2229 GMT (6:29 p.m. EDT)
Arianespace has confirmed a successful flight for the Ariane 5 rocket, marking its 66th success in a row since 2003.
2225 GMT (6:25 p.m. EDT)
The European Space Agency reports a successful acquisition of signal with the newly-launched MSG 4 weather satellite.
2222 GMT (6:22 p.m. EDT)
The MSG 4 weather satellite has deployed from the Ariane 5 rocket. Built by Thales Alenia Space, the spacecraft will operate at least seven years to track storms from geostationary orbit for Eumetsat, owner of Europe's weather observatories.
2213 GMT (6:13 p.m. EDT)
Plus+31 minutes. The Sylda dual-payload adapter has jettisoned, setting the stage for separation of MSG 4 at Plus+40 minutes, 20 seconds.
2210 GMT (6:10 p.m. EDT)
Separation of Star One C4 confirmed, beginning a 15-year mission for Brazil's Embratel Star One. Star One C4's 48 Ku-band transponders will beam direct-to-home television broadcasts over Brazil, Latin America and the continental United States.

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.

2207 GMT (6:07 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 Star One C4.
2206 GMT (6:06 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.
2203 GMT (6:03 p.m. EDT)
Plus+21 minutes. The upper stage will shut down at Plus+24 minutes, 49 seconds, after reaching a target orbit with a low point of 155 miles, a high point of 22,309 miles, and an inclination of 4 degrees.
2200 GMT (6:00 p.m. EDT)
Plus+18 minutes, 30 seconds. Altitude is 201 km and velocity is 8.37 km/s. After intentionally losing altitude in order to gain speed, the Ariane 5 is now climbing again.
2159 GMT (5:59 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 8.06 kilometers per second, or more than 18,000 mph.
2156 GMT (5:56 p.m. EDT)
Plus+14 minutes. This upper stage engine burn will last approximately 16 minutes. Ariane 5 is at an altitude of 193 kilometers. A tracking station on Ascension Island in the Atlantic Ocean has picked up signals from the Ariane 5.
2151 GMT (5:51 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.
2151 GMT (5:51 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 DirecTV 15 and Sky Mexico 1 satellites into orbit.

2150 GMT (5:50 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.
2147 GMT (5:47 p.m. EDT)
Plus+5 minutes, 30 seconds. Altitute is 160 km, downrange distance is 558 km and velocity is 3.37 km/s.
2145 GMT (5:45 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.
2144 GMT (5:44 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.
2143 GMT (5:43 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.
2142 GMT (5:42 p.m. EDT)
Liftoff of an Ariane 5 rocket on a dual-satellite delivery mission with Star One C4 and MSG 4!
2141 GMT (5:41 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.

2140 GMT (5:40 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.
2139 GMT (5:39 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.
2138 GMT (5:38 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.
2137 GMT (5:37 p.m. EDT)
Minus-5 minutes. All status panel lights remain green, indicating no problems right now that could prevent blastoff at 2142 GMT.
2136 GMT (5:36 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.
2135 GMT (5:35 p.m. EDT)
Minus-7 minutes and counting.

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 2142 GMT (5:42 p.m. EDT). Liftoff actually occurs even seconds later with ignition of the solid rocket boosters.

2134 GMT (5:34 p.m. EDT)
Minus-8 minutes. The synchronized countdown sequence is supposed to begin in one minute, transferring all control over to computers.
2132 GMT (5:32 p.m. EDT)
Minus-10 minutes. The synchronized launch sequence will begin in three minutes.
2130 GMT (5:30 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.
2122 GMT (5:22 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.

2112 GMT (5:12 p.m. EDT)
Minus-30 minutes. Today's launch will deliver the Star One C4 communications satellite and MSG 4 weather observatory to an orbit targeting a planned high point of 22,309 miles, a targeted low point of 155 miles and an inclination of 4 degrees.

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

The Star One C4 telecom satellite occupies the upper position in the rocket’s dual-payload berth. Owned by Embratel Star One of Brazil, Star One C4 will be positioned in geostationary orbit at 70 degrees west longitude, beginning a 15-year mission to broadcast HD television and other services across Brazil, Latin America and the United States

The 12,268-pound Star One C4 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, 16 seconds.

A Sylda 5 adapter will be discarded a few minutes later, revealing the 4,504-pound MSG 4 spacecraft, the mission’s other satellite passenger.

Manufactured by Thales Alenia Space in France, MSG 4 carries imaging and sounding instrumentation to track storms and weather systems across Europe, Africa, the Middle East and the Atlantic Ocean for Eumtesat, operator of Europe's institutional meteorological satellites.

It is fourth and last spacecraft in the Meteosat Second Generation series.

Separation of the MSG 4 spacecraft from the Ariane 5 rocket is scheduled at T+plus 40 minutes, 20 seconds.

The payloads have a combined mass of approximately 18,931 pounds, or 8,587 kilograms, including the barrel-shaped Sylda dual-payload adapter. Sign up to follow us on Twitter for the latest launch updates and space news.

2057 GMT (4:57 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:

2042 GMT (4:42 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.

1842 GMT (2:42 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)
Countdown operations are underway at the Guiana Space Center for today's launch of an Ariane 5 rocket with a Brazilian television broadcast satellite and a new European weather observatory.

Packaged inside the Ariane 5's nose cone are the Star One C4 communications satellite and the fourth spacecraft in Europe's Meteosat Second Generation weather satellite program.

Built by Space Systems/Loral, Star One C4 will beam television services directly into millions of homes and businesses across Brazil, Latin America and the United States. The 12,268-pound satellite is owned by Embratel Star One, a Brazilian television broadcaster, and will be positioned in geostationary orbit at 70 degrees west longitude.

Star One C4's co-passenger is MSG 4, the last in Europe's current series of weather satellites. The spin-stabilized weather observatory is made by Thales Alenia Space, and will be renamed Meteosat 11 once in orbit for its seven-year mission to track storms across Europe, Africa and the Middle East for Eumetsat.

The countdown began at 1012 GMT (6:12 a.m. EDT), with clocks programmed for liftoff of the Ariane 5 ECA rocket at 2142 GMT (5:42 p.m. EDT), or 6:42 p.m. local time at the launch site in French Guiana. The launch window extends for 37 minutes.

A check of electrical systems is scheduled to occur around 1412 GMT (10:12 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 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 1642 GMT (12:42 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 1742 GMT (1:42 p.m. EDT).

Chilldown conditioning of the Vulcain 2 first stage engine will occur at 1842 GMT (2:42 p.m. EDT), and a communications check between the rocket and ground telemetry, tracking and command systems is scheduled for 2032 GMT (3:32 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, 23 seconds after liftoff.

Once above the dense atmosphere, the launcher's payload fairing will fall away at an altitude of about 76 miles. The Ariane 5's first stage will shut down 8 minutes, 55 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 22,309 miles, a targeted low point of 155 miles and an inclination of 4 degrees.

The release of Star One C4 is scheduled for 28 minutes, 16 seconds, after liftoff. The rocket's barrel-shaped Sylda 5 dual-payload adapter will be jettisoned a few minutes later.

MSG 4 will separate from the lower portion of the payload stack at 40 minutes, 20 seconds.