BY JUSTIN RAY

Follow the countdown and launch of the Atlas 5 rocket carrying the Air Force's second Wideband Global SATCOM communications spacecraft. Reload this page for the latest on the launch.

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SATURDAY, APRIL 4, 2009

It was a perfect Friday night flight for the Atlas 5 rocket, completing a military mission that put an advanced communications spacecraft into the sky to serve U.S. forces deployed in Iraq and Afghanistan.

Read our launch story.

A full gallery of launch photos is available here.

0300 GMT (11:00 p.m. EDT Fri.)

Tonight's launch marked the 400th RL10 rocket engine to power a payload into space, Pratt & Whitney Rocketdyne says.

"Pratt & Whitney Rocketdyne is proud to be a key part of the Atlas 5 team that is supporting our armed forces with this launch," said Jim Maus, director, expendable propulsion systems, Pratt & Whitney Rocketdyne. "We are also pleased to reach a milestone in the RL10 program that is testament to the reliability and longevity of the RL10 engine and to the dedication of our employees toward mission success."

This is the 46th year of flight by the RL10 engine, the world's first liquid-hydrogen fueled rocket engine. The RL10 has placed more than 163 military, government and commercial satellites into orbit and powered historic space missions to nearly every planet in our solar system, Pratt & Whitney Rocketdyne said.

The engines have accumulated a record 734 in-space firings and over 2.2 million seconds of ground and flight operating time.

0245 GMT (10:45 p.m. EDT Fri.)

The post-launch plan for the WGS 2 spacecraft involves about two weeks of orbit raising maneuvers using its bi-propellant system, followed by approximately 38 days of xenon ion propulsion orbit raising to get into the desired circular location and right test slot, said Mark Spiwak, WGS program director at Boeing.

Deployments of the spacecraft's appendages and testing of the communications payload will follow over the subsequent month. Boeing hopes to hand the satellite to the Air Force for the military's own checkout program in June.

0140 GMT (9:40 p.m. EDT Fri.)

A beautiful photo of tonight's launch can be seen here.

0120 GMT (9:20 p.m. EDT Fri.)

"ULA congratulates the Air Force and our mission partners on the successful launch of WGS 2," said James Bell, United Launch Alliance's WGS mission manager for Atlas and Delta. "ULA is proud of its continuing role of providing reliable assured access to space for the Air Force's critical missions. WGS is a force multiplier for our troops in the field who defend America's freedom everyday."

0104 GMT (9:04 p.m. EDT Fri.)

T+plus 33 minutes. The Atlas 5 rocket has delivered the WGS 2 spacecraft into a good orbit.

0102 GMT (9:02 p.m. EDT Fri.)

T+plus 31 minutes, 48 seconds. SPACECRAFT SEPARATION! The Centaur upper stage has deployed the second Wideband Global SATCOM communications spacecraft for the U.S. Air Force, completing tonight's launch.

The sophisticated satellite promises to provide a major increase in communications capacity for the Defense Department. It is the second of six Boeing-built spacecraft that will upgrade the military's orbiting communications satellite infrastructure.

0102 GMT (9:02 p.m. EDT Fri.)

T+plus 31 minutes, 8 seconds. Centaur is in the spacecraft deploy position.

0100 GMT (9:00 p.m. EDT Fri.)

T+plus 29 minutes, 45 seconds. Centaur is maneuvering itself to the proper orientation for releasing WGS.

0059 GMT (8:59 p.m. EDT Fri.)

T+plus 28 minutes, 57 seconds. MECO 2. Main engine cutoff confirmed. Centaur has completed its second burn of the night. Release of the payload from the rocket is expected in about three minutes.

0059 GMT (8:59 p.m. EDT Fri.)

T+plus 28 minutes. Engine operating parameters remain normal.

0058 GMT (8:58 p.m. EDT Fri.)

T+plus 27 minutes, 30 seconds. It's a smooth burn. The vehicle is stable and body rates are normal.

0058 GMT (8:58 p.m. EDT Fri.)

T+plus 27 minutes. About two minutes left in this burn.

0056 GMT (8:56 p.m. EDT Fri.)

T+plus 25 minutes, 30 seconds. Engine continues to burn well for the second time tonight.

0055 GMT (8:55 p.m. EDT Fri.)

T+plus 24 minutes, 23 seconds. Ignition and full thrust! The Centaur's single Pratt & Whitney Rocketdyne RL10 engine has re-ignited to accelerate the WGS payload into a supersynchronous transfer orbit.

0054 GMT (8:54 p.m. EDT Fri.)

T+plus 23 minutes, 30 seconds. About one minute from engine start. Tank pressurization has started in preparation for ignition.

0053 GMT (8:53 p.m. EDT Fri.)

T+plus 22 minutes, 30 seconds. Centaur is operating correctly in the coast. Battery voltages and tank pressures are as expected.

0050 GMT (8:50 p.m. EDT Fri.)

T+plus 19 minutes. Centaur systems are reported normal during this coast period.

0048 GMT (8:48 p.m. EDT Fri.)

T+plus 17 minutes, 30 seconds. A map of the rocket's planned ground track for today's launch is available here.

0047 GMT (8:47 p.m. EDT Fri.)

T+plus 16 minutes, 10 seconds. Centaur power bus and battery voltages are normal, tank pressures are stable.

0046 GMT (8:46 p.m. EDT Fri.)

T+plus 15 minutes, 30 seconds. Cutoff occurred a few moments early given the extra performance from the first stage.

0046 GMT (8:46 p.m. EDT Fri.)

T+plus 15 minutes, 14 seconds. MECO 1. Centaur's main engine has shut down following its first burn tonight, achieving a preliminary orbit around Earth. The rocket will coast in this orbit for about nine minutes before the RL10 engine re-ignites.

0045 GMT (8:45 p.m. EDT Fri.)

T+plus 14 minutes. Systems on the Centaur still look normal.

0043 GMT (8:43 p.m. EDT Fri.)

T+plus 12 minutes, 15 seconds. The RL10 engine continues to fire normally, burning liquid hydrogen and liquid oxygen propellants.

0042 GMT (8:42 p.m. EDT Fri.)

T+plus 11 minutes. Centaur is 138 miles in altitude and 1,560 miles downrange, traveling at 15,600 mph.

0041 GMT (8:41 p.m. EDT Fri.)

T+plus 10 minutes, 45 seconds. The vehicle is flying right on course.

0041 GMT (8:41 p.m. EDT Fri.)

T+plus 10 minutes. A little more than five minutes remain in this burn of Centaur to reach a parking orbit around Earth.

0039 GMT (8:39 p.m. EDT Fri.)

T+plus 8 minutes, 45 seconds. Everything still looks good as the Centaur continues to fire.

0038 GMT (8:38 p.m. EDT Fri.)

T+plus 7 minutes, 40 seconds. The rocket is 135 miles in altitude and 849 miles downrange from the launch pad, traveling at 13,600 mph.

0038 GMT (8:38 p.m. EDT Fri.)

T+plus 7 minutes. Centaur is conducting a planned roll maneuver to align with NASA's Tracking and Data Relay Satellite System for communications.

0037 GMT (8:37 p.m. EDT Fri.)

T+plus 6 minutes, 30 seconds. Good performance, even a little better than expected, reported from the first stage.

0036 GMT (8:36 p.m. EDT Fri.)

T+plus 5 minutes, 40 seconds. Centaur engine readings look good as this 11-minute burn continues.

0035 GMT (8:35 p.m. EDT Fri.)

T+plus 4 minutes, 45 seconds. The two-halves of the Atlas 5 rocket nose cone encapsulating the WGS 2 spacecraft have separated.

0035 GMT (8:35 p.m. EDT Fri.)

T+plus 4 minutes, 35 seconds. Centaur has ignited. The RL10 engine is up and running at full thrust.

0035 GMT (8:35 p.m. EDT Fri.)

T+plus 4 minutes, 26 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.

0035 GMT (8:35 p.m. EDT Fri.)

T+plus 4 minutes, 20 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.

0034 GMT (8:34 p.m. EDT Fri.)

T+plus 3 minutes, 40 seconds. Atlas now weighs just 25 percent of what it did at liftoff.

0034 GMT (8:34 p.m. EDT Fri.)

T+plus 3 minutes. The RD-180 main engine continues to fire normally at its 95 percent throttle setting, burning a mixture of highly refined kerosene and liquid oxygen.

0033 GMT (8:33 p.m. EDT Fri.)

T+plus 2 minutes, 43 seconds. Atlas is 33 miles in altitude and 67 miles downrange, traveling at 4,900 mph.

0033 GMT (8:33 p.m. EDT Fri.)

T+plus 2 minutes, 22 seconds. The Aerojet-made solid rocket boosters have separated from the Atlas 5, having completed their job of adding a powerful kick at liftoff.

0033 GMT (8:33 p.m. EDT Fri.)

T+plus 2 minutes. Atlas now weighs half of what it did at liftoff.

0032 GMT (8:32 p.m. EDT Fri.)

T+plus 100 seconds. RD-180 is revving back to full throttle.

0032 GMT (8:32 p.m. EDT Fri.)

T+plus 95 seconds. Solid rocket booster burnout has occurred. But the spent motors will remain attached to the first stage for about 50 seconds, until the Atlas 5 reaches a point where the airborne dynamic pressure reduces to an allowable level for a safe jettison.

0032 GMT (8:32 p.m. EDT Fri.)

T+plus 88 seconds. Atlas is 12 miles in altitude, 11 miles downrange from the launch pad, traveling at 2,200 mph already.

0032 GMT (8:32 p.m. EDT Fri.)

T+plus 60 seconds. One minute into the climb uphill for the Atlas 5 rocket carrying the WGS 2 satellite. The vehicle has passed through the region of maximum aerodynamic pressure and broken the sound barrier.

0031 GMT (8:31 p.m. EDT Fri.)

T+plus 40 seconds. The main engine is easing back to two-thirds throttle as planned.

0031 GMT (8:31 p.m. EDT Fri.)

T+plus 30 seconds. The Atlas 5 rocket is making its thunderous departure from Cape Canaveral on the combined power of the RD-180 main engine and twin solid rocket boosters.

0031 GMT (8:31 p.m. EDT Fri.)

LIFTOFF! Liftoff of the Atlas 5 rocket on a mission to support our troops in Iraq and Afghanistan. And the vehicle has cleared the tower!

0030 GMT (8:30 p.m. EDT Fri.)

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

0030 GMT (8:30 p.m. EDT Fri.)

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

0030 GMT (8:30 p.m. EDT Fri.)

T-minus 1 minute. Now 60 seconds from liftoff of the second Wideband Global SATCOM spacecraft. The communications satellite will provide a major increase in capacity when it enters service this September for use by U.S. Central Command in Afghanistan, Iraq and other parts of Southwest Asia.

0029 GMT (8:29 p.m. EDT Fri.)

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

0029 GMT (8:29 p.m. EDT Fri.)

T-minus 1 minute, 50 seconds. Liquid oxygen and liquid hydrogen propellant topping to the Centaur upper stage is being secured.

0029 GMT (8:29 p.m. EDT Fri.)

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

0029 GMT (8:29 p.m. EDT Fri.)

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

0028 GMT (8:28 p.m. EDT Fri.)

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.

0028 GMT (8:28 p.m. EDT Fri.)

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

0027 GMT (8:27 p.m. EDT Fri.)

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

0027 GMT (8:27 p.m. EDT Fri.)

T-minus 4 minutes and counting. Clocks have resumed for the final minutes of this evening's countdown to launch the Atlas 5 rocket carrying the WGS 2 communications spacecraft. Liftoff is set to occur at 8:31 p.m.

0026 GMT (8:26 p.m. EDT Fri.)

Countdown clocks will resume in one minute. Still targeting a launch 31 minutes past the hour.

0025 GMT (8:25 p.m. EDT Fri.)

All systems are "go" to continue with the countdown for an on-time liftoff at 8:31 p.m.

0024 GMT (8:24 p.m. EDT Fri.)

Polling of the team by Atlas launch conductor Doug Lebo is underway.

0023 GMT (8:23 p.m. EDT Fri.)

The WGS 2 spacecraft is switching to internal battery power for launch.

0019 GMT (8:19 p.m. EDT Fri.)

Coming up in five minutes, the launch team will be polled for a "go" or "no go" to proceed with the count.

0017 GMT (8:17 p.m. EDT Fri.)

T-minus 4 minutes and holding. The countdown has entered the planned 10-minute hold to give the launch team a chance to review all systems before pressing ahead with liftoff.

0016 GMT (8:16 p.m. EDT Fri.)

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

0015 GMT (8:15 p.m. EDT Fri.)

The fuel-fill sequence for the first stage main engine is complete.

0014 GMT (8:14 p.m. EDT Fri.)

The first stage liquid oxygen tank is reported at flight level.

0010 GMT (8:10 p.m. EDT Fri.)

The RP-1 tank pressure decay has been deemed no problem for flight. A test was not able to determine if the very small leak is in the ground equipment or the vehicle. However, the leak is so tiny that it poses no concern and there's plenty of margin for launch, the anomaly team has concluded.

0006 GMT (8:06 p.m. EDT Fri.)

Flight control system final preps are complete. Now 25 minutes from launch time.

0005 GMT (8:05 p.m. EDT Fri.)

The first stage main engine fuel-fill sequence is beginning.

0001 GMT (8:01 p.m. EDT Fri.)

Thirty minutes to go. Today's launch will be the 15th for an Atlas 5 rocket and the second mission carrying a Wideband Global SATCOM (WGS) spacecraft.

"This mission represents another significant milestone for the Air Force's Atlas 5 program. This will be the first re-flight aboard the Atlas 5, essentially a repeat of the highly successful launch of the first WGS spacecraft on the 10th of October in 2007. As we speak, that spacecraft is on-orbit providing vital communications capabilities to our deployed forces," Col. Michael Moran, commander of the Atlas Group at the Air Force's Space and Missile Systems Center.

The Air Force has multi-satellite programs that fly repetitive rocket flights. WGS is the first such project on the relatively new Atlas 5 vehicle.

These WGS flights use the rocket's 421 configuration. The Common Core Booster first stage is outfitted with the RD-180 main engine, two solid rocket boosters are attached for added thrust at liftoff, the Centaur upper stage has a single RL10 cryogenic engine and the payload shroud is the four-meter diameter option.

FRIDAY, APRIL 3, 2009
2357 GMT (7:57 p.m. EDT)

Pre-launch check of the rocket's safety system has been completed.

2354 GMT (7:54 p.m. EDT)

The Centaur liquid hydrogen tank has reached flight level. The liquid oxygen tank achieved its flight level earlier.

2351 GMT (7:51 p.m. EDT)

Forty minutes from liftoff. The countdown clocks are heading to the T-minus 4 minute mark where a planned 10-minute hold will occur. Launch of Atlas 5 on the WGS satellite deployment mission remains scheduled for 8:31 p.m. EDT.

2349 GMT (7:49 p.m. EDT)

The anomaly team has been discussing a higher than expected tank pressure decay on the RP-1 system. A test is being performed to isolate where the pressure loss is occurring -- either the ground-side or the rocket-side.

2345 GMT (7:45 p.m. EDT)

The launch team is going through normal post-fueling valve and system checks.

2339 GMT (7:39 p.m. EDT)

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

2337 GMT (7:37 p.m. EDT)

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

2335 GMT (7:35 p.m. EDT)

Weather continues to be acceptable for launch this evening. There's now a 90 percent chance of favorable conditions.

At launch time, the latest outlook calls for scattered clouds at 5,000 and 10,000 feet, 7 miles of visibility, westerly winds 16-22 knots from 260 degrees and a temperature of 73-74 degrees F.

2334 GMT (7:34 p.m. EDT)

The Centaur liquid hydrogen tank is 70 percent loaded so far. The cryogenic propellant will be consumed with liquid oxygen by the stage's Pratt & Whitney-made RL10 engine.

2333 GMT (7:33 p.m. EDT)

The first stage liquid oxygen tank is 90 percent full now.

2331 GMT (7:31 p.m. EDT)

Launch is now just 60 minutes away. Here's a look at some stats about tonight's mission. This will be:

• The 15th launch of Atlas 5 since 2002
• The 14th to occur from Cape Canaveral
• The seventh under the United Launch Alliance banner
• The U.S. Air Force's sixth Atlas 5 mission
• The third rocket to fly in the 421 configuration
• The second Wideband Global SATCOM on Atlas

2326 GMT (7:26 p.m. EDT)

The first stage liquid oxygen tank is about three-quarters full.

2325 GMT (7:25 p.m. EDT)

The Centaur liquid oyxgen tank has reached flight level. The stage's liquid hydrogen tank is 10 percent full.

2319 GMT (7:19 p.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.

2311 GMT (7:11 p.m. EDT)

The Centaur engine chilldown sequence is being initiated.

2310 GMT (7:10 p.m. EDT)

First stage liquid oxygen tank is 40 percent full. 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-quarter minutes of flight today. The 25,000 gallons of RP-1 were loaded into the rocket during the first countdown a couple of weeks ago.

2305 GMT (7:05 p.m. EDT)

Liftoff of the Atlas 5 rocket remains on track for 8:31 p.m.

If you are heading out to Titusville or the beach 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.)

2303 GMT (7:03 p.m. EDT)

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

2301 GMT (7:01 p.m. EDT)

Now 90 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen is proceeding as planned and activities remain on schedule for a liftoff at 8:31 p.m. EDT.

2258 GMT (6:58 p.m. EDT)

The Centaur liquid oxygen tank is 80 percent full.

2257 GMT (6:57 p.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 13,000 gallons of the cryogenic propellant.

2256 GMT (6:56 p.m. EDT)

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

2251 GMT (6:51 p.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 13,000 gallons of the cryogenic propellant.

2251 GMT (6:51 p.m. EDT)

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

2247 GMT (6:47 p.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 about 50,000 gallons of cryogenic oxidizer for the RD-180 main engine.

2243 GMT (6:43 p.m. EDT)

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

2241 GMT (6:41 p.m. EDT)

The anomaly resolution team has concluded the first stage valve issue is not a problem. There is no constraint against continuing with the countdown.

2235 GMT (6:35 p.m. EDT)

Filling of the Centaur upper stage with about 4,300 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 perform a pair of firings tonight to deliver the WGS 2 satellite into the desired orbit.

2233 GMT (6:33 p.m. EDT)

The vent valve issue assessed a little while ago in the countdown has resurfaced. The anomaly resolution team is discussing.

2228 GMT (6:28 p.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.

2221 GMT (6:21 p.m. EDT)

T-minus 120 minutes and counting! The launch countdown is continuing for this evening's flight of the Atlas 5 rocket to deploy the Wideband Global SATCOM communications satellite for the U.S. Air Force.

Clocks have one more built-in hold planned at T-minus 4 minutes. That pause will last 10 minutes during which time the final "go" for launch will be given. All remains targeted for liftoff at 8:31 p.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.

2218 GMT (6:18 p.m. EDT)

All console operators in the Atlas Spaceflight Operations Center reported a "ready" status during the pre-fueling poll just completed by the launch conductor. The ULA launch director and Air Force mission director gave their approvals as well. Loading of cryogenic liquid oxygen and liquid hydrogen into the Atlas 5 rocket will be getting underway a short time from now.

2216 GMT (6:16 p.m. EDT)

Atlas launch conductor Doug Lebo is briefing his team on procedures before entering into the final two hours of the countdown. A readiness check of the team members is next.

2212 GMT (6:12 p.m. EDT)

Engineers have resolved a discussion involving an airborne vent valve on the rocket. The launch team has run a couple of tests and determined there is no issue in continuing with the countdown.

2200 GMT (6:00 p.m. EDT)

As this planned hold in the countdown continues, you're invited to check out our new High Definition video collection covering recent space shuttle missions and rocket launches. Newly added footage includes this week's sunrise rollout of Atlantis to pad 39A for the Hubble Space Telescope servicing flight. Shuttle Discovery's just-completed space station construction mission can be seen here.

The video is available to Spaceflight Now+Plus users.

2151 GMT (5:51 p.m. EDT)

T-minus 2 hours and holding. The countdown has just entered the first of two planned holds over the course of the evening that will lead to the 8:31 p.m. EDT launch of the Atlas AV-016 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 last for 10 minutes.

2140 GMT (5:40 p.m. EDT)

The hold-fire checks have been performed to ensure safety officers have the capability of halting the countdown if a problem occurs.

2136 GMT (5:36 p.m. EDT)

Workers are clearing the Complex 41 pad in advance of this evening's propellant loading and launch of the Atlas 5 rocket.

2135 GMT (5:35 p.m. EDT)

The launch weather team says the cloud coverage over Cape Canaveral will continue to scattered out as the frontal system slides further southward this evening. There are no showers left around the launch site and peak winds are 23 knots, which remain well below the 33-knot limit.

And with that outlook, the odds of acceptable launch weather have risen to 80 percent.

None of the weather rules are being violated right now and meteorologists expect good conditions for the rest of the evening.

2115 GMT (5:15 p.m. EDT)

At this point in today's countdown, the Atlas propulsion and hydraulic preps have been completed, along with a test of the rocket's guidance system. Ground crews report they have finished removing access platforms and handrails on the mobile launch platform and around the pad area.

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2056 GMT (4:56 p.m. EDT)

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.

Recently acomplished were the internal battery checks and testing of the C-band system used to track the rocket as it flies downrange.

2031 GMT (4:31 p.m. EDT)

Launch is just four hours away, but work to ready this Atlas 5 rocket to the carry the WGS 2 spacecraft has been underway since last summer.

The rocket was assembled inside the Vertical Integration Facility (VIF) starting with erection of the bronze first stage onto the mobile launch platform on June 27.

The first stage, known as a Common Core Booster, is 106 feet long and 12.5 feet in diameter. It is equipped with the kerosene-fueled RD-180 main engine.

The twin strap-on solid rocket boosters were attached on July 1 and 2. The 67-foot long, five-foot diameter composite graphite epoxy boosters are considered the world's longest single-segment SRBs. The motors provide the additional thrust needed to increase the Atlas 5's payload-carrying capacity.

Assembly continued with mating of the interstage adapter on July 7, the Centaur upper stage on July 9 and the boattail structure on July 10.

The hydrogen-fueled Centaur is equipped with a cryogenic RL10 engine, which will fire twice during launch to propel the payload into the desired orbit. The stage is about 40 feet long and 10 feet in diameter.

Technical issues delayed the launch into 2009, and the rocket remained stacked inside the VIF hangar at Complex 41.

The WGS spacecraft arrived at Cape Canaveral from its Boeing manufacturing factory in January. It underwent final pre-flight preparations at the Astrotech facility near Titusville, including electrical and mechanical checkouts, system functional testing and then loading of maneuvering propellant.

After being encapsulated in the Atlas nose cone shroud, the satellite was transported to the VIF for hoisting atop the rocket on March 4. The combined operations between the rocket and payload were accomplished over the following week.

An initial countdown attempt March 17 was scrubbed during fueling when a leak was detected on the Centaur liquid oxygen inlet valve. The rocket was rolled back to the VIF the following day for the subsequent replacement of that valve. More on the leak can be read here.

The Air Force held a readiness review this week and gave approval to proceed with today's launch.

1931 GMT (3:31 p.m. EDT)

Tonight's half-hour flight of the Atlas 5 rocket to deploy the Wideband Global SATCOM 2 communications spacecraft into supersynchronous transfer orbit begins with an 8:31 p.m. EDT blastoff from Cape Canaveral's Complex 41 pad.

The RD-180 engine ignites at T-minus 2.7 seconds, shooting a giant cloud of steam from the pad's main exhaust duct while undergoing a check to ensure its vital signs are healthy. The twin strap-on solid rocket boosters are lit at T+plus 0.8 seconds, leading to liftoff of the 19-story Atlas vehicle at T+plus 1.1 seconds.

The Aerojet-made solid boosters will burn for about 90 seconds to assist the RD-180 in propelling the rocket skyward. The SRB casings remain attached to the first stage for another 50 seconds until the rocket reaches a lower dynamic pressure region of flight.

After the spent boosters are jettisoned, the kerosene-fueled first stage will continue to fire until T+plus 4 minutes, 15 seconds. The bronze stage separates about six seconds later, leaving the hydrogen-fueled Centaur upper stage to ignite for an 11-minute burn that will inject itself and WGS 2 into a preliminary orbit.

Centaur completes its first burn over the Central Atlantic Ocean and enters a brief nine-minute coast. The Pratt & Whitney RL10 cryogenic engine then restarts for a four-and-a-half-minute firing to propel the WGS spacecraft into a highly elliptical orbit stretching from 220 nautical miles at its lowest point to no greater than 40,000 miles at its highest point and inclined 20.9 degrees to the equator.

The mission uses the "minimum residual shutdown" option in which the Centaur fires until either its fuel supply is exhausted or the altitude cap is reached, achieving an optimum transfer orbit.

Release of the WGS 2 satellite from the rocket to complete the launch is expected at T+plus 31 minutes and 39 seconds.

Ground controllers will maneuver the Boeing-built satellite into a circular geostationary orbit to begin preparations for entering service later this year. The craft's operational location will be around 60 degrees East longitude over the equator, according to Col. William Harding, vice commander of the Military Satellite Communications Systems Wing at the Space and Missile Systems Center in Los Angeles.

1901 GMT (3:01 p.m. EDT)

Countdown activities are proceeding as planned at Complex 41. Once the clocks started ticking at 1:31 p.m., power was applied to the Atlas first stage and Centaur upper stage. Final preps for the Centaur's liquid oxygen and liquid hydrogen systems then began, along with a test of the rocket's guidance system. Other functional checks and routine pre-flight testing are getting underway as the launch team targets an 8:31 p.m. EDT liftoff.

1831 GMT (2:31 p.m. EDT)

It's been a gray and rainy day at the Cape so far today. But the weather system producing those gloomy conditions is slowly moving out of the area. In fact, Air Force meteorologists have increased the odds of acceptable conditions for tonight's launch to 70 percent.

"A cold front is pushing through the peninsula with pre-frontal showers and thunderstorms over Central Florida. The pre-frontal thunderstorms are expected to clear the area by early afternoon with isolated showers persisting through mid-afternoon. A gradual clearing trend is expected by late afternoon into the evening," the weather team reported today.

"Ground winds are expected to remain gusting in the upper 20s with isolated gusts in the low 30s (230 ft) through the afternoon and trending down to the low to mid 20s during the window."

At launch time tonight, the outlook calls for scattered low- and high-level clouds, isolated showers in the area, 7 miles of visibility, westerly winds 18-24 knots from 260 degrees and a temperature of 72-73 degrees F.

The specific concerns causing the 30 percent chance of violating the launch weather rules will be cumulus clouds associated with lingering showers and the gusty ground winds.

If the launch slips to Saturday night for some reason, the weather looks even better. "In the event of a 24-hour delay, the frontal boundary is expected to be in South Florida with fair weather conditions behind the front. The primary concerns for a 24-hour delay are cumulus clouds," forecasters reported.

1731 GMT (1:31 p.m. EDT)

The countdown officially begins now for tonight's launch of the Atlas 5 rocket carrying the Wideband Global SATCOM spacecraft from Cape Canaveral, Florida.

"We look forward to delivering enhanced communication capability to the warfighter through this launch," said Lt. Col. Dave Hook, the Air Force launch director.

Liftoff is scheduled for 8:31 p.m. EDT from Complex 41. A 62-minute window extends to 9:33 p.m. EDT, giving mission managers time to deal with any technical troubles or weather problems that could hold up the launch.

This will be the 15th flight of the Atlas 5 vehicle, an evolved rocket capable of hoisting a wide range of payloads into space.

"The Atlas 5 is a derivative of previous Atlas launch vehicles and has a long and proud history of success," Hook said.

Tonight's mission will see the Atlas 5 fly in what's known as the 421 vehicle configuration. It's distinguished by a four-meter payload shroud, two solid rocket boosters and a single-engine Centaur upper stage.

"This launch vehicle, which employs two solid rocket boosters, will have a maximum thrust of over 1.7 million pounds when it leaves the pad," Hook said.

Roaring into the nighttime sky, the solid-fuel boosters give an added kick for the first 90 seconds, while the first stage's kerosene-fed main engine will continue firing through the initial four minutes of ascent. The Centaur upper stage with its single cryogenic engine then takes over, completing a pair of burns to reach the desired orbit with the 12,790-pound satellite payload.

1450 GMT (10:50 a.m. EDT)

Expecting the second try to be the charm, the Atlas 5 team is gearing up for countdown No. 2 to launch the Wideband Global SATCOM spacecraft. The massive satellite's mission will provide expansive new communications capacity for U.S. military forces in Iraq and Afghanistan.

Tonight's liftoff from Cape Canaveral is scheduled for 8:31 p.m. EDT.

Countdown clocks will start ticking at 1:31 p.m. EDT as the launch team begins powering up the rocket to commence standard pre-flight tests. Crews at the pad will make preparations to systems and equipment before the Complex 41 site is cleared of all personnel at 6:30 p.m.

A planned half-hour hold begins at 5:51 p.m. when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 6:18 p.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 6:38 p.m., followed by the first stage filling around 6:51 p.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 8:17 p.m. That 10-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 extends from 8:31 to 9:33 p.m. EDT.

And a reminder that if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message 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.)

THURSDAY, APRIL 2, 2009

A collection of photos showing the Atlas 5 rocket rolling out to the launch pad Thursday morning is posted here.

1340 GMT (9:40 a.m. EDT)

On the pad! The Atlas 5 rocket with the U.S. military's Wideband Global SATCOM communications satellite nestled inside its nose cone has completed the journey from the 30-story Vertical Integration Facility to Cape Canaveral's Complex 41 pad for Friday night's 8:31 p.m. EDT launch.

The United Launch Alliance-built rocket will fly to supersynchronous transfer orbit for deployment of the WGS 2 spacecraft.

A half-dozen WGS satellites are being constructed by Boeing to provide a major increase in communications capacity for the Defense Department and replace the satellite infrastructure in orbit today.

"The launch of WGS 2 continues what WGS 1 began in October 2007, augmenting and eventually replacing the aging legacy Defense Satellite Communications System, or DSCS, constellation, which has been the Department of Defense's backbone for satellite communications over the past two decades," said Col. William Harding, vice commander of the Military Satellite Communications Systems Wing at the Space and Missile Systems Center in Los Angeles.

"The WGS system provides a quantum leap in communications bandwidth for Soldiers, Sailors, Airmen and Marines, and is the DOD's highest capacity communications satellite."

WGS 1 entered service last year to cover the vast Pacific Command that spans the U.S. western coast all the way to Southeast Asia.

Following Friday's launch, the new WGS 2 satellite will be maneuvered into a circular geostationary orbit 22,300 miles above the planet where it can match the Earth's rotation and appear parked over one area of the globe. It should be ready for use by U.S. Central Command in September.

"We are putting WGS 2 over the CENTCOM area of responsibilities," Harding said. "It will be providing coverage for the warfighters currently in Afghanistan, Iraq and other parts of Southwest Asia."

And a reminder that if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message 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.)

1310 GMT (9:10 a.m. EDT)

Rollout underway! Two weeks after a leaky liquid oxygen valve postponed the Atlas 5 rocket's flight carrying a U.S. military communications satellite, the 19-story launcher is heading back to the launch pad this morning.

A pair of specially-made "trackmobiles" are pushing the Atlas 5 rocket's 1.4-million pound mobile launching platform along rail tracks for this 1,800-foot trip.

The two mobile trailers connected to the launching platform, which are part of the convoy during rollout, will be hooked up to power and communications systems at the pad. These trailers provide conditioned air to the payload and communications with the rocket. They are protected from the blast of launch by a concrete structure on the north-side of the platform.

Later today, 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. The undercarriages used to move the mobile platform will be disconnected and the "trackmobiles" pulled free this afternoon.

The launch countdown commences at 1:31 p.m. EDT tomorrow, some seven hours before liftoff time.

1230 GMT (8:30 a.m. EDT)

The Atlas 5 rocket will return to the launch pad this morning. Crews are making final preparations for the rollout from the Vertical Integration Facility to the Complex 41 pad.

The odds of acceptable weather for Friday night's launch have improved to 60 percent, meteorologists reported this morning.

"On launch day, the front to the north is expected to transit the peninsula and be over Central Florida by early afternoon. Isolated showers and thunderstorms during the early portion of the count are expected with a decreased threat by late afternoon. Ground winds gusting in the upper 20s to low 30s (230 ft) overnight and during the count and low 20s during the window are expected. The primary concerns for launch day are cumulus clouds, isolated thunderstorms and ground winds," the latest forecast says.

The revised outlook for launch time includes scattered clouds at 3,000 and 25,000 feet, isolated showers and thunderstorms around, visibility of 7 miles, west-southwesterly winds from 250 degrees at 18 to 24 knots and a temperature of 72-73 degrees F.

The backup launch opportunity on Saturday also has a bit better weather forecast. There's now an 80 percent chance of acceptable conditions then.

WEDNESDAY, APRIL 1, 2009

The Atlas 5 rocket and its military satellite deployment mission is scheduled for liftoff at 8:31 p.m. EDT Friday from Cape Canaveral. Rollout of the 19-story rocket to the launch pad will occur on Thursday morning.

Air Force weather forecasters say there's a 40 percent chance that conditions will be within limits for the launch. Here's the latest update from the weather team:

"An upper level trough and associated low pressure system in the Central U.S. will cause a series of frontal boundaries to persist over North Florida through Friday morning. Moisture and instability in the atmosphere are sufficient to produce isolated thunderstorms each afternoon and evening.

"For MLP roll on Thursday, a near stationary front is expected to be in North Florida with isolated pre-frontal showers and thunderstorms over the peninsula. Winds are expected to be gusting in the mid to upper teens during the morning and low 20s by late morning; however, winds are expected to remain below the 25-knot roll constraint through mid-afternoon.

"On launch day, the front to the North is expected to be begin transiting the peninsula and be over Central Florida. Isolated showers, thunderstorms, and ground winds gusting in the upper 20s to near 30 knots (230 ft) during the count and low to mid 20s during the window are expected. The primary concerns for launch day are cumulus clouds, isolated thunderstorms, and ground winds."

During Friday's 62-minute launch window, the outlook calls for scattered clouds at 3,000 feet and a broken deck of clouds at 25,000 feet, isolated showers and thunderstorms around, visibility of 7 miles, west-southwesterly winds from 250 degrees at 18 to 24 knots and a temperature of 71-72 degrees F.

Once the frontal system passes the Cape area, the odds of good weather on Saturday night jumps to 70 percent, if the launch is delayed to the backup date for some reason.

"In the event of a 24-hour delay, the frontal boundary is expected to be in South Florida with clearing conditions behind the front. The primary concerns for a 24-hour delay are cumulus clouds," forecasters say.

MONDAY, MARCH 30, 2009

An Atlas 5 rocket that will deploy a vital new communications satellite to support U.S military forces in Iraq and Afghanistan is aiming for a Friday evening launch from Cape Canaveral, now that a leaky liquid oxygen valve has been replaced.

Read our update story.

SUNDAY, MARCH 29, 2009

Tuesday evening's target launch date for the Atlas 5 rocket on a satellite-deployment mission for the U.S. military is being pushed back a few more days to complete analysis work, the Air Force said today.

A liquid oxygen valve leak on the Centaur upper stage scrubbed the original launch attempt on March 17. Since that time, the rocket was returned to the assembly building where the suspect valve was removed and replace.

A tentative new launch date of Friday, April 3 was announced today. But that date will depend on the completion of analysis to clear the rocket for flight, officials said.

Read our earlier status center coverage.