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.

FRIDAY, MARCH 27, 2009

There's a 70 percent chance of acceptable weather for Tuesday evening's launch of the Atlas 5 rocket from Cape Canaveral. Liftoff will be possible during a window of 8:32 to 9:35 p.m. EDT.

Here's the latest forecast from Air Force meteorologists for Monday's rollout of the rocket to the pad and Tuesday's countdown.

"A developing low pressure system in the Southeast will push a cold front into the big bend area Saturday evening and through Central Florida Sunday. Pre-frontal showers with isolated embedded thunderstorms are possible in advance of the front Saturday evening and with the frontal passage on Sunday. High pressure with fair weather conditions build in for (Mobile Launch Platform) roll with winds from the North gusting in the low teens (230 feet).

"On launch day, a slow return of the boundary from the South toward Central Florida with Southwesterly steering flow introduces a threat of isolated showers. The primary concerns for launch day are cumulus clouds and thick clouds.

"In the event of a 24-hour delay, a second cold front is expected in Northern Florida with continued return of the boundary from the South and increasing moisture introducing a threat of pre-frontal showers and an isolated thunderstorm. Winds increase with the front to our North. The primary concerns for a 24-hour delay are cumulus clouds, thick clouds, and gusty ground winds."

The odds of acceptable weather for launch on Wednesday is 60 percent.

TUESDAY, MARCH 24, 2009

The new target launch date for the Atlas 5 rocket is Tuesday, March 31. The evening's 63-minute launch window will extend from 8:32 to 9:35 p.m. EDT.

WEDNESDAY, MARCH 18, 2009

A day after a liquid oxygen valve leak on the Centaur upper stage scrubbed its launch, an Atlas 5 rocket has been wheeled off the Complex 41 pad and returned to the nearby assembly building.

It is just the second time in 14 missions that an Atlas 5 has rolled back to the 30-story Vertical Integration Facility for repairs in the wake of a scrub.

The Atlas 5 program at Cape Canaveral uses a "clean-pad" concept with minimal infrastructure and no service gantry at the launch pad. Instead, stacking of the rocket stages and pre-flight work is accomplished the VIF. A mobile launch platform transports the Atlas between the hangar and the pad.

Air Force officials said this launch will be delayed until at least next week. Technicians need to inspect the rocket, identify how to fix the leak and complete that work.

A new launch date hasn't been set, but the officials say the liftoff won't occur until sometime after next Tuesday's Delta 2 rocket flight from the Cape carrying a GPS satellite.

2100 GMT (5:00 p.m. EDT)

The Atlas 5 rocket has begun its return to the Vertical Integration Facility. Rollback begun a few minutes ago.

1430 GMT (10:30 a.m. EDT)

Ground crews at Complex 41 today are preparing to transport the Atlas 5 rocket back to the Vertical Integration Facility. The rollback to the 30-story assembly building will allow technicians to fix the liquid oxygen valve leak that scrubbed Tuesday night's countdown.

A new launch date has not been established yet. But guests attending the launch were told it will be delayed until after the planned March 24 liftoff of a Delta 2 rocket carrying a new Global Positioning System satellite from Cape Canaveral.

The Air Force-run Eastern Range that provides required tracking, communications and safety services for all rocket flights from the Florida spaceport needs about 48 hours between launches.

Sources say that work timelines show it's not possible to squeeze in the Atlas launch prior to the Delta.

Also on the Range schedule next week is tracking support for space shuttle Discovery's landing at the Kennedy Space Center runway on March 28. Typically, the Range needs about 24 hours to reset systems between a rocket launch and a shuttle landing.

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

A statement issued from ULA:

"The launch of the Atlas 5 carrying the WGS 2 satellite was scrubbed tonight after the launch team detected an anomalous leak rate in the Centaur upper stage oxidizer valve. The operation was secured for the evening and the team commenced the standard detanking procedure. Once the cause of the leak is understood, the team will begin planning for a new launch date in coordination with the U.S. Air Force and Eastern Range."

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

A United Launch Alliance spokesperson says the delay is expected to be longer than 48 hours. However, specific rescheduling plans have not been determined.

0020 GMT (8:20 p.m. EDT Tues.)

To recap, the Atlas 5 rocket's countdown was proceeding toward a liftoff at 9:24 p.m. EDT from Complex 41 at Cape Canaveral. Loading of liquid oxygen into the Centaur upper stage and the Atlas first stage was underway as scheduled.

A drop in temperature was reported on the Centaur, which prompted an examination to see what was causing the unexpected condition. The troubleshooters determined there was a liquid oxygen leak in the Centaur engine's oxidizer valve, effectively stopping the launch attempt.

A scrub was declared at 7:49 p.m. and the launch team began offloading the liquid oxygen that had been loaded into the rocket thus far.

The leak is being evaluated, and it's too early to tell what repairs will be required or when another launch attempt can be made.

TUESDAY, MARCH 17, 2009
2357 GMT (7:57 p.m. EDT)

It is not yet clear how long it will take to resolve this leak or when the launch can be rescheduled. We'll update this page as further information becomes available.

2352 GMT (7:52 p.m. EDT)

The launch team detected falling temperatures on the Centaur upper stage a short time ago. The anomaly resolution team was convened to assess the data. Engineers determined there was a liquid oxygen leak in the oxidizer valve. Officials quickly made the decision to stop the fueling operation, scrub the launch for tonight and begin draining the cryogenics from the rocket.

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

SCRUB. Tonight's launch attempt is being called off due to a problem experienced.

2348 GMT (7:48 p.m. EDT)

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

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

Engineers are discussing a couple of issues that have arisen. The countdown is proceeding and fueling is continuing while the issues are assessed.

2340 GMT (7:40 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.

2338 GMT (7:38 p.m. EDT)

The Centaur liquid oxygen tank is 20 percent full already.

2330 GMT (7:30 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.

2322 GMT (7:22 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.

2314 GMT (7:14 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 9:24 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 a short time from now.

2311 GMT (7:11 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 launch director and mission director gave their approvals as well. So the loading of cryogenic liquid oxygen and liquid hydrogen into the Atlas 5 rocket will be getting underway shortly.

2309 GMT (7:09 p.m. EDT)

The ULA launch conductor 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.

2244 GMT (6:44 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 9:24 p.m. EDT launch of Atlas. 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.

2235 GMT (6:35 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.

2230 GMT (6:30 p.m. EDT)

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

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

Guidance system testing and the flight control countdown preps are complete.

2153 GMT (5:53 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.

A test of the C-band system, which is used for tracking the rocket as it flies downrange, just finished.

Liftoff of the Atlas 5 rocket remains on track for 9:24 p.m.

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

Technicians have completed preps on the Centaur upper stage liquid oxygen and liquid hydrogen systems. Atlas propulsion and hydraulic preps also were accomplished.

The launch team is beginning internal battery checks. A test of the S-band telemetry relay system began a little while ago.

2124 GMT (5:24 p.m. EDT)

It is a busy day for space news. At 10:21 a.m. EDT today, Europe launched a slender, winged ion-driven satellite to glide through the upper atmosphere for nearly two years measuring Earth's gravity field with unprecedented precision. You can read more about that satellite here.

Space shuttle Discovery docked to the space station at 5:20 p.m. EDT for its mission to deliver the $300 million Starboard 6 solar array truss, the fourth and final power module for the international outpost. The 15.5-ton girder will be connected to the station's backbone by spacewalking astronauts this week. The latest updates on the shuttle flight are posted here.

And now the countdown at Cape Canaveral is entering the final hours before liftoff of the Atlas 5 rocket to deploy the U.S. military's Wideband Global SATCOM 2 communications satellite.

2024 GMT (4:24 p.m. EDT)

Launch is just five hours away, but work to ready this Atlas 5 rocket to the launch 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.

"It has been 11 months since the last Atlas 5 and a year since our last Department of Defense mission aboard Atlas. During that time, our integrated government and industry team has successfully worked through several issues to achieve the highest possible level of flight-worthiness for this launch vehicle. As always, mission success is our No. 1 priority," said Col. Michael Moran, commander of the Atlas Group at the Air Force's Space and Missile Systems Center.

"The launch team will continue to verify that all systems and mission plans meet our rigorous standards for flight readiness as we continue processing toward launch. Only when everything is 100 percent ready will we launch, not before."

1920 GMT (3:20 p.m. EDT)

Air Force meteorologists have increased the odds of acceptable conditions for tonight's launch to 70 percent.

The outlook calls for broken low- and mid-level clouds, isolated showers in the area, 7 miles of visibility, northeasterly winds from 030 degrees at 20 gusting to 25 knots and a temperature of 68-69 degrees F.

1825 GMT (2:25 p.m. EDT)

The countdown began moments ago for tonight's launch of the Atlas 5 rocket from Cape Canaveral, Florida. The vehicle is being powered up for the start of final pre-flight preparations.

The weather in Central Florida is looking ideal this afternoon, with partly cloudy skies, a light breeze and warm temperatures.

News organizations just finished setting up sound-activated cameras around the launch pad at Complex 41 to capture the liftoff. And stay tuned to this page as we'll be providing live countdown updates throughout the day.

1415 GMT (10:15 a.m. EDT)

A massive satellite that will boost communications for U.S. military forces in Iraq and Afghanistan will be launched into space tonight from Cape Canaveral atop an Atlas 5 rocket.

Liftoff from Complex 41 is scheduled for 9:24 p.m. EDT. Forecasters predict generally favorable weather for the launch of the second Wideband Global SATCOM communications spacecraft, known as WGS 2.

"I am extremely proud of the hard work and dedication of the launch team in working the long hours that have brought us to where we are today," said Col. Donald Robbins, commander of the Wideband SATCOM Group at the Air Force's Space and Missile Systems Center. "With the launch of WGS 2, we are continuing to provide increased wideband capabilities to the warfighter and also providing more capabilities with each WGS space vehicle than the entire on-orbit Defense Satellite Communications System constellation."

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

Countdown clocks will start ticking at 2:24 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 site is cleared of all personnel at 6:30 p.m.

A planned half-hour hold begins at 6:44 p.m. when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 7:11 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 7:30 p.m., followed by the first stage filling around 7:45 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 9:10 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.

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.)

MONDAY, MARCH 16, 2009

U.S. and allied forces in Iraq and Afghanistan, who face a shortage of wideband communications, will gain major additional capability pending the successful launch of the new Wideband Global SATCOM spacecraft on its Atlas 5 rocket.

Read our payload preview story.

1440 GMT (10:40 a.m. EDT)

On the pad! The Atlas 5 rocket has arrived at the Complex 41 pad for Tuesday's planned liftoff to deliver into orbit the U.S. Air Force's Wideband Global SATCOM 2 satellite.

The two mobile trailers connected to the launching platform, which were part of the convoy during this morning's rollout, soon 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 during the rollout and the countdown. They are protected from the blast of launch by a concrete structure on the north-side of the platform.

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.

Later this afternoon, the undercarriages used to move the mobile platform will be disconnected and the "trackmobiles" pulled free. The launch team then loads a highly refined kerosene propellant, called RP 1, into the rocket's first sage. Crews will secure the rocket and pad for the night.

The launch countdown commences at 2:25 p.m. EDT tomorrow, some seven hours before liftoff time.

1406 GMT (10:06 a.m. EDT)

Rollout begins! The United Launch Alliance Atlas 5 rocket that will carry into space a new communications satellite for the U.S. military just began a slow half-hour drive from the 30-story Vertical Integration Facility to Cape Canaveral's Complex 41 pad for Tuesday night's liftoff.

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.

With last night's successful departure of the space shuttle Discovery from the Kennedy Space Center, the Eastern Range at the Florida spaceport is working to reconfigure tracking, communications and safety systems to support the Atlas flight. The Range needs about 48 hours between launches, and the Atlas managers opted to delay their launch from Saturday to enable NASA to get the shuttle up over the weekend.

Air Force meteorologists are predicting a 60 percent chance of acceptable conditions at Tuesday's 9:24 p.m. EDT launch time. Strong winds and the thickness of cloud cover are the main worries.

The outlook calls for scattered clouds at 3,000 feet and broken decks at 10,000 and 25,000 feet, isolated showers, 7 miles of visibility, northeasterly winds from 040 degrees at 20-25 knots and a temperature of 68-69 degrees F.

"A cold front remains just north of the Florida peninsula. The front slowly tracks south into the peninsula today and is expected to slowly migrate through Central Florida on Tuesday. The front is expected to be south of Central Florida by the launch window; however, residual moisture and clouds are slow to clear. The pressure gradient tightens through the day with gusty winds in the mid 20s expected," forecasters reported today.

"On launch day, the boundary slowly migrates over the area. Cloudy conditions accompanied by isolated rainshowers are expected with a slow gradual clearing trend beginning early evening. There is a small threat of isolated thunderstorms associated with the front. Gusty winds associated with the front are expected during the afternoon and evening hours gusting in the mid 20s. The primary concerns for launch day are thick clouds and gusty ground winds.

"In the event of a 24-hour delay, the front remains to the south with residual moisture over the central peninsula. The primary concerns for a 24-hour delay are thick clouds and cumulus clouds associated with an isolated shower."

FRIDAY, MARCH 13, 2009

With NASA's decision to try launching space shuttle Discovery on Sunday evening, the upcoming flight of an Atlas 5 rocket carrying a military communications satellite from Cape Canaveral will go into standby mode under an agreement reached between the space agency and Air Force officials.

"We've just heard as we walked in that the Range schedule has been updated. The Atlas has moved to an indefinite date, which just means that they are going to slide behind us," NASA's Mission Management Team chairman Mike Moses told reporters at a news conference this morning.

"So thank you very much to the Air Force for allowing us to take this opportunity to launch before the Soyuz cutout."

The Atlas had been targeting a rollout to the launch pad today and a liftoff on Saturday night. But since the Eastern Range cannot support two back to back launches so close together, the rocket will give up its slot on the schedule to allow the shuttle to fly on Sunday.

The shuttle has to launch by Tuesday in order to carry out its mission, albeit a shortened one, before the next Russian Soyuz capsule flies from Baikonur Cosmodrome in Kazakhstan on March 26 with the space station's new resident crew.

The shuttle has to accomplish its mission at the station and leave before the Soyuz arrives to begin a busy handover period between the incoming and outgoing resident crews. If Discovery doesn't launch by Tuesday, the shuttle would remain grounded until the outgoing crew departs aboard its Soyuz in early April.

Under the current scenario, a shuttle liftoff on Sunday would allow the Atlas to launch as early as Tuesday evening.

The Tuesday launch window will extend from 9:24 to 10:00 p.m. EDT.

This morning's rollout from the Vertical Integration Facility to the pad at Complex 41 was cancelled. The rocket will remain safely tucked inside the assembly building for the weekend.

THURSDAY, MARCH 12, 2009

Despite NASA's hopes to get the gaseous hydrogen system repaired quickly enough for shuttle Discovery to launch Sunday, managers of the Atlas 5 rocket are keeping their options open to fly a military mission Saturday night as originally planned.

Officials said late Thursday that the Atlas 5 launch had not been delayed and workers were instructed to report for duty Friday morning in advance of the rocket's rollout to the pad at Cape Canaveral.

The Atlas is targeting a 10 a.m. trip from its assembly building to the Complex 41 pad in preparation for blastoff at 9:25 p.m. EDT Saturday to deploy into orbit the Air Force's second Wideband Global SATCOM communications spacecraft.

But this week's delay of the shuttle by a gaseous hydrogen leak and NASA's need to get Discovery in space within the next few days has raised the possibility of the Atlas waiting a few days before flying.

NASA wants to squeeze in Discovery's construction mission to the space station before the upcoming swapout of Russian Soyuz capsules and resident crews at the outpost. Just three launch opportunities remain on Sunday, Monday and Tuesday for Discovery or else the shuttle would be postponed until April 7, causing further impacts to downstream scheduling.

The tracking, communications and safety systems at the spaceport needs about two days between launches to reconfigure from one vehicle to another. That makes it impossible for the Atlas to launch on Saturday and the shuttle on Sunday.

Since the Atlas mission does not have the time sensitivity that the shuttle is facing, officials have said they would discuss delaying the rocket launch while NASA works to get Discovery airborne.

Shuttle workers are replacing hardware in the umbilical that vents gaseous hydrogen from the shuttle's external tank to plumbing on the launch pad. A leak in that system caused Wednesday's countdown to be scrubbed.

NASA said Thursday afternoon that "teams believe they have sufficient understanding of the hydrogen leak to continue toward a Sunday launch at 7:43 p.m."

A Sunday launch is preferred over Monday or Tuesday because it would allow Discovery's astronauts to perform three of the planned spacewalks while docked to the station. The shuttle's flight would have to be shortened further, given the Soyuz constraint, if launch occurs on Monday or Tuesday, thereby reducing the numbers of spacewalks that could be accomplished.

More information is expected Friday, pending negotiations between the shuttle and Atlas management teams.

If a Sunday evening launch of the shuttle is approved, that would lead to the Atlas lifting off on Tuesday.

Mission managers for the Atlas did hold the Launch Readiness Review on Thursday and reported the rocket, payload and support systems were in good shape for flight.

WEDNESDAY, MARCH 11, 2009

Saturday's planned launch of an Atlas 5 rocket carrying a military communications satellite from Cape Canaveral is in limbo while NASA sorts out when space shuttle Discovery will be ready for its next countdown.

A leaky gaseous hydrogen vent line at pad 39A that connects to the shuttle's external fuel tank caused NASA to scrub Wednesday's scheduled liftoff on a construction mission to the space station. Repairs will delay the launch until at least Sunday.

The Air Force-run Eastern Range that provides critical tracking, communications and safety services for all rocket and shuttle launches from the Florida spaceport needs about 48 hours between flights to reconfigure systems. An Atlas on Saturday night and the shuttle on Sunday wouldn't be feasible.

Atlas managers have said they will work with NASA to accommodate attempts to launch the shuttle, which is facing a Tuesday deadline to get off the ground or else wait until April due to the upcoming Russian Soyuz mission to the space station.

Whether the shuttle would be able to try again Sunday is not yet clear, leaving the possibility of Atlas flying on Saturday night as planned. NASA officials will meet Thursday to assess work schedules to determine how long it will take to get Discovery prepped for another launch shot.

"If we look like we can't go till Monday, we'll see if the Atlas can go in front of us and we'll go on the other side of it," Mike Moses, chairman of NASA's Mission Management Team told reporters during an evening news conference.

"Again, it's a little too early to tell. We really need to wait till tomorrow until the team comes back with that master schedule and we have a really good cut at what we really need to do to be ready for another launch attempt."

TUESDAY, MARCH 10, 2009

Once space shuttle Discovery flies away from the Kennedy Space Center, attention at the Florida spaceport will turn to Saturday night's planned launch of an Atlas 5 rocket carrying a sophisticated communications satellite to serve U.S. military forces in Iraq and Afghanistan.

The 19-story rocket, mounted atop a mobile launching platform, is scheduled for rollout Friday from the assembly building to the pad at Complex 41 to begin final pre-flight preparations.

Saturday's launch window extends from 9:25 to 10:03 p.m. EDT.

The United Launch Alliance-built rocket will perform a half-hour flight to supersynchronous transfer orbit for deployment of the Air Force's second Wideband Global SATCOM communications spacecraft, known as WGS 2.

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. A single WGS spacecraft provides more than 10 times the capacity of the entire DSCS constellation, and this allows the warfighter to process and receive data quicker than ever before."

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 this weekend'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."

Getting this new spacecraft into orbit is the responsibility of the Atlas 5 rocket, one the Air Force's next generation space-lift workhorses set to make its 15th flight.

"This mission represents the epitome of teamwork as ULA and the Air Force launch vehicle and payload teams have worked hand-in-hand to bring us to this point where we are only a few days away from launch," said James Bell, ULA's WGS mission manager.

"We anticipate our Atlas 5 vehicle performing exceptionally well, allowing WGS to provide the enhanced communications capabilities our troops in the field need to defend our great nation for decades to come."

The rocket will 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.

Roaring away from Cape Canaveral on nearly 1.7 million pounds of thrust, 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.

Weather forecasters predict an 80 percent chance of acceptable conditions for Saturday's launch. Cumulus clouds pose only a small threat of violating the liftoff rules.

"The peninsula remains under high pressure and fair weather with a cold front extending from the Carolinas into Louisiana by mid to late week. The front is expected to slowly drift south by launch day; however, remaining just north of the peninsula. In advance of the front, winds become southerly by the weekend with a gradual increase in moisture and a threat of isolated showers," the launch weather team reported Tuesday.

"For MLP roll, east-southeast winds prevail gusting in the mid-teens with a small threat of an isolated coastal shower. On launch day, the front remains near stationary just north of the peninsula with a gradual increase in moisture and a threat of isolated showers. No thunderstorms are expected. The primary concerns for launch day are cumulus clouds."

The launch time outlook calls for scattered clouds at 3,000 and 25,000 feet, possible isolated showers, visibility of 7 miles, southerly winds from 160 degrees at 14-18 knots and a temperature of 70-71 degrees F.

If the launch is delayed to Sunday night for some reason, there's a 70 percent chance of favorable weather.

"In the event of a 24-hour delay, the front encroaches into the panhandle with a slight increased threat of showers. The primary concerns for a 24-hour delay are cumulus clouds," the forecasters said.

Watch this page for continuing coverage and live reports during the countdown!