ULA's launch director has given the final approval to resume the countdown.

Polling of the team by Atlas launch conductor just occurred. All systems are GO for liftoff at 10:44 p.m. EDT!

Standing by for the final readiness check to be conducted. The launch team will be polled for a "go" or "no go" to proceed with the count.

The MMS satellites are switching to internal power for launch.

NASA launch director Omar Baez has polled his group and verified the agency team is ready for the flight of MMS.

Today marks the 53rd flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 52 missions since debuting in August 2002, the tally shows 19 flights dedicated to the Defense Department, 11 for NASA, 11 with spy satellites for the National Reconnaissance Office. and 11 commercial missions with communications and Earth-observing spacecraft.

The final planned weather briefing to launch officials just occurred and the odds of acceptable conditions today has improved to 90 percent. All conditions are favorable tonight.

Just 30 minutes until liftoff time.

A $1.1 billion science investigation involving four formation-flying satellites circling the Earth, each one identical, fitted with 25-instrument sensors and measuring 94 feet by 369 feet when fully deployed, will share a single Atlas 5 rocket launch Thursday night from Cape Canaveral to probe explosions in the magnetic field with millisecond speed.

Read our mission preview story.

T-minus 4 minutes and holding. The countdown has entered the planned 30-minute hold to give the launch team a chance to review all systems before pressing ahead with liftoff.

T-minus 5 minutes. Standing by to go into the final built-in hold.

Now 45 minutes from liftoff. See a preview of tonight's climb to orbit.

The fuel-fill sequence for the first stage main engine is starting.

Here's a look at some stats about today's mission. This will be:

• The 635th launch for Atlas program since 1957
• The 341st Atlas launch from Cape Canaveral
• The 224th mission of a Centaur upper stage
• The 201st use of Centaur by an Atlas rocket
• The 43rd Atlas 5 launch from Cape Canaveral
• The 456th production RL10 engine to be launched
• The 59th flight of the RD-180 main engine
• The 53rd launch of an Atlas 5 since 2002
• The 12th NASA mission aboard an Atlas 5
• The 81st Evolved Expendable Launch Vehicle flight
• The 94th United Launch Alliance flight overall
• The 63rd United Launch Alliance flight from Cape Canaveral
• The 45th Atlas 5 under United Launch Alliance
• The 35th 400-series flight of the Atlas 5
• The 4th Atlas 5 to fly in the 421 configuration
• The 2nd Atlas 5 launch of 2015

Now 60 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen has progressed very smoothly tonight and the countdown continues on schedule for a liftoff at 10:44 p.m. EDT.

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The liquid hydrogen tank in the Centaur upper stage just reached the 96 percent level. Topping is now beginning.

Fast-filling of the first stage liquid oxygen tank has been completed. Topping mode is now underway.

Centaur LH2 at 80 percent.

The first stage liquid oxygen tank has reached the 90 percent mark.

Centaur's liquid hydrogen tank is 30 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Aerojet Rocketdyne-made RL10 engine.

Chilldown of the liquid hydrogen system has been accomplished. The launch team has received the "go" to begin filling the Centaur upper stage with the supercold fuel.

First stage liquid oxygen tank is 50 percent full thus far. Chilled to Minus-298 degrees F, the liquid oxygen will be used with RP-1 kerosene by the RD-180 main engine on the first stage during the initial four minutes of flight today. The 25,000 gallons of RP-1 were loaded into the rocket yesterday.

The Centaur engine chilldown sequence is being initiated.

The first stage liquid oxygen tank has reached the 30 percent mark.

The Centaur liquid oxygen tank reached the 97 percent level. The topping off process is starting now.

The first stage liquid oxygen loading is transitioning from slow-fill to fast-fill mode.

And now at the 90 percent level on the Centaur upper stage's liquid oxygen tank.

The chilldown conditioning of liquid hydrogen propellant lines at Complex 41 is starting to prepare the plumbing for transferring the Minus-423 degree F fuel into the rocket. The Centaur holds about 12,300 gallons of the cryogenic propellant.

Centaur liquid oxygen is 60 percent loaded.

The conditioning of the systems for the first stage liquid oxygen tank have been completed. And a "go" has been given to begin pumping supercold liquid oxygen into the Atlas 5's first stage.

The Common Core Booster stage's liquid oxygen tank is the largest tank to be filled today. It holds 48,750 gallons of cryogenic oxidizer for the RD-180 main engine.

Passing the 10 percent level on the Centaur upper stage's liquid oxygen tank.

The Centaur liquid oxygen pad storage area has been prepped. The next step is conditioning the transfer lines, which is now beginning to prepare the plumbing for flowing the cryogenic oxidizer.

T-minus 120 minutes and counting! The launch countdown has resumed for today's flight of the Atlas 5 rocket on a mission to deploy the Magnetospheric Multiscale satellites for NASA.

Clocks have one more built-in hold planned at T-minus 4 minutes. During that pause, the final "go" for launch will be given. All remains targeted for liftoff at 10:44 p.m. EDT from Cape Canaveral's Complex 41.

In the next couple of minutes, chilldown thermal conditioning of the mobile launch platform upon which the rocket stands will begin. This is meant to ease the shock on equipment when supercold cryogenic propellants start flowing into the rocket.

All console operators have reported GO status during the pre-fueling readiness poll. The ULA launch director also voiced his approval for moving forward with the countdown as scheduled today.

Loading of cryogenic liquid oxygen and liquid hydrogen into the Atlas 5 rocket will be getting underway a short time from now.

The ULA launch conductor at the Atlas Spaceflight Operations Center is briefing his team on procedures before entering into the final two hours of the countdown.

NASA launch director Omar Baez has polled his group and verified the agency team is ready for the start of fueling.

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T-minus 2 hours and holding. The countdown has just entered the first of two planned holds over the course of the day that will lead to the 10:44 p.m. EDT launch of the Atlas 5 rocket.

This initial 30-minute pause was designed to give the team some margin in the countdown timeline to deal with technical issues or any work that could fall behind schedule before fueling starts. But all is going smoothly today, with officials not reporting any troubles in the count.

The final hold will occur at T-minus 4 minutes.

The final hands-on work has concluded at the launch pad and technicians are departing the complex. Safety officials will soon confirm that the surrounding danger area has been cleared of all workers for the remainder of the countdown.

Weather conditions remain "GO" at the present time.

The countdown has been underway for a couple of hours now and there's not any problems reported by NASA on the status of activities. Crews are wrapping up their work at the launch pad ahead of fueling. Weather is 80 percent favorable.

In today's first weather briefing to mission managers, all current conditions are observed GO for launch of the Atlas 5 rocket and odds for the 10:44 p.m. EDT liftoff time have improved to 80 percent favorable. The main concern that meteorologists are watching is the thickness of clouds and development of cumulus clouds that the rocket would fly through.

A low pressure system in north-central Gulf of Mexico will continues to slowly migrate east. Upper level moisture associated with the system will increase clouds over Central Florida through the evening hours. Also, southeasterly low level winds will create a coastal shower threat.

The outlook for the launch window now predicts scattered clouds at 3,000 and broken clouds at 28,000 feet, the chance of isolated showers, good visibility, southeasterly winds of 16 gusting to 20 knots, a temperature of 74 degrees F.

If the launch should slip to Friday at 10:40 p.m. EDT for some reason, there is a 70 percent chance of acceptable wether.

The countdown begins now for tonight's launch of the Atlas 5 rocket to deploy the four Magnetospheric Multiscale satellites for NASA.

Clocks are picking up the seven-hour sequence of work that will prepare the booster, payload and ground systems for blastoff at 10:44 p.m. EDT (0244 GMT).

Soon the launch team will begin powering up the rocket to commence standard pre-flight tests. Over the subsequent few hours, final preps for the Centaur's liquid oxygen and liquid hydrogen systems will be performed, along with a test of the rocket's guidance system and the first stage propulsion and hydraulic preps, internal battery checks and testing of the C-band system used to track the rocket as it flies downrange, plus a test of the S-band telemetry relay system.

A planned half-hour hold begins when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled to verify all is in readiness to start fueling the rocket for launch.

Supercold liquid oxygen begins flowing into the Centaur upper stage, followed by the first stage filling. Liquid hydrogen fuel loading for Centaur will be completed a short time later.

A final hold is scheduled at the T-minus 4 minute mark. That 30-minute pause will give everyone a chance to finish any late work and assess the status of the rocket, payload, Range and weather before proceeding into the last moments of the countdown.

The launch window extends from 10:44 to 11:14 p.m. EDT (0244-0314 GMT).

Riding the rails to the launch pad Wednesday morning, the Atlas 5 rocket was rolled out to Cape Canaveral's Complex 41 for liftoff Thursday night to deploy four NASA satellites that will probe Earth's magnetosphere for a high-priority research project.

Read our full story.

ASee a photo gallery.

Mission leaders Tuesday gave approval for rollout of the United Launch Alliance Atlas 5 rocket on Wednesday morning, leading to liftoff Thursday night of the Magnetospheric Multiscale science project for NASA.

Read our full story.

PREVIEW: A $1.1 billion science investigation involving four formation-flying satellites circling the Earth, each one identical, fitted with 25-instrument sensors and measuring 94 feet tall by 369 feet wide when fully deployed, will share a single Atlas 5 rocket launch Thursday night from Cape Canaveral to probe explosions in the magnetic field with millisecond speed.

Read our full story.