BY JUSTIN RAY

Follow the countdown and launch of the Lockheed Martin Atlas 5 rocket with NASA's Mars Reconnaissance Orbiter. Reload this page for the very latest on the mission.

1257 GMT (8:57 a.m. EDT)

Draining of the rocket's cryogenic tanks of the supercold fuels is now beginning following this morning's scrub.

1250 GMT (8:50 a.m. EDT)

SCRUB! Mars Reconnaissance Orbiter will not begin its 310 million mile voyage to the Red Planet today after a sensor problem cropped up during fueling of the Centaur upper stage with liquid hydrogen. The launch team will prepare for another attempt tomorrow, pending resolution of the problem. The window will be 7:43 to 9:43 a.m. EDT (1143-1343 GMT).

1249 GMT (8:49 a.m. EDT)

It appears this hydrogen system problem cannot be resolved in time for launch today. We're standing by for an official announcement.

1246 GMT (8:46 a.m. EDT)

T-minus 4 minutes and holding. The countdown has entered this final hold, which has been extended from the planned 10 minutes to 45 minutes due to the hydrogen fueling problem. Launch has been delayed to 9:35 a.m. EDT, the last possible minute that liftoff could occur today.

1244 GMT (8:44 a.m. EDT)

NEW LAUNCH TIME. Mission managers are planning to delay liftoff to the very end of today's launch window -- 9:35 a.m. EDT because of the Centaur upper stage liquid hydrogen fueling operations. There is a problem between sensors on the rocket and the software used to fuel the vehicle.

1241 GMT (8:41 a.m. EDT)

The launch weather officer just completed his last planned briefing to management before liftoff time. All weather conditions are "go" for launch.

1239 GMT (8:39 a.m. EDT)

Engineers are still working to determine how to work around a sensor problem that is causing a snag in liquid hydrogen fueling.

1238 GMT (8:38 a.m. EDT)

T-minus 12 minutes and counting. The countdown clocks are heading to the T-minus 4 minute mark where a planned 10-minute hold will begin at 8:46 a.m. EDT. That hold surely will be extended, however, due to delays getting the upper stage loaded with liquid hydrogen this morning. There is no estimate on how much this will impact liftoff time. The launch team has until 9:35 a.m. EDT to get the rocket off the ground today.

1235 GMT (8:35 a.m. EDT)

The flight control system preps are complete. Also, rocket's "pogo" suppressor has been charged for launch. This energy absorption device aboard is used to damp out oscillations.

1230 GMT (8:30 a.m. EDT)

The liquid hydrogen tank reached the five-percent level in the manual fueling mode vs. the automated software due to a sensor problem experienced a little while ago. However, the flow has been stopped again by the system. Engineers are assessing the situation.

1223 GMT (8:23 a.m. EDT)

The anomaly resolution team has devised a plan to manually load the upper stage liquid hydrogen tank to five-percent full coupled with some software resets to work around a sensor glitch.

1215 GMT (8:15 a.m. EDT)

Launch time is now 45 minutes away. Liquid hydrogen fueling remains on hold while engineers assess a sensor problem.

The liquid oxygen tanks in the first stage and Centaur upper stage are full.

1210 GMT (8:10 a.m. EDT)

Topping of the first stage liquid oxygen tank is underway.

We're awaiting further word on the liquid hydrogen system problem.

1200 GMT (8:00 a.m. EDT)

Time to launch is 60 minutes. The first stage liquid oxygen tank is almost three-quarters full.

1158 GMT (7:58 a.m. EDT)

Liquid hydrogen flow has stopped due to unexpected readings from sensor probes. The anomaly resolution team has begun discussing the situation.

1155 GMT (7:55 a.m. EDT)

The flight control final preparations are starting.

1151 GMT (7:51 a.m. EDT)

With the liquid hydrogen chilldown now complete, the super-cold fuel is beginning to fill the Centaur upper stage. The cryogenic propellant will be consumed with liquid oxygen by the stage's Pratt & Whitney-made RL10 engine that gives Mars Reconnaissance Orbiter the needed push to leave Earth orbit on the way to the Red Planet.

1150 GMT (7:50 a.m. EDT)

The Centaur engine chilldown has been initiated.

1148 GMT (7:48 a.m. EDT)

First stage liquid oxygen tank is now half full. 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.

1138 GMT (7:38 a.m. EDT)

The Centaur liquid oyxgen tank has reached flight level.

Meanwhile, a layer of ice and frost, which is perfectly normal, is forming on the first stage's bronze skin as the super-cold liquid oxygen continues to flow into the vehicle. That tank is 30 percent full.

1133 GMT (7:33 a.m. EDT)

The Centaur liquid oxygen tank has now reached the 95 percent level. Topping is starting.

1133 GMT (7:33 a.m. EDT)

Ten percent of the first stage liquid oxygen tank has been filled.

1128 GMT (7:28 a.m. EDT)

The chilldown conditioning of liquid hydrogen propellant lines at Complex 41 is now starting to prepare the plumbing for transferring the Minus-423 degree F fuel into the rocket.

1126 GMT (7:26 a.m. EDT)

The Centaur liquid oxygen tank is now 70 percent full.

1121 GMT (7:21 a.m. EDT)

The chilldown conditioning of the systems for the first stage liquid oxygen tank have been completed. And a "go" has been given to begin pumping super-cold liquid oxygen into the Atlas 5's first stage. The Atlas liquid oxygen tank is the largest tank to be filled today.

The propellant for the first stage -- the RP-1 kerosene -- was loaded aboard the rocket during an earlier practice countdown dress rehearsal.

1116 GMT (7:16 a.m. EDT)

The Centaur liquid oxygen tank is now 30 percent full.

1106 GMT (7:06 a.m. EDT)

Following the thermal conditioning of the transfer pipes, Centaur liquid oxygen tanking operations have begun at Cape Canaveral's Complex 41.

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 high-energy Centaur will perform two firings today to propel the MRO spacecraft on its way to Mars.

1100 GMT (7:00 a.m. EDT)

Now two hours from launch.

1056 GMT (6:56 a.m. EDT)

The Centaur liquid oxygen transfer line chilldown is starting in advance of loading the upper stage. This thermal conditioning prepares the plumbing for flowing the cryogenic oxidizer.

1055 GMT (6:55 a.m. EDT)

Weather conditions are slowly improving here at the Cape as the storms move well offshore. And the sun is rising on this launch day for Mars Reconnaissance Orbiter!

1050 GMT (6:50 a.m. EDT)

T-minus 120 minutes and counting! The countdown is running again for today's launch of the Atlas 5 rocket and Mars Reconnaissance Orbiter. Liftoff is scheduled to occur at 9:00 a.m. EDT, a 70-minute delay caused by bad weather interfering with the pre-launch chores at the pad during the night. There are 35 minutes remaining to spare in today's window to accommodate any further delays.

1045 GMT (6:45 a.m. EDT)

The pre-fueling poll of the launch team concluded with everyone announcing a "go" to press ahead.

1040 GMT (6:40 a.m. EDT)

Lockheed Martin is looking to resume the countdown from the T-minus 120 minute mark at 6:50 a.m. EDT following an hour and 40-minute hold. The pause was extended an hour and 10 minutes beyond the intended half-hour to catch up from the weather-related countdown delays.

Once the count starts ticking again, fueling operations will commence at Complex 41.

One final hold is built into the countdown at T-minus 4 minutes. That will last 10 minutes, leading to liftoff at 9 a.m. EDT.

1035 GMT (6:35 a.m. EDT)

This hold is going to be extended 10 more minutes, pushing back liftoff to 9:00 a.m. EDT (1300 GMT). The launch team still racing to recover from the delays experienced over the past few hours because of stormy weather in the area.

1034 GMT (6:34 a.m. EDT)

The launch team is receiving a briefing on procedures from the launch conductor before the countdown resumes from this hold at the T-minus 120 minute mark at 6:40 a.m. EDT. Next, the team will be polled to verify readiness for fueling the rocket with cryogenic liquid oxygen and liquid hydrogen.

1032 GMT (6:32 a.m. EDT)

NASA launch manager Chuck Dovale has polled his agency management team for a "go" to proceed with loading supercold rocket fuel into the Atlas vehicle. Weather conditions are the only issue at this time.

1031 GMT (6:31 a.m. EDT)

The blast danger area around the launch pad has been confirmed clear of all personnel for fueling.

1027 GMT (6:27 a.m. EDT)

There haven't been any significant technical snags worked during the countdown overnight. Things are simply running behind schedule due to the thunderstorms that moved through the launch site area, forcing workers to evacuate the pad because of lightning concerns. But the launch team is catching up and liftoff is now scheduled for 8:50 a.m. EDT. The launch can occur as late as 9:35 a.m. EDT this morning.

1010 GMT (6:10 a.m. EDT)

Launch complex 41 is being cleared of all personnel in readiness for the start of cryogenic fueling.

1005 GMT (6:05 a.m. EDT)

The built-in hold has been extended by another 30 minutes, pushing today's launch time back to no earlier than 8:50 a.m. EDT (1250 GMT).

0952 GMT (6:52 a.m. EDT)

NASA launch commentator George Diller says today's launch attempt is running about an hour to an hour-and-a-half behind schedule because of the overnight thunderstorm activity.

0940 GMT (5:40 a.m. EDT)

NEW LAUNCH TIME. The range has approved a new launch time of 8:20 a.m. EDT (1220 GMT).

0928 GMT (5:28 a.m. EDT)

The built-in hold has been extended by at least 30 minutes to allow the launch team time to catch up on launch preparations which have been held up by bad weather in the Cape Canaveral area. This is likely to push back the launch time by 30 minutes.

0917 GMT (5:17 a.m. EDT)

The lightning advisory has now been cleared, allowing workers to return to the launch pad and resume final pre-flight preparations.

0910 GMT (5:10 a.m. EDT)

T-minus 120 minutes and holding. The countdown has entered the scheduled 30-minute hold. The launch team builds holds into the count to provide margin to recover from work running behind schedule and address technical issues. Clocks are slated to resume at 5:40 a.m. EDT.

0908 GMT (5:08 a.m. EDT)

A lightning strike occurred within a mile of the launch pad a little while ago. The launch team has completed a series of re-test activities to ensure there were no problems caused by this event.

0855 GMT (4:55 a.m. EDT)

Range Safety is performing the hold-fire checks to ensure the countdown can be halted if a problem arises.

0845 GMT (4:45 a.m. EDT)

First stage hydraulic and propulsion preps are complete.

0811 GMT (4:11 a.m. EDT)

Testing of the Atlas 5 rocket's guidance system has been completed.

0806 GMT (4:06 a.m. EDT)

A number of launch weather rules are "no go" at this time as the thunderstorms slowly move over the spaceport.

0750 GMT (3:50 a.m. EDT)

The countdown is entering the final four hours to liftoff time. Checks of the vehicle's internal batteries are starting.

0744 GMT (3:44 a.m. EDT)

A test of the rocket's C-band tracking beacon system is complete. The lightning advisory remains in effect as a line of heavy storms drifts across the Cape.

0725 GMT (3:25 a.m. EDT)

Chilldown thermal conditioning of the first stage liquid oxygen storage area has been accomplished. This is the early precursor to pumping the super-cold oxidizer into the vehicle a couple of hours from now.

0715 GMT (3:15 a.m. EDT)

The countdown is continuing for the Atlas 5 launch. However, a lightning advisory has been issued for the Cape as a thunderstorm moves through the area. Hands-on work at the pad is being temporarily suspended as technicians seek cover from the storm.

0330 GMT (11:30 p.m. EDT Wed.)

ON THE PAD! The Atlas 5 rocket has arrived at the Complex 41 launch pad for Thursday's liftoff carrying the Mars Reconnaissance Orbiter.

The two mobile trailers connected to the launching platform, which were part of the convoy during tonight'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 roll and throughout 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 upcoming fueling of the booster.

Overnight, the Atlas first stage and Centaur upper stage are scheduled to be powered up, and testing of the rocket's guidance system will start along with flight control system preps. A variety of other standard countdown operations will continue into the predawn hours, including loading fueling into the vehicle.

Liftoff remains scheduled for 7:50 a.m. EDT (1150 GMT) at the opening of a one-hour, 45-minute launch window.

0246 GMT (10:46 p.m. EDT Wed.)

ROLLOUT BEGINS! NASA's Mars Reconnaissance Orbiter is taking its first step to departing Earth on a 310 million mile voyage to the Red Planet.

The MRO spacecraft, bolted atop the Lockheed Martin Atlas-Centaur rocket, has begun a slow drive from the 30-story Vertical Integration Facility at Cape Canaveral's Complex 41 to the launch pad. A pair of specially-made "trackmobiles" are pushing the Atlas 5 rocket's 1.4-million pound mobile launching platform along rail tracks for this 1,800-foot trip.

To learn more about the "clean pad" concept used by Lockheed Martin for the Atlas 5 program, see our complete story.

WEDNESDAY, AUGUST 10, 2005
2310 GMT (7:10 p.m. EDT)

COUNTDOWN START. The countdown is commencing at Cape Canaveral for tomorrow's launch to Mars.

The Atlas 5 rocket and Mars Reconnaissance Orbiter stand fully assembled inside Lockheed Martin's Vertical Integration Facility building at Complex 41. A mobile launching platform will wheel the vehicle to the launch pad tonight, emerging from the 30-story VIF just around 10:45 p.m. EDT (0245 GMT) for the 1,800-foot, half-hour move.

Once positioned on the pad, crews will power up the rocket at 12:50 a.m. (0450 GMT) and carry out a prescribed test schedule on various equipment aboard the Atlas 5 over the next several hours.

At 5:57 a.m. (0957 GMT), the loading of super-cold liquid oxygen into the Centaur upper stage begins. That will be followed by filling the first stage with liquid oxygen and Centaur with cryogenic liquid hydrogen fuel.

The countdown enters a planned 10-minute hold at the T-minus 4 minute mark at 7:36 a.m. (1136 GMT). The pause permits the launch team to catch up on late work and address any problems.

Veteran launch conductor Ed Christiansen, seated in the Atlas Spaceflight Operations Center four miles from the pad, will poll the various console operators to ensure all systems are functioning properly. The 'go' status is passed to launch director Jerry Jamison located above and behind the launch team in the management room. If there are no constraints, the count will resume at 7:46 a.m.

During the last four minutes to blastoff, the fuel systems will be secured, the fuel tanks inside the rocket pressurized, onboard systems switched from ground-fed power to internal batteries and the final arming performed. All of the activities culminate with ignition of the main engine in the count's last three seconds.

The RD-180 surges to life at 7:50 a.m. EDT (1150 GMT), building up thrust as the powerplant's vital signs are checked before explosive bolts blow to release the 740,000-pound rocket from three posts on the mobile launching platform upon which the vehicle had rested.

As the ascent begins, the rocket executes a series of tiny pitch, yaw and roll maneuvers to align itself with the desired trajectory that minimizes aerodynamic loads. The rocket will head eastward on a 104-degree flight azimuth.

The RD-180 will be firing at full throttle on the mixture of kerosene fuel and liquid oxygen to propel Mars Reconnaissance Orbiter on its trek to the Red Planet. About 100 seconds into flight, the engine thrust eases down to 95 percent throttle. About three-and-a-half minutes after liftoff, the engine begins a ramped throttle down to keep the acceleration forces on the vehicle at a constant 5 g's. Then just prior to engine shutdown, the throttle level is decreased to lessen the acceleration to 4.6 g's.

With the first stage fuel supply nearly exhausted four minutes into ascent, the main engine is shut down while traveling 10,000 mph about 63 miles above the Atlantic and 174 miles east of the launch pad.

The spent stage then drops away, leaving the cryogenic Centaur upper stage to ignite its Pratt & Whitney RL10 engine at T+plus 4 minutes, 19 seconds. Less than 10 seconds later, the rocket's nose cone that protected the MRO craft during atmospheric ascent is no longer needed and separates.

Centaur burns its engine for just under 10 minutes, reaching an elliptical parking orbit with a high point of 115 miles and low point of 92 miles at T+plus 13 minutes, 52 seconds.

The vehicle coasts in this orbit for a half-hour before the upper stage re-ignites the RL10 for a second firing reach a speed of 25,000 mph, boosting MRO out of Earth orbit for the seven-month cruise to Mars. The burn lasts five-and-a-half minutes, concluding around T+plus 52 minutes.

Centaur reorients itself and at T+plus 55 minutes deploys the satellite cargo. The exact timing of the second Centaur burn and spacecraft separation will be determined by when the Atlas lifts off in the available MRO launch window.

The discarded upper stage will perform another maneuver that ensures it won't recontact MRO or reach Mars.

2020 GMT (4:20 p.m. EDT)

GO FOR LAUNCH! The technical concern has been put to rest this afternoon, clearing the way for launch of the Lockheed Martin Atlas 5 rocket with NASA's Mars Reconnaissance Orbiter tomorrow. The Redundant Rate Gyro Units aboard the rocket were deemed healthy for flight following an analysis prompted by the failure of another RRGU back in the factory.

Watch this page for live updates tonight during the rocket's rollout to the launch pad, scheduled to start around 10:45 p.m. EDT, and early tomorrow during the final hours of the countdown to liftoff.

1640 GMT (12:40 p.m. EDT)

A round of engineering meetings this morning gave a tentative approval to proceed with launch tomorrow, a NASA spokesman says. A final review is scheduled to start around 3 p.m. EDT this afternoon to give the formal approval to enter the countdown this evening.

1445 GMT (10:45 a.m. EDT)

Lockheed Martin and NASA launch officials hope to put the Redundant Rate Gyro Unit issue to rest today, allowing the countdown to begin this evening and the Atlas 5 rocket to make its transfer to the launch pad from the assembly building.

The weather forecast for a Thursday morning launch remains favorable. See the full forecast here.

TUESDAY, AUGUST 9, 2005
1855 GMT (2:55 p.m. EDT)

Engineers are working to determine if there are any problems with a pair of Redundant Rate Gyro Units (RRGUs) aboard the Atlas 5 rocket that will launch the Mars Reconnaissance Orbiter. The units are part of the vehicle's flight control system.

The concern stems from an RRGU that failed factory testing while being readied for a future launch.

If the issue can be resolved promptly, liftoff will proceed on Thursday morning.

1615 GMT (12:15 p.m. EDT)

DELAY. Launch of the Mars Reconnaissance Orbiter has been postponed 24 hours due to a technical issue. More details are expected to be released by NASA soon.

The launch window on Thursday extends from 7:50 to 9:35 a.m. EDT (1150-1335 GMT).

The weather forecast calls for an 80 percent chance of acceptable conditions during the window. The minor worries will be thick clouds and cumulus clouds associated with isolated rainshowers in the area.

We'll update this page when more information is known.

MONDAY, AUGUST 8, 2005

NASA's two-and-a-half ton Mars Reconnaissance Orbiter, an instrument-laden spacecraft designed to capture an unprecedented level of detail about the Red Planet and help guide future missions, is awaiting launch Wednesday morning from Cape Canaveral Air Force Station, Florida.

Liftoff of the Lockheed Martin Atlas 5 rocket is slated for 7:54 a.m. EDT (1154 GMT) from Complex 41. A continuous window lasting an hour and 45 minutes -- until 9:39 a.m. EDT (1339 GMT) -- will be available for the launch to occur or else the mission must wait until Thursday.

Nearly an hour after liftoff, the rocket's Centaur upper stage dispatches the MRO probe on its seven-month, 310-million mile journey to Mars. A Japanese space agency tracking station acquires the craft's signal a few minutes later as an autonomous sequence of onboard events begin to unfurl the two power-generating solar arrays and deploy the 10-foot primary communications antenna.

MRO should arrive at Mars next March and start five months of aerobraking maneuvers to reach its science-collecting near-polar orbit stretching from 199 miles above the planet's surface at its furthest point to just 158 miles at the closest.

The $720 million mission's main science phase runs from November 2006 to December 2008, enabling the onboard cameras, spectrometer, climate sounder and subsurface radar to gather an unparalleled amount of data about Mars.

"We will keep pursuing a follow-the-water strategy with Mars Reconnaissance Orbiter," said Michael Meyer, Mars exploration chief scientist at NASA Headquarters. "Dramatic discoveries by Mars Global Surveyor, Mars Odyssey and the Mars Exploration Rovers about recent gullies, near-surface permafrost and ancient surface water have given us a new Mars in the past few years. Learning more about what has happened to the water will focus searches for possible Martian life, past or present."

The instruments on MRO will offer sharper focus than earlier spacecraft, giving scientists hope for revolutionary discoveries.

"Higher resolution is a major driver for this mission. Every time we look with increased resolution, Mars has said, 'Here's something you didn't expect. You don't understand me yet.' We're sure to find surprises," said Richard Zurek, the orbiter's project scientist from NASA's Jet Propulsion Laboratory.

Featuring the largest telescope to orbit another planet, MRO's high-resolution camera can spot rocks as small as three-feet across and surface layering that will be critical to Mars research as well as selecting safe but interesting sites for future landers.

"Mars Reconnaissance Orbiter is the next step in our ambitious exploration of Mars," said Douglas McCuistion, director of the Mars Exploration Program in NASA's Science Mission Directorate. "We expect to use this spacecraft's eyes in the sky in coming years as our primary tools to identify and evaluate the best places for future missions to land."

The launch team has Wednesday and Thursday to get MRO on its way. If the liftoff is delayed beyond that for some reason, the launch would likely wait until after a commercial Boeing Delta 4 rocket gets a couple of attempts to loft the GOES-N weather observatory from Cape Canaveral.

MRO must fly by September 5 in order to reach its destination due to alignment of Earth and Mars.

Final readiness reviews are underway today to ensure all systems are ready for the much-anticipated launch. The Atlas 5 rocket is fully assembled inside the Vertical Integration Facility hangar at Complex 41. It will be rolled to the launch pad atop a mobile platform just before 11 p.m. EDT Tuesday evening. The final hours of the countdown will see the vehicle fueled, tested and placed on internal control for flight.

The weather forecast calls for an 80 percent chance of favorable conditions during the launch window.

"For launch day, the launch complex will have a slight risk for isolated coastal showers during the morning hours," launch weather officer Clay Flinn reported today. "The primary concerns for launch day are thick clouds and cumulus clouds associated with isolated coastal showers.

"Conditions for a 24-hour delay are similar as well."

Watch this page for additional pre-launch coverage and live play-by-play reports during Wednesday morning's countdown.