Current updates
1648 GMT (12:48 p.m. EDT)
The first stage liquid oxygen tank and Centaur's liquid oxygen and liquid hydrogen tanks are reported at flight level.
1647 GMT (12:47 p.m. EDT)
T-minus 4 minutes and holding. The countdown has entered the planned 30-minute hold to give the launch team a chance to review all systems before pressing ahead with liftoff.
1646 GMT (12:46 p.m. EDT)
T-minus 5 minutes. Standing by to go into the final built-in hold.
1643 GMT (12:43 p.m. EDT)
The fuel-fill sequence for the first stage main engine is starting.
1636 GMT (12:36 p.m. EDT)
Now 45 minutes from liftoff. Today marks the 50th flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 49 missions since debuting in August 2002, the tally shows 17 flights dedicated to the Defense Department, 11 for NASA, 10 with spy satellites for the National Reconnaissance Office. and 11 commercial missions with communications and Earth-observing spacecraft.
1631 GMT (12:31 p.m. EDT)
Now 50 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 1:21 p.m. EDT (1721 GMT). The weather is forecast to be acceptable.

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

1615 GMT (12:15 p.m. EDT)
The U.S. Air Force's 45th Space Wing has issued a statement saying engineers conducted an overnight review of components used by the Atlas 5 rocket and the Antares launcher that disintegrated in a fireball seconds after liftoff Tuesday.

"Following the launch failure of the Antares at Wallops Flight Facility last night, the 45th Space Wing and the launch team evaluated the Atlas 5 launch vehicle for common components with the failed Antares launch vehicle," the Air Force said in a statement.

"Based on this evaluation the 45th Space Wing and the launch team have determined that these common components do not introduce any additional risk to the success of the Atlas 5 GPS mission."

1611 GMT (12:11 p.m. EDT)
The Atlas 5's first stage liquid oxygen tank is now in topping mode.
1610 GMT (12:10 p.m. EDT)
The Centaur's liquid hydrogen system has transitioned into topping mode. The cryogenic propellant will be consumed with liquid oxygen by the stage's Aerojet Rocketdyne-made RL10 engine.
1550 GMT (11:50 a.m. EDT)
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.
1548 GMT (11:48 a.m. EDT)
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.
1541 GMT (11:41 a.m. EDT)
The first stage liquid oxygen tank has reached the 30 percent mark.
1533 GMT (11:33 a.m. EDT)
The Atlas 5's liquid oxygen tank is reported at 10 percent full, and the Centaur oxidizer tank is 96 percent full. The topping off process for the Centaur LOX tank is starting now.
1520 GMT (11:20 a.m. EDT)
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.
1516 GMT (11:16 a.m. EDT)
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.

1502 GMT (11:02 a.m. EDT)
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.

1454 GMT (10:54 a.m. EDT)
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.
1451 GMT (10:51 a.m. EDT)
T-minus 120 minutes and counting! The launch countdown has resumed for today's flight of the Atlas 5 rocket on the GPS 2F-8 mission.

Clocks have one more built-in hold planned at T-minus 4 minutes. That pause will last 30 minutes during which time the final "go" for launch will be given. All remains targeted for liftoff at 1:21 p.m. EDT (1721 GMT) from Cape Canaveral's Complex 41, weather permitting.

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.

1448 GMT (10:48 a.m. EDT)
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 launch pad has been cleared in preparation for cryogenic tanking.

1421 GMT (10:21 a.m. EDT)
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 1:21 p.m. EDT (1721 GMT) 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.

The final hold will occur at T-minus 4 minutes.

1405 GMT (10:05 a.m. EDT)
The Air Force's launch weather officer reports there is still a 70 percent chance of favorable conditions at launch time. The primary concerns are with cumulus clouds and proton flux from elevated solar activity.

Today's launch window opens at 1:21 p.m. EDT (1721 GMT) and extends for 18 minutes.

1401 GMT (10:01 a.m. EDT)
Atlas first stage propulsion, pneumatic and hydraulic preps have been accomplished. And launch pad cameras have been verified configured for monitoring fueling operations.
1301 GMT (9:01 a.m. EDT)
Internal battery checks are getting started now.
1220 GMT (8:20 a.m. EDT)
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. The countdown continues on schedule for the 1:21 p.m. EDT launch.
1210 GMT (8:10 a.m. EDT)
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.
1011 GMT (6:11 a.m. EDT)
The countdown begins now for today's launch of the Atlas 5 rocket to deploy the Global Positioning System 2F-8 satellite.

Clocks are picking up the seven-hour sequence of work that will prepare the booster, payload and ground systems for blastoff at 1:21 p.m. EDT (1721 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 a test of the S-band telemetry relay system. The Complex 41 site will be cleared of all personnel at 10:16 a.m.

A planned half-hour hold begins at 10:31 a.m. when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 10:49 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 11:18 a.m., followed by the first stage filling around 11:31 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 12:57 p.m. That 20-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 opens at 1:21 and extends to 1:39 p.m. EDT (1721-1739 GMT).

An Atlas 5 rocket has been rolled out to its launch pad at Cape Canaveral for blastoff Wednesday to deploy a replacement satellite to strengthen the Global Positioning System for U.S. military forces and the worldwide economy. Read the full story.

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

The Launch Readiness Review was held today and reported all systems are proceeding for liftoff of the Atlas 5 rocket on Wednesday.

Liftoff is planned for 1:21 p.m. EDT, at the opening of an 18-minute launch window.

The rocket will be rolled from its assembly building to the launch pad at Cape Canaveral's Complex 41 on Tuesday morning. The countdown begins Wednesday at 6:11 a.m. EDT, followed by the start of fueling at 11:11 a.m. EDT.

The weather forecast calls for a 70 percent chance of acceptable conditions.

It will the 12th flight this year for United Launch Alliance and the fourth to deploy a modernized GPS satellite.

Forecasters are predicting a 70 percent chance of acceptable weather for launch of the Atlas 5 rocket on Wednesday.

"For MLP Roll on Tuesday, high pressure migrates east off the mid-Atlantic states with on-shore winds. There is a slight threat of a coastal shower during MLP roll and pre-launch preparations. No lightning is expected," forecasters report.

"On launch day, similar conditions are expected with high pressure off the mid-Atlantic states and light on-shore winds during the launch window. There is a slight threat of coastal showers during the count and window, slightly greater in the morning.

"Concerning solar weather, there is a significant, complex sunspot (2192) that poses a threat for X-class flares (45%) with the potential to elevate proton flux through the launch opportunities.

"The primary concerns for launch are the potential for elevated proton flux and Cumulus Clouds."

The outlook includes scattered low-level and high-level clouds, isolated coastal showers, good visibility, northeasterly winds of 10-14 knots and a temperature of 78 degrees F.

"In the event of a 24-hour delay, the next cold front encroaches into the Gulf Coast states with a gradual increase in moisture in advance of the front. The primary concerns for a 24-hour delay are Cumulus Clouds and the potential for elevated Proton Flux," forecasters say.

Launching new Global Positioning System navigation satellites at a rate not seen in 21 years, this year's fourth such deployment is coming up at midday Wednesday by an Atlas 5 rocket from Cape Canaveral.

Read our full story.

Current updates