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And the upper stage's HM7B engine is now firing to inject the Eutelsat 25B/Es'hail 1 and GSAT 7 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.
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 13,911-pound Eutelsat 25B/Es'hail 1 satellite, built by Space Systems/Loral of Palo Alto, Calif., has a joint mission for Paris-based operator Eutelsat and Es'hailSat, the state-owned satellite communications firm of Qatar.
The spacecraft's Ku-band and Ka-band payloads will beam video broadcasting and government enterprise services across a swath of North Africa and the Middle East.
The 5,842-pound GSAT 7 satellite will relay communications between Indian naval ships, covering the Indian subcontinent and neighboring waters with UHF, S-band, C-band and Ku-band transponders.
It will take nearly 35 minutes to inject the satellites into orbit and release them. The payloads have a combined mass of approximately 19,753 pounds. 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, or liquid oxygen 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.
The Ariane 5's twin solid rocket boosters are packed with propellant before they are assembled.
With the rocket now fully fueled for launch, the vehicle weighs 1.7 million pounds.
Check out a timeline of today's launch events.
The 17-story launcher arrived at the launch pad around midday after being towed from its final assembly building by a heavy-duty Titan truck.
After its arrival at the ELA-3 launch zone near the Atlantic coast of French Guiana, the Ariane 5 rocket was to be connected to the launch pad via electrical, communications and plumbing lines ahead of the start of the launch countdown early Thursday.
The launcher's payload fairing contains the Eutelsat 25B/Es'hail 1 and GSAT 7 communications satellites for satellite operators Eutelsat and Es'hailSat - of Paris and Qatar - and the Indian Navy.
It will mark the fourth of five planned Ariane 5 launches this year, and the 71st Ariane 5 flight overall since 1996.
Liftoff is scheduled for 2030 GMT (4:30 p.m. EDT; 5:30 p.m. local time) at the opening of a 50-minute launch window.
Final Ariane 5 countdown procedures are scheduled to begin around 0900 GMT (5 a.m. EDT). A check of electrical systems was scheduled to occur around 1300 GMT (9 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 at 1540 GMT (11:40 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 1640 GMT (12:40 p.m. EDT).
Chilldown conditioning of the Vulcain 2 first stage engine will occur at 1740 GMT (1:40 p.m. EDT), and a communications check between the rocket and ground telemetry, tracking and command systems is scheduled for 1920 GMT (3:20 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 and 23 seconds after liftoff.
Once above the dense atmosphere, the launcher's payload fairing will fall away at an altitude of about 66 miles. The Ariane 5's first stage will shut down 8 minutes, 46 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 20,969 mph miles to reach an orbit with a planned high point of 22,325 miles and a targeted low point of 154 miles.
The release of Eutelsat 25B/Es'hail 1 is scheduled for 27 minutes, 45 seconds. The rocket's barrel-shaped Sylda 5 dual-payload adapter will be jettisoned a few minutes later.
GSAT 7 will separate from the lower portion of the payload stack at 34 minutes, 26 seconds.
The dual-payload launch from the European-run Guiana Space Center is scheduled during a 50-minute window opening at 2030 GMT (4:30 p.m. EDT). Liftoff is set for 5:30 p.m. local time at the French Guiana spaceport located on the northern coast of South America.
Two communications satellites are enclosed inside the Ariane 5's 5.4-meter, or 17.7-foot, diameter payload fairing.
Built by EADS Astrium, the launcher will deploy the Eutelsat 25B/Es'hail 1 and GSAT 7 satellites about a half-hour after liftoff, injecting the payloads into a geostationary transfer orbit.
Arianespace, the Ariane 5's marketer and operator, announced today the mission passed its launch readiness review, the traditional status check on the rocket, its passengers and ground systems.
Rollout of the Ariane 5 rocket from its final assembly building to the ELA-3 launch pad is set to begin at about 1330 GMT (9:30 a.m. EDT; 10:30 a.m. local time). The 1.7-mile journey will take about an hour to complete, following rail tracks under the power of a heavy-duty tow truck named the Titan.
Once the 165-foot-tall Ariane 5 arrives at the launch pad, workers will connect the launcher and its mobile platform to electrical, fueling and communications systems before the start of the countdown early Thursday.
The mission's passengers are stacked inside the Ariane 5 payload fairing, separated by a Sylda dual-payload adapter. A communications satellite for Eutelsat and Qatar - dubbed Eutelsat 25B and Es'hail 1 by each of its owners - will ride to space in the upper position. The GSAT 7 communications satellite is in the lower spot.
Built under a strategic partnership between Eutelsat and the Es'hailSat, a startup operator owned by the Qatari government, the Eutelsat 25B/Es'hail 1 spacecraft will be positioned at 25.5 degrees east longitude to cover North Africa and the Middle East.
Eutelsat 2B/Es'hail 1 was built by Space Systems/Loral of Palo Alto, Calif., and its payload consists of Ku-band and Ka-band transponders, four deployable reflectors and four steerable spot beam antennas. The 6.3-metric ton satellite, designed for direct video broadcasting, enterprise communications and government services, will be a long-term replacement for Eutelsat 25A, which was launched on an Atlas 2A rocket in October 1998.
The GSAT 7 satellite is India's first military communications satellite, destined to link ships in the Indian Navy with command and control centers.
GSAT 7 will beam UHF, S-band, C-band and Ku-band services across India and neighboring waters from an operating post 22,300 miles over the equator at 74 degrees east longitude. The craft is designed for a seven-year mission.