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"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."
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.
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.
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.
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.
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.
Today's launch window opens at 1:21 p.m. EDT (1721 GMT) and extends for 18 minutes.
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).
Watch this page for live updates throughout the countdown and flight, plus live streaming video.
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.
"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.
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