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The CLIO satellite's final orbit has not been disclosed.
The weather is reported GO for launch!
The first stage is fueled by RP-1, a highly refined kerosene, and supercold liquid oxygen. The RD-180 main engine, made by NPO Energomash, generates about 860,000 pounds of thrust. Known as the Common Core Booster, or CCB, the stage measures 42 feet in length and 10 feet diameter.
The Centaur stage features a restartable Aerojet Rocketdyne RL10A-4-2 engine burning liquid hydrogen and liquid oxygen. The total thrust is 22,300 pounds.
Protecting the payload during atmospheric ascent is the payload fairing measuring 39 feet in length and 14 feet in diameter. It is an aluminum nose cone built in two halves.
The Atlas 5 stands 189 feet tall and weighs about 745,000 pounds at liftoff.
Four weather rules are currently being violated, including the surface electric field mill rule, the cumulus cloud rule, the attached anvil rule, and the lightning rule.
The launch window extends to 8:10 p.m. EDT (0010 GMT).
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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.
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.
Officials will likely choose between SpaceX, Boeing Co. and Sierra Nevada Corp., three companies already receiving federal funding to design human-rated spacecraft.
For details, check out our story.
Clocks have one more built-in hold planned at T-minus 4 minutes. That pause will last 15 minutes during which time the final "go" for launch will be given. All remains targeted for liftoff at 5:44 p.m. EDT (2144 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.
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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.
The Air Force weather office says conditions should be worst at the opening of this evening's window at 5:44 p.m. EDT (2144 GMT), with a possibility of improving weather toward the end of the window, which closes at 8:10 p.m. EDT (0010 GMT).
The prime concerns are with anvil clouds, thick clouds, cumulus clouds and lightning.
Overcast skies are forecast, with numerous thunderstorms in the area. The launch weather officer just briefed the ULA launch team, adding conditions may improve later in this evening's launch window.
Clocks are picking up the seven-hour sequence of work that will prepare the booster, payload and ground systems for blastoff at 5:44 p.m. EDT (2144 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 2:39 p.m. EDT.
A planned half-hour hold begins at 2:54 p.m. EDT when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 3:12 p.m. EDT to verify all is in readiness to start fueling the rocket for launch.
Supercold liquid oxygen begins flowing into the Centaur upper stage around 3:41 p.m. EDT, followed by the first stage filling around 3:54 p.m. EDT. 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 5:20 p.m. EDT. 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 5:44 p.m. EDT and extends to 8:10 p.m. EDT (2144-0010 GMT).
The launch of the CLIO mission is set for 5:44 p.m. EDT (2144 GMT). Tuesday's launch window extends for 2 hours, 26 minutes
The countdown will begin at 10:44 a.m. EDT (1434 GMT), leading to activation of the rocket, final testing and system preps. Cryogenic fueling operations start at approximately 3:41 p.m. EDT (1941 GMT).
"The ULA team is proud to launch the CLIO mission on an Atlas V 401, from Space Launch Complex 41, for Lockheed Martin Space Systems Company," said Jim Sponnick, ULA's vice president of Atlas and Delta programs.
This afternoon, ground crews will plug the rocket into the launch pad's propellant, pneumatic and electrical systems. The first stage will also be loaded with RP-1 kerosene fuel today.
The weather forecast from the Air Force's 45th Weather Squardon calls for a 60 percent chance of violating launch constraints.
Thunderstorms are expected to develop along the sea breeze on Florida's East Coast during Tuesday afternoon. The storms are expected to migrate slowly inland but remain near the coast, according to the official launch weather forecast issued by the U.S. Air Force's 45th Weather Squadron.
"Strengthening mid-level winds will cause late afternoon thunderstorms over the interior to drift back toward the east coast," forecasters wrote. "Likewise, upper-level winds with a westerly component will transport anvil clouds back toward the east coast. Thunderstorms will begin dissipating by early evening."
Elevated proton levels caused by solar activity could also prevent launch Tuesday.
The outlook calls for mostly cloudy skies with thunderstorms in the vicinity, southwest winds of 10 to 15 mph, a temperature between 81 and 84 degrees Fahrenheit, and moderate solar activity.
If launch is scrubbed Tuesday, conditions worsen slightly for Wednesday, with a 70 percent chance of weather violating launch commit criteria.
Watch this page for live updates throughout the countdown and flight, plus live streaming video.
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The 189-foot-tall satellite booster is moving to Cape Canaveral's Complex 41 for the launch of CLIO, a spacecraft built by Lockheed Martin Corp. shrouded inside the launcher's payload fairing. The satellite's purpose and orbit have not been disclosed, but officials acknowledge its mission is for an unidentified U.S. government customer.
The two-stage rocket and the CLIO spacecraft were put together inside the assembly building over the past six weeks 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 many other sites.
Liftoff is scheduled for 5:44 p.m. EDT, at the opening of a two-hour, 26-minute window that extends to 8:10 p.m. EDT (2144-0010 GMT).
The Atlas 5 rocket, making its 49th flight, will be rolled out to the launch pad on Monday from its vertical assembly building.
The vehicle, flying for the 25th time in the 401 configuration, features an RD-180 main engine, no solid rocket boosters, a single RL10 upper stage engine and a four-meter nose cone.
Enclosed within that shroud is CLIO, a clandestine satellite made by Lockheed Martin for a classified U.S. government user.
The CLIO system is built on commercial technology, including Lockheed Martin's A2100 satellite framework of solar arrays, propulsion system and core electronics. There are currently more than 40 A2100 spacecraft in orbit with more than 400 collective years of on-orbit service, including both commercial and U.S. government satellites.
"We're proud to support the CLIO system," said Rick Ambrose, executive vice president of Lockheed Martin Space Systems. "The A2100 bus gives our customers proven performance at an affordable cost, and we're pleased to add CLIO to the growing list of systems that are built on this venerable spacecraft."
The CLIO launch harkens back to September 2009 and launch of the equally mysterious PAN satellite produced by Lockheed Martin for the government.
"Lockheed Martin is the prime contractor for the PAN mission, which includes a commercial-based satellite and launch system solution for the U.S. government," said a Lockheed Martin statement released to Spaceflight Now at the time.
The launch will employ a two-burn Centaur ascent profile separated by a two-and-a-half hour coast period to deploy CLIO into orbit. Separation of the spacecraft to complete the launch is expected at two hours and 52 minutes after liftoff.
It will be the 88th mission for United Launch Alliance and 11th just this year.