This marks the 61st successful Ariane 5 flight in a row.
And the upper stage's HM7B engine is now firing to inject the Measat 3b and Optus 10 satellites into orbit.
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.
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.
Liftoff is set for 2205 GMT (6:05 p.m. EDT).
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.
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.
Liftoff is set for 2126 GMT (5:26 p.m. EDT).
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.
The satellites will use their on-board engines to raise their orbits and position themselves over the equator.
The 13,000-pound Measat 3b satellite, built by Airbus Defence and Space, will begin a 15-year operational mission beaming direct-to-home television services to Malaysia, Indonesia, India and Australia.
"Once operational, Measat 3b will be a key contributor to the Asian pay TV landscape, supporting direct-to-home television platforms serving more than 18 million households," said Paul Brown-Kenyon, CEO of Measat Satellite Systems, in a press release.
Based on the Airbus Eurostar E3000 satellite bus, the satellite carries 48 Ku-band transponders and will be positioned at 91.5 degrees east longitude.
The 7,209-pound Optus 10 spacecraft, manufactured by Orbital Sciences Corp., is owned by the Australian telecom operator Optus.
It will support direct television broadcasts, Internet, telephone and data transmission services to Australia, New Zealand and the Antarctic region.
The rocket will take more than 34 minutes to inject the satellites into orbit and release them. The payloads have a combined mass of approximately 22,262 pounds, or 10,098 kilograms, including the barrel-shaped Sylda dual-payload adapter.
Get an overview of the launch sequence.
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Here are some statistics on today's launch:
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.
Chilldown of liquid oxygen and liquid hydrogen propellant lines at the ELA-3 launch pad has begun. The chilldown procedure helps condition the ground plumbing before the cryogenic propellants are pumped inside the Ariane 5's first and second stages.
The countdown began as scheduled at 0951 GMT (5:51 a.m. EDT), followed by power-up of the rocket's computer and avionics systems for an electrical check beginning at 1351 GMT (9:51 a.m. EDT).
Workers finished their hands-on tasks on the launch pad, including the closure of doors, removal of safety barriers and configuring fluid lines for fueling. The ground team then evacuated the ELA-3 launch pad before the start of fueling.
The launch window extends for 62 minutes.
A check of electrical systems is scheduled to occur around 1351 GMT (9:51 a.m. EDT).
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 launch team will begin the process to fuel the rocket with super-cold liquid hydrogen and liquid oxygen propellants after 1621 GMT (12:21 p.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 2221 GMT (1:21 p.m. EDT).
Chilldown conditioning of the Vulcain 2 first stage engine will occur at 2321 GMT (2:21 p.m. EDT), and a communications check between the rocket and ground telemetry, tracking and command systems is scheduled for 2011 GMT (4:11 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 less than a minute into the mission. The Ariane 5's twin solid rocket boosters will jettison 2 minutes, 21 seconds after liftoff.
Once above the dense atmosphere, the launcher's payload fairing will fall away at an altitude of about 69 miles. The Ariane 5's first stage will shut down 8 minutes, 56 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 approximately 16 minutes, accelerating to a velocity of 20,929 mph, or more than 9.3 kilometers per second, to reach an orbit with a planned high point of 22,236 miles, a targeted low point of 155 miles and an inclination of 6 degrees.
The release of Measat 3b is scheduled for 26 minutes, 54 seconds. The rocket's barrel-shaped Sylda 5 dual-payload adapter will be jettisoned a few minutes later.
Optus 10 will separate from the lower portion of the payload stack at 34 minutes, 26 seconds.
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Mounted on top of a mobile launch platform, the 18-story rocket arrived in the Guiana Space Center's ELA-3 launch zone around midday Wednesday after a 1.7-mile rollout from the facility's final assembly building.
The rocket and launch platform were towed by a Titan tug powered by a 540-horsepower engine with dual transmission modes to control its movements with millimeter precision while running at full power.
The one-hour transfer of the launcher to the ELA-3 launch zone was to be followed by the careful positioning of the rocket's mobile launch platform over the flame trench, then the connection of the rocket with the launch pad's electrical, telemetry and propellant loading systems.
Ground crews planned to fill the Ariane 5 first stage's helium pressurization system later Wednesday.
The Ariane 5 rocket is set for its fourth flight of the year, with liftoff scheduled for 2121 GMT (5:21 p.m. EDT; 6:21 p.m. French Guiana time). The launch will mark the 75th Ariane 5 mission since its debut in 1996.
Two communications satellites are enclosed inside the Ariane 5 rocket's payload shroud.
The larger of the satellite pair, Measat 3b, will ride into the upper berth of the dual-payload composite. The Optus 10 spacecraft is nestled inside the Ariane 5's Sylda adapter, a device specially designed to accommodate to full-size communications satellites.
Owned by Measat, a Malaysian satellite operator, the Measat 3b spacecraft will expand direct-to-home television services in Malaysia, Indonesia, India and Australia.
The Optus 10 satellite will be operated by the Australian operator Optus to support direct television broadcasts, Internet, telephone and data transmission services to Australia, New Zealand and the Antarctic region.
Thursday's flight continues a rapid-fire launch cadence by Arianespace, which oversees Ariane 5, Soyuz and Vega missions from the European-run spaceport in South America.
Arianespace has up to eight launches scheduled for the second half of 2014, and Thursday's Ariane 5 liftoff is the fourth in the sequence, following a Soyuz launch for O3b Networks on July 10, an Ariane 5 flight July 29 to service the International Space Station, and the Aug. 22 liftoff of another Soyuz booster.
The Soyuz launch last month put two European Galileo navigation satellites in the wrong orbit, and investigators are still working out what went wrong.
The next Ariane 5 launch is expected Oct. 16, then a lightweight solid-fueled Vega launcher is on the manifest for Nov. 18 with the European Space Agency's Intermediate Experimental Vehicle.
Ariane 5 and Soyuz launches are on Arianespace's launch schedule in December, assuming a quick resumption of Soyuz flights after the Aug. 22 failure.
If all the missions get off as planned, Arianespace would launch 12 rockets this year, setting a record for launches from French Guiana.