A United Launch Alliance Atlas 5 rocket dispatched a $671 million gas-sniffing sleuth to Mars on Monday, taking the first step in a long-distance voyage across the solar system to survey the Martian atmosphere and decipher an enigma nearly as old as the solar system itself.

Read our full story.

United Launch Alliance has declared today's launch a success.

"United Launch Alliance is proud to be a part of this tremendous mission, working closely with the NASA Launch Services Program and MAVEN teams," said Jim Sponnick, vice president of ULA's Atlas and Delta programs. "Missions to Mars are very exciting and over the last decade, ULA launch systems have been entrusted to launch all of NASA's missions to the red planet, including the Spirit and Opportunity rovers and most recently the Mars Science Lab mission with the Curiosity rover."

The launch marks the 76th mission conducted by United Launch Alliance since it was established by parent companies Boeing Co. and Lockheed Martin Corp. in December 2006.

"In just a few days, the Centaur upper stage will celebrate its 50th anniversary since its first successful launch," Sponnick said in a statement. "Centaur has a rich heritage dating back to the beginning of human spaceflight and continues to reliably deliver critical science missions for NASA."

NASA confirms a good solar array deployment.

From the Launch Services Program Twitter account: "Spacecraft batteries are charging #MAVEN (NASA Integration Engineer reported to NASA Chief Engineer)"

From the Launch Services Program Twitter account: "NASA Integration Engineer reported to Chief Engineer that #MAVEN has star tracker lock and is slewing to communication attitude."

NASA's Launch Services Program has posted an update to Twitter: "MAVEN has seen a thermal rise in the solar arrays, but they are still waiting for current."

We spoke with NASA Administrator Charlie Bolden this morning about Mars, budget difficulties and his impressions of the first Orion capsule

You can see the interview here.

The MAVEN team says they are receiving telemetry from the spacecraft.

As we wait to hear an update on MAVEN's post-launch condition, NASA launch director Omar Baez says all indications MAVEN got a good ride from the United Launch Alliance Atlas 5 rocket.

"The flight of the Atlas and Centaur were flawless from what we could tell," Baez said. "Separation occurred on time, and all the mark events occurred on time.

"All we're waiting for now is for the spacecraft to go through their sequence and deploy their solar arrays, and that could happen any time in the [next few minutes]."

The post-launch sequence calls for MAVEN to power up its transmitter and contact ground stations in Perth and Canberra, Australia, with its status. MAVEN's solar panels, which stretch as long as a school bus, should be fully unfurled within about 15 minutes.

T+plus 52 minutes, 50 seconds. SPACECRAFT SEPARATION! The Mars Atmosphere and Volatile Evolution, or MAVEN, spacecraft has been deployed from the Atlas 5's Centaur upper stage. It arrives at Mars on Sept. 22, 2014.

Less than a minute to spacecraft separation. The Centaur is in the correct attitude for separation of MAVEN.

T+plus 49 minutes. The Centaur is maneuvering to a new orientation for release MAVEN at T+plus 52 minutes, 42 seconds.

T+plus 47 minutes. MECO 2. The Centaur has finished its last burn, completing the powered phase of the 41st Atlas 5 launch.

T+plus 45 minutes. Less than two minutes left in this burn to propel MAVEN on a hyperbolic escape trajectory.

T+plus 43 minutes. Centaur's RL10 engine continues a good firing.

T+plus 42 minutes. The engine is burning well. This is a planned five-and-a-half minutes firing by the Centaur's single Aerojet Rocketdyne RL10 engine.

T+plus 41 minutes, 30 seconds. Ignition and full thrust! The Centaur's single RL10 engine has re-ignited to accelerate the MAVEN spacecraft with enough velocity to escape the grip Earth's gravity.

T+plus 41 minutes. Centaur is getting pressurized again in preparation to re-ignite.

T+plus 40 minutes. The rocket stage is in the proper orientation for the burn.

T+plus 37 minutes. Centaur has despun from its coast configuration.

T+plus 35 minutes, 30 seconds. The vehicle is reorienting for the next engine firing.

T+plus 30 minutes. The flight path took the vehicle over the equatorial Atlantic Ocean before crossing Africa. Here's the planned track map.

T+plus 25 minutes. Check out Spaceflight Now's Facebook page for images from today's liftoff.

T+plus 16 minutes. The Centaur has reached a preliminary orbit very close to prelaunch predictions, which were targeting a perigee of 87 nautical miles and an apogee of 170 nautical miles.

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

T+plus 13 minutes, 50 seconds. MECO 1. Centaur's RL10 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 28 minutes before the RL10 engine re-ignites.

T+plus 13 minutes, 50 seconds. MECO 1. Centaur's RL10 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 28 minutes before the RL10 engine re-ignites.

About one minute remains in this first of two Centaur burns.

T+plus 11 minutes. The RL10 continues to perform well, burning liquid hydrogen and liquid oxygen propellants.

T+plus 9 minutes. All continues to go well in this burn.

T+plus 7 minutes, 15 seconds. Now 152 miles in altitude, 826 miles downrange from the launch pad, traveling at 12,500 mph.

T+plus 7 minutes, 15 seconds. Now 152 miles in altitude, 826 miles downrange from the launch pad, traveling at 12,500 mph.

T+plus 6 minutes, 50 seconds. The first stage delivered the expected performance.

T+plus 6 minutes, 40 seconds. The rocket is performing a planned roll to improve the link with NASA's orbiting Tracking and Data Relay Satellite System.

T+plus 5 minutes. Centaur engine readings look good as its first 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, 13 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, 10 seconds. Altitude is 35 miles, downrange distance is 70 miles and velocity is 6,000 mph

T+plus 2 minutes, 45 seconds. Reaction control system has been activated.

T+plus 2 minutes, 35 seconds. Atlas continues tracking on course.

T+plus 2 minutes, 30 seconds. Atlas now weighs half of what it did at liftoff.

T+plus 2 minutes, 15 seconds. Vehicle systems looking good.

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 NASA's newest Mars mission.

T+plus 40 seconds. The Atlas 5 is sending a thunderous roar across Florida's spaceport as it climbs into clouds over Cape Canaveral.

T+plus 15 seconds. The Atlas 5 rocket has cleared the tower on 860,000 pounds of thrust from the RD-180 main engine. Pitch, yaw and roll maneuvers are underway to put the rocket on the proper heading.

LIFTOFF! Liftoff of Atlas 5 and MAVEN, kicking off a voyage across the solar system probe the upper atmosphere of Mars!

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 55 seconds. Range is green.

T-minus 1 minute. Now 60 seconds away from MAVEN taking flight to the red planet.

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 the Mars-bound MAVEN spacecraft from Cape Canaveral, Fla.

The MAVEN spacecraft is running on internal power for launch. Standing by to resume the countdown at T-minus 4 minutes.

ULA and the Air Force have given their respective final approvals to resume the countdown.

Polling of the team by Atlas launch conductor just occurred. All systems are "go" for a liftoff today at 1:28 p.m. EST (1828 GMT).

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.

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

• The 623rd launch for Atlas program since 1957
• The 333rd Atlas to occur from Cape Canaveral
• The 212th mission for the Centaur upper stage
• The 189th use of Centaur by an Atlas rocket
• The 441st production RL10 engine to be launched
• The 47th flight of the RD-180 main engine
• The 41st launch of an Atlas 5 since 2002
• The 65th Evolved Expendable Launch Vehicle flight
• The 10th Atlas 5 launch for NASA
• The 35th Atlas 5 to occur from the Cape
• The 28th daytime Atlas 5 launch from the Cape
• The 76th United Launch Alliance flight overall
• The 10th ULA launch of the year
• The 33rd Atlas 5 under United Launch Alliance
• The 27th 400-series flight of the Atlas 5
• The 19th Atlas 5 to fly in the 401 configuration
• The 3rd Mars mission to launch on Atlas 5
• The 7th Atlas launch of 2013
• The 9th launch from Cape Canaveral in 2013

A poll of the NASA management team by NASA launch director Omar Baez has given approval to proceed with the launch of MAVEN. A poll of the ULA launch team is coming up in a few minutes.

T-minus 4 minutes and holding. The countdown has entered the planned 10-minute hold to give the launch team a chance to review all systems before pressing ahead with liftoff.

T-minus 5 minutes. Standing by to go into the final built-in hold. All of the Atlas 5's propellant tanks are reported at flight level.

Today marks the 41st flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 40 missions since debuting in August 2002, the tally shows 15 flights dedicated to the Defense Department, 9 commercial missions with communications spacecraft, 9 for NASA and 7 with spy satellites for the National Reconnaissance Office.

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.

The launch weather officer says all weather constraints are observed GO and expected to remain GO at the time of liftoff. The forecast worsens later in the two-hour window as thicker clouds move into the area.

The fuel-fill sequence for the first stage main engine is starting, and flight control final preps are complete.

The launch team has cleared a concern with a transducer on the Centaur pressurization system.

Just 30 minutes until liftoff time. Today's launch is the third Mars mission to blast off on an Atlas 5 rocket.

The Mars Atmosphere and Volatile Evolution, or MAVEN, mission will reach the red planet Sept. 22, 2014, if it launches today. MAVEN weighs exactly 2,458.83 kilograms fully fueled, and its fuel tank contains 431 gallons, or 3,626 pounds, of hydrazine.

Built by Lockheed Martin Corp., MAVEN has two wings of solar panels that will extend 37.5-feet tip-to-tip. The solar array deployment sequence should be complete within 15 minutes of MAVEN's deployment from the Atlas 5's Centaur upper stage.

MAVEN carries eight instruments and nine sensors to probe the upper atmosphere of Mars from an elliptical orbit stretching from a peak altitude of 3,850 miles to a low point 93 miles from the Martian surface, where MAVEN will survey the Martian upper atmosphere to determine what processes caused the atmosphere to thin out and cool off in the ancient past, dooming any potential life on the planet.

MAVEN will join other operating spacecraft at Mars, including NASA's Curiosity and Opportunity rovers on the surface, NASA's Odyssey and Mars Reconnaissance Orbiter in orbit, and the European Space Agency's Mars Express orbiter.

If the launch goes at the opening of the window today, MAVEN will deploy from the upper stage of the Atlas 5 rocket about 52 minutes after liftoff.

Check out a timeline of the launch. The times are accurate if MAVEN launches at the beginning of today's launch window.

Following separation from the launch vehicle, MAVEN will activate its radio transmitter and radio its status to ground stations in Perth and Canberra, Australia. Then MAVEN will extend its solar panels and point them toward the sun to generate electricity.

Open-loop testing of the Atlas 5's flight termination system is complete.

Launch is now 45 minutes away. The Atlas 5 rocket stands 188 feet tall and weighs 742,359 pounds fully loaded with propellant.

This launch will mark the seventh Atlas 5 launch of the year and the third Atlas 5 flight of the year for NASA.

Earlier this year, an Atlas 5 rocket launched NASA's Tracking and Data Relay Satellite K from Florida on Jan. 30, followed by a launch Feb. 11 from California with the Landsat 8 Earth observation satellite. On March 19, an Atlas 5 launched the U.S. Air Force's second Space Based Infrared System Geosynchronous satellite, or SBIRS GEO 2, for missile early-warning detection.

A new GPS navigation satellite launched from Cape Canaveral on an Atlas 5 rocket May 15, followed July 19 by the launch of the U.S. Navy's second Mobile User Objective System, or MUOS 2, satellite designed to facilitate communications with troops on the move.

The Atlas 5's most recent launch Sept. 18 lofted the Air Force's third Advanced Extremely High Frequency, or AEHF 3, satellite for strategic nuclear-hardened communications.

The first stage liquid oxygen tank and the Centaur's liquid hydrogen tank have reached the 95 percent levels, and topping to flight level has started.

Final open-loop testing of the Atlas 5's destruct system is getting underway now.

The weather officer reports all constraints are now GREEN, but there is still a 40 percent chance of violating weather rules at the time of launch.

Centaur's liquid hydrogen tank is 90 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Aerojet Rocketdyne-made RL10 engine.

The weather officer reports the field mill rule is currently NO GO. This is a measurement of electrical charges in the atmosphere and is a concern due to the potential for rocket-triggered lightning during launch.

Now 60 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen is progressing smoothly as the countdown continues on schedule for a liftoff at 1:28 p.m. EST (1828 GMT). Weather remains GO, and the launch window runs until 3:28 p.m. EST (2028 GMT)

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Centaur liquid hydrogen has reached the 10-percent level, and Atlas first stage liquid oxygen is at 70 percent.

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

First stage liquid oxygen tank is 50 percent full thus far. Chilled to Minus-298 degrees F, the liquid oxygen will be used with RP-1 kerosene by the RD-180 main engine on the first stage during the initial four minutes of flight today. The 25,000 gallons of RP-1 were loaded into the rocket Oct. 29 during a practice countdown.

The Centaur engine chilldown sequence is being initiated.

Frost is building up on the Atlas 5 first stage as super-cold liquid oxygen is pumped aboard the rocket.

Centaur liquid oxygen is now at flight level.

The Centaur liquid oxygen tank reached the 92.4 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 75 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.

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

Alignment of the Atlas 5's guidance computer is complete.

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 5,420-pound MAVEN spacecraft onto an interplanetary trajectory today.

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.

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.

T-minus 120 minutes and counting! The launch countdown has resumed for today's flight of the Atlas 5 rocket on the NASA mission to deploy the MAVEN spacecraft on a 10-month journey to Mars.

Clocks have one more built-in hold planned at T-minus 4 minutes. That pause will last 10 minutes during which time the final "go" for launch will be given. All remains targeted for liftoff at 1:28 p.m. EST (1828 GMT) 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.

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.

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The launch weather officer just briefed the Atlas 5 launch team, and all conditions at the launch site are currently observed go for launch.

There is still a 40 percent chance of violating weather rules at the time of launch due to disturbed weather, thick clouds and cumulus clouds.

The outlook for the launch calls for scattered clouds at 2,500 feet, broken clouds at 12,000 feet, and an overcast deck at 24,000 feet with isolated showers in the area. Winds will be out of the south-southeast at 10 knots gusting to 15 knots, and temperature will be approximately 79 degrees Fahrenheit.

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 1:28 p.m. EST (1828 GMT) launch of the Atlas 5 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.

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.

The weather status board shows all launch criteria still green and GO. The forecast for the 1:28 to 3:28 p.m. EST (1828-2028 GMT) window continues to reflect a 60 percent chance of favorable conditions.

The outlook for the launch window now predicts scattered clouds at 2,000 and 12,000 feet, a broken deck at 26,000 feet, isolated showers in the area, good visibility, southerly winds of 20 gusting to 24 knots, a temperature of 73 degrees F and humidity level of 80 percent.

Today's launch window opens at 1:28 p.m. EST and closes at 3:28 p.m. EST (1828-2028 GMT), United Launch Alliance has planned launch opportunities at five-minute intervals throughout the window, including the opening and closing seconds.

Based on the rotation of Earth and the alignment of planets, the trajectory to Mars changes throughout the launch window. So engineers pre-developed 1,000 trajectory programs to load into the Atlas 5's guidance computer in case the launch delays into the window.

"For MAVEN, we've got a two-hour launch opportunity every day," said Jim Sponnick, ULA's vice president of Atlas and Delta programs. "And within those two hours, we have an opportunity to launch once every five minutes. It's those opportunities each day over the launch window make up those 1,000 trajectories. It's fairly consistent with how we have designed, developed and validated trajectories and mission designs for previous missions to Mars."

Today's launch window opens at 1:28 p.m. EST and closes at 3:28 p.m. EST (1828-2028 GMT), United Launch Alliance has planned launch opportunities at five-minute intervals throughout the window, including the opening and closing seconds.

Based on the rotation of Earth and the alignment of planets, the trajectory to Mars changes throughout the launch window. So engineers pre-developed 1,000 trajectory programs to load into the Atlas 5's guidance computer in case the launch delays into the window.

"For MAVEN, we've got a two-hour launch opportunity every day," said Jim Sponnick, ULA's vice president of Atlas and Delta programs. "And within those two hours, we have an opportunity to launch once every five minutes. It's those opportunities each day over the launch window make up those 1,000 trajectories. It's fairly consistent with how we have designed, developed and validated trajectories and mission designs for previous missions to Mars."

Now entering the final four hours to the United Launch Alliance Atlas 5 rocket flight with the MAVEN payload for NASA. This will be the civilan space agency's 10th use of Atlas 5 over the past eight years.

At this point in the countdown, the launch team is conducting checks of the internal batteries aboard the vehicle.

"ULA is proud to provide NASA's ride to Mars for this great science mission, ULA's program manager for NASA missions. "Over the last decade, ULA vehicles have successfully launched all of NASA's missions to the red planet, including the Spirit and Opportunity rovers, and then most recently, just a couple of years ago, the Mars Science Lab with the Curiosity rover."

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 tandem 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. The Mars Science Laboratory with the Curiosity rover left Earth on Nov. 26, 2011. The twin Radiation Belt Storm Probes were sent aloft on Aug. 30, 2012, followed by NASA's TDRS K communications satellite on Jan. 30, 2013. Most recently, an Atlas 5 rocket deployed the joint NASA-USGS Landsat 8 Earth observation satellite on Feb. 11, 2013.

Now comes the launch of MAVEN at 1:28 p.m. EST (1828 GMT).

"MAVEN will be our seventh Mars mission since 2001," Thorp said. "Our first launches to Mars occurred 49 years ago with the Mariner 3 and Mariner 4 spacecraft in 1964. We used the Atlas-Agena at the time.

"One of those 1964 launches was Mariner 4," Thorp said. "It was Earth's first successful mission to Mars. It was a flyby mission, and if I recall correctly, it returned 21 images as it flew by. I think it's an amazing testament to how far spacecraft and instrument design has come when you compare that data to the amount of data that the MAVEN spacecraft is going to return."

The Atlas-Centaur rocket has been powered up at Complex 41 and the launcher's guidance system has been put through preflight checks.

The C-band and S-band systems have also been tested in the countdown. They are used for vehicle tracking and telemetry relay, respectively. The countdown continues for launch at 1:28 p.m. EST (1828 GMT).

The countdown begins now for today's launch of the Atlas 5 rocket to deploy NASA's Mars Atmosphere and Volatile Evolution, or MAVEN, mission. The 18-story Atlas 5 rocket will release MAVEN on a high-speed Earth escape trajectory heading toward the red planet.

NASA officials report that nothing is amiss and activities are progressing as planned for flight.

Clocks are picking up the seven-hour sequence of work that will prepare the booser, payload and ground systems for blastoff at 1:28 p.m. EST (1828 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.

NASA managers will arrive on console by 9 a.m. EST (1400 GMT) to oversee the final phase of the countdown. The Complex 41 site will be cleared of all personnel at 10:33 a.m. EST (1533 GMT).

A planned half-hour hold begins at 10:48 a.m. EST (1548 GMT) when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 11:15 a.m. EST (1615 GMT) to verify all is in readiness to start fueling the rocket for launch.

Supercold liquid oxygen begins flowing into the Centaur upper stage around 11:35 a.m. EST (1635 GMT), followed by the first stage filling around 11:48 a.m. EST (1648 GMT). 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 1:14 p.m. EST (1814 GMT). That 10-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 1:28 to 3:28 p.m. EST (1828-2028 GMT).

The Atlas 5 launch team is taking the day off ahead of launch day Monday, with countdown activities scheduled to kick off at 6:28 a.m. EST (1128 GMT).

In the early phase of the seven-hour countdown, the United Launch Alliance team will switch on the Atlas 5's systems, put the rocket through preflight tests and prepare for loading of cryogenic propellant into the two-stage launcher.

Fueling operations start at 11:20 a.m. EST (1620 GMT), beginning with the thermal conditioning of the Atlas 5's mobile launch platform and chilldown of the fluid lines leading from ground tanks to the rocket.

The super-cold propellants will begin flowing into the Atlas 5 a few minutes later, starting with liquid oxygen for the Centaur upper stage, then moving into liquid oxygen loading into the Atlas 5's bronze first stage, and then finally the liquid hydrogen fuel for the Centaur's RL10 engine will be pumped aboard.

The Atlas 5's first stage fuel tank was filled with about 25,000 gallons of RP-1 fuel, a highly refined kerosene, during the rocket's prelaunch fueling test Oct. 29.

An Atlas 5 rocket is geared up for launch from Cape Canaveral on Monday with a NASA spacecraft dreamed up a decade ago to help solve the puzzle of how Mars cooled off and dried up sometime long ago, likely killing off any life that may have existed there.

Read our full story.

We have posted a photo gallery of the Atlas 5's rollout.

Topped with NASA's next Mars mission, the first devoted to study of the Martian atmosphere, the United Launch Alliance Atlas 5 rocket has arrived at its launch pad after completing the first one-third of a mile of the journey to the red planet.

The voyage resumes Monday at 1:28 p.m. EST (1828 GMT) with liftoff of the two-stage Atlas 5 rocket. Monday's launch window extends for two hours.

The countdown will begin at 6:28 a.m. EST Tuesday (1128 GMT), leading to activation of the rocket, final testing and system preps. Fueling operations start at 11:35 a.m. EST (1635 GMT).

"United Launch Alliance is proud to provide NASA’s ride to Mars for this very critical science mission," said Jim Sponnick, ULA's vice president of Atlas and Delta programs. "Over the last decade, ULA launch systems have successfully launched all of NASA's missions to the red planet, including the Spirit and Opportunity rovers and most recently the Mars Science Lab mission with the Curiosity rover."

The weather forecast from the Air Force's 45th Weather Squardon calls for 40 percent chance of violating launch constraints.

The outlook predicts scattered, broken and overcast cloud decks at 2,500, 10,000 and 24,000 feet with showers and isolated rainshower activity. Winds will be out of the southwest at 10 to 15 knots with a temperature of approximately 77 degrees Fahrenheit.

The primary concerns for Wednesday morning are violating the thick cloud, disturbed weather and the cumulus cloud rules.

If the launch gets pushed back to Tuesday or Wednesday, a cold front pushes into the area, bringing a chance for thunderstorms and windier conditions. The chance of violating weather rules Tuesday and Wednesday is 60 percent and 70 percent, respectively.

Watch this page for live updates throughout the countdown and flight, plus live streaming video.

We will also be tweeting updates during the countdown, so sign up for our Twitter feed to keep up with the Atlas 5/AEHF 3 mission and all the latest space news. U.S. readers can also receive test messages on your cellphone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

The Atlas 5 rocket is nearing its destination at Complex 41.

The United Launch Alliance Atlas 5 rocket's rollout to the pad is underway!

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 188-foot-tall satellite booster is moving to Cape Canaveral's Complex 41 for the launch of MAVEN, NASA's next Mars orbiter.

The two-stage rocket and the MAVEN spacecraft were put together inside the assembly building over the past 36 days 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.

It is rollout morning for the Atlas 5 rocket, which is about to emerge from its 30-story Vertical Integration Facility for a quick trip to the launch pad.

A United Launch Alliance Atlas 5 rocket will roll to its oceanfront launch pad at Cape Canaveral Air Force Station on Saturday morning for blastoff Monday with a $671 million Mars orbiter dedicated to atmospheric research.

Rollout is due to begin at 10 a.m. EST (1500 GMT), with the Atlas 5 rocket riding on rails from the 30-story Vertical Integration Facility to the launch pad at Complex 41. The journey should take about a half-hour to complete.

The rollout begins the final day-and-a-half of launch preparations for the Atlas 5, which is making its 41st flight since debuting in August 2002 with the launch of a commercial communications satellite for Eutelsat. It also marks the 76th rocket flight conducted under the auspices of United Launch Alliance since 2006.

Affixed to this Atlas 5 launcher is NASA's MAVEN spacecraft, a 5,420-pound platform equipped with instruments to sniff the contents of the Martian upper atmosphere and monitor the planet's response to solar storms.

The goal is to learn how Mars transformed from a warmer, wetter planet into the barren world of today.

Monday's launch is scheduled for a two-hour window opening at 1:28 p.m. EST (1828 GMT).

The 188-foot-tall Atlas 5 is flying in the "401" configuration with a four-meter payload fairing manufactured at ULA's facility in Harlingen, Texas, no solid-fueled boosters provided and a single RL10 upper stage engine provided by Aerojet Rocketdyne.

Built by Lockheed Martin Corp., MAVEN is embarking on a 10-month cruise to the red planet, with arrival set for Sept. 22, 2014, if the launch occurs Monday.

Monday's rollout comes after 36 days of assembly and testing of the Atlas 5 rocket inside the Vertical Integration Facility.

Stacking of the Atlas 5 rocket began with the standing up of the launcher's first stage inside the VIF on Oct. 11. Technicians hoisted the rocket's Centaur upper stage into position Oct. 14.

The MAVEN spacecraft, encapsulated inside the Atlas 5's payload fairing, topped off the rocket Nov. 8.

This morning's launch readiness review gave a "go" to continue launch preparations for Monday's liftoff of a NASA spacecraft destined to probe the upper atmosphere of Mars.

The review concluded there were no technical issues standing in the way of Monday's launch at 1:28 p.m. EST (1828 GMT) at the opening of a two-hour window.

The decision clears the way for the 1,800-foot move of the Atlas 5 rocket from its Vertical Integration Facility to the launch pad at Cape Canaveral's Complex 41 on Saturday. The half-hour rollout should begin at 10 a.m. EST (1500 GMT).

Once on the pad, the Atlas 5 and its mobile launch platform will be connected to the facility's fluid and electrical supplies Saturday afternoon. Sunday is scheduled to be a crew rest day before the countdown begins early Monday.

The weather forecast issued this morning by Air Force meteorologists hasn't changed much from the outlook released yesterday.

There continues to be a 60 percent chance of favorable weather for launch Monday, but conditions worsen Tuesday and Wednesday with only a 40 percent and 30 percent probability of acceptable weather those days.

The main worries Monday are still cumulus clouds, disturbed weather and thick clouds.

Weather forecasters expect to be watching rain showers and thick clouds during Monday's countdown to launch NASA's MAVEN mission to Mars, meteorologists reported Thursday.

The forecast for Monday calls for a 40 percent chance of violating the Atlas 5 rocket's weather constraints, with the primary threat coming from cumulus clouds, thick clouds and disturbed weather associated with rain showers, according to meteorologists with the U.S. Air Force's 45th Weather Squadron.

The Atlas 5's two-hour launch window opens at 1:28 p.m. EST (1828 GMT).

"On launch day, high pressure migrates east into the western Atlantic in advance of the next cold front along the Eastern seaboard and extending into the Florida panhandle," forecasters wrote in a weather synopsis. "Moisture increases through the day with isolated showers developing in the morning and increasing through the afternoon. The bulk of the weather associated with the advancing cold front is expected to remain just to the north with a shower threat over Central Florida.

"There is a low lightning threat during the count. Winds are expected from the south with gusts in the mid to upper teens [at 230 feet]. The primary concerns for launch day are cumulus clouds, disturbed weather associated with showers, and thick clouds with the quite moist atmosphere."

The outlook calls for scattered clouds at 2,500 feet, broken clouds at 10,000 feet and an overcast cloud deck at 24,000 feet with isolated showers in the area. Winds will be out of the south at 12 to 16 knots, temperatures are forecast to be around 76 degrees Fahrenheit, and visibility is predicted to be 7 miles.

Forecasters expect the weather to deteriorate over Central Florida on Tuesday and Wednesday, with a 60 percent chance of exceeding weather rules Tuesday and a 70 percent chance Wednesday.

"In the event of a 24-hour delay, the aforementioned cold front advances into Central Florida during the count and becomes near stationary in the Central Florida vicinity. Showers are likely during the count with a small thunderstorm threat.

"In the event of a 48-hour delay, the cold front remains near stationary just to the south with a disturbed weather pattern over Central Florida. The pressure gradient tightens Tuesday evening with gusty winds for the 48 hour delay," the forecast says.

Conditions are expected to be favorable for Saturday's rollout of the 189-foot-tall Atlas 5 to the launch pad at Cape Canaveral's Complex 41. The rollout is scheduled to begin at 10 a.m. EST (1500 GMT) and will take about a half-hour to complete.

A launch readiness review is on tap Friday to give final approval for the rollout.

NASA's MAVEN spacecraft, the centerpiece of a $671 million mission to study the atmosphere of Mars, reached its penultimate stop before liftoff when technicians transported the delicate probe to the Atlas 5 rocket's seaside launch complex Friday.

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Gearing up for the launch of NASA's MAVEN mission to Mars next month, United Launch Alliance ran through a practice countdown, loaded propellant into an Atlas 5 rocket, and rehearsed launch procedures Tuesday to work out the kinks before the real launch day Nov. 18.

It turns out there wasn't much to resolve. No issues were reported in the countdown, which aborted moments before a mock launch time of 1 p.m. EDT (1700 GMT).

The ULA launch team has finished assembling and testing MAVEN's Atlas 5 rocket, and technicians moved the two-stage booster from its Vertical Integration Facility to the launch pad at Complex 41 on Monday.

The rocket is missing its payload and nose shroud, which will be added to the Atlas 5 next week.

Controllers powered up the Atlas 5 early Tuesday, tested flight and ground systems required for the countdown, and filled the rocket with kerosene and cryogenic propellants.

The wet dress rehearsal is a risk reduction exercise for the Atlas 5 and its support team. The test is designed to expose any potential anomalies that could show up on launch day, giving engineers time to fix the problems.

With the rehearsal complete, ULA will return the Atlas 5 rocket to the barn to await the arrival of MAVEN, which is finishing up preparations in a clean room at Kennedy Space Center's Payload Hazardous Servicing Facility.

Workers pumped fuel into the spacecraft last week, and plans call for MAVEN to be lifted and mated Friday to the adapter that will connect the orbiter to the Atlas 5 rocket's Centaur upper stage.

MAVEN will be enclosed inside the rocket's four-meter diameter payload fairing beginning Saturday, with the second half of the clamshell-like aerodynamic shroud to be added Monday.

The payload will be trucked to the towering Atlas 5 integration building Nov. 6 to be hoisted and attached to the top of the 189-foot-tall rocket.

Launch is on schedule for Nov. 18 in a two-hour window opening at 1:28 p.m. EST (1828 GMT).

A team of technicians with specialized skills filled the propellant tank inside NASA's Mars-bound MAVEN spacecraft with 3,626 pounds of hydrazine fuel Friday to help steer the mission toward its ultimate destination in orbit around the red planet.

Inside the Kennedy Space Center's Payload Hazardous Servicing Facility, ground crews suited up in protective suits with self-contained breathing equipment and pumped 431 gallons of toxic propellant into the spacecraft's fuel tank.

The all-day operation completed Friday evening, and technicians were pressurizing the propellant tank following the fueling procedure.

The Mars Atmosphere and Volatile Evolution, or MAVEN, spacecraft will use much of the propellant Sept. 22, 2014, when the Lockheed Martin-built probe will fire its main engine to enter orbit around the red planet.

Subsequent rocket burns will lower MAVEN's orbit to reach an operational perch ranging in altitude from 3,850 miles to 93 miles from the Martian surface, where MAVEN will survey the Martian upper atmosphere to determine what processes caused the atmosphere to thin out and cool off in the ancient past, dooming any potential life on the planet.

The fuel will also maintain MAVEN's course from Earth to Mars once it is released from the upper stage of a United Launch Alliance Atlas 5 rocket following its Nov. 18 liftoff.

And the propellant will boost MAVEN into a higher orbit around Mars at the end of its science mission to serve as a communications relay between Earth and rovers on the surface, NASA said.

Friday's fueling activity came after engineers put the spacecraft through a spin balance test earlier this week, in which MAVEN was placed on a rotation fixture and spun up to determine its exact mass properties. The data is critical for achieving an on-target launch and accurate navigation on its cruise to the red planet.

With fuel on-board, MAVEN is weighs about as much as a fully-loaded SUV.

With functional testing of MAVEN's systems and instruments completed, and with fueling now completed, attention will turn to getting the orbiter ready for attachment to its United Launch Alliance Atlas 5 rocket.

Over the next week, workers will mate MAVEN with the Atlas 5's payload adapter and enclose the spacecraft inside the Atlas 5's four-meter-diameter payload fairing.

MAVEN is scheduled to be transported Nov. 7 to the Atlas 5 rocket inside the Vertical Integration Facility adjacent to its launch pad at Cape Canaveral's Complex 41.

The arrival of the payload fairing will top off assembly the 189-foot-tall Atlas 5 rocket, which began Oct. 11 with the hoisting of the launcher's kerosene-fueled first stage booster.

ULA added rocket's Centaur upper stage Oct. 14, then powered up the two-stage rocket for testing.

United Launch Alliance on Friday began assembling the Atlas 5 rocket assigned to launch NASA's MAVEN mission in November and send the orbiter on a 10-month cruise to Mars to help decipher the red planet's thinning atmosphere.

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Engineers returned to work on NASA's next Mars mission at the Kennedy Space Center on Thursday after receiving an emergency exception under federal law to continue launch preparations for a $671 million orbiter to probe the red planet's atmosphere.

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Without funding to pay for numerous programs and research, engineers began shutting down work on a $671 million Mars science orbiter at the Kennedy Space Center on Tuesday, halting critical preparations ahead of the mission's narrow interplanetary launch window in November.

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NASA showcased the Mars Atmosphere and Volatile Evolution, or MAVEN, spacecraft to news media inside a secure, contaminant-free clean room at the Kennedy Space Center on Friday.

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The team in charge of NASA's $671 million Mars orbiter due for liftoff in November says the project is on schedule and on budget for launch during an immovable 20-day interplanetary window this fall.

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