the program and for the Department with the upcoming third launch of five MUOS satellites," said Navy Capt. Joe Kan, MUOS program manager.

"To deliver a satellite like MUOS is no small task and the team worked around the clock and through every holiday. We are honored to do so, because we know that our warfighters never stop in their own mission to keep us safe," said Iris Bombelyn, vice president of Narrowband Communications at Lockheed Martin.

The rocket is flying the 551 vehicle configuration for the fifth time in 52 flights. The version features two stages, five solid rocket boosters and a five-meter-diameter nose cone. It is powered off the launch pad by an RD AMROSS RD-180 main engine and the Centaur upper stage is equipped with an Aerojet Rocketdyne RL10C.

Countdown clocks begin ticking Tuesday at 12:33 p.m. EST, leading to activation of the rocket, final testing and system preps. Fueling operations start at 5:33 p.m.

The day's available launch opportunity begins at 7:43 p.m. and closes at 8:27 p.m. EST to send the rocket on an easterly heading into an equatorial orbit.

There is an 80 percent chance of acceptable weather, a 20 percent improvement over earlier forecasts.

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

See our earlier MUOS 3 coverage.

And see our Atlas archive for further information.

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

And check out Spaceflight Now's Facebook page!

The odds of acceptable weather conditions on Tuesday night for the Atlas 5 rocket launch have improved to 70 percent. Previous forecasts has projected a 60 percent chance of good weather. The main concern is cloud thickness.

Preparations for the launch continue to move forward. Rollout of the rocket from its assembly building to the launch pad is scheduled for Monday.

PREVIEW: The 200th Atlas-Centaur rocket, history some 52 years in the making, will blast off Tuesday from Cape Canaveral to deliver a U.S. Navy mobile communications satellite into orbit.

Read our full story.

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