2123 GMT (5:23 p.m. EDT)
Arianespace chairman and CEO Stephane Israel confirms today's launch was a success, extending the Ariane 5's streak to 67 successful flights in a row since 2003.
2117 GMT (5:17 p.m. EDT)
The Intelsat 34 communications satellite has deployed from the Ariane 5 rocket. Built by Space Systems/Loral, the spacecraft will operate at least 15 years, beaming Internet connectivity to airliners and ships in the North Atlantic and video services to Latin America.
2105 GMT (5:05 p.m. EDT)
Plus+31 minutes. The Sylda dual-payload adapter has jettisoned, setting the stage for separation of Intelsat 34 at Plus+41 minutes, 54 seconds.
2102 GMT (5:02 p.m. EDT)
Separation of Eutelsat 8 West B confirmed, beginning a 15-year mission for Paris-based Eutelsat. The satellite carries 40 active Ku-band transponders and 10 C-band transponders to broadcast HD and Ultra HD television over the Middle East and North Africa.

The Ariane 5's Sylda 5 dual payload adapter will be separated next, revealing the MSG 4 spacecraft for its deployment in a few minutes.

2059 GMT (4:59 p.m. EDT)
Plus+25 minutes, 10 seconds. The rocket's second stage shut down as scheduled. The upper stage is now maneuvering into the correct orientation for deployment of Eutelsat 8 West B.
2058 GMT (4:58 p.m. EDT)
Plus+24 minutes. The rocket is surpassing a speed of 20,500 mph. Shutdown of the upper stage is about a minute from now. A tracking station in Malindi, Kenya, is now in contact with Ariane 5.
2055 GMT (4:55 p.m. EDT)
Plus+21 minutes. The upper stage will shut down at Plus+24 minutes, 54 seconds, after reaching a target orbit with a low point of 156 miles, a high point of 22,273 miles, and an inclination of 4.7 degrees. The rocket is now being tracked by a ground station in Libreville, Gabon.
2052 GMT (4:52 p.m. EDT)
Plus+18 minutes, 30 seconds. Altitude is 186 km and velocity is 8.37 km/s. After intentionally losing altitude in order to gain speed, the Ariane 5 is now climbing again.
2051 GMT (4:51 p.m. EDT)
Plus+17 minutes. Everything is going well with the burn of the upper stage HM7B engine as the Ariane 5 races across the Atlantic Ocean at 8.12 kilometers per second, or more than 18,000 mph.
2048 GMT (4:48 p.m. EDT)
Plus+14 minutes. This upper stage engine is about 5 minutes into a planned 16-minute burn. Ariane 5 is at an altitude of 160 kilometers. A tracking station on Ascension Island in the Atlantic Ocean has picked up signals from the Ariane 5.
2043 GMT (4:43 p.m. EDT)
Plus+9 minutes, 45 seconds. The Ariane 5 has passed over the horizon from Kourou and is now out of range of the Galliot tracking station near the launch pad.
2043 GMT (4:43 p.m. EDT)
Plus+9 minutes, 15 seconds. The main cryogenic stage's Vulcain engine has cut off and the spent stage has separated. It will fall back into the atmosphere into the Atlantic Ocean off the west coast of Africa.

And the upper stage's HM7B engine is now firing to inject the Eutelsat 8 West B and Intelsat 34 satellites into orbit.

2042 GMT (4:52 p.m. EDT)
Plus+8 minutes. Now in range of a communications station in Natal, Brazil, the launcher is about to shut down its first stage and ignite its cryogenic upper stage. Downrange distance is now about 1,200 km, altitude is 160 km and velocity is 5.68 km/s.
2039 GMT (4:39 p.m. EDT)
Plus+5 minutes. Altitude is 157 km, downrange distance is 458 km and velocity is 3.02 km/s.
2037 GMT (4:37 p.m. EDT)
Plus+3 minutes, 30 seconds. Separation of the rocket's nose cone has been confirmed. The Ariane 5 core stage will continue burning until about Plus+9 minutes into the mission.
2036 GMT (4:36 p.m. EDT)
Plus+2 minutes, 30 seconds. The solid rocket boosters have been jettisoned from the Ariane 5 rocket's core stage after consuming approximately 480 metric tons of propellant. The liquid-fueled Vulcain 2 main engine continues to fire to propel the vehicle and its satellite payload to space.
2035 GMT (4:35 p.m. EDT)
Plus+60 seconds. The vehicle is on the proper heading as it rides the power of the twin solid rocket boosters and main stage liquid-fueled engine.
2034:08 GMT (4:34:08 p.m. EDT)
Liftoff of an Ariane 5 rocket on a dual-satellite delivery mission with Eutelsat 8 West B and Intelsat 34!
2033 GMT (4:33 p.m. EDT)
Minus-1 minute. A fast-paced series of events leading to launch will begin at Minus-37 seconds when the automated ignition sequence is started. The water suppression system at the launch pad will start at Minus-30 seconds. At Minus-22 seconds, overall control will be given to the onboard computer. The Vulcain main engine will be readied for ignition with hydrogen chilldown starting at Minus-18 seconds.

The residual hydrogen burn flares will fire beneath the Vulcain engine at Minus-6 seconds to burn away any free hydrogen gas. At Minus-3 seconds, onboard systems take over and the two inertial guidance systems go to flight mode. Vulcain main engine ignition occurs at Minus-0 seconds with checkout between Plus+4 and 7 seconds. If there are no problems found, the solid rocket boosters are ignited at Plus+7.0 seconds for liftoff at Plus+7.3 seconds.

2033 GMT (4:33 p.m. EDT)
Minus-2 minutes. The Vulcain main engine supply valves are being opened. And the ground valves for engine chilldown are being closed.
2031 GMT (4:31 p.m. EDT)
Minus-3 minutes. The scheduled launch time has been loaded into the rocket's main computer system. The main stage tank pressures should now be at flight level.
2030 GMT (4:30 p.m. EDT)
Minus-4 minutes. Pressurization is now underway for the main cryogenic stage's liquid oxygen and hydrogen tanks. Also, final pyrotechnic arming is starting.
2039 GMT (4:29 p.m. EDT)
Minus-5 minutes. All status panel lights remain green, indicating no problems right now that could prevent blastoff at 2034 GMT.
2038 GMT (4:28 p.m. EDT)
Minus-6 minutes. Liquid hydrogen and liquid oxygen supplies of the main cryogenic stage are being verified at flight level. Also, the pyrotechnic line safety barriers are being armed.
2037 GMT (4:27 p.m. EDT)
Minus-7 minutes and counting. The countdown has resumed after a 24-minute hold to complete resolution of a problem from earlier in the countdown that delayed the start of fueling.

The synchronized sequence has started. Computers are now in control of this automated final phase of the launch countdown to prepare the rocket and ground systems for liftoff. There are three computers running the countdown - one aboard the Ariane 5 and two redundant computers at the launch complex.

The launch time is set for 2034:01 GMT (4:34:01 p.m. EDT). Liftoff actually occurs even seconds later with ignition of the solid rocket boosters.

2006 GMT (4:06 p.m. EDT)
The launch window this evening extends to 2156 GMT (5:56 p.m. EDT). The problem appears to be with ground infrastructure at the Guiana Space Center.
2003 GMT (4:03 p.m. EDT)
Minus-7 minutes and holding. The countdown has stopped before entering the synchronized sequence due to a problem.
2002 GMT (4:02 p.m. EDT)
Minus-8 minutes. The synchronized countdown sequence is supposed to begin in one minute, transferring all control over to computers.
2000 GMT (4:00 p.m. EDT)
Minus-10 minutes. The synchronized launch sequence will begin in three minutes.
1958 GMT (3:58 p.m. EDT)
Minus-12 minutes. All parameters, including weather and technical readiness, are reporting green on the status board inside the Jupiter control room at the Guiana Space Center.
1950 GMT (3:50 p.m. EDT)
Minus-20 minutes. The Synchronized Sequence is being prepped for activation. This computer-run sequence assumes control of the countdown at the Minus-7 minute mark to perform the final tasks to place the rocket and pad systems in launch configuration.

At Minus-4 seconds, the rocket's onboard computer will take over control of main engine start, health checks of the powerplant and solid rocket booster ignition commanding for liftoff.

1940 GMT (3:40 p.m. EDT)
Minus-30 minutes. Today's launch will deliver the Eutelsat 8 West B and Intelsat 34 communications satellites to an orbit targeting a planned high point of 22,273 miles, a targeted low point of 156 miles and an inclination of 4.7 degrees.

The satellites will use their on-board engines to raise their orbits and position themselves over the equator.

The Eutelsat 8 West B telecom satellite occupies the upper position in the rocket’s dual-payload berth. Owned by Paris-based Eutelsat, the satellite is beginning a 15-year service life broadcasting high-definition and Ultra HD television services over the Middle East and North Africa. It will also provide C-band telecom support over Africa and South America, according to Eutelsat.

The 12,747-pound Eutelsat 8 West B spacecraft, built by in Palo Alto, California by Space Systems/Loral, will deploy first from the Ariane 5’s payload stack at T+plus 28 minutes, 17 seconds.

A Sylda 5 adapter will be discarded a few minutes later, revealing the 7,275-pound Intelsat 34 spacecraft, the mission’s other satellite passenger.

Manufactured by Thales Alenia Space in France, Intelsat 34 will broadcast direct-to-home television to Brazil, distribute video services across Latin American for programmers like HBO and Fox, and provide broadband connectivity for travelers in the air and at sea over the North Atlantic.

Separation of the Intelsat 34 spacecraft from the Ariane 5 rocket is scheduled at T+plus 41 minutes, 54 seconds.

The payloads have a combined mass of approximately 21,874 pounds, or 9,922 kilograms, including the barrel-shaped Sylda dual-payload adapter. Sign up to follow us on Twitter for the latest launch updates and space news.

1925 GMT (3:25 p.m. EDT)
Minus-45 minutes. All parameters continue to look good for launch in 45 minutes. A communications check between ground stations and the rocket should have concluded by now.

Some statistics on today's flight:

1910 GMT (3:10 p.m. EDT)
Minus-60 minutes. The Ariane 5's first and second stages are now loaded with cryogenic liquid hydrogen and liquid oxygen propellants.

The 17.7-foot-diameter first stage's Vulcain 2 engine burns 149.5 metric tons, or about 329,000 pounds, of liquid oxygen and 25 metric tons, or about 55,000 pounds, of liquid hydrogen. The cryogenic upper stage's HM7B engine consumes about 14.7 metric tons, or more than 32,000 pounds, of oxygen and hydrogen.

The fluids are stored at super-cold temperatures and naturally boil off in the warm tropical atmosphere in French Guiana. More propellant is slowly pumped into the rocket for most of the countdown to replenish the cryogenic fuel.

The topping sequence ends in the final few minutes of the countdown as the fuel tanks are pressurized and the fueling system is secured.

Built by a consortium of European contractors led by Safran in Vernon, France, the Vulcain 2 engine generates up to 300,000 pounds of thrust during its 9-minute firing. It burns about 320 kilograms, or 705 pounds, of liquid oxygen and liquid hydrogen propellant per second.

The engine's nozzle has an exit diameter of 2.1 meters, or about 6.9 feet. It weighs more than 4,600 pounds and its liquid oxygen and liquid hydrogen turbopumps spin at 12,300 rpm and 35,800 rpm, respectively.

The Vulcain 2 replaced the Vulcain engine used on the initial version of the Ariane 5. The newer engine produces 20 percent more thrust.

The Ariane 5's upper stage is powered by an HM7B engine, a modified version of the HM7 engine used on the upper stage of the Ariane 4 rocket. The 364-pound HM7B engine is manufactured by Airbus Defence and Space in Ottobrunn, Germany.

The HM7B engine produces more than 14,500 pounds of thrust in vacuum.

The Ariane 5 configuration with a Vulcain 2 engine and HM7B-powered cryogenic upper stage is known as the Ariane 5 ECA.

The Ariane 5's twin solid rocket boosters are packed with propellant near the launch site in French Guiana before they are assembled and positioned on each side of the cryogenic core stage.

With the rocket now fully fueled for launch, the vehicle weighs 1.7 million pounds. At liftoff, the rocket produces 2.9 million pounds of thrust.

1710 GMT (1:10 p.m. EDT)
Minus-3 hours. The Ariane 5 rocket's first and second stages, known by the French acronyms EPC and ESC-A, are being filled with liquid oxygen and liquid hydrogen. The first stage Vulcain 2 engine and the upper stage HM7B engine both consume the super-cold propellants.

The cryogenic propellant will be gradually pumped inside the rocket to maintain proper levels as the fuel evaporates over the rest of the countdown.

1300 GMT (9:00 a.m. EDT)
The clock is ticking toward today's launch of an Ariane 5 rocket on the workhorse booster's fourth mission of 2015.

The 18-story rocket is carrying communications satellites for Eutelsat and Intelsat, a total payload package that tips the scales at nearly 21,900 pounds -- more than 9.9 metric tons -- including adapter hardware used to connect the spacecraft to the rocket.

Liftoff is set for 2010 GMT (4:10 p.m. EDT) at the opening of a 106-minute launch window. The launch is scheduled for 5:10 p.m. local time at the Ariane 5's launch base in French Guiana.

Officials are hopeful weather will cooperate given the Ariane 5's long launch window, but meteorologists will watch for developing storms at the fringe of Hurricane Danny, which is churning in the Atlantic Ocean to the north of French Guiana.

The final countdown began at 0840 GMT (4:40 a.m. EDT). Four hours later, at 1240 GMT (8:40 a.m. EDT), the launch team was scheduled to go through a check of the rocket's electrical systems.

Workers will also put finishing touches on the launch pad, including the closure of doors, removal of safety barriers and configuring fluid lines for fueling. The flight program for today's launch will be loaded into the rocket's computer.

The launch team will begin the process to fuel the rocket with super-cold liquid hydrogen and liquid oxygen propellants around 1510 GMT (11:10 a.m. EDT). First, ground reservoirs will be pressurized, then the fuel lines will be chilled down to condition the plumbing for the flow of super-cold liquid hydrogen and liquid oxygen, which are stored at approximately minus 423 degrees Fahrenheit and minus 298 degrees Fahrenheit, respectively.

It will take approximately two hours to fill the Ariane 5 core stage tanks.

A similar procedure for the Ariane 5's cryogenic upper stage will commence at 1610 GMT (12:10 p.m. EDT).

Chilldown conditioning of the Vulcain 2 first stage engine will occur at 1710 GMT (1:10 p.m. EDT), and a communications check between the rocket and ground telemetry, tracking and command systems is scheduled for 1900 GMT (3:00 p.m. EDT).

The computer-controlled synchronized countdown sequence will begin seven minutes before launch to pressurize propellant tanks, switch to on-board power and take the rocket's guidance system to flight mode.

The Vulcain 2 engine will ignite as the countdown clock reaches zero, followed by a health check and ignition of the Ariane 5's solid rocket boosters seven seconds later to send the 1.7 million-pound launcher skyward.

Five seconds after blastoff, the rocket will begin pitching east from the ELA-3 launch pad, surpassing the speed of sound 48 seconds into the mission. The Ariane 5's twin solid rocket boosters will jettison 2 minutes, 23 seconds after liftoff.

Once above the dense atmosphere, the launcher's payload fairing will fall away at an altitude of about 68 miles, or 110 kilometers. The Ariane 5's first stage will shut down 8 minutes, 51 seconds after liftoff, followed moments later by stage separation and ignition of the hydrogen-fueled cryogenic HM7B upper stage engine.

The rocket's upper stage will fire for nearly 16 minutes, accelerating to a velocity of 21,000 mph, or more than 9.3 kilometers per second, to reach an orbit with a planned high point of 22,273 (35,845 kilometers) miles, a targeted low point of 156 miles (252 kilometers) and an inclination of 4.7 degrees.

The release of Eutelsat 8 West B is scheduled for 28 minutes, 17 seconds, after liftoff. The rocket's barrel-shaped Sylda 5 dual-payload adapter will be jettisoned a few minutes later.

Intelsat 34 will separate from the lower portion of the payload stack at 41 minutes, 54 seconds.

Eutelsat 8 West B, based on Space Systems/Loral's 1300-series spacecraft design, is heading for an operating post in geostationary orbit at 8 degrees west longitude, where it will broadcast HD and Ultra HD television over the Middle East and North Africa.

The multi-purpose Intelsat 34 satellite will be positioned at 55.5 degrees west and beam broadband Internet to airliners and ships transiting the North Atlantic. It will replace two aging satellites in Intelsat's fleet and also broadcast TV into Brazilian homes and distribute video across Latin America.

Ground crews towed a towering Ariane 5 rocket to its launch pad in French Guiana on Wednesday for final flight preparations before Thursday's launch with spacecraft for two of the world's top commercial satellite operators.

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