1230 GMT (7:30 a.m. EST)
The sharp-sighted Gaia observatory, carrying the largest camera ever flown in space, rocketed into a predawn sky from French Guiana on Thursday to survey a billion stars and reveal the structure of the Milky Way in finer detail than any mission before.

Read our full story.

1054 GMT (5:54 a.m. EST)
ESA says Gaia's thermal sunshield, extending the size of half a tennis court, has deployed.
1010 GMT (5:10 a.m. EST)
Gaia is running through its post-launch automated sequence nominally, according to ESA. These activities include the priming and pressurization of the propulsion system, deployment of the 10-meter-diameter sunshield and the severing of carbon-fiber connections between the probe's payload and service modules.
0959 GMT (4:59 a.m. EST)
European controllers in Darmstadt, Germany, have received the first signals from Gaia.
0954 GMT (4:54 a.m. EST)
Gaia separation! The 4,484-pound spacecraft has been released from the Fregat upper stage. The critical deployment of Gaia's thermal sunshield is coming up in a few minutes.
0949:19 GMT (4:49:19 a.m. EST)
Shutdown of the Fregat engine. This concludes the powered phase of the launch of Gaia, with separation due in about five minutes. The Fregat engine will fire once more after deployment of Gaia to drain its propellant tank for the normal post-launch passivation procedure.
0946:19 GMT (4:46:19 a.m. EST)
T+plus 34 minutes. The Fregat engine is in the final portion of its second burn, flying nearly 13,000 km downrange from French Guiana, at an altitude of 477 km and a velocity of 9.4 kilometers per second.
0934:19 GMT (4:34:19 a.m. EST)
T+plus 22 minutes. The Fregat main engine is firing again, this time for a burn lasting 15 minutes, 34 seconds.
0925:19 GMT (4:25:19 a.m. EST)
T+plus 13 minutes. Arianespace has received confirmation of the completion of the first Fregat upper stage burn and the rocket is now in orbit. The second burn comes at 0933 GMT (4:33 a.m. EST) after a coast phase lasting 8 minutes, 47 seconds.
0921:19 GMT (4:21:19 a.m. EST)
T+plus 9 minutes. The Soyuz third stage and Fregat upper stage have now separated.
0920:34 GMT (4:20:34 a.m. EST)
T+plus 8 minutes, 15 seconds. The third stage RD-0124 engine is firing as planned. Altitude is 181 km and downrange distance is 1,376 km.
0917:44 GMT (4:17:44 a.m. EST)
T+plus 5 minutes, 25 seconds. At an altitude of 102 miles and a velocity of 8,500 mph, the Soyuz rocket's second stage has shut down and separated. Third stage ignition is also confirmed.
0916:09 GMT (4:16:09 a.m. EST)
T+plus 3 minutes, 50 seconds. The rocket's 13.5-foot ST-type nose fairing has jettisoned now that the launcher is out of the lower atmosphere.
0914:19 GMT (4:14:19 a.m. EST)
T+plus 2 minutes. The four strap-on boosters of the Soyuz rocket have separated at an altitude of approximately 37 miles. The core stage continues firing.
0912:19 GMT (4:12:19 a.m. EST)
LIFTOFF of the Soyuz with Gaia, a "discovery machine" to put a billion stars on the map.
0912:04 GMT (4:12:04 a.m. EST)
T-minus 15 seconds. Ignition of the Soyuz rocket's engines.
0911:19 GMT (4:11:19 a.m. EST)
T-minus 1 minute. The Soyuz will transition to internal power 40 seconds before liftoff.
0910:19 GMT (4:10:19 a.m. EST)
T-minus 2 minutes. The upper umbilical mast servicing the Soyuz rocket's six satellite payloads is being disconnected from the launcher.
0908:19 GMT (4:08:19 a.m. EST)
T-minus 4 minutes. The exact liftoff time is 0912:19 GMT (4:12:19 a.m. EST; 6:12:19 a.m. local time).
0906:19 GMT (4:06:19 a.m. EST)
T-minus 6 minutes. The launch key has been installed inside the launch control center, beginning the Soyuz rocket's synchronized countdown sequence.
0904:19 GMT (4:04:19 a.m. EST)
T-minus 8 minutes. The Soyuz rocket family has flown 1,812 times since the 1950s, and this is the sixth time the venerable launcher will fly from outside the territory of the former Soviet Union.

After liftoff today, the Soyuz rocket's communications signals will be picked up by ground stations near the French Guiana launch site, in Brazil, on Ascension Island, in Africa, Mauritius, and Perth, Australia.

0902:19 GMT (4:02:19 a.m. EST)
T-minus 10 minutes. All systems are reporting a "go" status for an on-time launch this morning.

It is currently 6:02 a.m. in French Guiana.

0854 GMT (3:54 a.m. EST)
The Gaia spacecraft, built by a European-wide industrial consortium led by EADS Astrium, should be switching to internal battery power at this point in the countdown. The live video stream provided by Arianespace and ESA should begin in a few minutes.
0842 GMT (3:42 a.m. EST)
T-minus 30 minutes. The launch team has loaded more than 500,000 pounds of liquid oxygen and kerosene propellant into the rocket this evening, plus hydrogen peroxide to drive the engines' gas turbines and liquid nitrogen to keep the propellant tanks pressurized.

The Soyuz countdown sequence begins 6 minutes, 10 seconds prior to liftoff, then the Fregat upper stage will transition to internal power five minutes before launch.

The umbilical arm servicing the upper stage and payloads will pull away at T-minus 2 minutes, 25 seconds. The Soyuz rocket is operating on internal power at T-minus 40 seconds, and the final servicing mast retracts from the rocket 20 seconds later.

The ignition sequence of the Soyuz rocket's kerosene-fueled core stage and four strap-on boosters begins 17 seconds before liftoff, and all engines should be at full thrust three seconds before launch.

0820 GMT (3:20 a.m. EST)
The 174-foot-tall mobile gantry is being retracted to launch position about 260 feet away from the Soyuz rocket.
0812 GMT (3:12 a.m. EST)
One hour until launch.

The European-funded, Russian-built pad is located about eight miles northwest of the Ariane 5 and Vega launch pads at the Guiana Space Center. Engineers selected the Soyuz launch site based on terrain, geology and a location away from Ariane facilities to ensure they did not interfere with each other.

It took three years and cost European governments $800 million to build the Soyuz launch facility, which is known by its French acronym ELS. Other than the 17-story mobile servicing tower and four lightning masts, the launch pad is modeled after the Soyuz launch pad at the Baikonur Cosmodrome in Kazakhstan.

This morning's launch is the sixth Soyuz to fly from ELS.

The Soyuz pad includes blue and yellow umbilical arms and hold-down petals at the base of the rocket. On the back side of the pad is a deep flame trench dug out of granite bedrock. The facility also houses living quarters for Russian workers and a launch control center.

The Soyuz site lies closer to the town of Sinnamary than Kourou, which is more typically associated with the spaceport.

0755 GMT (2:55 a.m. EST)
After riding the Soyuz rocket and Fregat upper stage to space, Gaia will be released on its own approximately 43 minutes after liftoff.

Almost immediately, ground controllers expect to acquire the first radio signals from Gaia through a pair of ground stations in New Norcia and Perth in Australia.

Then begins an hour-long series of key post-launch deployments and activations. The first thing to do will be to unfurl a 10-meter-diameter thermal sunshield.

The sunshield is composed of 12 frames of carbon fiber tubes hinged at the base of the spacecraft. The frames are covered with gold multilayer insulation blankets. Fully deployed, Gaia covers half the size of a tennis court.

The sunshield is also covered with solar cells to generate electricity for Gaia.

"Ten minutes after the separation from the Fregat upper stage, we deploy the shield," said Giuseppe Sarri, ESA's Gaia project manager, in an interview with Spaceflight Now. "We have two kinds of solar arrays placed on the bottom of the spacecraft in fixed panels. Then there are also a certain number of panels on the sunshield, so those are deployable. It will take some time to deploy, about 20 or 30 minutes."

There is a backup mechanism to deploy the sunshield, which launches folded up against the spacecraft's main body, in case the primary method fails.

The shield keeps Gaia's sensitive camera, the largest ever flown in space, at a chilly minus 148 degrees Fahrenheit for its five-year mission.

"We have another quite critical thing to release, which is the connection of the payload module to the service module. Since we have to do very precise measurements of the stars, the telescope and the optical bench has to very stable from a thermal point of view," Sarri said.

Three delicate fiberglass struts connect Gaia's payload and service modules throughout the mission, but engineers had to add stronger carbon-fiber struts to withstand the intense shaking of launch.

But the carbon-fiber struts are efficient heat conductors, and that's a bad thing from the perspective of scientists. Gaia's sensitive focal plane must be maintained at just the right temperature to make its measurements.

So soon after the sunshield is unfolded, a spring mechanism will release the carbon-fiber struts so the fiberglass connections are the only structural link between the two major parts of the Gaia spacecraft.

Other milestones in the first hour after separation include priming of Gaia's propulsion system and initial commanding of the spacecraft from the ground control center in Darmstadt, Germany, according to David Milligan, Gaia's operations manager.

The first major course-correction burn with Gaia's rocket engines will occur about 27 hours after launch to put the probe on course to arrive at its observing post at the L2 Lagrange point more than 900,000 miles from Earth. Gaia will get there around Jan. 7.

0742 GMT (2:42 a.m. EST)
90 minutes until launch. Topping of the Soyuz propellant tanks with liquid oxygen and liquid nitrogen pressurant continues, and filling of the rocket's kerosene tanks is complete.

The next milestone in the countdown will be retraction of the Soyuz rocket's mobile gantry. Engineers are currently configuring the servicing tower to move to a point about 260 feet from the Soyuz rocket.

0732 GMT (2:32 a.m. EST)
Everything is on track for an on-time launch at 0912:19 GMT (4:12:19 a.m. EST).

The Soyuz rocket with Gaia on-board is a modernized version of the venerable Russian launcher, with an upgraded RD-0124 third stage engine and digital control system. It also has a flight termination system that can receive commands from safety officials on the ground in the event of a mishap, a key difference between the Soyuz rockets flying from French Guiana and Russian launch sites.

The Soyuz launching this morning is known as the Soyuz ST-B or Soyuz 2-1b configuration.

After liftoff, the rocket will go through pitch and roll programs to align with an easterly trajectory from the launch pad near Sinnamary, French Guiana. After a nearly 10-minute flight powered by the Soyuz rocket's three core stages, a Fregat-MT upper stage will take over for a pair of burns to propel Gaia on a course toward the L2 Lagrange point more than 900,000 miles from Earth.

See our launch timeline for specific details on the launch sequence.

If Gaia blasts off this morning, it will arrive in orbit around the L2 point between Jan. 6 and Jan. 9, with the first scientific data to be available a few weeks later.

0620 GMT (1:20 a.m. EST)
Fueling of the three Soyuz core stages continues with no problems.

Some statistics on today's flight:
0555 GMT (12:55 a.m. EST)
The launch team has begun loading propellant into the three core stages of the Soyuz rocket. The fueling process should be complete about two hours before launch.

The State Commission meeting of senior launch officials concluded earlier tonight with a "go" for the final countdown.

0350 GMT (10:50 p.m. EST on Wed.)
The launch team has completed electrical checks after turning on the Soyuz rocket's avionics system. Everything is on track for the start of fueling shortly after 0500 GMT (12 a.m. EST; 2 a.m. local time).

Launch is set for 0912:19 GMT (4:12:19 a.m. EST; 6:12:19 a.m. local time), shortly before sunrise at the Guiana Space Center.

See our countdown timeline for more details on the activities scheduled over the next few hours.

0312 GMT (10:12 p.m. EST on Wed.)
With six hours left in the overnight countdown, no problems are reported with the Soyuz rocket, Gaia spacecraft or ground systems.

The latest check of weather conditions at the Guiana Space Center this evening show favorable low-level and high-altitude winds, and there is no threat of lightning in the area.

The Russian State Commission will convene in the next couple of hours to review launch preparations and give the "go" to inject kerosene and liquid oxygen propellants into the 151-foot-tall Soyuz rocket, which is housed inside a 174-foot-tall mobile gantry about 8 miles northwest of the spaceport's Ariane 5 and Vega launch facilities.

The European Space Agency is sending the $1.2 billion Gaia mission to space Thursday to catalog a billion stars, promising to be a prolific discoverer of planets, asteroids and supernovas for generations of astronomers.

With a billion-pixel camera at the heart of the mission, Gaia is billed as the most sensitive telescope ever put into space.

Its twin telescopes will see an average of 40 million stars every day over Gaia's five-year mission, which begins Thursday with a launch atop a Russian Soyuz rocket from the Guiana Space Center on the northern coast of South America.

Read our full story.

1950 GMT (2:50 p.m. EST)
Officials at a launch readiness review today gave the "go" to proceed with the final countdown for Thursday morning's liftoff of a Soyuz rocket from French Guiana with Europe's Gaia spacecraft, embarking on a five-year mission to plot the locations of a billion stars.

The preliminary weather forecast is favorable, with little chance high winds or precipitation will violate launch constraints. Another weather briefing is set for approximately 0100 GMT (8 p.m. EST).

Launch remains set for 0912:19 GMT (4:12:19 a.m. EST; 6:12:19 a.m. local time) from the Guiana Space Center in South America.

Engineers ran through a complete countdown and launch rehearsal today for Thursday morning's launch of the European Space Agency's Gaia mission on a Soyuz rocket.

Already positioned on the launch pad in French Guiana, the Soyuz launcher and Gaia spacecraft were activated for today's rehearsal, along with ground control networks in French Guiana and Germany, which will oversee the launch sequence and the critical early operations after Gaia's deployment.

Those early milestones include the deployment of Gaia's thermal sunshield and solar arrays, and the decoupling of carbon-fiber struts connecting the probe's payload and service modules.

These activities are crucial since Gaia's instruments are sensitive to temperature. The sunshield maintains the thermal conditions for the mission's star survey campaign, and the carbon-fiber struts are required to withstand the shaking and force of launch. The carbon-fiber struts are conductors of heat, so engineers plan to explosively sever the structural connections soon after launch.

Launch remains set for 0912:19 GMT (4:12:19 a.m. EST; 6:12:19 a.m. local time) from the Guiana Space Center in South America.

A team of Russian and European technicians working in the Amazon jungle transferred a Soyuz rocket and a sharp-eyed astronomical observatory to the launch pad this weekend, connecting the launcher and payload together for liftoff Thursday on a $1.2 billion mission to create an atlas of the galaxy.

The rollout operations began Saturday at sunrise at the Guiana Space Center in South America. Russian workers moved the three-stage rocket on a specialized transporter-erector system riding on rail tracks.

See the photos.

The European Space Agency's Gaia spacecraft, fueled and ready to begin a mission mapping a billion stars, was closed up inside the nose shroud of a Soyuz rocket Thursday as the probe begins its final week of launch preparations.

Gaia is set to blast off Dec. 19 at 0912:18 GMT (4:12:18 a.m. EST; 6:12:18 a.m. local time) from the Guiana Space Center on the northern coast of South America.

The 4,484-pound spacecraft will launch on the sixth Russian Soyuz rocket to fly from French Guiana, and it's the first major European science payload to launch on Soyuz from its new home in South America.

With a budget of more than $1.2 billion, accounting for investments from ESA and supporting institutions across Europe, Gaia is on the cusp of launching 13 years after it was approved by ESA decision-makers.

After a boost from the Soyuz rocket and its Fregat-MT upper stage, Gaia is heading for the L2 Lagrange point, an operating post about one million miles from the night side of Earth. L2 is a thermally-stable position where gravity from the Earth and sun balance a satellite's motion.

Once Gaia sets up shop at L2, it will open the apertures of its twin telescopes and begin a five-year mission scanning the galaxy, picking up light from the billion brightest stars in the sky. By repeatedly observing the stars, Gaia will obtain location fixes and motion estimates, giving scientists an unprecedented snapshot of the positions, movement and characteristics of stars in the Milky Way.

Astronomers will use the information to plot the galaxy's structure, history and evolution.

Gaia also promises to discover supernovas, asteroids and planets around other stars.

In the last few weeks, technicians have filled Gaia with propellant and mounted the cylinder-shaped spacecraft on its Fregat-MT upper stage, which will do the last bit of lifting to propel Gaia away from Earth toward L2.

On Thursday, ground crews enclosed Gaia and the Fregat stage inside the Soyuz rocket's ST payload fairing based on the aerodynamic shroud used by Arianespace's Ariane 4 rocket.

Russian technicians will roll the three-stage Soyuz launcher, already assembled horizontally, the 700-meter (2,300-foot) distance from its integration building to the launch pad Saturday morning.

Once the rocket arrives on the launch pad, it will be hydraulically lifted vertical and suspended over the facility's cavernous flame trench.

The launch pad is a near-clone of the Soyuz complex at the Baikonur Cosmodrome in Kazakhstan, with a couple of important exceptions.

The French Guiana launch pad, known by the French acronym ELS, features a 174-foot-tall mobile gantry to allow workers to install payloads on the Soyuz in a vertical orientation. Satellites are attached to the Soyuz horizontally at Baikonur.

And the Soyuz pad in South America includes four towers to protect the rocket from lightning at the jungle spaceport.

The launch pad's mobile gantry will be moved into position around the Soyuz later Saturday for the arrival of Gaia inside the payload fairing on a special ground transporter. Cranes will hoist the spacecraft on top of the launcher Saturday evening.

Several days of final systems testing and a launch readiness review are scheduled before Thursday morning's launch, which is set for approximately 20 minutes before sunrise in French Guiana.