Shot into orbit Saturday by a gleaming white and gold Atlas 5 rocket, the U.S. Air Force's second top secret X-37B space plane will push the boundaries of the craft's design and could stay in orbit longer than its predecessor, according to military officials.

The military isn't divulging what the space plane carries, but it could be shepherding high-tech Air Force experiments, spy sensors and other research payloads.

Read our full launch story.

And see the launch photo gallery from cameras at the pad and our gallery of pictures taken at the Kennedy Space Center Press Site.

The X-37B spaceplane was built Boeing. Work on the program occurs at the company's Huntington Beach, Seal Beach, and El Segundo sites in California. Boeing has issued its post-launch release that includes some quotes:

"History was made in December when the X-37B became the United States' first unmanned vehicle to return from space and land on its own," said Craig Cooning, vice president and general manager of Boeing Space & Intelligence Systems. "The success of that mission validated this reusable and effective way to test new technologies in space and return them for examination.

"Today, we took another important step with the successful launch of the second OTV, enabling the RCO to further experiment with the vehicle and its ability to operate in low-Earth orbit," Cooning continued. "Close teamwork between the Air Force Rapid Capabilities Office, the United Launch Alliance Atlas team, and the 45th Space Wing at Cape Canaveral Air Force Station made this launch a success."

"This program provides a test capability that was difficult to achieve through other means, the ability to examine how highly complex technologies will perform in space before they are made operational," said Deputy Under Secretary of the Air Force for Space Programs Richard McKinney. "But right now our focus is on the X-37B itself, and this second flight is important to our further understanding of its capabilities."

This flight of a second vehicle in the X-37B program will expand the knowledge gained from operating the first spaceplane in orbit last year, the Air Force says.

"We look forward to testing enhancements to the landing profile," said Lt. Col. Troy Giese, X-37B program manager for the Air Force Rapid Capabilities Office.

"The X-37B really is a remarkable scientific and aerospace achievement," he said. "We'll also be looking at the performance of its advanced thermal protection systems and tiles, solar power systems and environmental modeling - all important system capabilities for a space vehicle that we want to be able to bring back and then re-launch quickly."

This flight could last longer than last year's OTV 1 mission, Giese also said.

"We may extend the mission to enhance our understanding of the OTV capabilities," he said, "especially since the performance data from the first flight suggest that the vehicle could have gone beyond the 270-day requirement."

The Air Force has issued its post-launch press release:

"Launch is a very demanding business and having what appears to be a successful launch is always welcome news," said Deputy Under Secretary of the Air Force for Space Programs Richard McKinney, adding he is pleased with the vehicle's initial status reports. "It is important to remember that this is an experimental vehicle; that this is just the second launch; and that we have just started what is a very systematic checkout of the system."

Rocket-maker United Launch Alliance hails today's flight of the Atlas 5 rocket as a success, boosting the second Orbital Test Vehicle into space for the X-37B program and the Air Force's Rapid Capabilities Office.

"The ULA team is proud to have played a critical role in successfully launching both of these important missions of the Orbital Test Vehicle for the Air Force RCO," said Jim Sponnick, ULA vice president, Mission Operations.

"It took a tremendous amount of teamwork to successfully launch both vehicles in less than a year. I am confident that the information collected by the Rapid Capabilities Office from these missions will lead to even bigger and bolder missions in the future.

"Congratulations to the combined Air Force and ULA launch team and our many mission partners that made today's successful launch possible."

And with that, the launch will go into a cloak of secrecy. Confirmation that the Orbital Test Vehicle has separated from the Centaur won't be announced live.

T+plus 18 minutes, 30 seconds. Centaur's main engine has completed its burn today to achieve orbit.

T+plus 15 minutes, 50 seconds. The RL10 is performing well, burning liquid hydrogen and liquid oxygen propellants.

T+plus 15 minutes, 30 seconds. The rocket is 194 miles in altitude, some 2,138 miles downrange and traveling 15,495 mph.

In the webcast you are seeing a graphical depiction of the Centaur rocket firing its main engine to reach orbit. But the live audio from the telemetry engineer and information about the flight's progress has stopped.

T+plus 6 minutes, 30 seconds. All appears to be proceeding well in the Atlas 5 rocket's climb to orbit.

T+plus 5 minutes, 10 seconds. Centaur has ignited! The RL10 engine is up and running at full thrust for its firing to propel X-37B to orbit.

T+plus 5 minutes, 2 seconds. The Atlas 5's Common Core Booster first stage has been jettisoned, and the Centaur upper stage's liquid hydrogen and liquid oxygen systems are being readied for engine start.

T+plus 4 minutes, 55 seconds. BECO. Booster Engine Cutoff is confirmed as the RD-180 powerplant on the first stage completes its burn. Standing by to fire the retro thrusters and separate the spent stage.

T+plus 3 minutes, 43 seconds. The two-halves of the Atlas 5 rocket nose cone encapsulating the X-37B spaceplane have separated. The Orbital Test Vehicle is now exposed to space. Also jettisoned was the Forward Load Reactor, a two-piece deck that rings the Centaur stage to support the bulbous fairing during launch.

T+plus 3 minutes, 30 seconds. RD-180 is performing well as the rocket climbs away from the planet.

T+plus 3 minutes, 18 seconds. The rocket is 47 miles in altitude, some 46 miles downrange and traveling 4,048 mph.

T+plus 2 minutes, 30 seconds. Everything looking very good.

T+plus 1 minutes, 45 seconds. The RD-180 main engine continues to fire normally, burning a mixture of highly refined kerosene and liquid oxygen.

T+plus 90 seconds. Mach 1.

T+plus 75 seconds. Atlas is 3.5 miles in altitude, 0.3 miles downrange from the launch pad and traveling 1,186 mph.

T+plus 60 seconds. One minute into the ascent as the Atlas 5 climbs away from the Cape into beautiful blue skies.

T+plus 45 seconds. Good engine performance.

T+plus 40 seconds. A slow and steady rise for the Atlas booster, delivering a thunderous roar across the Florida spaceport.

T+plus 15 seconds. The vehicle has cleared the towers at Complex 41 on 860,000 pounds of thrust from the RD-180 main engine.

LIFTOFF! Liftoff of the Atlas 5 rocket and the second experimental X-37B, America's miniature military space shuttle.

T-minus 20 seconds. "Go Atlas" and "Go Centaur" was just called by launch team during a final status check.

T-minus 40 seconds. Centaur's liquid oxygen and liquid hydrogen tanks are stable at flight pressures.

T-minus 1 minute. Now 60 seconds from the launch that will put the second X-37B spaceplane into orbit to build upon last year's technology demonstrations.

T-minus 90 seconds. The safety system has been armed.

T-minus 1 minute, 45 seconds. Liquid oxygen and liquid hydrogen propellant topping to the Centaur upper stage is being secured.

T-minus 1 minute, 55 seconds. The launch sequencer has been commanded to start.

T-minus 2 minutes. The Atlas first stage and Centaur upper stage are now switching from ground power to internal batteries.

T-minus 2 minutes, 30 seconds. The first stage RP-1 kerosene fuel tank and the liquid oxygen have stepped up to proper flight pressure levels.

T-minus 3 minutes. The Atlas first stage liquid oxygen replenishment is being secured so the tank can be pressurized for launch.

T-minus 3 minutes, 50 seconds. The ground pyrotechnics have been enabled.

T-minus 4 minutes and counting. Clocks have resumed for the final minutes of today's countdown to launch the Atlas 5 rocket carrying X-37B. Liftoff is set to occur at 5:46 p.m. EST.

The Orbital Test Vehicle nestled inside the nose of the Atlas 5 rocket is switching to internal power for launch.

Countdown clocks will resume in one minute.

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The ULA launch director has given his "go" for today's liftoff.

Polling of the team by Atlas launch conductor has occurred. All systems are reported "go" to continue with the countdown for liftoff at 5:46 p.m. EST.

Standing by for the launch team readiness polls.

The Atlas 5 rocket stands 196 feet tall and weighs 757,380 pounds at launch.

Here's a look at some stats about today's mission. This will be:

• The 606th launch for Atlas program since 1957
• The 319th Atlas to occur from Cape Canaveral
• The 24st launch of an Atlas 5 since 2002
• The 20th Atlas 5 to occur from the Cape
• The 16th Atlas 5 under United Launch Alliance
• The 3rd Atlas 5 to fly in the 501 configuration
• The 11th Department of Defense launch of Atlas 5
• The 1st Atlas launch of 2011

No problems being reported by the launch team. Countdown continues to sit in the hold period at T-minus 4 minutes, waiting for the launch window to open at 5:46 p.m. EST.

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The chance of good weather has improved to 70 percent favorable now. Meteorologists say there's only a slight concern for a cloud violation and ground winds are well within limits now.

The launch time outlook now calls for just some scattered low and mid clouds, some broken high clouds, coastal showers, east-southeasterly winds of 12 gusting to 16 knots and a temperature of 71 degrees F.

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 twice. The first was the original X-37B launch last April and then a California flight in September deployed a classified satellite for the National Reconnaissance Office.

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 Russian-designed 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 tried-and-true RL10 engine for 22,300 pounds of thrust 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 lasting through approximately T+plus 17 minutes, 30 seconds to loft X-37B into the proper orbit around the planet.

The first stage liquid oxygen tank and Centaur's liquid oxygen and liquid hydrogen tanks are reported at flight level.

T-minus 4 minutes and holding. The countdown has entered the planned 40-minute hold to give the launch team a chance to review all systems and assess the weather before pressing ahead with liftoff.

T-minus 5 minutes. Standing by to go into the final built-in hold.

The ground winds continue to remain within limits. It's the clouds moving in and out of the local area that pose the main concern for launching the Atlas 5 rocket in this upcoming window today.

The odds of acceptable weather for today's launch have improved to 60 percent favorable.

The outlook now calls for scattered low-level and broken mid-level clouds, occasional high overcast, coastal showers, east-southeasterly winds of 14 gusting to 18 knots and a temperature of 71 degrees F.

Now 60 minutes from liftoff. All three of Atlas/Centaur rocket's cryogenic tanks have entered topping mode to flight levels as the countdown proceeds to the scheduled launch time of 5:46 p.m. EST.

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The particular clouds that were violating the launch weather rules have moved through. Weather is "green" again.

Fast-filling of the first stage liquid oxygen tank has been completed. Topping mode is now underway.

The liquid hydrogen tank in the Centaur upper stage just reached the 97 percent level. Topping is now beginning.

The first stage liquid oxygen tank is passing the 80 percent mark.

Now 90 minutes from launch.

Centaur's liquid hydrogen tank is 60 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Pratt & Whitney Rocketdyne-made RL10 engine.

And the weather conditions just went "red" due to the cloud cover moving over Cape Canaveral.

Sixty percent of the Atlas liquid oxygen tank has been filled so far.

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.

First stage liquid oxygen tank is 40 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 previously.

The Centaur engine chilldown sequence is being initiated.

Upper stage liquid oxygen has reached flight level.

Just under two hours from launch. Weather remains "go" at this time.

The first stage liquid oxygen flow rate is switching from slow-fill to fast-fill mode.

The Centaur liquid oxygen tank reached the 95 percent level. The topping off process is starting now.

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.

Now three-quarters full on Centaur liquid oxygen.

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.

Fifty percent of the Centaur liquid oxygen tank has been filled so far.

The Centaur liquid oxygen tank is approach a quarter-full already.

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.

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.

T-minus 120 minutes and counting! The launch countdown has resumed for today's flight of the Atlas 5 rocket with the Orbital Test Vehicle No. 2 aboard.

Clocks have one more built-in hold planned at T-minus 4 minutes. That pause will last a revised 40 minutes in length, during which time the final "go" for launch will be given. All remains targeted for liftoff at 5:46 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.

The launch conductor at the Atlas Spaceflight Operations Center just held a pre-fueling readiness poll and all console operators reported a "ready" status.

The ULA launch director also voiced his approval for proceeding with the countdown at 3:06 p.m.

Loading of cryogenic liquid oxygen and liquid hydrogen into the Atlas 5 rocket begins shortly.

With the repairs successfully finished, the Complex 41 pad and the surrounding danger area have been cleared of all workers for the remainder of the countdown.

The final members of the pad crew are leaving the pad.

Repairs have been completed at the launch and leak checks were performed successfully. The effort involved removing and replacing a bad regulator valve in the system that supplies a helium purge to the rocket's interstage adapter.

A check of the weather shows the winds at Complex 41 having eased down into the teens after being up above 20 knots through most of the day thus far. So the current weather is acceptable.

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The new timeline calls for the countdown to resume ticking at 3:06 p.m. EST from the T-minus 2 hour mark. The planned built-in hold at the T-minus 4 minute mark will last for 40 minutes under this revised game plan today, all leading to the 5:46 p.m. EST launch.

Countdown clocks remain in this extended hold at T-minus 2 hours as the launch team targets the new 5:46 p.m. EST liftoff time.

NEW LAUNCH TIME. The clock has run out on getting the new regulator installed and still getting the Atlas 5 rocket launched during the initial opportunity today. So officials have pushed the launch to the day's second window that opens at 5:46 p.m. EST.

The pad team has updated that there's still some work left to finish on the regulator installation job.

The new regulator has been installed successfully at the pad. Leak checks are underway now.

NEW LAUNCH TIME. Liftoff is being shifted back by 10 minutes to 4:19 p.m. EST. Those 10 minutes are being inserted into this current hold in the countdown, giving the pad crew additional time to finish the bad purge regulator removal and replacement.

Now 10 minutes remaining in this hold. A readiness check of the launch team is coming up shortly. Once the pad is cleared of all workers, the hazardous fueling operations can begin to load the rocket with supercold liquid oxygen and liquid hydrogen this afternoon.

To give the pad team time to complete the regulator replacement and preserve plans for an on-time launch today, the countdown timeline is being juggled a bit. This current hold at T-minus 2 hours will be extended an additional 10 minutes prior to the start of fueling operations. Those 10 minutes will be take out of the planned T-minus 4 minute hold period, which was supposed to be 20 minutes long but now will be 10 minutes. All of that math means the countdown clocks can hit T-0 as scheduled at 4:09 p.m. EST.

The failed purge regulator is being removed and replaced with a spare already available at the pad. This work and the re-testing of the system is estimated to take 30 to 60 minutes, officials say.

The troubleshooting has isolated the problem causing the purge leak at the launch pad. Engineers are assessing the game plan to fix it.

T-minus 2 hours and holding. The countdown has just entered the first of two planned holds over the course of the afternoon that will lead to the 4:09 p.m. EST launch of the Atlas rocket. This initial pause lasts 30 minutes, giving 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 and will last for 20 minutes.

Technicians continue working at the launch pad to examine the purge leak problem. They are hoping to get the situation fixed shortly so that fueling operations can begin on schedule.

So far, all the rainshowers are staying offshore today. Winds at the launch pad are persistant at about 20 knots and the gusts are expected to pick up into the mid-20s this afternoon. The exact limit depends on the specific direction the wind is blowing.

Troubleshooting steps are being developed for the purge leak.

The guidance system testing and internal battery checks have been accomplished.

The hazard area roadblocks around the launch site's safety perimeter are being established now. And the launch team has started configuring the pad's water deluge system.

The anomaly team is assessing a regulator leak in the system that provides a helium purge to the rocket's interstage adapter area.

United Launch Alliance officials say if liftoff doesn't happen today on this second attempt for some reason, they would give the team Sunday to rest and make a third try on Monday afternoon.

Now four hours away from the planned liftoff time. The weather team is monitoring the winds very closely today as the speeds hover near the launch limit.

Testing of the C-band beacon used to track the rocket as it flies downrange and the S-band telemetry relay system are being conducted at this point in the countdown.

Early countdown activities are going well this morning. Flight control preps complete and the operational test is underway. And the Atlas pneumatics, propulsion and hydraulic preps are in work.

The weather picture for today's launch is shaping up like forecasters have been advertising. The latest update still gives a 40 percent chance of acceptable liftoff conditions. That's a 60 percent chance of winds and clouds again being a problem.

The outlook for this afternoon calls for scattered low-level and broken mid-level clouds, occasional high overcast, isolated coastal showers but perhaps not the widespread amount as yesterday, east-southeasterly winds of 20 gusting to 25 knots and a temperature of 71 degrees F.

The second countdown just commenced for today's shot at launching the X-37B spaceplane aboard an Atlas 5 rocket from Cape Canaveral, Florida. Liftoff is scheduled for 4:09 p.m. EST (2109 GMT) from Complex 41 at the spaceport.

The launch team is beginning to power up the rocket just like yesterday for the repeat of standard pre-flight tests. Over the next few hours, final preps for the Centaur's liquid oxygen and liquid hydrogen systems will be performed once again, 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. The Complex 41 site will be cleared of all personnel just after 1 p.m. EST.

A planned half-hour hold begins when the count reaches T-minus 120 minutes at 1:19 p.m. Near the end of the hold at 1:46 p.m., 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 at about 2:06 p.m., followed by the first stage filling with LOX and then liquid hydrogen fuel loading for Centaur.

A final hold is scheduled at the T-minus 4 minute mark at 3:45 p.m. 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 for liftoff.

Tomorrow's launch time for the Atlas 5 rocket is 4:09 p.m. EST (2109 GMT).

Draining of the cryogenic liquid oxygen and liquid hydrogen from the Atlas vehicle has begun as the launch team safes and secures the rocket for this 24-hour launch postponement.

SCRUB. The U.S. military's second experimental X-37B spaceplane, the covert prototype for an unmanned reusable shuttle, will not launch today due to bad weather at Cape Canaveral.

Gusty winds and cumulus clouds prevented a liftoff in the first of two available launch windows at 3:50 p.m. EST today for the Atlas 5 rocket booster to carry the Orbital Test Vehicle No. 2 into space.

After waiting for the second window that would open at 5:27 p.m. EST, the weather remained solidly "red" for launch and left managers with no choice but call a postponement.

Another shot will be made Saturday afternoon. But the weather outlook remains iffy at best. The odds of acceptable conditions are 40 percent due to a continuation of similar weather.

The outlook includes scattered clouds at 3,000, 10,000 and 25,000 feet, isolated coastal showers, southeasterly winds of 20 gusting to 24 knots and a temperature of 71 degrees F.

If the launch should slip to Sunday, the forecast is just 30 percent favorable because of a more extensive cloud cover and isolated thunderstorms.

This launch window runs to 5:37 p.m. EST.

NEW TIME. The hold will be extended five more minutes for 5:32 p.m. EST liftoff. But clouds and high winds remain out of limits.

The readiness poll confirms there are no technical problems in the countdown. It's just the weather that's preventing a launch today.

The launch conductor's readiness poll is coming up at 5:20 p.m. We'll be standing by for the official decision on today's countdown.

All in all, the odds of acceptable weather in the upcoming launch opportunity today officially stand at 10 percent. The cumulus cloud rule and the various points of criteria that govern cloud cover for a rocket launch are "no go" and expected to remain that way for the foreseeable future this evening at the Florida spaceport.

The outlook for the 5:27 p.m. launch time calls for continued scattered low and mid clouds, a broken deck of high clouds, isolated coastal rainshowers, easterly winds of 20 gusting to 26 knots and a temperature of 63 degrees.

Weather forecasters say conditions are expected to remain "no go" for the rest of today's launch opportunity. There's not much hope of anything changing for the better.

For the next chance of launching the Atlas 5 booster with X-37B today, the team will be polled for a readiness check at 5:20 p.m., followed by resumption of the countdown at 5:23 p.m. EST for liftoff at 5:27 p.m. EST (2227 GMT).

The launch window stretches 10 minutes to 5:37 p.m. EST. If the rocket isn't airborne by then, the flight would be scrubbed for the day.

Check out this photo! A rainbow appeared above the Atlas 5 launch pad as rainshowers approached Cape Canaveral a little bit ago.

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NEW LAUNCH TIME. And now a lengthy extension of this hold has been ordered while the Atlas 5 rocket sits on the launch pad to await the next liftoff opportunity that opens today at 5:27 p.m. EST for carrying X-37B into the desired orbit.

Weather is "no go" for this initial 10-minute launch window today. Atlas will wait for next shot opening at 5:27 p.m. EST today.

As expected, the cumulus cloud rule has gone "red" due to showers marching toward shore from the Atlantic. That joins the "red" winds as constraints for liftoff in this first of two launch opportunities today.

NEW TIME. Extending this countdown hold five more minutes to target 4:00 p.m. EST launch, weather permitting.

And still expecting the cumulus clouds to be a problem by 3:48 p.m.

NEW LAUNCH TIME. Liftoff being pushed back to 3:55 p.m. EST due to high ground winds at the pad.

Polling of the team by Atlas launch conductor is occurring.

Still waiting the weather and whether there's any shot of launch happening at 3:50 p.m. before the clouds arrive.

United Launch Alliance says this initial launch window extends for 10 minutes from 3:50 to 4:00 p.m. EST. If weather prohibits an on-time launch, there is a backup opportunity available from 5:27 to 5:37 p.m. EST today.

The first stage liquid oxygen tank and Centaur's liquid oxygen and liquid hydrogen tanks are reported at flight level.

Those approaching rainshowers are about 10 miles away. The cumulus clouds associated with this weather is expected to violate the launch criteria and keep the countdown in a holding pattern until conditions improve. At least that's what the forecast is predicting will happen.

T-minus 4 minutes and holding. The countdown has entered the planned 20-minute hold to give the launch team a chance to review all systems and assess the weather before pressing ahead with liftoff.

T-minus 5 minutes. Standing by to go into the final built-in hold.

The fuel-fill sequence for the first stage main engine has been completed.

Weather has even gotten more pessimistic for an on-time launch due to a line of showers headed right for the pad. The inbound weather should violate the cumulus cloud rule shortly. And with that situation, there's now a 90 percent chance of weather prohibiting liftoff at 3:50 p.m. EST.

Thirty minutes from liftoff. The countdown clocks are heading to the T-minus 4 minute mark where a planned 20-minute hold will occur. Launch of Atlas 5 remains scheduled for 3:50 p.m. EST.

Now 40 minutes till launch. Will this current hole in the weather last long enough for the Atlas to fly on schedule?

The cumulus cloud rule just went "green," meaning current conditions aren't violating that criteria. However, there's more weather off the coast moving this way.

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The outlook for launch calls for scattered low and mid clouds, a broken deck of high clouds, isolated coastal rainshowers, easterly winds of 20 gusting to 26 knots and a temperature of 70 degrees.

The odds of weather allowing the Atlas 5 rocket today have gotten worse. Meteorologists now say there's only a 20 percent chance of acceptable conditions this afternoon. The cumulus cloud rule remains "no go" and winds are very strong.

Now 60 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen is proceeding as planned for a liftoff at 3:50 p.m. EST.

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Fast-filling of the first stage liquid oxygen tank has been completed. Topping mode is now underway.

The liquid hydrogen tank in the Centaur upper stage just reached the 97 percent level. Topping is now beginning.

The first stage liquid oxygen tank is passing the 80 percent mark.

Centaur's liquid hydrogen tank is 40 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Pratt & Whitney Rocketdyne-made RL10 engine.

The first stage liquid oxygen has reached the 70 percent level.

Weather continues to be troublesome due to the cloud cover and strong winds blowing across the Cape this afternoon.

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.

Half of the Atlas liquid oxygen tank has been filled so far.

The Centaur engine chilldown sequence is being initiated.

First stage liquid oxygen tank is 40 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 earlier.

Winds had eased a bit earlier during the rainshowers. But the gusts are picking up again.

Upper stage liquid oxygen has reached flight level.

The first stage liquid oxygen flow rate is switching from slow-fill to fast-fill mode.

The Centaur liquid oxygen tank reached the 95 percent level. The topping off process is starting now.

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.

Now three-quarters full on Centaur liquid oxygen.

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.

Fifty percent of the Centaur liquid oxygen tank has been filled so far.

The Centaur liquid oxygen tank has reached the 20 percent mark.

Live streaming video of the Atlas now available on this page. Hit reload to bring in the player.

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.

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.

The field mill weather rule, which governs the electrical potential in the air, is back to "green" status. However, the weather remains "no go" due to cumulus clouds.

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. 2 aboard.

Clocks have one more built-in hold planned at T-minus 4 minutes. That pause will last 20 minutes during which time the final "go" for launch will be given. All remains targeted for liftoff at 3:50 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.

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.

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The Complex 41 pad and the blast danger area have been cleared of all workers for the remainder of the countdown.

T-minus 2 hours and holding. The countdown has just entered the first of two planned holds over the course of the afternoon that will lead to the 3:50 p.m. EST launch of the Atlas rocket. This initial pause lasts 30 minutes, giving 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 and will last for 20 minutes.

The current hope is the cloud and electrical concerns should clear by about 3:15 p.m. EST, forecasters are projecting. But as the showers move out, the winds will increase and become a possible concern. Overall, there's still only a 30 percent chance of acceptable weather for launch today.

The guidance system testing and flight control operational test have been accomplished.

Internal battery checks have been performed and the Atlas propulsion and hydraulic preps are now reported complete. Also, a test of the C-band system used to track the rocket as it flies downrange has been conducted.

Atlas engineers have done the due diligence and reported to management that there is no concern for their rocket stemming from this morning's Taurus launch failure in California. The two rockets aren't similar and don't share commonality.

It's become a wet afternoon at the launch pad. Two weather rules are "red" currently because of cumulus clouds and the electric potential in the area as these showers move through. But forecasters hope the conditions will improve later in the countdown.

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 twice. The first was the original X-37B launch last April and then a California flight in September deployed a classified satellite for the National Reconnaissance Office.

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 Russian-designed 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 tried-and-true RL10 engine for 22,300 pounds of thrust 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.

The hazard area roadblocks around the launch site's safety perimeter are being established now. And the launch team has started configuring the pad's water deluge system.

The weather team is keeping close tabs on showers, some packing intense rainfall, that are moving ashore from the Atlantic. Skies are overcast and winds are brisk.

No technical issues are being reported in the countdown and technicians have just completed final preps on the Centaur upper stage liquid oxygen and liquid hydrogen systems. The launch team will soon begin a test of the S-band telemetry relay system.

Now entering the final five hours of the United Launch Alliance Atlas 5 rocket's countdown for liftoff carrying the Orbital Test Vehicle No. 2. The day's early activities are going well and clocks remain aimed toward a 3:50 p.m. EST launch.

The latest launch weather forecast still gives only a 30 percent chance of acceptable conditions this afternoon due to gusty winds and violation of the cumulus clouds.

The outlook calls for scattered clouds, isolated showers in the area, easterly winds of 20 gusting to 26 knots and a temperature of 70 degrees F.

The countdown clocks have started ticking for today's launch of the X-37B spaceplane aboard an Atlas 5 rocket from Cape Canaveral, Florida.

The launch team has powered up the rocket to commence standard pre-flight tests. Over the next 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. The Complex 41 site will be cleared of all personnel three hours before launch.

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 two hours before launch, followed by the first stage filling with LOX and then liquid hydrogen fuel loading for Centaur.

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 for liftoff.

Officials have formally announced today's target launch time will be 3:50 p.m. EST (2050 GMT). The duration of the launch window wasn't disclosed.

An Atlas 5 rocket crowned with a covert U.S. Air Force spaceplane rolled to a Florida launch pad Thursday to begin final preps for blastoff with a secret cache of military experiments.

When the 20-story launcher blasts off, it will be hauling a stubby-winged spaceship called the Orbital Test Vehicle several hundred miles above Earth. Also known as the X-37B, the spaceplane is carrying a number of classified experiments inside its cargo bay, which is about the size of the bed of a pickup truck.

Resembling a mini-space shuttle, the OTV's purpose is shrouded in secrecy, but the vehicle itself is not classified. It features a powerful main engine to change its orbit, uses a solar array for power production and is covered in ceramic heat-resistant tiles to protect the craft during re-entry.

Read our launch preview.

And see a beautiful gallery of photos covering the Atlas 5 rocket's rollout to the launch pad.

Check out this photo gallery showing the X-37B spacecraft being readied for launch.

An Atlas 5 rocket was rolled to its Cape Canaveral pad this afternoon for Friday's launch of the U.S. military's second experimental X-37B spaceplane, the prototype for an unmanned reusable space shuttle.

Riding atop its mobile launching platform, the United Launch Alliance-built rocket left the Vertical Integration Facility for the 1,800-foot trip along rail tracks to Complex 41.

The 20-story-tall vehicle, with the Orbital Test Vehicle-Flight 2 payload enclosed within the bulbous nose cone, reached the pad to begin final pre-flight activities.

Within the next hour, the auto couplers between the pad and platform will be engaged to route umbilical connections from the ground to the rocket for tomorrow's fueling of the booster with cryogenic propellants.

Ground crews will spend a few hours conducting various test sequences and removing the undercarriages used to transport the launch platform before securing the rocket for a quiet evening.

The countdown begins Friday morning, leading to blastoff sometime after 3:39 p.m. EST. The day's available launch opportunity hasn't been publicly announced.

So join us tomorrow on this page for live countdown and launch coverage!

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The rocket is passing through the pad entrance gate.

Rollout begins! The United Launch Alliance Atlas 5 rocket that will boost the U.S. military's experimental X-37B spaceplane into orbit just began a slow half-hour drive from the 30-story Vertical Integration Facility to Cape Canaveral's Complex 41 pad for Friday's liftoff.

A pair of specially-made "trackmobiles" are carrying the Atlas 5 rocket's 1.4-million pound mobile launching platform along rail tracks for this third-of-a-mile trip.

Technicians assembled and tested the rocket inside the Vertical Integration Facility. The Atlas 5 is designed to spend minimal time at the actual launch pad, which does not include a service gantry like other sites.

At Cape Canaveral's Complex 41, the giant sliding doors on the Vertical Integration Facility are open and the Atlas 5 rocket appears ready to emerge for today's rollout to the launch pad.

The towering booster plans to blast off sometime Friday afternoon, if the weather allows. Forecasters continue to give only a 30 percent chance that conditions will be acceptable for launch, or a 70 percent chance of a weather scrub. Strong winds and cumulus clouds are the two areas for concern.

The outlook for the subsequent days:

• Saturday: 40 percent favorable with continued wind and cumulus cloud concerns
  
  
• Sunday: 30 percent favorable due to concerns for cumulus clouds, disturbed weather and thick clouds
  
  
• Monday: 80 percent favorable with only a slight concern for thick clouds

Officials ended up not revealing the launch time on Wednesday afternoon as originally planned. The time is being kept secret until Friday morning.

The odds of "no go" weather for Friday's X-37B spaceplane launch atop an Atlas 5 rocket from Cape Canaveral have increased to 70 percent because of cloud and high wind worries.

The latest outlook issued this morning called for scattered cumulus clouds at 3,000 feet, scattered high cirrus at 25,000 feet, isolated showers in the area, 7 miles of visibility, easterly winds of 22 gusting to 28 knots and a temperature of 70-71 degrees F.

The forecast for Saturday's backup launch opportunity is marginally better.

Officials are expected to announce the exact launch time later today. For now, they say liftoff will happen sometime after 3:39 p.m. EST.

Air Force weather forecasters say gusty ground winds and cumulus clouds overhead could hold up Friday's scheduled launch of the Atlas 5 rocket carrying the military's second X-37B spaceplane.

The odds say there's a 60 percent chance of violating the weather rules during the launch period that opens at 3:39 p.m. EST.

"High pressure builds into the peninsula on Wednesday with the front stalling in South Florida and the Florida Straits. Low level moisture coupled with strengthening Easterly winds introduces a morning isolated coastal shower threat each day through launch day," meteorologists said today.

For the rocket's rollout to the pad on Thursday, "winds are expected from the east-northeast gusting in the mid 20s," meteorologists say.

"On launch day, the morning coastal shower threat and tight pressure gradient persist with easterly winds gusting in the mid 20s. The primary concerns for launch day are gusty ground winds and cumulus clouds.

"In the event of a 24-hour delay, high pressure retreats to the east with a persistent tight pressure gradient and breezy southeasterly winds in advance of the next cold front on Sunday. The primary concerns for a 24-hour delay are gusty ground winds and cumulus clouds."

The second Orbital Test Vehicle, the U.S. military's secretive mini space shuttle, arrived at the Atlas 5 rocket's assembly hangar this morning for mounting atop the launcher.

Liftoff is scheduled for March 4 from Cape Canaveral's Complex 41.

Already shrouded within in the aerodynamic nose cone for launch, the OTV 2 spaceplane was hauled across the Cape overnight from its processing area to the Atlas' Vertical Integration Facility.

A heavy-duty crane hoisted the payload into the towering VIF around 8:30 a.m. and cautiously positioned above the two-stage launcher for mating later in the day.

The next couple of weeks will see a full array of combined systems testing and final readiness reviews conducted before giving a "go" to fly.

The United Launch Alliance rocket will be rolled to the nearby launch pad aboard a mobile platform the day before liftoff. Comprised of the kerosene-fueled first stage, the single-engine cryogenic Centaur upper stage, the large Swiss-made nose cone and no strap-on solid motors, this Atlas is known as the 501 configuration.

The Atlas was wheeled to the pad for a countdown dress rehearsal earlier this month, rolling out February 3, undergoing a simulated launch day and fueling exercise on February 4 and then returning to the VIF on February 5.

The mission follows the successful first OTV test flight last year that began with an April 22 launch from Florida, more than seven months spent in orbit and then concluded with a pinpoint landing at Vandenberg Air Force Base in California on December 3.

Air Force officials hailed the first OTV mission as a complete success in demonstrating the unmanned craft's ability to operate in orbit, carrying out in-space operations and then autonomously returning to Earth for a runway landing.

The upcoming flight of the second spaceplane will continue developing the X-37B program's capabilities. However, exactly what is being carried within the craft's payload bay is classified.

This will be the Atlas rocket's first of five planned launches this year. Further missions include a National Reconnaissance Office payload on April 12, a new-generation missile warning satellite in early May, NASA's Juno probe bound for Jupiter in August and the Mars Science Laboratory rover in November.

Two months after a sister ship successfully concluded a demo flight, the U.S. military's second robotic X-37B space plane is in Florida preparing for blastoff March 4 on another secret mission, according to an Air Force official.

The 29-foot-long, 14-foot-wide space plane is being readied for flight in a processing facility near its launch site at Cape Canaveral, Fla., according to Lt. Col. Troy Giese, the X-37B program manager in the Air Force Rapid Capabilities Office.

Like its predecessor last year, the X-37B will lift off inside the nose cone of an Atlas 5 rocket. The flight is scheduled for March 4, but the Air Force has not released a time for the blastoff.

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