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1430 GMT (10:30 a.m. EDT)
T-minus 5 minutes. Standing by to go into the final built-in hold.
1425 GMT (10:25 a.m. EDT)
Now 40 minutes till launch. Rumbling away from the planet on nearly a million pounds of thrust, the Atlas 5 rocket will be flying in a basic, two-stage configuration without any added strap-on solid motors. The vehicle sports a voluminous nose cone that encapsulates the X-37B spaceplane during the atmospheric ascent before being shed.

In technical speak, this is the Atlas 5-501 configuration that has successfully flown five times. The first was the original X-37B launch in April 2010.

With the liftoff thrust not considerably more than the rocket's weight, this Atlas will display a slow and majestic rise trailing only a flickering golden flame from its RD-180 main engine.

Once above the launch pad, the rocket sets sail for the eastward trek downrange over the Atlantic Ocean, constantly gaining speed as its double-nozzle engine gulps 25,000 gallons of kerosene fuel and 50,000 gallons of superchilled liquid oxygen in just four-and-a-half minutes.

The bronze first stage, its propellants depleted and job now completed, then jettisons with the help of tiny thrusters. Some 106.5 feet long and 12.5 feet around, the stage is discarded to fall back into the open sea.

The cryogenic Centaur upper stage ignites moments after shedding the lower booster, lighting the RL10 engine to continue clawing toward orbit.

Covered with insulating foam, this stage stretches 41.5 feet in length and 10 feet in diameter. Centaur must perform the full burn to loft X-37B into the proper orbit around the planet.

1419 GMT (10:19 a.m. EDT)
The fuel-fill sequence for the first stage main engine is starting.
1415 GMT (10:15 a.m. EDT)
Atlas 5 represents the culmination of evolution stretching back several decades to America's first intercontinental ballistic missile. At the dawn of the space age, boosters named Atlas launched men into orbit during Project Mercury and became a frequent vehicle of choice to haul civil, military and commercial spacecraft to orbit.

Topped with the high-energy Centaur upper stage, Atlas rockets have been used since the 1960s to dispatch ground-breaking missions for NASA, including the Surveyors to the Moon, Mariner flights to Mars, Venus and Mercury, and the Pioneers that were the first to visit Jupiter and beyond.

In its newest era, the Atlas 5 rocket sent the Mars Reconnaissance Orbiter to the red planet in 2005, propelled the New Horizons probe toward Pluto and the solar system's outer fringes in 2006, doubled up with the dual Lunar Reconnaissance Orbiter and LCROSS impactor to the Moon in 2009, hurled Juno to Jupiter in August 2011 and dispatched the car-sized Curiosity rover on the Mars Science Lab mission in November 2013.

Today marks the 54th flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 53 missions since debuting in August 2002, the Atlas 5 has flown 19 flights dedicated to the Defense Department, 12 for NASA, 11 with spy satellites for the National Reconnaissance Office. and 11 commercial missions with communications and Earth-observing spacecraft.

1405 GMT (10:05 a.m. EDT)
Now 60 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen is complete as the countdown continues as planned for a liftoff at 11:05 a.m. EDT.
1355 GMT (9:55 a.m. EDT)
Fast-filling of the first stage liquid oxygen tank has been completed. Topping mode is now underway.
1354 GMT (9:54 a.m. EDT)
The liquid hydrogen tank in the Centaur upper stage just reached the 97 percent level. Topping is now beginning.
1345 GMT (9:45 a.m. EDT)
Centaur's liquid hydrogen tank is 40 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Aerojet Rocketdyne-made RL10 engine.
1341 GMT (9:41 a.m. EDT)
The first stage liquid oxygen has reached the 70 percent level.
1336 GMT (9:36 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.
1335 GMT (9:35 a.m. EDT)
Launch of the Atlas 5 rocket is just 90 minutes away.

If you are heading out to the beach or Port Canaveral to watch the launch, sign up for our Twitter feed to get occasional countdown updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

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

1333 GMT (9:33 a.m. EDT)
Half of the Atlas liquid oxygen tank has been filled so far.
1327 GMT (9:27 a.m. EDT)
Upper stage liquid oxygen has reached flight level.
1325 GMT (9:25 a.m. EDT)
First stage liquid oxygen tank is 30 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-and-a-half minutes of flight today. The 25,000 gallons of RP-1 were loaded into the rocket yesterday.
1320 GMT (9:20 a.m. EDT)
The Centaur liquid oxygen tank reached the 96 percent level. The topping off process is starting now.
1315 GMT (9:15 a.m. EDT)
The first stage liquid oxygen flow rate is switching from slow-fill to fast-fill mode.
1312 GMT (9:12 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 12,700 gallons of the cryogenic propellant.
1311 GMT (9:11 a.m. EDT)
Now three-quarters full on Centaur liquid oxygen.
1306 GMT (9:06 a.m. EDT)
The chilldown 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 49,000 gallons of cryogenic oxidizer for the RD-180 main engine.

1305 GMT (9:05 a.m. EDT)
Fifty percent of the Centaur liquid oxygen tank has been filled so far.
1258 GMT (8:58 a.m. EDT)
The Centaur liquid oxygen tank has reached the 20 percent mark.
1250 GMT (8:50 a.m. EDT)
Filling of the Centaur upper stage with about 4,150 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 provide the thrust to put X-37B into orbit.

1243 GMT (8:43 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.
1235 GMT (8:35 a.m. EDT)
T-minus 120 minutes and counting! The launch countdown is continuing on schedule for today's flight of the Atlas 5 rocket with the Orbital Test Vehicle No. 4 aboard.

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 11:05 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.

1233 GMT (8:33 a.m. EDT)
After briefing his team on procedures before entering into the final two hours of the countdown, the launch conductor at the Atlas Spaceflight Operations Center just held a pre-fueling readiness poll. All console operators reported a "ready" status.

The ULA launch director also voiced his approval for proceeding with the countdown.

Loading of cryogenic liquid oxygen and liquid hydrogen into the Atlas 5 rocket will be getting underway a short time from now.

1215 GMT (8:15 a.m. EDT)
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1205 GMT (8:05 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 11:05 a.m. EDT launch of the Atlas rocket. The holds give the team some margin in the countdown timeline to deal with technical issues or any work that is running behind. The final hold is scheduled to occur at T-minus 4 minutes.
1130 GMT (7:30 a.m. EDT)
LAUNCH TIME. A pair of 10-minute launch windows will be avaialble for the X-37B to take flight aboard the Atlas 5 rocket today. Liftoff is targeted for 11:05 a.m. EDT (1505 GMT). A backup opportunity begins at 12:42 p.m. EDT (1642 GMT).
0840 GMT (4:40 a.m. EDT)
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 launch period stand at 60 percent favorable. The main concern that meteorologists are watching are cumulus and anvil clouds and lightning potential.

The outlook predicts scattered clouds at 3,000 and 30,000 feet, isolated showers, good visibility, southwesterly winds of 10 gusting to 13 knots, a temperature in the low 80s degrees F.

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.

0800 GMT (4:00 a.m. EDT)
The countdown is beginning for today’s launch of the Atlas 5 rocket to haul the Air Force’s X-37B reusable spaceplane back into orbit.

Clocks are picking up the seven-hour sequence of work that will prepare the booster, payload and ground systems for blastoff some time after 10:45 a.m. EDT (1445 GMT).

The exact launch time is yet to be announced.

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 GPS metric tracking system used to follow the rocket as it flies downrange, plus a test of the S-band telemetry relay system.

A planned 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 unclassified launch period extends to 2:45 p.m. EDT (1845 GMT).

TUESDAY, MAY 19, 2015
A United Launch Alliance Atlas 5 rocket topped by the Air Force's X-37B miniature space shuttle rolled onto the perch at the launch pad Tuesday morning, poised for liftoff Wednesday from Cape Canaveral's Complex 41 to deliver the spaceplane into orbit.

Liftoff of the Orbital Test Vehicle mission No. 4 is slated for some time after 10:45 a.m. EDT. The exact time remains classified until launch morning.

The 20-story-tall booster was wheeled out aboard a mobile platform, emerging from the assembly building where the rocket’s two stages and the payload were integrated over the past few weeks.

The slow drive from the 30-story Vertical Integration Facility to the launch pad, which began shortly after 9 a.m., 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 launch will deliver the X-37B into a low-Earth orbit. Deployment of the spacecraft occurs less than 20 minutes after liftoff.

On Monday, officials from United Launch Alliance and the Air Force gathered for the Launch Readiness Review. All systems were verified ready to proceed with today's rollout and the start of launch pad operations.

The rocket is flying the 501 vehicle configuration for the sixth time in 54 flights. The version features two stages and a five-meter-diameter nose cone. It is powered off the launch pad by an RD AMROSS RD-180 main engine. The Centaur upper stage is equipped with an Aerojet Rocketdyne RL10.

Countdown clocks begin ticking seven hours before launch, leading to activation of the rocket, final testing and system preps.

The day’s launch period begins at 10:45 a.m.and closes at 2:45 p.m. EDT. The usable launch window, which remains classified, falls within that broad, four-hour period.

There is a 60 percent chance of acceptable weather, with conditions expected to be more favorable earlier in the period than later.

MONDAY, MAY 18, 2015
PREVIEW: An experiment-carrying, reusable mini space shuttle operated by the U.S. Air Force will be boosted into Earth orbit Wednesday atop a United Launch Alliance Atlas 5 rocket.

It will be the fourth flight of the military’s X-37B program, also known as Orbital Test Vehicle mission No. 4. The project is run inside the Pentagon at the Rapid Capabilities Office.

Liftoff is scheduled for some time during a four-hour, unclassified period that extends from 10:45 a.m. to 2:45 p.m. EDT (1445-1845 GMT) from Complex 41 at Cape Canaveral Air Force Station in Florida. The actual target time will be announced on launch day.

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