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The rocket is currently flying in range of a tracking site in Perth, Australia.
Deployment of the DZZ-HR spacecraft is set for 0230 GMT (10:30 p.m. EDT) at the T+plus 55-minute, 29-second mark.
A ground station in Saint-Hubert, Quebec, is now tracking the rocket.
The synchronized launch sequence takes over the countdown about four minutes prior to liftoff. The computer-controlled final sequence checks thousands of parameters in the final steps of the countdown.
After liftoff, Vega will clear the pad's four lightning towers and pitch north from the Guiana Space Center, heading over the Atlantic Ocean and surpassing the speed of sound in about 30 seconds.
The Vega's solid-fueled P80FW first stage, producing a maximum of 683,000 pounds of thrust, burns out 113 seconds after liftoff, giving way to the launcher's Zefiro 23 second stage at an altitude of about 33 miles.
After a 102-second burn, the second stage consumes its propellant 3 minutes, 36 seconds after launch and separates. The Vega's third stage, the Zefiro 9A motor, ignites 3 minutes, 51 seconds into the mission.
A few seconds later, Vega's 8.5-foot-diameter payload fairing will jettison.
Vega's third stage fires for more than two minutes, turning off and separating 6 minutes, 14 seconds after liftoff.
The fourth stage, known as AVUM, ignites its liquid-fueled Ukrainian engine 6 minutes, 21 seconds into the mission, burning more than five minutes to reach a transfer orbit above Earth.
After coasting for about 41 minutes, the AVUM fourth stage will fire again to reach a circular 466-mile-high (750-kilometer) orbit with an inclination of 98.5 degrees.
Separation of the DZZ-HR payload, also known as KazEOSat 1, is expected 55 minutes, 29 seconds into the flight.
For more details, check out the launch timeline.
So far in the countdown, Vega's systems have been powered on and launch controllers have checked communications, tracking and command links between the rocket and ground facilities at the Guiana Space Center.
Officials confirm the launch pad's 16-story mobile gantry is now in its launch position about 260 feet from the rocket. The mobile gantry was retracted about 90 minutes later in the countdown last night to avoid the problem with a payload ventilation hose that scrubbed Monday's launch attempt.
The 270-square-mile space center, run by the French space agency, CNES, and the European Space Agency, is located on the jungle coastline of French Guiana, situated on the northeast corner of South America.
The Vega launch pad, known by its French acronym SLV, is about 1 kilometer southwest of the Ariane 5 launch complex. It was built on the former site of ELA-1, the home of Ariane 1, Ariane 2 and Ariane 3 launchers from 1979 until 1989.
Construction of the Vega launch pad began in 2004, including the building of a new 16-story mobile gantry weighing some 1000 metric tons. A fixed umbilical mast standing 105 feet tall provides air conditioning to the Vega's payload.
Workers also added four lightning towers at the pad to protect the Vega rocket from thunderstorms.
Unlike the Ariane 5 rocket, the Vega's stages are stacked on the pad inside the mobile gantry, which provides protection of the launcher from weather at the spaceport.
Vega's countdown is managed from Guiana Space Center's prime control center less than a mile from the launch pad, the same building where Ariane 5's countdown is controlled.
The initial weather briefing shows conditions on the ground and at high altitude are acceptable for launch tonight. A further check of weather is scheduled for T-minus 4 hours.
Liftoff is set for an instantaneous opportunity at 0135:15 GMT (9:35:15 p.m. EDT).
Countdown operations are set to begin around 1750 GMT (1:50 p.m. EDT).
"Arianespace observed a non-compliant behavior of the payload's ventilation umbilical during countdown after the gantry's withdrawal.
According to its quality policy, Arianespace has decided to perform an inspection of its facilities.
This inspection will take a few hours and a decision regarding the launcher's flightworthiness will be made tomorrow."
The launch time each day remains fixed at 0135:15 GMT (9:35:15 p.m. EDT).
The synchronized launch sequence takes over the countdown about four minutes prior to liftoff. The computer-controlled final sequence checks thousands of parameters in the final steps of the countdown.
After liftoff, Vega will clear the pad's four lightning towers and pitch north from the Guiana Space Center, heading over the Atlantic Ocean and surpassing the speed of sound in about 30 seconds.
The Vega's solid-fueled P80FW first stage, producing a maximum of 683,000 pounds of thrust, burns out 113 seconds after liftoff, giving way to the launcher's Zefiro 23 second stage at an altitude of about 33 miles.
After a 102-second burn, the second stage consumes its propellant 3 minutes, 36 seconds after launch and separates. The Vega's third stage, the Zefiro 9A motor, ignites 3 minutes, 51 seconds into the mission.
A few seconds later, Vega's 8.5-foot-diameter payload fairing will jettison.
Vega's third stage fires for more than two minutes, turning off and separating 6 minutes, 14 seconds after liftoff.
The fourth stage, known as AVUM, ignites its liquid-fueled Ukrainian engine 6 minutes, 21 seconds into the mission, burning more than five minutes to reach a transfer orbit above Earth.
After coasting for about 41 minutes, the AVUM fourth stage will fire again to reach a circular 466-mile-high (750-kilometer) orbit with an inclination of 98.5 degrees.
Separation of the DZZ-HR payload, also known as KazEOSat 1, is expected 55 minutes, 29 seconds into the flight.
For more details, check out the launch timeline.
Italy is the largest contributor to the lightweight booster, funding more than 58 percent of Vega's budget. France was the second-largest partner, responsible for 25 percent of the work.
Five other ESA member states participated in the Vega program, including Belgium, Spain, the Netherlands, Switzerland and Sweden.
European officials say the Vega program is aimed at the institutional market for research satellites, particularly Earth observation spacecraft such as the DZZ-HR, or KazEOSat 1, satellite launching tonight. Many ESA remote sensing payloads now launch on Russian rockets, which are composed of retired Soviet-era ballistic missile stages.
If today's flight goes as planned, the next flight of Vega is scheduled for this fall with the European Space Agency's Intermediate Experimental Vehicle, a lifting body re-entry prototype.
So far in the countdown, Vega's systems have been powered on and launch controllers have checked communications, tracking and command links between the rocket and ground facilities at the Guiana Space Center.
Officials confirm the launch pad's 16-story mobile gantry is now in its launch position about 260 feet from the rocket.
The 270-square-mile space center, run by the French space agency, CNES, and the European Space Agency, is located on the jungle coastline of French Guiana, situated on the northeast corner of South America.
The Vega launch pad, known by its French acronym SLV, is about 1 kilometer southwest of the Ariane 5 launch complex. It was built on the former site of ELA-1, the home of Ariane 1, Ariane 2 and Ariane 3 launchers from 1979 until 1989.
Construction of the Vega launch pad began in 2004, including the building of a new 16-story mobile gantry weighing some 1000 metric tons. A fixed umbilical mast standing 105 feet tall provides air conditioning to the Vega's payload.
Workers also added four lightning towers at the pad to protect the Vega rocket from thunderstorms.
Unlike the Ariane 5 rocket, the Vega's stages are stacked on the pad inside the mobile gantry, which provides protection of the launcher from weather at the spaceport.
Vega's countdown is managed from Guiana Space Center's prime control center less than a mile from the launch pad, the same building where Ariane 5's countdown is controlled.
Over the next few hours, the 98-foot-tall launcher's avionics systems, computer and transmitters will be activated for testing.
See our countdown timeline for more details.
The weather outlook is fair. An initial check of surface conditions and high-altitude winds shows no problems for liftoff tonight and the countdown is proceeding toward launch at 0135:15 GMT (9:35:15 p.m. EDT).
We will have live streaming video coverage of the launch on this page.
Liftoff from the European-run Guiana Space Center on the northeast shore of South America is set for 0135:15 GMT Tuesday (9:35:15 p.m. EDT; 10:35:15 p.m. local time Friday), kicking off a 55-minute mission to deliver the DZZ-HR high-resolution Earth-watching satellite to orbit for the government of Kazakhstan.
The satellite will be renamed KazEOSat 1 after reaching its assigned orbit.
Tipping the scales at 1,830 pounds, or 830 kilograms, DZZ-HR will supply Kazakh government authorities with very high-resolution satellite imagery for security, environmental, and search and rescue applications.
The optical telescope on-board the spacecraft will take images with a maximum resolution of 1 meter, or about 3.3 feet, sharp enough to spot cars.
Built by Airbus Defence & Space, the spacecraft was bundled in an agreement with the European aerospace contractor which included training of Kazakh satellite controllers. Kazakhstan also selected Airbus Defence & Space, formerly known as EADS Astrium, to supply equipment for a new satellite integration and test center in Astana.
"Like the most advanced space systems that we have built, namely Gaia, Herschel and Spot 6, KazEOSat 1 benefits from cutting-edge technologies, new avionics, silicon carbide, a large aperture and an agile platform," said Francois Auque, head of space systems at Airbus. "Furthermore, through our training program, we have transferred our knowledge to the engineers from Kazakhstan who will operate the satellite, enabling Kazakhstan to control the satellite completely autonomously, and collect and use the images provided by the satellite."
Arianespace, the operator of Vega launches from French Guiana, signed a contract with Airbus in June 2012 for the launch of DZZ-HR.
The countdown is set to begin Monday at 1750 GMT (1:50 p.m. EDT).
The four-stage launcher's avionics system will be powered up at 1955 GMT (3:55 p.m. EDT) for testing. At 2005 GMT (4:05 p.m. EDT), the rocket's guidance system will be activated and Vega's telemetry transmitters will be switched on.
The flight program will be loaded into the launcher's computer at 2040 GMT (4:40 p.m. EDT), followed by power-up of of the rocket's safety systems beginning at 2045 GMT (4:45 p.m. EDT). The navigation system will be aligned for flight at 2115 GMT (5:15 p.m. EDT).
The 45-minute process to retract the launch pad's 16-story mobile service gantry begins at 2255 GMT (6:55 p.m. EDT). The moveable tower will be parked about 260 feet from the rocket for liftoff.
Controllers will report the rocket is ready for the final countdown at 0101 GMT (9:01 p.m. EDT), leading to the start of the computer-controlled synchronized countdown sequence about four minutes prior to liftoff.
After vaulting off the launch pad, the solid-fueled rocket will tilt on a northerly trajectory over the Atlantic Ocean bound for an orbit over Earth's poles. The launcher will accelerate through the speed of sound in about 30 seconds, then release its first stage P80FW motor about 2 minutes after liftoff. Vega's second and third stages, both powered by solid rocket motors, will fire to accelerate the rocket to nearly 17,000 mph, leaving the launcher's Ukrainian fourth stage engine to adjust the rocket's orbit for deployment of the DZZ-HR satellite at T+plus 55 minutes, 29 seconds.