"ULA and our mission partners are honored to work with the outstanding NASA team and we are proud of the vitally important data relay capabilities that were safely delivered today," said Jim Sponnick, United Launch Alliance vice president of Atlas and Delta programs. "With 43 successful missions spanning a decade of operational service and launched with a one-launch-at-a-time focus on mission success, the Atlas 5 continues to provide reliable, cost-effective launch services for our nation's most complex and valued payloads.

"Atlas and TDRS have supported each other for almost 20 years, and all three of the second generation satellites, now known as TDRS 8, 9, and 10, launched on Atlas vehicles in 2000 and 2002," said Sponnick. "While we were integrating those spacecraft onto Atlas in the late 1990s, we also developed a new TDRS-compatible transmitter so that Atlas could use the TDRS constellation to receive and distribute the launch vehicle telemetry relay during flight. We are now also using TDRS services for our Delta 2 and Delta 4 programs."

"We're all thrilled," said NASA launch director Tim Dunn. We had a "successful spacecraft separation and most importantly a successful spacecraft acquisition by our ground telemetry. We've confirmed that we've got a healthy spacercaft so far, so the launch team is thrilled, and you can imagine the TDRS L spacecraft team is even more thrilled."

Signals from the TDRS L spacecraft have been acquired from a ground station in Dongara, Western Australia, verifying the satellite is alive following tonight's launch.

T+plus 106 minutes, 2 seconds. SPACECRAFT SEPARATION! The Tracking and Data Relay Satellite L has been released from the Centaur upper stage, completing this evening's launch to replenish NASA's communications network in space!

T+plus 104 minutes. Centaur is in separation attitude and is now spinning up to a rotation rate of 30 degrees per second before deploying TDRS L.

Telemetry indicates the rocket achieved a good orbit.

The vehicle has a reached a geosynchronous transfer orbit, a highly elliptical orbit with one end of the orbit at geosynchronous altitude. This is the typical drop-off point for communications satellites. TDRS L will be left to maneuver itself into a circular perch 22,300 miles above the Pacific.

T+plus 102 minutes. Centaur is turning itself to the proper orientation for releasing the first payload.

MECO 2. Main engine cutoff confirmed. Centaur has completed its second burn of the night!

The RL10 engine has ignited for second time to boost the TDRS L spacecraft into a higher orbit.

The Centaur stage's RL10 engine will restart at 11:13 p.m. EST (0413 GMT) for a 63-second burn. Spacecraft separation is expected at 11:19 p.m. EST (0419 GMT).

T+plus 35 minutes. The flight path took the vehicle over the equatorial Atlantic Ocean before crossing Africa. Here's the planned track map.

T+plus 25 minutes. Check out Spaceflight Now's Facebook page for images from today's liftoff.

T+plus 21 minutes. The Centaur has reached a preliminary orbit very close to prelaunch predictions.

T+plus 18 minutes, 20 seconds. MECO 1. Centaur's RL10 main engine has shut down following its first burn today, achieving a preliminary orbit around Earth. The rocket will coast in this orbit for approximately 82 minutes before the RL10 engine re-ignites.

At MECO-1, the vehicle will be settling into a parking orbit of approximately 15,450 statute miles at apogee, 115 statute miles at perigee and inclination of 26.5 degrees. A lengthy coast is initiated to reach the proper conditions for restarting the Centaur and inject the spacecraft into the proper transfer orbit.

About three minutes remain in this first of two Centaur burns.

The Centaur has attained enough velocity to reach orbit.

T+plus 10 minutes, 30 seconds. The RL10 continues to perform well, burning liquid hydrogen and liquid oxygen propellants. The Antigua downrange tracking station has acquired the Atlas 5 rocket.

Altitude is now 184 miles, downrange distance is 1,472 miles and velocity is 14,039 mph.

T+plus 9 minutes. All continues to go well in this burn.

T+plus 7 minutes, 15 seconds. Now 166 miles in altitude, 797 miles downrange from the launch pad, traveling at 12,112 mph.

T+plus 7 minutes, 5 seconds. The first stage delivered the expected performance.

T+plus 5 minutes. Centaur engine readings look good as its first burn gets underway.

T+plus 4 minutes, 30 seconds. The two halves of the four-meter-diameter Atlas 5 rocket nose cone encapsulating the spacecraft have separated.

T+plus 4 minutes, 222 seconds. Centaur has ignited! The RL10 engine is up and running at full thrust to power the vehicle into orbit.

T+plus 4 minutes, 14 seconds. The Atlas 5's Common Core Booster has been jettisoned, completing the first stage of flight, and the Centaur upper stage's liquid hydrogen and liquid oxygen systems are being readied for engine start.

T+plus 4 minutes, 4 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, 45 seconds. Atlas now weighs just a quarter of what it did at liftoff.

T+plus 3 minutes. Altitude is 52 miles, downrange distance is 57 miles and velocity is 5,325 mph.

T+plus 2 minutes, 45 seconds. Reaction control system has been activated.

T+plus 2 minutes, 35 seconds. Atlas continues tracking on course.

T+plus 2 minutes, 30 seconds. Atlas now weighs half of what it did at liftoff.

T+plus 2 minutes, 15 seconds. Vehicle systems looking 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 100 seconds. Now passing through the region of maximum aerodynamic pressure on the vehicle as its accelerates through the dense lower atmosphere.

T+plus 85 seconds. All looks good aboard Atlas as it passes Mach 1.

T+plus 60 seconds. One minute into the ascent of NASA's TDRS L spacecraft.

T+plus 40 seconds. The Atlas 5 is sending a thunderous roar across Florida's spaceport as it climbs into a clear night sky over Cape Canaveral.

T+plus 15 seconds. The Atlas 5 rocket has cleared the tower on 860,000 pounds of thrust from the RD-180 main engine. Pitch, yaw and roll maneuvers are underway to put the rocket on the proper heading.

LIFTOFF! Liftoff of the Atlas 5 rocket on a mission to reinforce NASA's space-based tracking and communications network.

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

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

T-minus 55 seconds. Range is green.

T-minus 1 minute. Now 60 seconds away from launch of NASA's next tracking station in the sky.

T-minus 90 seconds. The rocket's 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 are enabled.

T-minus 4 minutes and counting. Clocks have resumed for the final minutes of today's countdown to launch the Atlas 5 rocket with the Tracking and Data Relay Satellite L spacecraft from Cape Canaveral, Fla.

Polling of the team by Atlas launch conductor just occurred. All systems are "go" for a liftoff today at 9:33 p.m. EST (0233 GMT).

NEW LAUNCH TIME. Liftoff is now targeting for 9:33 p.m. EST (0233 GMT).

Deputy NASA launch director Omar Baez says the data dropouts from the TDRS L spacecraft occurred after the ground team began configuring the payload for launch. During most of the countdown, engineers monitor data from the spacecraft through a hard line via the Atlas 5 rocket and launch pad, but the team switches the data source to a radio link shortly before launch to track the satellite's condition through ascent.

To get around the dropout issue, the launch team will stay with the hard line data link until liftoff, Baez said, then switch to the radio telemetry path.

The launch team is investigating the "data hits" seen from the TDRS L spacecraft in order to determine whether to proceed with launch this evening.

Tonight's launch window extends until 9:45 p.m. EST (0245 GMT). The countdown must resume by 9:41 p.m. EST in order to launch tonight.

The countdown is holding at T-minus 4 minutes. The current plan is to hold here for about 10 minutes if the spacecraft telemetry problem can be resolved.

The launch team reports there was a data dropout in telemetry coming from the TDRS L spacecraft, causing the payload team to report a "no go" to pick up the countdown. This means launch will not occur at 9:05 p.m. EST (0205 GMT).

DELAY. The countdown clock will not resume as scheduled.

The TDRS L is verified to be on internal power. Standing by to resume the countdown at T-minus 4 minutes.

ULA and the Air Force have given their respective final approvals to resume the countdown.

Polling of the team by Atlas launch conductor just occurred. All systems are "go" for a liftoff today at 9:05 p.m. EST (0205 GMT).

Standing by for the final readiness check to be conducted. The launch team will be polled for a "go" or "no go" to proceed with the count.

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

• The 625th launch for Atlas program since 1957
• The 334th Atlas to occur from Cape Canaveral
• The 214th mission for the Centaur upper stage
• The 191st use of Centaur by an Atlas rocket
• The 443rd production RL10 engine to be launched
• The 49th flight of the RD-180 main engine
• The 43rd launch of an Atlas 5 since 2002
• The 67th Evolved Expendable Launch Vehicle flight
• The 11th Atlas 5 launch for NASA
• The 36th Atlas 5 to occur from the Cape
• The 8th nighttime Atlas 5 launch from the Cape
• The 78th United Launch Alliance flight overall
• The 1st ULA launch of the year
• The 35th Atlas 5 under United Launch Alliance
• The 28th 400-series flight of the Atlas 5
• The 20th Atlas 5 to fly in the 401 configuration
• The 12th Tracking and Data Relay Satellite launch
• The 5th TDRS built by Boeing
• The 5th TDRS launched on an Atlas vehicle
• The 2nd Atlas 5 launch of TDRS
• The 1st Atlas 5 launch of 2014
• The 2nd launch from Cape Canaveral in 2014

A poll of the NASA management team by NASA launch director Tim Dunn has given approval to proceed with the launch of TDRS L. A poll of the ULA launch team is coming up in a few minutes.

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.

T-minus 5 minutes. Standing by to go into the final built-in hold. All of the Atlas 5's propellant tanks are reported at flight level.

The TDRS L spacecraft is transitioning to internal battery power at this time.

The fuel-fill sequence for the RD-180 first stage engine is complete.

Today marks the 43rd flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 42 missions since debuting in August 2002, the tally shows 15 flights dedicated to the Defense Department, 9 commercial missions with communications spacecraft, 10 for NASA and 8 with spy satellites for the National Reconnaissance Office.

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

The fuel-fill sequence for the first stage main engine is starting, and flight control final preps are complete.

The launch weather officer says all weather constraints are observed GO and expected to remain GO at the time of liftoff.

Just 30 minutes until liftoff time. Today's launch is the second TDRS spacecraft to launch on an Atlas 5 rocket.

The Tracking and Data Relay Satellite L will be delivered to an orbit with a perigee, or low point, of 3,007 miles and an apogee, or high point, of 22,238 miles. The target inclination at spacecraft separation is 25.5 degrees.

Built by Boeing Co., the TDRS L spacecraft is the second of three third-generation tracking and data relay satellites for NASA's space-based communications network, which links mission controllers with orbiting spacecraft such as the International Space Station, the Hubble Space Telescope and a host of scientific research platforms.

TDRS L will use its on-board propulsion system to boost itself into a circular orbit about 22,300 miles high inclined about 7 degrees to the equator. The satellite is scheduled to arrive in the circular geosynchronous orbit about 10 days after launch to begin several months of testing before NASA takes control of the satellite and it enters the operational constellation.

The spacecraft will extend its two power-generating solar array wings soon after arriving in its 22,300-mile-high operating orbit, along with antenna booms.

TDRS L is the 12th TDRS satellite to be launched since 1983. Two of the spacecraft have been retired and one was lost on the space shuttle Challenger accident in 1986.

TDRS L will deploy from the Atlas 5's Centaur upper stage about 106 minutes after liftoff. If launch goes on time, spacecraft separation is scheduled for 10:51 p.m. EST (0351 GMT).

Check out a timeline of the launch.

Following separation from the launch vehicle, TDRS L will activate its radio transmitter and radio its status to ground controllers through a tracking station at Dongora, Western Australia. Then the satellite will point its stowed solar panels toward the sun to generate electricity and unfurl its two large 15-foot mesh antennas.

Open-loop testing of the Atlas 5's flight termination system is complete.

Launch is now 45 minutes away. The Atlas 5 rocket stands 191 feet tall and weighs 746,061 pounds fully loaded with propellant.

This launch will mark the first Atlas 5 launch of the year.

The Atlas 5's most recent launch Nov. 18 lofted NASA's Mars Atmosphere and Volatile Evolution, or MAVEN, mission to the red planet.

United Launch Alliance plans nine Atlas 5 missions this year. Two of those launches will be with NASA payloads, including tonight's launch of TDRS L.

The next Atlas 5 launch is set for March 25 with a classified payload for the National Reconnaissance Office.

The first stage liquid oxygen tank and the Centaur's liquid hydrogen tank have reached the 95 percent levels, and topping to flight level has started.

And final open-loop testing of the Atlas 5's destruct system is getting underway now.

Centaur's liquid hydrogen tank is 90 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Aerojet Rocketdyne-made RL10 engine.

Now 60 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen is progressing smoothly as the countdown continues on schedule for a liftoff at 9:05 p.m. EST (0205 GMT). Weather remains GO, and the launch window runs until 9:45 p.m. EST (0245 GMT)

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The Atlas 5's first stage liquid oxygen tank is 80 percent full.

Centaur liquid hydrogen has reached the 10-percent level, and Atlas first stage liquid oxygen is at 70 percent.

First stage liquid oxygen tank is 50 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 minutes of flight today. The 25,000 gallons of RP-1 were loaded into the rocket yesterday.

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.

The Centaur engine chilldown sequence is being initiated.

Frost is building up on the Atlas 5 first stage as super-cold liquid oxygen is pumped aboard the rocket.

Centaur liquid oxygen is now at flight level.

The Centaur liquid oxygen tank reached the 92.4 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 12,325 gallons of the cryogenic propellant.

Now at 70 percent on Centaur liquid oxygen.

The 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 48,745 gallons of cryogenic oxidizer for the RD-180 main engine.

Centaur liquid oxygen is 50 percent loaded.

Passing the 20 percent level on the Centaur upper stage's liquid oxygen tank.

Alignment of the Atlas 5's guidance computer is complete.

Filling of the Centaur upper stage with about 4,075 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 two firings to propel the 7,615-pound TDRS L spacecraft into geosynchronous transfer 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.

All console operators have reported GO status during the pre-fueling readiness poll. The ULA launch director also voiced his approval for moving forward with the countdown as scheduled today.

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

T-minus 120 minutes and counting! The launch countdown has resumed for today's flight of the Atlas 5 rocket with the Tracking and Data Relay Satellite L for NASA.

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 9:05 p.m. EST (0205 GMT) 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 ULA launch conductor at the Atlas Spaceflight Operations Center is briefing his team on procedures before entering into the final two hours of the countdown.

NASA launch director Tim Dunn has polled his team and all consoles reported they were ready to proceed with fueling of the Atlas 5 rocket this evening.

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T-minus 2 hours and holding. The countdown has just entered the first of two planned holds over the course of the morning that will lead to the 9:05 p.m. EST (0205 GMT) launch of the Atlas 5 rocket.

This initial pause was designed to give the team some margin in the countdown timeline to deal with technical issues or any work that could fall behind schedule before fueling starts.

The final hold is scheduled to occur at T-minus 4 minutes.

The final hands-on work has wrapped up at the launch pad and technicians are departing the complex. Safety officials just confirmed that the surrounding danger area has been cleared of all workers for the remainder of the countdown.

Final flight preparation steps have been completed by the launch team, and holdfire checks with the Air Force range are being conducted now.

The weather status board shows all launch criteria still green and GO. The forecast for the 9:05 to 9:45 p.m. EST (0205-0245 GMT) window continues to reflect a 90 percent chance of favorable conditions.

The outlook for launch time calls for scattered clouds at 3,000 feet and 20,000 feet, north-northwest wings at 10 to 15 knots, good visibility, and a temperature of 46 degrees Fahrenheit.

Now entering the final four hours to the United Launch Alliance Atlas 5 rocket flight with the TDRS L payload for NASA. This will be the civilian space agency's 11th use of Atlas 5 over the past eight years.

At this point in the countdown, the launch team is conducting checks of the internal batteries aboard the vehicle.

"The voice and data communications that the TDRS constellation brings back to Earth every day from the International Space Station, Hubble Space Telescope and many of our on-orbit NASA science spacecraft touch each of our lives," said Tim Dunn, NASA's launch director for the TDRS L mission.

"NASA has a terrific record flying on Atlas 5," Dunn said. "We've successfully launched 10 missions on this rocket, missions to Pluto, Jupiter, the moon, the sun, the radiation belts and three spacecraft to Mars. TDRS L will be the 11th NASA mission on Atlas 5 and the 43rd Atlas 5 launch overall."

The first NASA mission sent the Mars Reconnaissance Orbiter to the red planet on Aug. 12, 2005. The New Horizons probe on a three-billion-mile voyage to fly by Pluto and explore the frigid edge of the solar system followed on Jan. 19, 2006. Then came the tandem launch of the Lunar Reconnaissance Orbiter and the LCROSS impactor to the Moon's South Pole on June 18, 2009. The Solar Dynamics Observatory was placed into orbit on Feb. 11, 2010. The Juno spacecraft to study the planet Jupiter was launched on Aug. 5, 2011. The Mars Science Laboratory with the Curiosity rover left Earth on Nov. 26, 2011. The twin Radiation Belt Storm Probes were sent aloft on Aug. 30, 2012, followed by NASA's TDRS K communications satellite on Jan. 30, 2013. An Atlas 5 rocket deployed the joint NASA-USGS Landsat 8 Earth observation satellite on Feb. 11, 2013. Most recently, NASA's MAVEN mission to Mars launched on an Atlas 5 on Nov. 18, 2013.

Now comes the launch of TDRS L at 9:05 p.m. EST (0205 GMT).

"Our ULA team started working with NASA to integrate this latest generation of TDRS spacecraft onto Atlas a little more than four years ago," said Vernon Thorp, director of NASA programs at United Launch Alliance.

The Atlas-Centaur rocket has been powered up at Complex 41 and the launcher's guidance system has been put through preflight checks.

The C-band and S-band systems have also been tested in the countdown. They are used for vehicle tracking and telemetry relay, respectively. The countdown continues for launch at 9:05 p.m. EST (0205 GMT).

The countdown begins now for today's launch of the Atlas 5 rocket to deploy NASA's Tracking and Data Relay Satellite L, a fresh spacecraft for the agency's space-based communications network linking orbiting spacecraft with mission controllers.

NASA officials report that nothing is amiss and activities are progressing as planned for flight.

Clocks are picking up the seven-hour sequence of work that will prepare the booser, payload and ground systems for blastoff at 9:05 p.m. EST (0205 GMT).

Soon the launch team will begin powering up the rocket to commence standard pre-flight tests. Over the subsequent few hours, final preps for the Centaur's liquid oxygen and liquid hydrogen systems will be performed, along with a test of the rocket's guidance system and the first stage propulsion and hydraulic preps, internal battery checks and 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.

NASA managers will arrive on console by about 4:30 p.m. EST (2130 GMT) to oversee the final phase of the countdown. The Complex 41 site will be cleared of all personnel at 6:10 p.m. EST (2310 GMT).

A planned half-hour hold begins at 6:25 p.m. EST (2335 GMT) when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 6:52 p.m. EST (2352 GMT) to verify all is in readiness to start fueling the rocket for launch.

Supercold liquid oxygen begins flowing into the Centaur upper stage around 7:12 p.m. EST (0012 GMT), followed by the first stage filling around 7:15 p.m. EST (0015 GMT). 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 8:51 p.m. EST (0151 GMT). 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 9:05 to 9:45 p.m. EST (0205-0245 GMT).

It's launch day at Cape Canaveral, where an Atlas 5 rocket is being prepared for liftoff tonight at 9:05 p.m. EST (0205 GMT) to replenish NASA's space-based network of tracking and data relay satellites to keep mission control in touch with the International Space Station and a host of other orbiting spacecraft.

The 191-foot-tall rocket is mounted atop Cape Canaveral's Complex 41 launch pad, where 35 Atlas 5 boosters have lifted off since August 2002 delivering missions into space for NASA, the U.S. Air Force, the National Reconnaissance Office and commercial customers.

The countdown is scheduled to begin around 2 p.m. EST (1900 GMT) with power-up of the Atlas 5's computer and avionics systems.

We have posted photos of the Atlas 5's half-hour rollout to the launch pad Wednesday morning.

The United Launch Alliance Atlas 5 rocket has arrived at its launch pad after completing the half-hour rollout from the Vertical Integration Facility.

The launch of NASA's newest Tracking and Data Relay Satellite -- TDRS L -- is set for 9:05 p.m. EST Thursday (0205 GMT Friday). Thursday's launch window extends for 40 minutes..

The countdown will begin at 1:55 p.m. EST (1855 GMT), leading to activation of the rocket, final testing and system preps. Cryogenic fueling operations start at approximately 7:02 p. m. EST (0002 GMT).

"The ULA team is proud to be the launch provider for the Tracking Data and Relay Satellite-L (TDRS-L) mission," said Jim Sponnick, ULA's vice president of Atlas and Delta programs. "The TDRS system is the third generation space-based communication system used to provide tracking, telemetry, command and high-bandwidth data-return services for the National Aeronautics and Space Administration's Space Network."

The weather forecast from the Air Force's 45th Weather Squardon calls for just a 10 percent chance of violating launch constraints.

This afternoon, ground crews will plug the rocket into the launch pad's propellant, pneumatic and electrical systems. The first stage will also be loaded with RP-1 kerosene fuel today.

Watch this page for live updates throughout the countdown and flight, plus live streaming video.

We will also be tweeting updates during the countdown, so sign up for our Twitter feed to keep up with the Atlas 5/TDRS L mission and all the latest space news. U.S. readers can also receive test messages on your cellphone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

The Atlas 5 rocket is nearing its destination at Complex 41.

The United Launch Alliance Atlas 5 rocket's rollout to the pad is underway!

This slow, half-hour drive from the 30-story Vertical Integration Facility to Cape Canaveral's Complex 41 pad uses a pair of specially-made "trackmobiles" to carry the rocket's 1.4-million pound mobile launching platform along rail tracks for the 1,800-foot trip.

The 191-foot-tall satellite booster is moving to Cape Canaveral's Complex 41 for the launch of TDRS L, an advanced communications spacecraft to connect mission control with orbiting missions such as the International Space Station, the Hubble Space Telescope and a host of Earth observation and research platforms.

The two-stage rocket and the TDRS L spacecraft were put together inside the assembly building over the past 40 days in preparation for this rollout event. The Atlas 5 is designed to spend minimal time at the launch pad, which does not include a service gantry like many other sites.

It is rollout morning for the Atlas 5 rocket, which is about to emerge from its 30-story Vertical Integration Facility for a quick trip to the launch pad.

The weather outlook has slightly improved for Thursday night's launch window, now showing a 90 percent chance of acceptable conditions.

The only weather concern is with thick clouds.

"On launch day, high pressure is over the peninsula with a relatively dry cold front pushing into North Florida with a slight increase in mid-level moisture," the Air Force weather team says. "Winds from the north-northwest with gusts in the mid-teens during the count and low to mid-teens (230 feet) during the window. The primary concern for launch, although slight, is Thick Clouds associated with a limited increase in moisture in advance of the next cold front overnight Thursday/Friday morning."

The forecast calls for scattered clouds at 3,000 feet, 15,000 feet and 30,000 feet with a launch time temperature of 46-47 degrees Fahrenheit. Winds will be out of the northwest between 12 and 16 knots.

In the event of a 24-hour delay, there is an increased chance of high winds and clouds violating launch weather rules, resulting in a 30 percent chance of conditions preventing liftoff.

United Launch Alliance rocket technicians will roll an Atlas 5 booster to its seaside launch pad this morning ahead of a scheduled liftoff Thursday night.

NASA's TDRS L communications satellite, equipped with mesh antennas and S-band, Ku-band and Ka-band data links, is folded up inside the Atlas 5's four-meter-diameter payload fairing.

The rocket's 1,800-foot rollout will begin at 10 a.m. EST (1500 GMT), following rail tracks from the 30-story Vertical Integration Facility to the Complex 41 launch pad. The 1.4-million-pound mobile launch platform will be propelled by "trackmobiles" for the half-hour journey.

ULA started assembling the Atlas 5 rocket for the TDRS L mission Dec. 13 with stacking of the 106-foot-long bronze first stage. The Centaur upper stage, covered with a white thermal coating, was lifted on top of the rocket Dec. 14.

Built by Boeing Co., the 3.8-ton TDRS L spacecraft was added to the rocket Jan. 13.

The 191-foot-tall Atlas 5 is flying in the "401" configuration with a four-meter payload fairing manufactured at ULA's facility in Harlingen, Texas, no solid-fueled boosters provided and a single RL10 upper stage engine provided by Aerojet Rocketdyne.

The Atlas 5 is making its 43rd flight since debuting in August 2002. Thursday night's liftoff will mark the 36th Atlas 5 launch from Cape Canaveral.

Launch is scheduled for a 40-minute window opening at 9:05 p.m. EST Thursday (0205 GMT Friday).

A fresh satellite for NASA's communications network is set for launch from Florida's Space Coast on Thursday to bolster voice and data links between mission control, the International Space Station and a fleet of orbiting research observatories.

Read our full story.

Air Force weather officials are predicting partly cloudy skies and favorable weather for Thursday night's launch of an Atlas 5 rocket with a NASA tracking and data relay satellite.

The 19-story booster is set for liftoff from Cape Canaveral Air Force Station at 9:05 p.m. EST Thursday (0205 GMT Friday) at the opening of a 40-minute window.

There is an 80 percent chance of acceptable conditions at launch time, according to the U.S. Air Force's 45th Weather Squadron.

Meteorologists expect mostly dry conditions this week, with a cold front set to pass over Central Florida on Tuesday evening. During the Atlas 5 rocket's rollout to the pad at 10 a.m. EST Wednesday, officials expect brisk northwest winds of nearly 30 knots.

"On launch day, high pressure is over Gulf of Mexico as another relatively dry cold front pushes into North Florida with an increase in mid and upper level moisture," forecasters wrote in a weather summary. "Winds continue to trend down during the day from the north-northeast with gusts in the low to mid-teens."

The primary concern for launch is thick clouds in the Atlas 5's flight path.

The forecast calls for scattered clouds at 3,000, 15,000 and 30,000 feet, northeast winds at 12 to 16 knots, and a temperature of about 55 degrees Fahrenheit.

In the event of a 24-hour delay, the weather Friday night is slightly less favorable, with a 30 percent chance of violating launch weather rules.

Officials gave the green light Friday to continue launch preparations for next week's liftoff of a United Launch Alliance Atlas 5 rocket with a NASA tracking and data relay satellite.

NASA and ULA managers met Friday for a flight readiness review to discuss the status of the rocket and the Boeing-built TDRS L payload, which is joining NASA's fleet of communications satellites designed to beam radio and data between the ground and the International Space Station, the Hubble Space Telescope and other missions, ensuring near-constant links between mission control and orbiting spacecraft.

Launch is scheduled from Cape Canaveral's Complex 41 launch pad at 9:05 p.m. EST on Jan. 23 (0205 GMT on Jan. 24), the opening of a 40-minute window.

Technicians attached the 3.8-ton TDRS L satellite to the Atlas 5 rocket Monday. Since then, launch crews have put the vehicle through a combined systems test to verify data and communications connections between the satellite and launcher.

The launch team held a full-up mission dress rehearsal Thursday to practice countdown procedures. The flight readiness review Friday gave the all-clear to continue tracking toward launch next Thursday night.

Officials will meet again Tuesday for the launch readiness review to give the go-ahead for rollout of the 191-foot-tall two-stage booster from its integration building to the launch pad Wednesday morning.

The preliminary weather outlook is favorable for Thursday night's launch window.

NASA's next-generation communications satellite was mounted atop the Atlas 5 rocket Monday, kicking off the final days of preparations to send the science-relay spacecraft into orbit.

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NASA invited media inside the clean room housing the agency's new Tracking and Data Relay Satellite L on Friday, giving reporters and photographers access to the spacecraft as it awaits launch later this month.

See the photos from the clean room.

Technicians at Cape Canaveral stood up the first stage of an Atlas 5 booster Friday, maintaining a near-continuous string of launch campaigns for United Launch Alliance's workhorse rocket, officials said.

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