A mission to answer questions as old as the space age launched from Cape Canaveral this morning as two armored satellites were sent into hellish orbits that submerge the craft in Earth's punishing radiation belts.

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"We have never before sent such comprehensive and high-quality instruments to study high radiation regions of space," said Barry Mauk, RBSP project scientist at the Johns Hopkins University's Applied Physics Laboratory. "RBSP was crafted to help us learn more about, and ultimately predict, the response of the radiation belts to solar inputs."

Both RBSP spacecraft have successfully extended their power-generating solar panels.

"The excitement of seeing the spacecraft in orbit and beginning to perform science measurements is like no other thrill," said Richard Fitzgerald, RBSP project manager. "The entire RBSP team, from across every organization, worked together to produce an amazing pair of spacecraft."

"The ULA team and our many mission partners are very proud of our role in delivering the twin RBPS spacecraft to orbit to conduct research about our space weather and gather important data that impacts our everyday life on Earth," said Jim Sponnick, United Launch Alliance's vice president for Mission Operations.

"The successful launch of this mission is a tribute to the partnerships with the highly skilled and professional teams from NASA's Launch Services Program and The Johns Hopkins University Applied Physics Laboratory."

"The Radiation Belt Storm Probes are now successfully separated," said NASA launch director Tim Dunn. "We also have good telemetry coming down from both spacecraft. As you can imagine, our spacecraft customer from the Applied Physics Laboratory is thrilled right now. Obviously, the spacecraft will be in early orbit checkout for a couple of months, but we're all thrilled, just excited as can be."

The Centaur upper stage jut executed a third burn that dropped the low point of its orbit into the atmosphere. The discarded rocket body will be take a fiery plunge back to Earth at the end of this lengthy orbit, ridding itself from becoming space junk.

NASA launch director Tim Dunn has declared today's ascent of the Atlas 5 rocket as a success, deploying the RBSP spacecraft into their respective orbits around the Earth to study the mysteries of the radiation belts. Both satellites are sending good data to ground controllers.

T+plus 91 minutes, 11 seconds. PROBE B SEPARATION! The lower satellite in the Radiation Belt Storm Probe stack has been released from the Centaur upper stage, completing this morning's launch!

Now 90 minutes since leaving the launch pad at Cape Canaveral as the rocket nears the western coast of the Americas from the Pacific at an altitude of 1,955 miles.

T+plus 89 minutes, 10 seconds. The rocket's turn is complete, now beginning the spin up to 5 revolutions per minute.

T+plus 87 minutes. Centaur is reorienting to the Probe B separation attitude.

T+plus 86 minutes, 35 seconds. The 140-second thruster burn is complete.

T+plus 86 minutes. The rocket is 1,422 miles in altitude.

T+plus 84 minutes. The Centaur upper stage's settling thrusters are on, nudging the launcher into a slightly higher altitude as requested for the Radiation Belt Storm Probe mission to put the two craft in separate orbits.

T+plus 81 minutes. Now 857 miles over the Pacific, south of Hawaii.

T+plus 79 minutes, 3 seconds. PROBE A SEPARATION! The upper satellite in the Radiation Belt Storm Probe stack has been released to fly free from its sister, embarking on its independent mission of exploring the harshest region of near-Earth space.

T+plus 77 minutes, 20 seconds. Spin up to 30 degrees per second is being achieved by the rocket stage.

T+plus 75 minutes. Centaur is turning itself to the proper orientation for releasing the first payload.

T+plus 74 minutes, 12 seconds. MECO 2. Main engine cutoff confirmed. Centaur has completed its second burn of the morning!

T+plus 73 minutes, 10 seconds. Centaur's RL10 engine continues a good firing.

T+plus 72 minutes, 50 seconds. Passing an altitude of 375 miles.

T+plus 72 minutes, 20 seconds. Bus and battery voltages, tank pressures and other system measurements look good.

T+plus 72 minutes. About two minutes are left in the burn to reach the planned orbit to release Probe A.

T+plus 71 minutes. The engine is burning well. This is a planned four-and-a-half-minute firing by the Centaur's single Pratt & Whitney Rocketdyne RL10 engine.

T+plus 70 minutes, 30 seconds. Vehicle is under good control.

T+plus 69 minutes, 32 seconds. Ignition and full thrust! The Centaur's single RL10 engine has re-ignited to accelerate the RBSP payload into the planned deployment orbit.

T+plus 68 minutes, 25 seconds. Centaur is getting pressurized again in preparation for the burn.

T+plus 67 minutes. Centaur has despun from its coast configuration. Altitude 360 miles.

T+plus 66 minutes. The vehicle is reorienting for the engine firing.

T+plus 64 minutes. Centaur is 353 miles in altitude as it skirts along the northern coast of Australia some five minutes away from the next burn.

One hour since launch and still 31 minutes left to go in the ascent sequnce. The Atlas 5 rocket will inject the instrument-laden satellites into highly elliptical orbits, inclined 10 degrees to the equator, to begin a two-year mission plunging into and out of the radiation belts discovered by James Van Allen at the dawn of the space age.

T+plus 55 minutes. Nearing the northwestern coast of Australia. Current altitude is 305 miles.

T+plus 50 minutes. See our Facebook page for quick images of this morning's launch!

T+plus 45 minutes. Centaur now cruising 222 miles over the Indian Ocean in its parking orbit.

T+plus 37 minutes. The vehicle has crossed Africa and will soon pass over Madagascar at altitude of 161 miles.

T+plus 34 minutes. Centaur's onboard systems are stable in this coast period continues.

We're now 30 minutes into this Atlas 5 rocket launch with the Radiation Belt Storm Probes that begins a mission 50 years in the making to unravel mysteries about the harshest region of near-Earth space.

The twin craft, beefed up with protective hardening, will take unprecedented measurements inside the hearts of the radiation belts to provide fundamental data about the behavior and character of these swaths of space that can harm spacecraft and astronauts alike.

T+plus 25 minutes. The flight path is taking the vehicle over the equatorial Atlantic Ocean, just off the western coast of Africa. Here's the planned track map.

T+plus 21 minutes. All vehicle parameters still reported normal.

T+plus 16 minutes. That first burn by Centaur inserted the rocket into an orbit with a high point of 373 statute miles, a low point of 104 statute miles and inclination of 25.5 degrees.

T+plus 15 minutes. The rocket is performing its turn to the proper position for the parking orbit coast.

T+plus 13 minutes, 41 second. MECO 1. Centaur's main engine has shut down following its first burn today, achieving a preliminary orbit around Earth. The rocket will coast in this orbit for nearly an hour before the RL10 engine re-ignites.

T+plus 13 minutes, 15 seconds. Everything is normal on Centaur and RL10 engine with a half-minute to go in this burn.

T+plus 12 minutes, 45 seconds. Centaur remains on course and looking good.

T+plus 11 minutes, 33 seconds. The rocket is 152 miles in altitude, some 1,783 miles downrange and traveling at 15,640 mph.

T+plus 11 minutes. RL10 engine parameters still look good.

T+plus 10 minutes, 45 seconds. About three minutes are left in this burn of Centaur.

T+plus 9 minutes, 25 seconds. Centaur is 159 miles in altitude, 1,270 miles downrange from the launch pad, traveling at 13,989 mph.

T+plus 9 minutes. The RL10 continues to perform well, burning liquid hydrogen and liquid oxygen propellants.

T+plus 8 minutes, 30 seconds. All systems reported stable as the Centaur fires to reach an initial Earth orbit.

T+plus 7 minutes, 40 seconds. Now 129 miles in altitude, 875 miles downrange from the launch pad, traveling at 12,844 mph.

T+plus 6 minutes, 20 seconds. The vehicle tracking right on the proper trajectory.

T+plus 5 minutes, 25 seconds. Now 106 miles in altitude, 429 miles downrange from the launch pad, traveling at 11,794 mph.

T+plus 5 minutes. Centaur engine readings look good as this burn gets underway.

T+plus 4 minutes, 32 seconds. The two halves of the four-meter-diameter Atlas 5 rocket nose cone encapsulating the spacecraft have separated.

T+plus 4 minutes, 24 seconds. Centaur has ignited! The RL10 engine is up and running at full thrust to power the vehicle into orbit.

T+plus 4 minutes, 14 seconds. The Atlas 5's Common Core Booster has been jettisoned, completing the first stage of flight, and the Centaur upper stage's liquid hydrogen and liquid oxygen systems are being readied for engine start.

T+plus 4 minutes, 5 seconds. BECO. Booster Engine Cutoff is confirmed as the RD-180 powerplant on the first stage completes its burn. Standing by to fire the retro thrusters and separate the spent stage.

T+plus 3 minutes, 45 seconds. Atlas now weighs just a quarter of what it did at liftoff.

T+plus 3 minutes, 30 seconds. The main engine is performing well as the rocket climbs away from the planet.

T+plus 2 minutes. Now 31 miles in altitude, 50 miles downrange from the launch pad, traveling at 5,083 mph.

T+plus 2 minutes, 50 seconds. Reaction control system has been activated.

T+plus 2 minutes, 20 seconds. Atlas now weighs half of what it did at liftoff.

T+plus 2 minutes. Now 11 miles in altitude, 8 miles downrange from the launch pad.

T+plus 1 minutes, 45 seconds. The RD-180 main engine continues to fire normally, burning a mixture of highly refined kerosene and liquid oxygen.

T+plus 100 seconds. Now passing through the region of maximum aerodynamic pressure on the vehicle as its accelerates through the dense lower atmosphere.

T+plus 85 seconds. All looks good aboard Atlas as it passes Mach 1.

T+plus 60 seconds. One minute into the ascent of two NASA satellites atop the 19-story-tall Atlas 5 rocket, bound for a highly elliptical orbit around Earth.

T+plus 40 seconds. The Atlas 5 is lighting up the night with its thunderous departure on an easterly trajectory from Cape Canaveral.

T+plus 15 seconds. The Atlas 5 is on its way, riding atop 860,000 pounds of thrust from the RD-180 main engine as pitch, yaw and roll maneuvers are performed to put the rocket on the proper heading.

LIFTOFF! Liftoff of the Atlas 5 rocket launching the Radiation Belt Storm Probes on a mission 50 years in the making! And the vehicle has cleared the tower!

T-minus 20 seconds. "Go Atlas" and "Go Centaur" was just called by launch team during a final status check.

T-minus 40 seconds. Centaur's liquid oxygen and liquid hydrogen tanks are stable at flight pressures.

T-minus 1 minute. Now 60 seconds away from the Atlas 5 rocket rising before the sun this morning at Cape Canaveral.

T-minus 90 seconds. The rocket's safety system has been armed.

T-minus 1 minute, 45 seconds. Liquid oxygen and liquid hydrogen propellant topping to the Centaur upper stage is being secured.

T-minus 1 minute, 55 seconds. The launch sequencer has been commanded to start.

T-minus 2 minutes. The Atlas first stage and Centaur upper stage are now switching from ground power to internal batteries.

T-minus 2 minutes, 30 seconds. The first stage RP-1 kerosene fuel tank and the liquid oxygen have stepped up to proper flight pressure levels.

T-minus 3 minutes. The Atlas first stage liquid oxygen replenishment is being secured so the tank can be pressurized for launch.

T-minus 3 minutes, 50 seconds. The ground pyrotechnics are enabled.

T-minus 4 minutes and counting. Clocks have resumed for the final minutes of today's countdown to launch the Atlas 5 rocket with NASA's Radiation Belt Storm Probes to take unprecedented measurements inside the hearts of the radiation belts and provide fundamental data about the behavior and character of these swaths of space that can harm satellites and astronauts alike.

The ULA launch director has given his "go" to resume the countdown as scheduled.

Both RBSP spacecraft are running on internal power for launch.

Polling of the team by Atlas launch conductor just occurred. All systems are "go" for a liftoff today at 4:05 a.m. EDT.

The Radiation Belt Storm Probes are switching to internal power for launch.

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.

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Twenty minutes from liftoff. Weather is cloudy but GO for launch.

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

• The 614th launch for Atlas program since 1957
• The 326th Atlas to occur from Cape Canaveral
• The 203rd mission for the Centaur upper stage
• The 180th use of Centaur by an Atlas rocket
• The 32nd launch of an Atlas 5 since 2002
• The 52nd Evolved Expendable Launch Vehicle flight
• The 28th Atlas 5 to occur from the Cape
• The 24th Atlas 5 under United Launch Alliance
• The 21st 400-series flight of the Atlas 5
• The 13th Atlas 5 to fly in the 401 configuration
• The 7th NASA launch on Atlas 5
• The 4th Atlas launch of 2012

The first stage liquid oxygen tank and Centaur's liquid oxygen and liquid hydrogen tanks are topped at flight level.

T-minus 4 minutes and holding. The countdown has entered the planned 25-minute hold to give the launch team a chance to review all systems before pressing ahead with liftoff. The hold is longer than usual this morning, as the team built in an additional 15 minutes to the countdown to deal with any issues or catch up on work during the overnight operation.

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

The countdown remains on target for liftoff at 4:05 a.m. EDT (0805 GMT) when today's 20-minute launch opportunity opens.

The kerosene-fueled first stage will be burn for the first four minutes of flight before separating to allow the cryogenic Centaur upper stage to ignite for a 9-minute initial burn to reach a preliminary parking orbit. The nose cone is jettisoned four-and-a-half minutes after launch.

The rocket settles into a coast across the equatorial Atlantic, Africa, the Indian Ocean and flies over the northern shores of Australia before restarting the RL10 upper stage engine 69 minutes into flight for a four-and-a-half-minute burn that accelerates RBSP to the intended highly elliptical, low inclination orbital perch.

The upper satellite is deployed 79 minutes after liftoff into an orbit with a high point of 19,042 statute miles, a low point of 371 miles and inclination titled 10 degrees to the equator.

After the upper stage uses its thrusters to nudge the orbit's apogee a bit higher, the bottom RBSP craft is released from Centaur 91 minutes after liftoff into an orbit with a high point of 19,082 statute miles, a low point of 373 miles and inclination titled 10 degrees to the equator.

The instrument-laden probes will operate independently to collect measurements from separate points in space about the behavior of the radiation belts, the impacts from solar storms on Earth and the dynamics of space physics.

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

Weather odds just jumped up to 90 percent favorable for this morning's launch window.

The outlook for launch predicts scattered clouds at 2,000 feet, a broken deck at 31,000 feet, good visibility, south-southeasterly winds of 10 gusting to 14 knots, a temperature of 78 degrees F and humidity level of 91 percent.

A reminder that if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

And check out Spaceflight Now's Facebook page!

Pre-launch checks of the rocket's safety system have been completed.

The weather forecast has been updated and odds of acceptable conditions at launch time have improved to 80 percent.

The outlook for launch predicts scattered clouds at 3,000 feet, a broken deck at 31,000 feet, good visibility, south-southeasterly winds of 10 gusting to 14 knots, a temperature of 78 degrees F and humidity level of 91 percent.

Now 60 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen is nearly complete as the countdown continues as planned for a liftoff at 4:05 a.m. EDT. Weather remains GO and no technical problems are being reported.

If you are heading out to the beach or Port Canaveral to watch the launch, sign up for our Twitter feed to get occasional countdown updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

And if you are need tips on picking a good viewing spot, check out this authoritative guide on where to go.

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

The liquid hydrogen tank in the Centaur upper stage just reached the 97 percent level. Topping is now beginning.

Passing the 80 percent mark on the first stage liquid oxygen tank.

Centaur's liquid hydrogen tank is 50 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Pratt & Whitney Rocketdyne-made RL10 engine.

First stage liquid oxygen tank is 60 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 after rollout.

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.

Now 90 minutes till launch of the Radiation Belt Storm Probes to explore the extremes of space weather! There are no technical problems being reported in the countdown, NASA says, and the current weather conditions are green.

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

The Centaur engine chilldown sequence is being initiated.

Centaur liquid oxygen is now at flight level.

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

The Centaur liquid oxygen tank reached the 95 percent level. The topping off process is starting now.

Now at 90 percent on Centaur liquid oxygen.

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 12,325 gallons of the cryogenic propellant.

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 48,745 gallons of cryogenic oxidizer for the RD-180 main engine.

Centaur liquid oxygen is 50 percent loaded.

Atlas 5 represents the culmination of evolution stretching back several decades to America's first intercontinental ballistic missile. At the dawn of the space age, boosters named Atlas launched men into orbit during Project Mercury and became a frequent vehicle of choice to haul civil, military and commercial spacecraft to orbit.

Topped with the high-energy Centaur upper stage, Atlas rockets have been used since the 1960s to dispatch ground-breaking missions for NASA, including the Surveyors to the Moon, Mariner flights to Mars, Venus and Mercury, and the Pioneers that were the first to visit Jupiter and beyond.

In its newest era, the Atlas 5 rocket sent the Mars Reconnaissance Orbiter to the red planet in 2005, propelled the New Horizons probe toward Pluto and the solar system's outer fringes in 2006, doubled up with the dual Lunar Reconnaissance Orbiter and LCROSS impactor to the Moon in 2009, hurled Juno to Jupiter last August and dispatched the car-sized Curiosity rover on the Mars Science Lab mission in November.

Today marks the 32nd flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 31 missions since debuting in August 2002, the tally shows 10 flights dedicated to the Defense Department, 9 commercial missions with communications spacecraft, six with spy satellites for the National Reconnaissance Office and six for NASA.

Passing the 20 percent level on the Centaur upper stage's liquid oxygen tank.

Filling of the Centaur upper stage with about 4,075 gallons of liquid oxygen has begun at Cape Canaveral's Complex 41 following the thermal conditioning of the transfer pipes.

The liquid oxygen -- chilled to Minus-298 degrees F -- will be consumed during the launch by the Centaur's single RL10 engine along with liquid hydrogen to be pumped into the stage a little later in the countdown. The Centaur will perform two firings to propel the RBSP satellites into their intended orbit this morning.

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 on schedule for today's flight of the Atlas 5 rocket on the NASA mission to deploy the Radiation Belt Storm Probes.

Clocks have one more built-in hold planned at T-minus 4 minutes. That pause will last 25 minutes during which time the final "go" for launch will be given. All remains targeted for liftoff at 4:05 a.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.

T-minus 2 hours and holding. The countdown has just entered the first of two planned holds over the course of the morning that will lead to the 4:05 a.m. EDT launch of the Atlas rocket.

This initial 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 very smoothly this morning.

The final hold is scheduled to occur at T-minus 4 minutes.

The final hands-on work has wrapped up at the launch pad and technicians have departed the complex. Safety officials just confirmed that the surrounding danger area has been cleared of all workers for the remainder of the countdown.

Hold-fire checks were just performed with the Eastern Range to ensure safety personnel can hold the countdown if necessary.

So far, so good on the weather tonight. The status board shows all launch criteria still green and GO. The forecast for the 4:05 to 4:25 a.m. EDT window continues to reflect a 70 percent chance of favorable conditions.

The "attached anvil cloud rule" is red in the current observations.

But the launch weather officer still predicts a 60 percent chance of acceptable weather at the 4:07 a.m. EDT liftoff time for the Atlas 5 rocket.

The outlook for launch predicts scattered clouds at 3,000 feet, a broken deck at 22,000 feet, good visibility, south-southeasterly winds of 10 gusting to 14 knots, a temperature of 78 degrees F and humidity level of 91 percent.

Guidance system testing has been accomplished as the countdown progresses smoothly today at Cape Canaveral.

Two heavily shielded 1,400-pound satellites set for launch early Thursday will fly in tandem through Earth's energetic Van Allen radiation belts in a two-year $686 million mission to probe the structure of the huge doughnut-shaped zones, how they are affected by titanic solar storms and the threat they pose to astronauts and costly spacecraft.

In so doing, the twin Radiation Belt Storm Probes are expected to shed light on fundamental physics governing the behavior of fields and particles across the cosmos and on the more immediate impacts of space weather, which can cripple satellites, disrupt power grids, scramble communications and interfere with Global Positioning System navigation beacons.

"We've been waiting for this mission for decades," said Project Scientist Richard Fitzgerald of the Johns Hopkins University Applied Physics Laboratory.

Said Nicola Fox, the deputy project scientist at APL: "The difficult thing for us to work out is why they're changing, why they change at different times to seemingly similar drivers. We know that variations in the sun create strong geomagnetic storms here at Earth. But what we don't understand is how we really, truly respond to them."

"It's almost like making a cake," she told reporters earlier this week. "You know all the ingredients, but you're not quite sure of the proportions of each piece."

Read our preview story.

Now entering the final four hours to the United Launch Alliance Atlas 5 rocket flight with the Radiation Belt Storm Probes payload for NASA. This will be the civilan space agency's seventh use of Atlas 5 over the past seven years.

The first NASA mission sent the Mars Reconnaissance Orbiter to the red planet on Aug. 12, 2005. The New Horizons probe on a three-billion-mile voyage to fly by Pluto and explore the frigid edge of the solar system followed on Jan. 19, 2006. Then came the dual launch of the Lunar Reconnaissance Orbiter and the LCROSS impactor to the Moon's South Pole on June 18, 2009. The Solar Dynamics Observatory was placed into orbit on Feb. 11, 2010. The Juno spacecraft to study the planet Jupiter was launched on Aug. 5, 2011. And most recently was the Mars Science Laboratory with the Curiosity rover leaving Earth on Nov. 26, 2011.

Now comes RBSP at 4:05 a.m. EDT.

In tonight's first weather briefing to mission managers, all current conditions are observed GO for launch of the Atlas 5 rocket and odds for the 4:05 a.m. EDT liftoff time are 70 percent favorable.

The outlook for the launch window has deleted isolated showers from the forecast. Meteorologists now predict scattered clouds at 3,000 and a broken deck at 22,000 feet, good visibility, south-southeasterly winds of 10 gusting to 14 knots, a temperature of 78 degrees F and humidity level of 91 percent.

The countdown has begun for the overnight launch of the Atlas 5 rocket to deploy NASA's Radiation Belt Storm Probes. The twin craft, beefed up with protective hardening, will take unprecedented measurements inside the hearts of the radiation belts to provide fundamental data about the behavior and character of these swaths of space that can harm satellites and astronauts alike.

Clocks are picking up the seven-hour, 15-minute sequence of work that will prepare the booser, payload and ground systems for blastoff at 4:05 a.m. EDT (0805 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. The Complex 41 site will be cleared of all personnel at 12:55 a.m.

A planned half-hour hold begins at 1:10 a.m. when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 1:37 a.m. to verify all is in readiness to start fueling the rocket for launch.

Supercold liquid oxygen begins flowing into the Centaur upper stage around 1:57 a.m., followed by the first stage filling around 2:10 a.m. 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 starting at 3:36 a.m. That 25-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 4:05 to 4:25 a.m. EDT (0805-0825 GMT).

Forecasters have improved the odds of acceptable weather for Thursday's 4:05 a.m. EDT launch of the Atlas rocket to 70 percent. Cumulus clouds and cloud thickness are the two areas of concern.

"Although some drying is expected today, afternoon and evening thunderstorms are expected. Over the next 24 hours, a trough pushes into South Georgia and North Florida and becomes stationary. For launch, the stalled trough to the north causes concerns for isolated showers and thick clouds during the count and window," Air Force meteorologists report.

The outlook at launch time calls for scattered clouds at 3,000 feet, a broken deck at 22,000 feet, isolated showers in the area, good visibility, south-southeasterly winds of 10 gusting to 14 knots, a temperature of 78 degrees F and relative humidity of 91 percent.

The Eastern Range has confirmed the Atlas launch reservation on its schedule. And officials report pre-flight preparations are proceeding on the proper timeline this morning.

Here's our photo gallery showing the Atlas 5's trip to the pad on Tuesday afternoon.

On the pad! Completing the first third-of-a-mile on the trek to probe the hearts of the radiation belts, the Atlas 5 rocket has arrived at the Complex 41 pad for Thursday's predawn ascent to deploy NASA's RBSP spacecraft.

"The dramatic dynamics of Earth's radiation belts caused by space weather are highly unpredictable," said Barry Mauk, RBSP project scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. "One of the fundamental objectives of the RBSP mission is to use Earth's magnetosphere as a natural laboratory to understand generally how radiation is created and evolves throughout the universe. There are many mysteries that need to be resolved."

The Atlas 5 rocket will inject the instrument-laden satellites into highly elliptical orbits, inclined 10 degrees to the equator, to begin a two-year mission plunging into and out of the radiation belts discovered by James Van Allen at the dawn of the space age.

"Both the twin spacecraft and the entire RBSP team are eager to begin their exploration of one of the most dangerous parts of space near our planet," said Richard Fitzgerald, RBSP project manager at APL.

The two mobile trailers connected to the launching platform, which were part of the convoy during this morning's rollout, soon will be hooked up to power and communications systems at the pad. These trailers provide conditioned air to the payload and communications with the rocket during the rollout and the countdown. They are protected from the blast of launch by a concrete structure on the north-side of the platform.

Within the next hour, the auto couplers between the pad and platform will be engaged to route umbilical connections from the ground to the rocket for tomorrow's fueling of the booster with cryogenic propellants.

Later today, the undercarriages used to move the mobile platform will be disconnected and the "trackmobiles" pulled free.

The rocket had spent the past two days in the assembly hangar to ride out the tropical storm, and engineers took the opportunity to remove and replace the rocket's tracking beacon that had exhibited "out-of-family" characteristics during the first launch attempt last Friday. Although the unit was declared acceptable for flight, the delay and rollback for Isaac gave the team time to swap it out.

Ground crews will secure the rocket for a quiet evening and most of Thursday leading into the launch opportunity. Countdown clocks will start ticking about seven hours before liftoff time.

Thursday's 20-minute launch window opens at 4:05 a.m. and closes at 4:25 a.m. EDT.

Here's a timeline of the countdown's key events:

 HR:MM..Eastern...Event
T-6:20...8:50pm...Countdown begins with rocket power up
T-5:30...9:40pm...Weather briefing
T-4:55..10:15pm...Start clearing assembly building area
T-4:20..10:50pm...C-band tracking beacon testing
T-3:40..11:30pm...S-band telemetry link checks
T-2:55..12:15am...Establish blast danger area roadblocks
T-2:20..12:50am...Weather briefing
T-2:15..12:55am...Clear the pad
T-2:00...1:10am...T-120 minutes and holding (for 30min)
T-2:00...1:35am...Launch conductor briefing to team
T-2:00...1:37am...Readiness poll for fueling
T-2:00...1:40am...Resume countdown
T-1:50...1:50am...Centaur LOX transfer line chilldown
T-1:43...1:57am...Begin Centaur liquid oxygen loading
T-1:30...2:10am...Begin Atlas first stage LOX loading
T-1:25...2:15am...Centaur LH2 transfer line chilldown
T-1:10...2:30am...Centaur RL10 engine chilldown
T-1:02...2:38am...Begin Centaur liquid hydrogen loading
T-0:40...3:00am...FLight termination system final test
T-0:16...3:24am...RD-180 engine fuel fill sequence
T-0:10...3:30am...Weather briefing
T-0:04...3:36am...T-4 minutes and holding (for 25min)
T-0:04...3:58am...Readiness poll for launch
T-0:04...4:01am...Resume countdown
T-0:00...4:05am...LAUNCH

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The weather forecast remains 60 percent favorable for launch from Cape Canaveral at 4:05 a.m. EDT on Thursday.

Cumulus clouds and cloud thickness are the two areas of concern.

The outlook calls for scattered clouds at 3,000 feet, a broken deck at 22,000 feet, isolated showers in the area, good visibility, southerly winds of 10 gusting to 15 knots and a temperature of 78 degrees F.

The United Launch Alliance Atlas 5 rocket has emerged from its vehicle assembly building for the journey to the pad, a milestone move for the program about to make its 32nd trip to space.

Riding atop a mobile launching platform, the 191-foot-tall satellite booster is moving to Cape Canaveral's Complex 41 for launch of NASA's Radiation Belt Storm Probes.

"We are proud to serve a critical role in delivering one-of-a-kind NASA payloads to orbit in support of the global science community," said Vern Thorp, ULA's program manager for NASA missions.

This will be the seventh NASA launch using the Atlas 5 rocket.

The rocket's rollout to the pad has begun!

This slow half-hour drive from the 30-story Vertical Integration Facility to Cape Canaveral's Complex 41 pad uses a pair of specially-made "trackmobiles" to carry the rocket's 1.4-million pound mobile launching platform along rail tracks for the 1,800-foot trip.

The two-stage rocket and the RBSP spacecraft were put together inside the assembly building in preparation for this rollout event. The Atlas 5 is designed to spend minimal time at the launch pad, which does not include a service gantry like other sites.

The rocket had been rolled back on Sunday to hide out from Tropical Storm Isaac. Now the vehicle is heading out to Complex 41 for Thursday's launch.


Read our earlier status center coverage.