Mission managers met this afternoon in another Launch Readiness Review for the Radiation Belt Storms Probes and affirmed plans to try again Thursday to fly the Atlas 5 rocket and deploy the twin NASA spacecraft.

Liftoff is targeted for 4:05 a.m. EDT (0805 GMT) at the opening of a 20-minute window.

The rocket will ride the rails back to the launch on Tuesday after safely waiting out the tropical weather from Isaac that has been drenching Cape Canaveral. Rollout is expected around 2 p.m. EDT (1800 GMT).

The long-range weather forecast is predicting a 60 percent chance of acceptable conditions to launch the Atlas 5 rocket from Cape Canaveral at 4:05 a.m. EDT on Thursday.

Thick clouds and gusty winds 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, southwesterly winds of 16 gusting to 23 knots and a temperature of 78 degrees F.

The odds improve to 70 percent favorable for Friday, according to the launch weather team, with winds calming and cloud thickness the only worry for violating liftoff criteria.

Marking only the sixth delay-and-rollback scenario in 28 flights from Cape Canaveral, the Atlas 5 rocket returned to the assembly hangar ride out the impending passage of Hurricane Isaac.

The Atlas originally moved from the nearby Vertical Integration Facility to the Complex 41 pad Wednesday morning. After experiencing back-to-back scrubs on Friday morning to double-check the rocket's tracking beacon system and Saturday morning for thunderstorms, officials decided to put the rocket back into the VIF while the high winds from the tropical weather moves through the area.

Launch is rescheduled for next Thursday at 4:05 a.m. EDT (0805 GMT).

The Atlas 5 program at the Florida spaceport uses a "clean-pad" concept with minimal infrastructure. Instead, stacking of the rocket stages and pre-flight work is accomplished inside the user-friendly confines of the VIF building. The mobile launch platform transports the Atlas between the hangar and the pad, a distance of just 1,800 feet.

Four times in the history of the Atlas 5 program has a rocket been forced to roll back for technical reasons. Those instances were driven by circumstances that required hardware removal-and-replacement jobs be performed in the 30-story-tall VIF.

The previous rollbacks occurred on the Hellas Sat mission in 2003 to replace data instrumentation unit, the WGS 2 flight in 2009 to replace a leaky liquid oxygen valve, Intelsat 14 also in 2009 to replace an ordnance control assembly and most recently in June on NROL-38 to remove a faulty ground support equipment duct in the environmental control system.

One other rollback occurred earlier this February due to an extended wait for Range availability after weather thwarted initial launch opportunities on the MUOS 1 flight.

Rollback is underway out at Complex 41, as the United Launch Alliance Atlas 5 team wheels the rocket to the Vertical Integration Facility to avoid standing exposed on the pad during Hurricane Isaac.

The trackmobiles are positioning the undercarriages beneath the Atlas 5 rocket and the mobile launch platform in preparation for this morning's departure from the pad and rollback to the Vertical Integration Facility.

ROLLBACK. The Atlas 5 rocket will be rolled back from the launch pad to its assembly hangar to avoid the threat of high winds buffeting Cape Canaveral in the coming days as the wide swath of Isaac, now a tropical storm but expected to develop into a hurricane, skirts by Florida.

Launch will be postponed until next Thursday, Aug. 30, at 4:05 a.m. EDT (0805 GMT).

Officials decided it was prudent to return the rocket aboard its mobile launch platform to the Vertical Integration Facility to wait out the storm in the safe confines of the assembly building. Atlas 5 rockets are meant to spend minimal time atop the Complex 41 pad, which is an "open air" site with no service gantry.

Since the rocket was fueled with supercold liquid hydrogen and liquid oxygen for this morning's launch attempt, the vehicle must be allowed to warm up for 24 hours before the rollback can begin.

The latest forecast path from the National Hurricane Center predicts Hurricane Isaac will pass through the Florida Keys late Sunday and early Monday, be due west of Tampa early Tuesday and make landfall in the Florida Panhandle late Tuesday or early Wednesday.

The initial launch attempt for the Radiation Belt Storm Probes on Friday morning was scrubbed due to "out-of-family" readings from the C-band beacon on the rocket that is used for tracking the vehicle as it flies downrange. After thorough analysis, officials determined the system was acceptable to launch as-is.

This morning's second countdown was scrubbed for thunderstorms in the local area that violated a number of the launch rules.

Managers initially told the launch team to posture for another shot on Sunday, but a subsequent meeting to assess Isaac resulted in the rollback decision.

Launch managers will be convening a meeting at 6 a.m. EDT to determine whether to attempt the liftoff on Sunday or not. They will assess the Atlas 5 rocket and Radiation Belt Storm Probes, as well as the weather forecast. The wildcard is Tropical Storm Isaac, which is expect to develop into a hurricane and bring high winds to the Cape Canaveral area over the next few days.

If the weather predictions indicate the rocket should be returned to its assembly building to ride out the storm, those rollback procedures need to begin soon.

The launch team is stepping into standard post-scrub procedures to safe the Atlas 5 rocket and offload the supercold liquid oxygen and liquid hydrogen from the vehicle.

The outlook for tomorrow's launch window calls for broken decks of clouds at 3,000 and 22,000 feet, isolated coastal showers, good visibility, easterly winds of 18 gusting to 25 knots, a temperature of 78 degrees F and humidity level of 97 percent.

The concerns will be cumulus clouds and liftoff winds.

The team will set up for another launch attempt tomorrow at 4:07 a.m. EDT. But the early weather outlook is only 40 percent favorable for Sunday's window.

SCRUB. Today's flight of the United Launch Alliance Atlas 5 rocket carrying NASA's Radiation Belt Storm Probes has been scrubbed due to nearby thunderstorms at Cape Canaveral. The 20-minute launch opportunity has run out, leaving no further time to wait on conditions to improve this morning.

Just about out of time this morning.

Field mill, cumulus cloud, attached anvil cloud and lightning rules all remain red.

Count would have to resume by 4:23 a.m.

And the last potential hold extension has been taken, putting liftoff no sooner than the very close of today's window at 4:27 a.m. EDT.

The launch team has been instructed to maintain readiness and that a re-poll will not be conducted before resuming the count, if weather somehow improves rapidly. The launch director will give approval to pick up the clock.

A fourth weather rule has gone red -- the field mills -- that measure the electrical potential in the air.

Five more minutes added to the hold clock, pushing launch back to 4:22 a.m. EDT.

Another five minutes have been added to the hold, tentatively pushing liftoff to 4:17 a.m. EDT if the weather should clear. Today's launch window runs to 4:27 a.m. EDT to put the RBSP spacecraft into their desired science orbit.

The team is adding five-minute increments to the hold clock at T-minus 4 minutes while waiting on the weather.

There is no estimate on when the weather might come back into acceptable limits. The team is patiently waiting.

Polling of the team by Atlas launch conductor just occurred. All systems are "go" for liftoff with the exception of the weather. The countdown will not resume at 4:03 a.m. as originally planned.

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

Weather remains "no go" due to violations of the cumulus cloud, attached anvil cloud and lightning rules associated with nearby thunderstorms. The team is waiting to see if the weather will improve to permit the Atlas 5 rocket to launch some time within today's 20-minute window that extends to 4:27 a.m. EDT.

A third weather rule has gone "red" -- the attached anvil cloud criteria - due to these thunderstorm cells.

Twenty minutes from liftoff, weather permitting.

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.

Meteorologists are watching thunderstorm cells to the south that are close enough to violate the launch rules governing cumulus cloud and lightning rules. They are moving to the west with a bit of northward motion as well. The weather team is sticking with 60 percent chance of acceptable weather for launch at 4:07 a.m. EDT.

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.

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

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

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The weather status board has gone "red" due to cumulus clouds and lightning.

Meteorologists keep managers advised when rules go "red" and "green" throughout the countdown.

A check of the current weather shows all conditions remain "go" at the present time. The forecast is unchanged and odds of acceptable weather at launch time are 60 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 4:07 a.m. EDT.

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.

Centaur's liquid hydrogen tank is 37 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.

Interrogations of the C-band tracking beacon on the Atlas 5 rocket have been going well in this morning's countdown, NASA launch director Tim Dunn says.

After yesterday's scrub, troubleshooting revealed the transponder aboard the rocket was producing slightly "out-of-family" readings that engineers ultimately determined were acceptable for flight as-is. The rocket team and the Range got comfortable with the beacon performance and concluded it was safe to launch the Atlas without needing any hardware replacements.

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 Centaur engine chilldown sequence is being initiated.

The first stage liquid oxygen tank has reached the 20 percent mark.

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.

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.

Now at 70 percent on Centaur liquid oxygen.

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.

Now entering the final two 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 4:07 a.m. EDT.

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 4:07 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.

Weather is back to a green status again. The anvil rule is no longer being violated by the current conditions.

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.

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 4:07 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.

Flashes of lightning are prevalent in the skies around Cape Canaveral tonight as weather to the southeast of the spaceport moves ashore from the Atlantic. The northern fringes of that disturbed weather is skirting just south of the launch site at this hour.

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 calls for scattered clouds at 3,000 feet, a broken deck at 22,000 feet, isolated coastal showers, good visibility, easterly winds of 12 gusting to 16 knots, a temperature of 78 degrees F and humidity level of 91 percent.

Two heavily shielded 1,400-pound satellites set for launch early Saturday 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.

In tonight's first weather briefing to mission managers, the odds of good conditions to launch the Atlas 5 rocket remain at 60 percent for the launch window, with the main concern being cumulus clouds from coastal rain showers.

The outlook for launch time now calls for scattered decks of clouds at 3,000 and 22,000 feet, isolated coastal showers around, good visibility, easterly winds of 12 gusting to 16 knots, a temperature of 78 degrees F and humidity level of 91 percent.

The second shot at sending the United Launch Alliance Atlas 5 rocket into orbit to deploy NASA's Radiation Belt Storm Probes is getting underway tonight. The countdown clocks are scheduled to start ticking right now to begin the seven-hour, 15-minute sequence of work that will prepare the rocket, payload and ground systems for Saturday's predawn blastoff at 4:07 a.m. EDT (0807 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 12:57 a.m.

A planned half-hour hold begins at 1:12 a.m. when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 1:39 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 1:59 a.m., followed by the first stage filling around 2:12 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 3:38 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 4:07 to 4:27 a.m. EDT (0807-0827 GMT).

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

 HR:MM..Eastern...Event
T-6:20...8:52pm...Countdown begins with rocket power up
T-5:30...9:42pm...Weather briefing
T-4:55..10:17pm...Start clearing assembly building area
T-4:20..10:52pm...C-band tracking beacon testing
T-3:40..11:32pm...S-band telemetry link checks
T-2:55..12:17am...Establish blast danger area roadblocks
T-2:20..12:52am...Weather briefing
T-2:15..12:57am...Clear the pad
T-2:00...1:12am...T-120 minutes and holding (for 30min)
T-2:00...1:37am...Launch conductor briefing to team
T-2:00...1:39am...Readiness poll for fueling
T-2:00...1:42am...Resume countdown
T-1:50...1:52am...Centaur LOX transfer line chilldown
T-1:43...1:59am...Begin Centaur liquid oxygen loading
T-1:30...2:12am...Begin Atlas first stage LOX loading
T-1:25...2:17am...Centaur LH2 transfer line chilldown
T-1:10...2:32am...Centaur RL10 engine chilldown
T-1:02...2:40am...Begin Centaur liquid hydrogen loading
T-0:40...3:02am...FLight termination system final test
T-0:16...3:26am...RD-180 engine fuel fill sequence
T-0:10...3:32am...Weather briefing
T-0:04...3:38am...T-4 minutes and holding (for 25min)
T-0:04...4:00am...Readiness poll for launch
T-0:04...4:03am...Resume countdown
T-0:00...4:07am...LAUNCH

Launch of the Atlas 5 rocket will be attempted again early Saturday morning, officials have decided, after troubleshooting of the C-band beacon problem.

"The decision followed a series of meetings to evaluate "out-of-family" readings in the signal of a C-Band tracking device linking the Atlas 5 rocket and ground-based range equipment that caused a scrub early Friday morning," NASA said in a statement.

Liftoff is targeted for 4:07 a.m. EDT (0807 GMT) at the opening of a 20-minute launch window. Weather forecasters predict a 60 percent chance of acceptable conditions on Saturday.

The beacon is used to track the rocket as it travels downrange from Cape Canaveral en route to orbit. The Air Force-controlled Eastern Range views the beacon as a mandatory system that must be working properly before the vehicle can be cleared for launch, given the implications for public safety.

After a fairly smooth countdown overnight, clocks entered the pre-planned hold at T-minus 4 minutes ahead of the planned 4:07 a.m. EDT launch. But the situation went downhill as the Range declared a "red" status because the beacon readings were showing an unexplained "frequency drift," NASA launch director Tim Dunn said after the scrub.

With very limited time to examine the situation, mission managers were forced to call a scrub as the 20-minute launch window was expiring.

But NASA announced this afternoon that the launch was rescheduled to try again on Saturday.

NASA says the Atlas 5 rocket will give it another shot on Saturday morning at 4:07 a.m. EDT. We'll post more details as they become available.

The preliminary weather forecast for Saturday's launch window calls for scattered decks of clouds at 3,000 and 22,000 feet, isolated coastal rain showers, good visibility, easterly winds of 12 gusting to 16 knots and a temperature of 78 degrees F. There is a 60 percent chance of acceptable launch conditions, with cumulus clouds the main concern.

NASA launch director Tim Dunn says the scrub was called after a problem with the C-band beacon for tracking the rocket exhibited a problem late in the countdown. That beacon, which is a mandatory component that must be working properly for launch to occur, was showing an unexpected "frequency drift" and there wasn't time to evaluate the cause before the today's window ran out. The system is used by the Eastern Range to follow the rocket's progress as it flies away from Cape Canaveral.

If the problem can be understood or the root cause quickly identified, the launch team will be postured to try the countdown again tomorrow. If hardware needs replaced, the delay could be longer, Dunn said.

The launch team is stepping into standard post-scrub procedures to safe the Atlas 5 rocket and offload the supercold liquid oxygen and liquid hydrogen from the vehicle.

Tomorrow's launch window is 20 minutes in duration opening at 4:07 a.m. EDT.

SCRUB! Today's attempt to launch NASA's Radiation Belt Storm Probes aboard an Atlas 5 rocket from Cape Canaveral has been postponed 24 hours because of a technical problem with the tracking beacon. The trouble could not be resolved in time for liftoff during the morning's launch window.

A re-poll of the launch team just occurred, but the Range remains NO GO due to the C-band beacon issue.

Due to collision avoidance cutouts with orbiting objects, it appears 4:25 a.m. EDT would be the final minute of today's launch opportunity to send the rocket on its way to space.

NEW LAUNCH TIME. Liftoff has been retargeted for 4:25 a.m. EDT, pending resolution of the tracking beacon problem.

There is no estimate on how long this hold will last. But today's launch window ends at 4:27 a.m. EDT.

So the launch window is now open for today's launch. But the countdown remains holding at T-minus 4 minutes due to a technical problem.

The anomaly team is being convened to talk about this issue, which appears to be with the tracking beacon.

The RBSP spacecraft remain on internal power for this extended hold.

So the team is standing by for launch clearance from the Range. Today's launch window extends about 20 minutes.

Polling of the team by Atlas launch conductor just occurred. A Range problem was reported and the countdown will not resume, delaying liftoff at least a few minutes.

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

NASA launch director Tim Dunn has completed a poll and verified the space agency's advisory team is ready for liftoff.

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

Twenty minutes from liftoff. No reports of anything amiss technically and the weather is GO for launch at 4:07 a.m. EDT.

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.

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.

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Current weather conditions remain "green" for launch. Skies are clearing out and there's no little concern for violating the thick cloud rule. The odds of launchable weather have improved to 90 percent.

The latest forecast for launch time now scattered decks of clouds at 3,000 and 22,000 feet, good visibility, northeasterly winds of 5 knots and a temperature of 78 degrees F.

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

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

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

A check of the current weather shows all conditions remain "go" at the present time.

The first stage liquid oxygen tank is above 80 percent full now.

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 4:07 a.m. EDT.

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.

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.

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

Passing the 50 percent level on the Centaur upper stage's liquid hydrogen tank.

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

The first stage liquid oxygen tank has reached the 20 percent mark.

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.

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

The weather status board is all "green" again. The earlier "red" condition for the cloud thickness rule has passed. The outlook for today's launch window remains 70 percent favorable.

Now 90 minutes till launch!

The Centaur engine chilldown sequence is being initiated.

The anomaly team that was looking at the liquid oxygen fill and drain valve on the first stage reports that when the launch team was cycling that valve earlier they got erroneous indications that the valve had not opened and closed as anticipated. However, further examination of other data confirms the valve is working and the indicator readings were false.

The team has suitable workaround options and can resume fueling operations with no impact to today's countdown and launch.

Centaur liquid oxygen is now at flight level.

The current configuration of the fill and drain valve is closed. The team is not actively flowing liquid oxygen into the first stage.

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

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 anomaly team has been convened to discuss a possible issue with the liquid oxygen fill and drain valve for the first stage.

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.

Now exactly two hours until launch!

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 4:07 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.

Weather has just gone "red" for some cloud cover passing overhead that violates the thickness rule for launch. However, forecasters expect this condition to clear shortly and still predict a 70 percent chance of acceptable weather at launch time.

Meteorologists keep managers advised when rules go "red" and "green" throughout the countdown. This current thick cloud rule violation won't hold up fueling or other operations for now, but obviously would need be "green" by the launch window.

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.

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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 4:07 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.

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

A check of the weather shows all of the rules are "go" at the current time. The outlook for launch time remains 70 percent favorable due to thick clouds.

In the pre-fueling briefing to mission managers, the latest forecast still calls for scattered decks of clouds at 3,000 and 22,000 feet, good visibility, southeasterly winds of 5 knots and a temperature around 78 degrees F.

Testing of the vehicle's guidance system is complete.

Just four hours left to go!

If you are in the local Cape Canaveral area and would like to watch today's Atlas 5 rocket launch in person, check out this authoritative viewing guide on where to go.

Among the activities currently underway at this point in the countdown, the launch team is completing pre-flight preps on the cryogenic systems in preparation for fueling and running standard tests on the C-band system used to track the rocket as it flies downrange and the S-band system used for telemetry relay from vehicle.

In the initial portion of the count, power up of the Atlas first stage and the Centaur upper stage occurred and guidance system testing was started.

And earlier this hour, the twin RBSP spacecraft were powered up and the "aliveness" testing commenced to ensure both satellites are ready to fly.

Now entering the final five hours in the countdown 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 4:07 a.m. EDT.

In the first weather briefing of tonight's countdown, the odds of good conditions to launch the Atlas 5 rocket have remained the same from the most recent outlook issued Thursday morning. Meteorologists continue to project a 70 percent chance of acceptable weather for the launch window, with only a slight concern for thick clouds.

The outlook for launch time now calls for scattered decks of clouds at 3,000 and 22,000 feet, no rain or showers, good visibility, southeasterly winds of 5 knots, a temperature around 78 degrees F and humidity level of 95 percent.

Welcome to liftoff night for the United Launch Alliance Atlas 5 rocket carrying NASA's Radiation Belt Storm Probes on 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 measures 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.

At Cape Canaveral, the countdown clocks are starting to tick right now, beginning the seven-hour, 15-minute sequence of work that will prepare the rocket, payload and ground systems for Friday's predawn blastoff at 4:07 a.m. EDT (0807 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 12:57 a.m.

A planned half-hour hold begins at 1:12 a.m. when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 1:39 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 1:59 a.m., followed by the first stage filling around 2:12 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 3:38 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 4:07 to 4:27 a.m. EDT (0807-0827 GMT).

Here's a photo gallery showing the Atlas 5's trip to the pad on Wednesday morning.

Two heavily shielded 1,400-pound satellites set for launch early Friday 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.

Read our preview story.

It's the final day before the scheduled launch of the Atlas 5 rocket and the team is resting before their late-night call to duty. The countdown will begin a little before 9 p.m. EDT in advance of the 4:07 a.m. EDT liftoff tomorrow.

The latest forecast continues to show a 70 percent chance of allowable weather conditions to launch the Atlas 5, with only thick clouds a potential concern for meteorologists.

"A persistent upper level trough remains over the Eastern U.S. with the Bermuda High pressure ridge to the south, and a weak front in North Florida. Southerly to south-southwesterly surface winds will become southeasterly with the seabreeze development. Southwesterly flow in the steering levels will favor the East coast for afternoon and evening thunderstorms. Some thunderstorms may be severe. Lingering cloud cover from afternoon and evening thunderstorms is the main concern for launch," forecasters report.

"Our primary concern for launch Friday morning is a thick cloud rule violation."

The outlook for the 20-minute launch window calls for scattered low- and high-level clouds, good visibility, southeasterly winds of 5 knots and a temperature around 78 degrees F.

We will begin our live play-by-play launch coverage at 1 a.m. EDT and streaming video of the mission starts on this page at 1:30 a.m. EDT.

Check out today's gallery of photos showing the Atlas 5 rocket poised atop Complex 41!

Weather forecasters have improved the chances of favorable conditions during Friday's launch window that opens at 4:07 a.m. EDT. The new odds call for a 70 percent chance of good weather. The possibility of clouds violating the thickness criteria poses the only concerns.

The outlook at launch time includes scattered low-level clouds at 3,000 feet, a deck of scattered high clouds at 22,000 feet, good visibility of 7 miles, southeasterly winds of 5 knots and a temperature around 78 degrees F.

"A persistent upper level trough remains over the Eastern U.S., and the Bermuda High pressure ridge is still to the south. Southwest flow will continue over the area, and the convergence of this flow with the afternoon sea breeze will continue to cause afternoon thunderstorms. Some thunderstorms may be severe," Air Force meteorologists reported this morning.

"On Thursday, a weak front will move into north Florida. Afternoon thunderstorms will again occur, dissipating by the evening hours, and lingering cloud cover is the main concern for launch. Meteorological models are indicating less chance for cloud cover; therefore, our probability of violating weather constraints decreased to 30 percent. Our primary concern for launch Friday morning is a thick cloud rule violation."

In the event the launch slips 24 hours to Saturday morning, the forecast is 60 percent favorable due to cumulus cloud worries.

"Friday evening, easterly flow will set in as the front washes out and Tropical Storm Isaac moves into the Caribbean near Hispaniola, potentially as a category 1 hurricane. The primary concerns for a Saturday morning launch will be coastal showers causing a cumulus cloud violation," the latest forecast says.

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 Friday'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. And RP 1, a highly refined kerosene fuel, will be loaded aboard the Atlas 5 rocket's first stage.

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.

Friday's 20-minute launch window opens at 4:07 a.m. and closes at 4:27 a.m. EDT.

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

 HR:MM..Eastern...Event
T-6:20...8:52pm...Countdown begins with rocket power up
T-5:30...9:42pm...Weather briefing
T-4:55..10:17pm...Start clearing assembly building area
T-4:20..10:52pm...C-band tracking beacon testing
T-3:40..11:32pm...S-band telemetry link checks
T-2:55..12:17am...Establish blast danger area roadblocks
T-2:20..12:52am...Weather briefing
T-2:15..12:57am...Clear the pad
T-2:00...1:12am...T-120 minutes and holding (for 30min)
T-2:00...1:37am...Launch conductor briefing to team
T-2:00...1:39am...Readiness poll for fueling
T-2:00...1:42am...Resume countdown
T-1:50...1:52am...Centaur LOX transfer line chilldown
T-1:43...1:59am...Begin Centaur liquid oxygen loading
T-1:30...2:12am...Begin Atlas first stage LOX loading
T-1:25...2:17am...Centaur LH2 transfer line chilldown
T-1:10...2:32am...Centaur RL10 engine chilldown
T-1:02...2:40am...Begin Centaur liquid hydrogen loading
T-0:40...3:02am...FLight termination system final test
T-0:16...3:26am...RD-180 engine fuel fill sequence
T-0:10...3:32am...Weather briefing
T-0:04...3:38am...T-4 minutes and holding (for 25min)
T-0:04...4:00am...Readiness poll for launch
T-0:04...4:03am...Resume countdown
T-0:00...4:07am...LAUNCH

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Now getting centered up on the pad.

The rollout convoy is passing through the pad perimeter fence.

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 immediately after marking the 10th anniversary of its first mission yesterday.

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 over the past five weeks 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 Atlas 5 rocket will be hauled out to the launch pad this morning, riding the rails from its assembly building to Complex 41 for the Radiation Belt Storm Probes mission.

First motion for the rollout is targeted for 8 a.m. EDT (1200 GMT). Watch this page for updates!

The meeting of senior NASA, United Launch Alliance and Air Force officials this afternoon formally granted clearance to the Atlas 5 rocket and the Radiation Belt Storm Probes for blastoff Friday morning at 4:07 a.m. EDT from Cape Canaveral, Florida.

The Launch Readiness Review, running a day late so technicians could conduct precautionary checks on the main engine steering system, concluded that the rocket is in good shape for a successful ascent.

Riding atop the mobile launch platform, the Atlas will be wheeled out of its assembly building and onto the the Complex 41 pad Wednesday morning starting at 8 a.m. EDT, a bit earlier than previously advertised to avoid any weather problems. The launch countdown begins Thursday evening at 8:52 p.m. EDT.

The Launch Readiness Review later this afternoon will make the "go/no go" on clearing the Atlas 5 rocket for rollout to the pad tomorrow for Friday morning's planned liftoff of NASA's Radiation Belt Storm Probes.

Testing of the hydraulic steering system for the main engine was completed last night, NASA reports, and examination of that data is underway this morning to present a recommendation to the LRR.

"At this point in the analysis, there appear to be no obvious problems with the system on the Atlas 5 that will launch RBSP," a NASA spokesperson says.

The latest weather forecast remains unchanged from the outlook presented yesterday, with a 60 percent chance of of good conditions Friday morning.

While the Radiation Belt Storm Probes sat quietly inside the Atlas 5 rocket nose cone today, awaiting Friday morning's launch, technicians at the base of the booster were running confidence-building tests on the main engine hydraulic steering system.

The checks were ordered after a test anomaly occurred Saturday night on another RD-180 first stage main engine at the United Launch Alliance rocket factory in Decatur, Alabama.

"The RD-180 under test at Decatur experienced an anomaly in the actuator system that moves the engine for steering in flight. The combined NASA and ULA technical team jointly agreed yesterday that additional testing and assessment is required to verify the same anomalous condition is not present on RBSP's launch vehicle engine hardware," said Tim Dunn, NASA launch director.

Officials believe RBSP's Atlas is good to fly, but opted to run the engine cycling tests just to make sure.

"The testing that we are doing today will build up some separation between this engine and the engine in the factory where we saw the issue," said Vernon Thorp, ULA's program manager of NASA missions.

"We think we have a good hydraulic system on the engine for the RBSP mission," he added. "We're not going to take anything for granted or make any assumptions we don't have to. We're going to go in and do this screening test test."

All of the data and results will be sorted through on Tuesday before the Launch Readiness Review is convened in the afternoon to formally clear the mission to proceed with rollout of the Atlas 5 to the pad Wednesday.

Meanwhile, the weather forecast is calling for a 60 percent chance of acceptable conditions during Friday morning's launch window opening at 4:07 a.m. EDT.

"On Thursday, a weak front will move into the area. Afternoon thunderstorms will again occur, dissipating by the evening hours. Still, lingering cloud cover is the main concern for launch. Our primary concern for launch is a thick cloud rule violation," forecasters reported.

The launch time outlook calls for scattered low clouds, a broken deck of high clouds, good visibility, easterly winds of 5 knots and a temperature of 78 degrees F.

Precautionary checks to ensure good confidence in the RD-180 main engine to power the Atlas 5 rocket carrying NASA's Radiation Belt Storm Probes will delay this week's launch by 24 hours, slipping the predawn blastoff to Friday at 4:07 a.m. EDT (0807 GMT).

NASA and rocket-supplier United Launch Alliance said Sunday night in a joint statement that "an anomalous engine condition was identified during testing of another Atlas vehicle at the Factory in Decatur, Ala.," and the one-day launch delay would be necessary for teams to verify that a similar condition does not exist on the RBSP rocket's engine.

Weather forecasters are predicting a 60 percent chance of favorable weather conditions for Friday's 20-minute launch window.

The pre-launch news conference and the mission science briefing will proceed on Monday afternoon as originally planned. We'll have live streaming video of the events starting at 1 p.m. EDT (1700 GMT).

Members of the launch team gathered for the mission dress rehearsal today to practice running through the countdown scripts and procedures to be used during next Thursday's real deal.

Activities remain on track at Complex 41 for blastoff at 4:08 a.m. EDT (0808 GMT).

The Launch Readiness Review will be held Monday morning, followed by the pre-launch news conference at 1 p.m. EDT (1700 GMT). Rollout of the Atlas 5 to the launch pad will occur Tuesday morning.

Devoid of any constraints, the Atlas 5 rocket and Radiation Belt Storm Probes passed their Flight Readiness Review with flying colors today as preparations remain on scheduled for launch next Thursday. Officials gathered for the meeting at Kennedy Space Center this morning and concluded the review quicker than expected with issues of note.

A pair of hardened spacecraft that will explore the harsh environment of Earth's radiation belts and the the United Launch Alliance Atlas 5 rocket that will provide the singular power to hurl the two satellites into looping orbits will undergo a combined checkout Tuesday.

The Integrated Systems Test for the rocket and the twin Radiation Belt Storm Probes will be run to confirm the booster and payload are ready for their departure from the launch pad next week.

The tip-to-tail electrical checkout verifies systems throughout the 19-story vehicle and stacked satellites are working properly ahead of the predawn blastoff Aug. 23.

RBSP and the Atlas have been operating on separate paths during manufacturing and assembly. But they finally met last Friday, getting bolted together in preparation for the launch. Current operations are being performed in the Vertical Integration Facility next to the Complex 41 pad at Cape Canaveral. Rollout of the rocket to the pad atop its mobile platform is planned for next Tuesday.

The satellites were readied for flight at the commercially-run Astrotech satellite processing campus in Titusville. After the two spacecraft were placed one on top of the other, then encapsulated in the rocket's two-piece nose cone in the cleanroom and placed aboard board a trailer-like transporter for the drive to the rocket's assembly building.

Last Friday's middle-of-the-night trek crossed the river, went northward through Kennedy Space Center, passed by the Vehicle Assembly Building and the old space shuttle launch pad before cruising down along the beach to the Atlas facility for hoisting atop the two-stage rocket.

See a photo gallery of the payload going to the VIF and being lifted to the rocket.

An initial state-of-health check was performed on the satellites after the big move and the onboard batteries were charged over the weekend.

Next Thursday's liftoff is scheduled for 4:08 a.m. EDT (0808 GMT) at the opening of a 20-minute launch opportunity.

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 lineup of Atlas 5 rocket launches this month -- a national security satellite deployment from California and a NASA space weather research project from Florida -- will flip-flop in order to give the Air Force more time to sort out Range instrumentation troubles that stalled the West Coast blastoff originally planned Aug. 2.

That puts the Aug. 23 launch of the Radiation Belt Storm Probes from Cape Canaveral next in the firing order. The rocket has been assembled and is scheduled to be topped by the tandem spacecraft on Thursday.

At Vandenberg Air Force Base, meanwhile, engineers continue their analysis and troubleshooting to understand a problem with the Western Range that cropped up late in the countdown on Aug. 2. The issue scrubbed the rocket's attempt to launch that day and the vehicle remains grounded awaiting a new liftoff date.

"During (the) countdown, Western Range Operations Control Center safety personnel identified an issue in the Mission Flight Control Center. The center processes radar, optical and telemetry data from instrumentation sites located on and off Vandenberg and displays and processes commands for the mission flight control officer, whose main responsibility is to terminate the flight of the launch vehicle should we need to for public safety reasons," the Air Force said in a statement to Spaceflight Now.

"Because public safety is the primary concern of the 30th Space Wing, we stopped countdown once the issue was identified."

Hoping the Range trouble would be fixed sooner rather than later, officials tentatively retargeted the launch for Aug. 14 or 15, the last opportunities to perform the California mission without impacting the Florida flight on Aug. 23.

The time necessary to complete the review of data from one launch before granting clearance to the next one requires about a week between missions.

But in meetings this week, the decision was made to stand down from the California launch for now and focus on getting the Aug. 23 flight performed for NASA.

"The Mission Flight Control Center is a complex and interdependent center comprised of 80 plus servers, 8 operating systems and thousands of software processes," the Air Force says.

The work to resolve the problem "will require in-depth engineering support to not only identify the cause of the malfunction but to correct, test and certify the center for operations."

Although the Vandenberg launch does not have a new launch date, the math says Aug. 30 would be the soonest it could occur, pending resolution with the Range.

Whenever that rocket does launch, it will carry a classified payload for the National Reconnaissance Office known as NROL-36.

The United Launch Alliance Atlas 5 rocket that will hurl two satellites into orbit next month to probe the harsh environment of Earth's radiation belts and understand the extremes of space weather began taking shape Friday at Cape Canaveral.

At the Vertical Integration Facility adjacent to Cape Canaveral's Complex 41 launch pad, technicians hoisted the giant first stage onto the mobile launching platform as the stacking operations got underway to assemble the vehicle for its planned Aug. 23 blastoff with NASA's Radiation Belt Storm Probes mission.

Friday's operation featured the first stage, known as the Common Core Booster, being rotated vertical, maneuvered into the 30-story VIF building and then the 106.6-foot-long stage stood upright on the mobile platform.

Bronze in color and 12.5 feet in diameter, the stage is equipped with a dual-nozzle RD-180 main engine that will burn kerosene fuel and supercold liquid oxygen during the initial four minutes of flight.

Stacking was delayed by two days due to unfavorable weather conditions.

Once the first stage was locked into position, the interstage adapter was scheduled to be set in place. This barrel-like structure tapers from the first stage's large diameter to the skinner Centaur upper stage that will be installed on Monday.

The payload and nose cone, already encapsulated together, come to the VIF for installation in early August to complete the 19-story-tall vehicle for flight in the Atlas' 401 configuration.

Rollout to the launch complex occurs Aug. 21 as the 1.4-million pound platform rides the rail tracks 1,800 feet from the VIF to the pad.

The seven-and-a-half-hour countdown operation begins at 8:38 p.m. EDT on Aug. 22, leading to cryogenic liquid oxygen and liquid hydrogen fueling operations in the overnight hours for a planned blastoff at 4:08 a.m. EDT Aug. 23. The day's launch window stretches 20 minutes.

There is another Atlas 5 launch between now and then, however. At California's Vandenberg Air Force Base, an Aug. 2 blastoff is planned with a classified payload for the National Reconnaissance Office.

Also coming up later this year for the busy Atlas program is the Oct. 26 deployment of Pentagon's X-37B orbital spaceplane taking its third voyage and launch of NASA's next Tracking and Data Relay Satellite on Dec. 6.