Return to current status
FRIDAY, DECEMBER 4, 2015
2310 GMT (6:10 p.m. EST)
To recap, gusty ground winds that could pose a safety risk as the United Launch Alliance Atlas 5 rocket climbs away from the pad caused three last-minute holds in the countdown tonight. In the end, available time in the launch window ran out and liftoff had to be delayed until Saturday.

"We just did not feel comfortable launching tonight," said Vern Thorpe, ULA's program manager for NASA missions.

2303 GMT (6:03 p.m. EST)
"It was a gallant effort," launch director Bill Cullen says. "Try again tomorrow."

Launch will be possible beginning at the opening of a 30-minute window at 5:10:38 p.m. EST.

Weather is 30 percent favorable, with ground winds and clouds the issues.

2300 GMT (6:00 p.m. EST)
The launch team is going through safing procedures.
2259 GMT (5:59 p.m. EST)
HOLD and SCRUB. Ground winds went out of limits and the launch window now closed.
2259:12 GMT (5:59:12 p.m. EST)
T-minus 4 minutes and counting. Clocks have resumed for the final minutes of today's countdown to launch the Atlas 5 rocket carrying Cygnus resupply ship for the space station.
2258:12 GMT (5:58:12 p.m. EST)
Countdown clocks will resume in one minute.
2257 GMT (5:57 p.m. EST)
Team readiness poll re-conducted and all console operators are set to proceed. This will be the final attempt of the night.
2246 GMT (5:46 p.m. EST)
DELAY. Liftoff has been targeted for the end of today's launch window at 6:03:12 p.m. EST.
2245 GMT (5:45 p.m. EST)
The ground winds limit was tripped, NASA commentator Mike Curie says.
2244 GMT (5:44 p.m. EST)
HOLD!
2244:12 GMT (5:44:12 p.m. EST)
T-minus 4 minutes and counting. Clocks have resumed for the final minutes of today's countdown to launch the Atlas 5 rocket carrying Cygnus resupply ship for the space station.
2243:12 GMT (5:43:12 p.m. EST)
Countdown clocks will resume in one minute.
2242 GMT (5:42 p.m. EST)
The team has been re-polled and the GO has been given to proceed with liftoff at 5:48:12 p.m.
2235 GMT (5:35 p.m. EST)
The available launch opportunities today:

* 5:48:12 p.m. EST
* 5:55:42 p.m. EST
* 6:03:12 p.m. EST

2232 GMT (5:32 p.m. EST)
DELAY. The target launch time has shifted to the opportunity at 5:48:12 p.m. EST.
2230 GMT (5:30 p.m. EST)
HOLD at T-minus 3 minutes, 9 seconds.
2229 GMT (5:29 p.m. EST)
T-minus 3 minutes, 50 seconds. The ground pyrotechnics have been enabled.
2229:12 GMT (5:29:12 p.m. EST)
T-minus 4 minutes and counting. Clocks have resumed for the final minutes of today's countdown to launch the Atlas 5 rocket carrying Cygnus resupply ship for the space station.
2228:12 GMT (5:28:12 p.m. EST)
Countdown clocks will resume in one minute.
2227 GMT (5:27 p.m. EST)
The ULA launch director has given the permission to fly today.
2227 GMT (5:27 p.m. EST)
Polling of the team by Atlas launch conductor has occurred. All systems -- and weather -- are reported "go" to continue with the countdown for liftoff at 5:33 p.m. EST.
2224 GMT (5:24 p.m. EST)
Standing by for the launch team readiness polls.

A reminder that if you will be away from your computer but would like to receive countdown updates, sign up for our Twitter feed to get text messages on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

2222 GMT (5:22 p.m. EST)
The Atlas 5 rocket stands 194 feet tall and weighs nearly 740,000 pounds at launch.
2220 GMT (5:20 p.m. EST)
No problems being reported by the launch team. Countdown continues to sit in the hold period at T-minus 4 minutes, waiting for the launch window to open at 5:33:12 p.m. EST.
2217 GMT (5:17 p.m. EST)
Cygnus is on internal power and ready for launch.
2212 GMT (5:12 p.m. EST)
The Cygnus freighter nestled inside the nose of the Atlas 5 rocket is switching to internal power for launch.
2210 GMT (5:10 p.m. EST)
Here's a look at some stats about today's mission. This will be:
2206 GMT (5:06 p.m. EST)
Weather is observed GO at the current time for launch of the Atlas 5 rocket.
2203 GMT (5:03 p.m. EDT)
Thirty minutes from liftoff of the Atlas 5 rocket on its first mission for the International Space Station.
2159 GMT (4:59 p.m. EST)
T-minus 4 minutes and holding. The countdown has entered the planned 30-minute hold to give the launch team a chance to review all systems and assess the weather before pressing ahead with liftoff.
2158 GMT (4:58 p.m. EST)
T-minus 5 minutes. Standing by to go into the final built-in hold.
2153 GMT (4:53 p.m. EST)
Now 40 minutes till launch. Rumbling away from the planet on nearly a million pounds of thrust, the Atlas 5 rocket will be flying in a basic, two-stage configuration without any added strap-on solid motors. The vehicle sports an aluminum, 14-foot-diameter nose cone that encapsulates the Cygnus spacecraft during the atmospheric ascent before being shed.

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

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

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

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

Covered with insulating foam, this stage stretches 41.5 feet in length and 10 feet in diameter. Centaur must perform a lengthy burn to loft Cygnus into the proper orbit around the planet.

2147 GMT (4:47 p.m. EST)
The fuel-fill sequence for the first stage main engine is starting.
2143 GMT (4:43 p.m. EST)
Atlas 5 represents the culmination of evolution stretching back several decades to America's first intercontinental ballistic missile. At the dawn of the space age, boosters named Atlas launched men into orbit during Project Mercury and became a frequent vehicle of choice to haul civil, military and commercial spacecraft to orbit.

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

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

Today marks the 60th flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 59 missions since debuting in August 2002, the Atlas 5 has flown 23 flights dedicated to the Defense Department, 12 for NASA, 12 with spy satellites for the National Reconnaissance Office and 12 commercial missions with communications and Earth-observing spacecraft.

2133 GMT (4:33 p.m. EST)
Now 60 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen is complete as the countdown continues as planned for a liftoff at 5:33 p.m. EST.
2128 GMT (4:28 p.m. EST)
Fast-filling of the first stage liquid oxygen tank has been completed. Topping mode is now underway.
2121 GMT (4:21 p.m. EST)
The liquid hydrogen tank in the Centaur upper stage just reached the 96 percent level. Topping is now beginning.
2118 GMT (4:18 p.m. EST)
The first stage liquid oxygen has reached the 80 percent level.
2111 GMT (4:11 p.m. EST)
Centaur's liquid hydrogen tank is 30 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Aerojet Rocketdyne-made RL10 engine.
2104 GMT (4:04 p.m. EST)
Half of the Atlas liquid oxygen tank has been filled so far.
2103 GMT (4:03 p.m. EST)
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.
2055 GMT (3:55 p.m. EST)
First stage liquid oxygen tank is 30 percent full thus far. Chilled to Minus-298 degrees F, the liquid oxygen will be used with RP-1 kerosene by the RD-180 main engine on the first stage during the initial four minutes of flight today. The 25,000 gallons of RP-1 were loaded into the rocket Wednesday.
2050 GMT (3:50 p.m. EST)
Upper stage liquid oxygen has reached flight level.
2047 GMT (3:47 p.m. EST)
The first stage liquid oxygen flow rate is switching from slow-fill to fast-fill mode.
2044 GMT (3:44 p.m. EST)
The Centaur liquid oxygen tank reached the 96 percent level. The topping off process is starting now.
2039 GMT (3:39 p.m. EST)
The chilldown conditioning of liquid hydrogen propellant lines at Complex 41 is starting to prepare the plumbing for transferring the Minus-423 degree F fuel into the rocket. The Centaur holds about 12,700 gallons of the cryogenic propellant.
2036 GMT (3:36 p.m. EST)
The chilldown conditioning of the systems for the first stage liquid oxygen tank have been completed. And a "go" has been given to begin pumping supercold liquid oxygen into the Atlas 5's first stage.

The Common Core Booster stage's liquid oxygen tank is the largest tank to be filled today. It holds 49,000 gallons of cryogenic oxidizer for the RD-180 main engine.

2035 GMT (3:35 p.m. EST)
Sixty percent of the Centaur liquid oxygen tank has been filled so far.
2030 GMT (3:30 p.m. EST)
Learn more about the process United Launch Alliance uses to fuel Atlas 5 rockets.
2026 GMT (3:26 p.m. EST)
The Centaur liquid oxygen tank has reached the 20 percent mark.
2018 GMT (3:18 p.m. EST)
Filling of the Centaur upper stage with about 4,150 gallons of liquid oxygen has begun at Cape Canaveral's Complex 41 following the thermal conditioning of the transfer pipes.

The liquid oxygen -- chilled to Minus-298 degrees F -- will be consumed during the launch by the Centaur's single RL10 engine along with liquid hydrogen to be pumped into the stage a little later in the countdown. The Centaur will provide the thrust to put Cygnus into orbit.

2011 GMT (3:11 p.m. EST)
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.
2003:12 GMT (3:03:12 p.m. EST)
T-minus 2 hours and counting! The launch countdown is continuing on schedule for today's flight of the Atlas 5 rocket to deploy the Cygnus cargo freighter for Orbital ATK.

Clocks have one more built-in hold planned at T-minus 4 minutes. During that pause the final "go" for launch will be given. All remains targeted for liftoff at 5:33:12 p.m. EST from Cape Canaveral's Complex 41.

In the next couple of minutes, chilldown thermal conditioning of the mobile launch platform upon which the rocket stands will begin. This is meant to ease the shock on equipment when supercold cryogenic propellants start flowing into the rocket.

2001 GMT (3:01 p.m. EST)
After briefing his team on procedures before entering into the final two hours of the countdown, the launch conductor at the Atlas Spaceflight Operations Center just held a pre-fueling readiness poll.

All console operators reported a "ready" status.

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

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

1953 GMT (2:53 p.m. EST)
Man stations for cryogenic tanking. That's the word to the launch team.
1940 GMT (2:40 p.m. EST)
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1933 GMT (2:33 p.m. EST)
T-minus 2 hours and holding. The countdown has just entered the first of two planned holds over the course of the day that will lead to the 5:33 p.m. EST launch of the Atlas rocket. The holds give the team some margin in the countdown timeline to deal with technical issues or any work that is running behind. The final hold is scheduled to occur at T-minus 4 minutes.
1924 GMT (2:24 p.m. EST)
Crews are departing the launch pad following completion of all hands-on work today. Fueling operations for the launch vehicle will be starting in about an hour.
1920 GMT (2:20 p.m. EST)
The weather outlook at Cape Canaveral continues to show a 40 percent chance of acceptable launch conditions. With a drying trend in the mid-level atmosphere, there is a lessening concern for thick clouds at launch time. The primary concerns, however, are cumulus clouds coming ashore from the ocean and gusty ground winds.
1805 GMT (1:05 p.m. EST)
The early portion of the countdown has been going smoothly. There are no issues being reported in the count and activities are on schedule.
1614 GMT (11:14 a.m. EST)
In today's first weather briefing to mission managers, all current conditions are observed GO for launch of the Atlas 5 rocket and the odds for the launch window now stand at 40 percent favorable. Meteorologists will be watching the cloud cover and thickness and ground winds during the launch window.

The outlook predicts a broken decks of clouds at 2,000 feet, 10,000 feet and 24,000 feet, showers in the area, good visibility, northeasterly winds of 22 gusting to 26 knots and a temperature of 70 degrees F.

1513 GMT (10:13 a.m. EST)
A new countdown just began ticking at Cape Canaveral for today's rescheduled launch of the Atlas 5 rocket and the Cygnus cargo freighter bound for the International Space Station.

It is been three years -- a span of 26 flights -- since an Atlas 5 rocket did not launch on the first attempt after being fueled.

Clocks are picking up the seven-hour sequence of work that will prepare the booster, payload and ground systems for blastoff.

The available launch opportunities today:

* 5:33:12 p.m. EST
* 5:40:42 p.m. EST
* 5:48:12 p.m. EST
* 5:55:42 p.m. EST
* 6:03:12 p.m. EST

The launch team will soon begin powering up the rocket to commence standard pre-flight tests. Over the subsequent few hours, final preps for the Centaur's liquid oxygen and liquid hydrogen systems will be performed, along with a test of the rocket's guidance system and the first stage propulsion and hydraulic preps, internal battery checks and testing of the GPS metric tracking system used to follow the rocket as it flies downrange, plus a test of the S-band telemetry relay system.

A planned 30-minute hold begins when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled to verify all is in readiness to start fueling the rocket for launch.

Supercold liquid oxygen begins flowing into the Centaur upper stage, followed by the first stage filling. Liquid hydrogen fuel loading for Centaur will be completed a short time later.

A final 30-minute hold is scheduled at the T-minus 4 minute mark. That pause will give everyone a chance to finish any late work and assess the status of the rocket, payload, Range and weather before proceeding into the last moments of the countdown.

THURSDAY, DECEMBER 3, 2015
The odds of acceptable launch weather conditions at Cape Canaveral look relatively bleak for the Atlas 5 rocket over the next few days. The Air Force meteorologists predict:

* 30 percent favorable conditions on Friday, with 30-minute window opening at 5:33 p.m. EST (2233 GMT), due to cumulus clouds, cloud thickness and ground winds.

* 30 percent favorable conditions on Saturday, with 30-minute window opening at 5:10 p.m. EST (2210 GMT), due to ground winds and cumulus clouds.

* 40 percent favorable conditions on Sunday, with 30-minute window opening at 4:44 p.m. EST (2144 GMT), due to ground winds and cumulus clouds.

After scrubbing Thursday evening's launch attempt by persistent rain and clouds, forecasters issued this outlook:

"Cloudy conditions with an elevated rainshower threat persists through mid-afternoon Friday as the stationary front remains stalled in South Florida. By late afternoon, an upper level trough advances into the Gulf of Mexico with some limited thinning of could cover from north to south. The bulk of showers transit south of the Spaceport with an isolated rainshower threat persisting over Central Florida. The pressure gradient strengthens by late afternoon with northeast winds gusting in the mid-20s during the window," forecasters said late Thursday.

2315 GMT (6:15 p.m. EST)
ULA launch director Bill Cullen to launch conductor Scott Barney in calling the scrub: "We'll have to scrub for today. Your performace has been top notch, very professional, great discipline, good teamwork across the board. Please set up for a 24-hour recycle."
2311 GMT (6:11 p.m. EST)
SCRUB. Foul weather at Cape Canaveral courtesy of a passing cool front bringing disturbed conditions and thick clouds across Central Florida has forced managers to call off today's launch attempt for the Atlas 5 rocket with the commercial Cygnus cargo freighter bound for the International Space Station.

It is the first time since 2012 that an Atlas 5 launch has been postponed after fueling had occurred. The vehicle was on a run of 26 consecutive missions to fuel-and-launch on the first attempt.

The weather deteriorated throughout the countdown today, falling from a 60 percent chance of good weather to just 30 percent and then 10 percent. The front draped over the Cape violated the launch rules throughout the final phases of the count and 30-minute launch window.

Liftoff is reset for Friday during a 30-minute launch window opening at 5:33:11 p.m. EST (2233:11 GMT). However, similar weather is forecast, with a 70 percent chance of conditions violating the launch rules.

If the launch does go tomorrow, rendezvous and berthing of the Cygnus cargo freighter at the International Space Station would happen on Monday morning.

There are three suitable days remaining to launch this week that would lead to favorable rendezvous opportunities before the International Space Station's traffic pattern gets complicated:

December 4
Launch: 5:33 p.m. EST

December 5
Launch: 5:10 p.m. EST

December 6
Launch: 4:44 p.m. EST

2305 GMT (6:05 p.m. EST)
Nothing new to report on the weather. Countdown remains holding.
2255 GMT (5:55 p.m. EST)
DELAY. Liftoff has been targeted for the end of today's launch window at 6:25:45 p.m. EST.
2250 GMT (5:50 p.m. EST)
DELAY. Liftoff continues to hold on the cloudy and rainy weather. The target launch time has shifted to the next opportunity at 6:10:45 p.m. EST.
2248 GMT (5:48 p.m. EST)
The hold has been extended again. No technical issues with the Atlas vehicle, but weather still miserable.
2240 GMT (5:40 p.m. EST)
DELAY. Liftoff will not happen at 5:55:45 p.m. EST due to unacceptable local weather. The target launch time has shifted to the next opportunity at 6:03:15 p.m. EST.
2236 GMT (5:36 p.m. EST)
T-minus 4 minutes and holding. The countdown has entered the planned 15-minute hold to give the launch team a chance to review all systems and assess the weather before pressing ahead with liftoff.

But at this time the weather is NO GO due to clouds and disturbed conditioned.

2235 GMT (5:35 p.m. EST)
T-minus 5 minutes. Standing by to go into the final built-in hold.
2228 GMT (5:28 p.m. EST)
Weather remains NO GO with little hope of clearing the violation of the launch rules.
2226 GMT (5:26 p.m. EST)
The launch director has instructed the team to not perform the fuel-fill sequence for the first stage main engine at this time, pending weather.
2225 GMT (5:25 p.m. EST)
Here's a look at some stats about today's mission. This will be:
2215 GMT (5:15 p.m. EST)
Now 40 minutes till launch. Rumbling away from the planet on nearly a million pounds of thrust, the Atlas 5 rocket will be flying in a basic, two-stage configuration without any added strap-on solid motors. The vehicle sports an aluminum, 14-foot-diameter nose cone that encapsulates the Cygnus spacecraft during the atmospheric ascent before being shed.

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

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

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

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

Covered with insulating foam, this stage stretches 41.5 feet in length and 10 feet in diameter. Centaur must perform a lengthy burn to loft Cygnus into the proper orbit around the planet.

2210 GMT (5:10 p.m. EST)
Atlas 5 represents the culmination of evolution stretching back several decades to America's first intercontinental ballistic missile. At the dawn of the space age, boosters named Atlas launched men into orbit during Project Mercury and became a frequent vehicle of choice to haul civil, military and commercial spacecraft to orbit.

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

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

Today marks the 60th flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 59 missions since debuting in August 2002, the Atlas 5 has flown 23 flights dedicated to the Defense Department, 12 for NASA, 12 with spy satellites for the National Reconnaissance Office and 12 commercial missions with communications and Earth-observing spacecraft.

2207 GMT (5:07 p.m. EST)
Fast-filling of the first stage liquid oxygen tank has been completed. Topping mode is now underway.
2203 GMT (5:03 p.m. EST)
The liquid hydrogen tank in the Centaur upper stage just reached the 96 percent level. Topping is now beginning.
2159 GMT (4:59 p.m. EST)
Weather is now 90% NO GO for a launch today.
2154 GMT (4:54 p.m. EST)
The first stage liquid oxygen has reached the 80 percent level.
2153 GMT (4:53 p.m. EST)
Centaur's liquid hydrogen tank is 40 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Aerojet Rocketdyne-made RL10 engine.
2150 GMT (4:50 p.m. EST)
The available launch opportunities today:

* 5:55:45 p.m. EST
* 6:03:15 p.m. EST
* 6:10:45 p.m. EST
* 6:25:45 p.m. EST

The shot at 6:18 p.m. was eliminated by a collision avoidance blackout period.

2142 GMT (4:42 p.m. EST)
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.
2140 GMT (4:40 p.m. EST)
Half of the Atlas liquid oxygen tank has been filled so far.
2135 GMT (4:35 p.m. EST)
First stage liquid oxygen tank is 40 percent full thus far. Chilled to Minus-298 degrees F, the liquid oxygen will be used with RP-1 kerosene by the RD-180 main engine on the first stage during the initial four minutes of flight today. The 25,000 gallons of RP-1 were loaded into the rocket Wednesday.
2129 GMT (4:29 p.m. EST)
Upper stage liquid oxygen has reached flight level.
2123 GMT (4:23 p.m. EST)
The Centaur liquid oxygen tank reached the 96 percent level. The topping off process is starting now.
2121 GMT (4:21 p.m. EST)
The first stage liquid oxygen flow rate is switching from slow-fill to fast-fill mode.
2115 GMT (4:15 p.m. EST)
Sixty percent of the Centaur liquid oxygen tank has been filled so far.
2112 GMT (4:12 p.m. EST)
The chilldown conditioning of the systems for the first stage liquid oxygen tank have been completed. And a "go" has been given to begin pumping supercold liquid oxygen into the Atlas 5's first stage.

The Common Core Booster stage's liquid oxygen tank is the largest tank to be filled today. It holds 49,000 gallons of cryogenic oxidizer for the RD-180 main engine.

2107 GMT (4:07 p.m. EST)
Learn more about the process United Launch Alliance uses to fuel Atlas 5 rockets.
2105 GMT (4:05 p.m. EST)
The Centaur liquid oxygen tank has reached the 20 percent mark.
2057 GMT (3:57 p.m. EST)
Filling of the Centaur upper stage with about 4,150 gallons of liquid oxygen has begun at Cape Canaveral's Complex 41 following the thermal conditioning of the transfer pipes.

The liquid oxygen -- chilled to Minus-298 degrees F -- will be consumed during the launch by the Centaur's single RL10 engine along with liquid hydrogen to be pumped into the stage a little later in the countdown. The Centaur will provide the thrust to put Cygnus into orbit.

2048 GMT (3:48 p.m. EST)
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.
2040:45 GMT (3:40:45 p.m. EST)
T-minus 2 hours and counting! And the countdown has resumed from the lengthened T-minus 2 hour hold. It was extended a bit while the launch team backed out of the high-wind precautions that had been taken on the Centaur upper stage tanks. Earlier, officials had been worried about incoming storms.

The launch conductor at the Atlas Spaceflight Operations Center has completed a pre-fueling readiness poll. All console operators reported a "ready" status now.

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

Clocks have one more built-in hold planned at T-minus 4 minutes. During that pause the final "go" for launch will be given for liftoff at 5:55:45 p.m. EST from Cape Canaveral's Complex 41.

The hold at T-minus 4 minutes will be shortened to make up for this unplanned extension at T-minus 2 hours.

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.

2026 GMT (3:26 p.m. EST)
This 30-minute hold has been extended a few minutes to put the Centaur tank pressures in the normal configuration. They had been put in a high-wind state to brace for any storms this afternoon.

Meanwhile, the odds for launch time have improved to 40 percent favorable.

2015 GMT (3:15 p.m. EST)
Man stations for cryogenic tanking. That's the word to the launch team.
2010 GMT (3:10 p.m. EST)
A reminder that if you will be away from your computer but would like to receive countdown updates, sign up for our Twitter feed to get text messages on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)
1955 GMT (2:55 p.m. EST)
T-minus 2 hours and holding. The countdown has just entered the first of two planned holds over the course of the day that will lead to the 5:55 p.m. EST launch of the Atlas rocket. The holds give the team some margin in the countdown timeline to deal with technical issues or any work that is running behind. The final hold is scheduled to occur at T-minus 4 minutes.
1945 GMT (2:45 p.m. EST)
The weather outlook at has deteriorated to only a 30 percent chance of acceptable conditions for launch today due to the cool front conditions over Florida.
1940 GMT (2:40 p.m. EST)
Crews are departing the launch pad following completion of all hands-on work this afternoon.

The early portion of the countdown has been going smoothly. There are no issues being reported in the count and activities are on schedule.

1630 GMT (11:30 a.m. EST)
In today's first weather briefing to mission managers, all current conditions are observed GO for launch of the Atlas 5 rocket and odds for the launch window stand at 60 percent favorable. Meteorologists will be watching is the cloud cover and thickness.

"A cold (cool) front pushed into Central Florida overnight and is slowly advancing toward South Florida where it should stall this evening. High moisture and cloudy conditions will persist over Central Florida today and tomorrow with a threat of isolated showers behind the front," forecasters say.

"The primary concerns for launch are Thick Clouds, Cumulus Clouds, and Disturbed Weather."

The outlook predicts a broken decks of clouds at 2,500 feet, 14,000 feet and 28,000 feet, isolated showers in the area, good visibility, northerly winds of 18 gusting to 22 knots and a temperature of 69 degrees F.

The Atlas-Centaur rocket has been powered up at Complex 41 and guidance system testing is getting started for today's launch, as the countdown progresses as planned.

1530 GMT (10:30 a.m. EST)
The countdown is beginning for today’s launch of the Atlas 5 rocket to haul the Cygnus cargo freighter into orbit for the International Space Station.

Clocks are picking up the seven-hour sequence of work that will prepare the booster, payload and ground systems for blastoff at 5:55:45 p.m. EST (2255:45 GMT).

The launch team will begin powering up the rocket to commence standard pre-flight tests. Over the subsequent few hours, final preps for the Centaur's liquid oxygen and liquid hydrogen systems will be performed, along with a test of the rocket's guidance system and the first stage propulsion and hydraulic preps, internal battery checks and testing of the GPS metric tracking system used to follow the rocket as it flies downrange, plus a test of the S-band telemetry relay system.

A planned hold begins when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled to verify all is in readiness to start fueling the rocket for launch.

Supercold liquid oxygen begins flowing into the Centaur upper stage, followed by the first stage filling. Liquid hydrogen fuel loading for Centaur will be completed a short time later.

A final hold is scheduled at the T-minus 4 minute mark. That pause will give everyone a chance to finish any late work and assess the status of the rocket, payload, Range and weather before proceeding into the last moments of the countdown.

The launch window extends 30 minutes to 6:25 p.m. EST (2325 GMT).

WEDNESDAY, DECEMBER 2, 2015
Taking aim at the International Space Station with a cargo-laden resupply ship, a United Launch Alliance Atlas 5 rocket moved from the assembly building to the pad this morning for Thursday's liftoff.

Launch is targeted for 5:55:45 p.m. EST at the opening of a 30-minute window, leading to rendezvous with the station early Sunday morning.

The Atlas 5 is topped by Orbital ATK's commercial Cygnus supply freighter, which is filled with over 7,300 pounds of provisions, equipment and science gear for the station and its residents.

The United Launch Alliance booster was wheeled out aboard a mobile platform, emerging from the Cape Canaveral facility where the rocket's two stages and the payload were integrated over the past three weeks.

The slow drive from the 30-story Vertical Integration Facility to the launch pad, which began at 10 a.m., used a pair of specially-made “trackmobiles” to carry the rocket's 1.4-million pound mobile launching platform along rail tracks for the 1,800-foot trip.

The 194-foot-tall booster will launch the Cygnus craft into a 143-mile to begin its chase of the station. Deployment of the spacecraft from the rocket occurs 21 minutes after liftoff.

The rocket is flying the 401 vehicle configuration. The version features two stages and a four-meter-diameter nose cone. It is powered off the launch pad by an RD AMROSS RD-180 main engine. The Centaur upper stage is equipped with an Aerojet Rocketdyne RL10C-1.

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

There is a 60 percent chance of acceptable launch weather.

There are four suitable days to launch this week that would lead to favorable rendezvous opportunities before the International Space Station's traffic pattern gets complicated:

December 3
Launch: 5:55:45 p.m. EST
ISS arrival: Dec. 6 @ 5:30 a.m. EST

December 4
Launch: 5:33:08 p.m. EST
ISS arrival: Dec. 7 @ 6:15 a.m. EST

December 5
Launch: 5:10:35 p.m. EST
ISS arrival: Dec. 9 @ 6:10 a.m. EST

December 6
Launch: 4:44:53 p.m. EST
ISS arrival: Date/time to be confirmed

Based upon an on-time launch Thursday, this is the timeline for Cygnus from liftoff through capture by the space station's robotic arm. See our timeline.

TUESDAY, DECEMBER 1, 2015
LRR: Mission managers today granted approval to proceed with Wednesday's rollout of the Atlas 5 rocket and Thursday's launch to deliver the commercial Cygnus cargo ship into orbit to resupply the International Space Station.

Read our full story.

MONDAY, NOVEMBER 30, 2015
TIMELINE: Follow the Atlas 5 rocket’s ascent into orbit from Cape Canaveral’s Complex 41 launch pad with the Orbital ATK Cygnus resupply ship for the International Space Station. Launch is scheduled for Thursday at 5:55 p.m. EST (2255 GMT).

See our full timeline.

SUNDAY, NOVEMBER 29, 2015
PREVIEW: Lending a helping hand to resume the stalled U.S. supply chain to the International Space Station, a United Launch Alliance Atlas 5 rocket will send a commercial Cygnus cargo craft in pursuit of the outpost Thursday.

Read our full story.