Mission Status Center

The weather forecast for Friday's launch calls for another 40 percent chance of acceptable conditions, with anvil and cumulus cloud concerns again. Those odds do not factor in upper level winds, since that issue is determined by the rocket engineering team and not the local weather folks.

The specifics tomorrow include scattered clouds at 3,000 and 10,000 feet, a broken deck at 25,000 feet, isolated showers in the area, good visibility, westerly winds from 270 degrees at 10 knots and a temperature around 78 degrees F.

The launch team is progressing with standard post-scrub safing of the vehicle. Draining of the first stage liquid oxygen is getting underway. The RP-1 fuel will be offloaded and the service gantry put back around the vehicle later today.

So GRAIL will spend an extra day on Earth. Launch will be rescheduled for tomorrow at 8:33:25 a.m. EDT. The second window is 9:12:31 a.m. EDT.

SCRUB! Upper level winds will not allow liftoff today.

NASA launch director Tim Dunn says the team continues to deal with upper level winds.

Now 10 minutes away from the final launch window for today.

No cloud or rain issues for the upcoming launch time, forecasters say.

The launch weather officer is about to give an update to managers.

The GRAIL spacecraft remain in good shape while waiting out this extended hold in the count.

Additional weather balloons are being sent up to gather data on the wind speeds and directions that the Delta 2-Heavy rocket would experience during ascent. That information is then compared against the vehicle's flight profile to ensure conditions are within the rocket's capability to safely fly through.

Two precise moments in time are available daily for liftoff to occur during GRAIL's mission launch window. The dual opportunities each day are driven by which azimuth the rocket is sent on -- either 93 or 99 degrees. The window closes in October to ensure GRAIL's mapping is completed before next June's lunar eclipse that is expected to be lethal to the satellites.

For today's launch, liftoff was planned for 8:37:06 a.m. EDT (1237:06 GMT). Since the winds aloft were not acceptable, the team will load the different flight azimuth program aboard the Delta and now shoot for liftoff at 9:16:12 a.m. EDT (1316:12 GMT).

A complete listing of the launch times can be found in our window chart.

The wind analysis team in Denver reported to management here at the Cape that the high-altitude situation would not improve in time for launch at 8:37 a.m. EDT. So the countdown is being retargeted to the second of today's two available shots to send GRAIL on its way to the Moon.

HOLD EXTENDED. Liftoff has been pushed back to the second launch opportunity today at 9:16 a.m. EDT due to upper level winds.

No problems reported during the readiness poll. Just waiting on latest upper level wind data.

The launch team is being polled for the final time before liftoff to confirm systems are ready for the Delta 2 rocket and GRAIL spacecraft.

NASA launch director Tim Dunn has polled his advisory team and the space agency is "go" to continue with the countdown.

There is a "red" for high-altitude wind conditions. However, the count will proceed while waiting for final balloon before the 8:37 a.m. launch opportunity.

Now 10 minutes to go. Still shooting for the Moon at 8:37 a.m. EDT.

T-minus 4 minutes and holding. The countdown has entered the final planned built-in hold. This is a scheduled 10-minute pause leading to today's liftoff time of 8:37:06 a.m. for the Delta 2 rocket.

The first stage kerosene fuel tank is being pressurized for flight.

The upper level wind conditions are being loaded into the Delta 2 rocket's flight computer for launch.

Twenty minutes from launch. The GRAIL satellites will become the 225th and 226th primary payloads put into space by the venerable Delta 2 rocket over the past 22 years. Here's a look at some other stats about today's mission. This will be:

• The 356th Delta rocket launch since 1960
• The 232nd Delta launch with NASA involvement
• The 259th Delta launch from Complex 17
• The fifth Delta of 2011
• The 150th Delta 2 rocket mission since 1989
• The 49th Delta 2 mission overseen by NASA
• The 110th Delta 2 rocket to fly from Cape Canaveral
• The 48th Delta 2 launch from Cape's pad 17B
• The 6th Delta 2 rocket to fly in 'Heavy' configuration

Weather remains "go" across the board right now. Showers developing to the north are not close enough to the launch pad to be a problem right now.

T-minus 15 minutes and counting. Countdown clocks are running again following the planned 20-minute hold. The count will continue to the T-minus 4 minute mark where another hold is scheduled. Launch remains set to occur at precisely 8:37:06 a.m. local (1237:06 GMT).

A poll of the launch team confirms everyone is ready to press ahead with the countdown after this pre-planned 20-minute hold.

Now just 35 minutes away from blastoff. The current weather still looks encouraging for an on-time launch this morning.

T-minus 15 minutes and holding. Clocks have entered the first of two planned hold periods during the final portion of the Terminal Countdown. This pause will last 20 minutes in duration.

These holds are designed to give the launch team a chance to deal with any problems and catch up on work that could be running behind schedule.

Now moving into data link checks to verify good telemetry connections.

The first stage engine steering checks are complete.

The second stage engine slews just finished. First stage tests are starting.

The launch team is beginning the "slew" or steering checks of the first and second stage engines. These are gimbal tests of the nozzles on the first stage main engine and twin vernier engines and second stage engine to ensure the rocket will be able to steer itself during launch.

Sixty minutes to launch. As this latest scientific adventure to the Moon nears liftoff, here's some thoughts by Maria Zuber, GRAIL's principal investigator:

"The Moon has played a central role in the human imagination and psyche, so much so that when humans reached out and landed on the Moon it really became a defining event for civilization.

"Since the dawn of the space age in 1959, there have been 109 missions that have targeted the Moon, 12 humans have walked on the surface of the Moon, we've brought back 842 pounds of rocks and soil that are still being analyzed today. As we speak, there are three spacecraft that are orbiting the Moon and collecting science observations."

"Given all of this activity, do we know everything that we'd like to know about Moon? In short, the answer is no.

"Trying to understand how the Moon formed and how it evolved over its history is one of the things we're trying to address with the GRAIL mission, but also how the Moon is an example of how terrestrial planets in general have formed."

Checks of the rocket's safety system will be conducted in the next few minutes of the countdown.

Loading of the Delta 2 rocket's first stage liquid oxygen tank has been accomplished. The filling process took 27 minutes, ending at 7:22 a.m. The tank will be replenished through the countdown to replace the super-cold liquid oxygen that naturally boils away.

The rocket now stands fully fueled for liftoff. The vehicle's first stage was successfully loaded with RP-1 kerosene fuel along with the liquid oxygen over the past hour-and-a-half. The second stage was filled with its storable nitrogen tetroxide and Aerozine 50 fuels last Friday. The nine strap-on booster rockets use solid propellants.

The first stage liquid oxygen tank just reached the 95 percent full level. The "rapid load" valve was closed, with the slower "fine load" phase continuing to fill the tank.

Liftoff is just 90 minutes away. No technical issues are being reported in the countdown this morning. The only concern for today's launch of the Delta 2 rocket with GRAIL is the weather, specifically anvil and cumulus clouds violating the liftoff safety rules.

Ten minutes into the flow. LOX loading is an approximate 25-minute process to fill the first stage liquid oxygen tank.

LOX loading begins! Cryogenic liquid oxygen, chilled to Minus-298 degrees F, has started flowing from the storage reservoir at Complex 17, through plumbing and into the bottom of the Delta 2 rocket. The LOX will be consumed by the first stage main engine during the first four-and-a-half minutes of flight along with the 10,000 gallons of RP-1 kerosene already loaded aboard the vehicle.

The official "go" has been given to start first stage liquid oxygen loading.

The launch team is performing the preparatory steps for filling the liquid oxygen tank.

NASA launch director Tim Dunn has completed a poll to confirm the agency management team is "go" for liquid oxygen loading.

All of the weather rules are "go" for launch at this time. But the outlook for today's window still calls for only a 40 percent chance of acceptable conditions due to clouds.

The official forecast includes scattered clouds at 3,000 and 12,000 feet, broken at 20,000 feet, isolated showers in the area, good visibility, southwesterly winds from 240 degrees at 10 knots and a temperature around 78 degrees F.

Now two hours from launch.

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The launch team has completed work to turn on and configure the Delta's onboard guidance computer.

The next milestone in the count will be loading super-cold cryogenic liquid oxygen into the first stage starting about 30 minutes from now. The kerosene and liquid oxygen will be consumed by the stage's Pratt & Whitney Rocketdyne RS-27A main engine and twin vernier steering thrusters during the initial four-and-a-half minutes of flight.

The first stage fuel tank of the Delta 2 rocket has been fully loaded for today's planned 8:37 a.m. EDT launch. The tank was filled with a highly refined kerosene, called RP-1, during a 19-minute, 42-second process that concluded at 6:22 a.m.

Rapid-loading of the RP-1 tank has concluded with 9,800 gallons having been pumped into the rocket. Fine load is continuing.

The launch team has computed that the full load for the first stage fuel tank will be 9,988 gallons.

Once the tank is filled to the 98 percent level, or 9,800 gallons, the "rapid load" valve will be closed and the slower "fine load" phase will continue to top off the tank.

First stage propellant loading has passed the 5,000-gallon mark. This process to load the kerosene fuel takes about 20 minutes.

Fueling begins! About 10,000 gallons of the kerosene propellant, called RP-1, are pumping into the base of the rocket from storage tanks at pad 17B as fueling of the Delta 2's first stage begins for today's launch.

Preparations for loading the Delta 2 rocket's first stage RP-1 fuel tank are beginning. After verifying valves, sensors, flow meters and equipment are ready, the highly refined kerosene fuel will start flowing into the vehicle a few minutes from now.

The first stage helium and nitrogen systems have been pressurized.

The rocket's control system and guidance computer are being turned on.

Workers have cleared the Complex 17 area in advance of the hazardous portion in today's launch countdown. The pad clear status will allow the start of activities such as pressurizing the helium and nitrogen storage tanks inside the rocket's first and second stages, along with the second stage fuel and oxidizer tanks.

Countdown begins! The three-hour Terminal Countdown sequence has been initiated for today's launch of the Delta 2-Heavy rocket with NASA's Moon-bound GRAIL gravity mappers.

Liftoff from pad 17B is scheduled for 8:37:06 a.m. EDT (1237:06 GMT). If the launch does not occur at that precise moment, another instantaneous opportunity is available at 9:16:12 a.m. EDT (1316:12 GMT).

Between now and the launch time, the rocket's guidance system will be activated, onboard helium and nitrogen tanks brought up to pressure, the kerosene fuel and liquid oxygen supplies loaded into the first stage, checks performed on the safety system and steering tests conducted on the engines.

The launch team has reported for duty. A readiness check just occurred to verify all consoles are "go" for starting the countdown 9 minutes from now. Workers have finished their hands-on chores at pad 17B and cleared the complex for the rest of today's countdown.

T-minus 150 minutes and holding. The countdown is entering the first of the planned holds that will occur over the course of the morning. This first pause is 60 minutes long.

The later holds -- at the T-minus 20 and T-minus 4 minute points -- will give the launch team some time to catch up on any work running behind and verify all is in readiness for liftoff. The holds will last 20 minutes and 10 minutes in duration, respectively.

Launch remains targeted for 8:37 a.m. EDT (1237 GMT) from Cape Canaveral, Florida.

Check out our photo collection of the Delta 2 rocket taken during gantry rollback.

The Delta 2-Heavy rocket has been unveiled from the mobile servicing gantry at Cape Canaveral's Space Launch Complex 17B. The cocoon-like tower was wheeled away to get activities underway for liftoff in the morning.

Check out some iPhone pictures taken during tower rollback on our Facebook page!

Ground teams will spend the next few of hours getting the pad configured and secured in advance of the Terminal Countdown picking up.

The count will include fueling of the first stage with highly refined kerosene starting about 6:20 a.m. and supercold liquid oxygen propellants at 7:10 a.m. EDT.

A standard hold at the T-minus 4 minute point will give launch managers the opportunity to survey the status before countdown clocks progress into the quickly paced final minutes that see the vehicle switch to internal power, the liquid oxygen tank pressurized and systems armed.

The main engine and six of the nine strap-on solid rocket motors ignite at liftoff to begin GRAIL's ride off the planet. The Delta 2-Heavy features slightly larger solid motors, originally developed for the more-powerful Delta 3 rocket.

"We will lift off with the combined thrust of about 1.3 million pounds from six of the SRBs and the main engine on the core stage," said Vernon Thorp, United Launch Alliance's program manager for NASA missions.

Arcing eastward over the Atlantic Ocean, the six solid boosters lit on the ground extinguish 77 seconds into flight as the remaining three motors are ignited moments later. The half-dozen spent boosters are jettisoned to fall harmlessly into the sea. The air-lit motors burn out and separate two minutes, 40 seconds after liftoff.

Once the solids are discarded, the kerosene-powered main engine will continue pushing the rocket to the edge of space. The spent stage then jettisons at T+plus 4 minutes, 31 seconds to let the hydrazine-fueled second stage propel the vehicle the rest of the way into an initial orbit.

The two-piece nose cone that shrouded GRAIL the trek through the atmosphere is shed at T+plus 4 minutes, 41 seconds just moments after the second stage comes to life.

Seven minutes and 10 seconds into the ascent, the second stage completes its initial firing. The Delta settles into 90-nautical-mile parking orbit inclined 28.5 degrees, flying along a trajectory that cruises above the central Atlantic before passing Africa, the Indian Ocean and skirting over northern Australia.

The second stage reignites its engine while crossing the Pacific some 68 minutes after blastoff for a four-and-a-half-minute firing that accelerates the rocket out of Earth orbit.

Deployment of GRAIL-A occurs within view of the Vandenberg tracking station in California at T+plus 82 minutes. After the second stage reorients itself, GRAIL-B is released at T+plus 90 minutes within the Whites Sands tracking station coverage zone.

"We anticipate that our Delta 2 vehicle will perform exceptionally well, placing GRAIL on an accurate trans-lunar trajectory," said Thorp.

"We are proud to support NASA science missions, playing a critical role by delivering these one-of-a-kind spacecraft in support of the global science community. This mission represents the culmination of years of hard work by NASA, Lockheed Martin's spacecraft team and our ULA launch team."

After the GRAIL twins separate from the Delta vehicle, the second stage will perform another burn to deplete remaining propellants and place the rocket body into an orbit between Earth and Mars, Thorp said.

The gantry has arrived in its launch position a safe distance away from the rocket.

After a short pause, the tower is in motion again.

The service tower is clear of the Delta 2-Heavy rocket as it continues to retract on rails on the pad's western side.

This gantry was used starting on April 7 to stack the two-stage vehicle atop the pad's launch mount, plus attach the nine strap-on solid rocket motors and the GRAIL payload. The tower also provided the primary weather protection and worker access to the rocket during its stay at the oceanside complex.

But now it's time to reveal the 12-story-tall Delta for this final planned rocket launch from Complex 17 after 55 years of flights.

Rollback of mobile tower has begun!

Good evening from Cape Canaveral's Complex 17 where reporters and photographers have gathered to cover tonight's rollback of the mobile service tower from the Delta 2-Heavy rocket. The gantry will begin moving once workers finish buttoning up the rocket and accomplishing all their preps. There is no specific time the tower must roll, so the schedule is a bit fluid.

Whenever it does happen, we'll have live streaming video.

The xenon spotlights have been turned on to shine brightly upon the Delta 2 rocket at pad 17B. The gantry-like mobile service tower will be retracted by ground crews a later tonight to reveal the launcher for the final hours before blastoff carrying the two GRAIL satellites on their way to the Moon. Liftoff remains on target for 8:37 a.m. EDT.

It is a peaceful day at Cape Canaveral's Complex 17 where the Delta 2-Heavy rocket stands enclosed within the protective mobile service gantry awaiting tomorrow's blastoff to the Moon.

Check out some photos taken at the pad this morning on our Facebook page!

Technicians will begin arriving around 8 p.m. EDT tonight for final engineering walkdown inspections of the vehicle and work to ready the tower-on-wheels for retraction to reveal the 12-story rocket.

The actual moving of the gantry is expected to occur sometime between 10:30 p.m. and midnight EDT, weather permitting.

Watch this page for live video and updates during tower rollback as the countdown operations kick off!

The weather forecast remains unchanged for tomorrow's launch, with meteorologists still predicting a 40 percent chance of acceptable conditions for liftoff to occur on schedule.

"A deep unstable southwesterly flow pattern continues to dominate the weather pattern across the region allowing warm, moist tropical air to flow across Central Florida. Additionally, a stationary surface trough is positioned across the Florida peninsula stretching from the Eastern Gulf of Mexico through central and northern Florida. This pattern will result in active and unstable conditions for the remainder of the week. Frequent scattered showers and thunderstorms are likely during the afternoon/evening hours Wednesday through Friday," this morning's forecast says.

"For (pad gantry) roll, west winds gusting 15-20 knots are expected during roll. Additionally, late evening thunderstorm/lightning activity may delay the beginning of the roll.

"On launch day, winds at Complex 17 are expected from the West gusting in the upper teens during the count and through the window. The primary concern for launch is anvil clouds moving over the Cape from persistent thunderstorm and lightning activity in the eastern Gulf and Florida west coast. Cumulus clouds and precipitation may also be a concern associated with the surface boundary draped across the region.

"In the event of a 24-hour delay, similar conditions are expected with winds from the west-southwest gusting in the upper teens. The primary concerns for a 24-hour delay are anvil clouds, cumulus clouds and precipitation."

Everything is progressing on schedule for Thursday's planned launch of the Delta 2-Heavy rocket from Cape Canaveral to propel the twin GRAIL spacecraft on their trek to the Moon.

"This morning we held a successful Launch Readiness Review and did receive concurrence from NASA management, as well as spacecraft, launch vehicle and Range agencies to proceed into the countdown," said NASA launch director Tim Dunn.

"Today at Complex 17, we did our final Range Safety beacon checks, our final pre-launch slewing of the engines and our final azimuth update. All of those tests went well."

The only potential worry is the weather. Forecasters say there's only a 40 percent chance of acceptable conditions during the two available launch opportunities at 8:37 and 9:16 a.m. EDT (1237 and 1316 GMT).

"Weather is always an issue regardless of what the mission is," said Joel Tumbiolo, launch weather officer from the 45th Weather Squadron.

A stationary weather front draped across Florida with moisture funneling across the state will bring unstable conditions for the next several days. The possible violations to launch rules would be thunderstorm anvil clouds drifting from the west or cumulus clouds developing in the local area, Tumbiolo said.

When the Delta rocket does blast off, it will fly a 90-minute ascent to hurl the twin spacecraft out of Earth orbit. The duo have plotted an indirect route to the Moon, eventually entering lunar orbit on New Year's Eve and New Year's Day, respectively, to begin an unprecedented gravity mapping expedition.

"GRAIL is, simply put, a journey on the center of the Moon. It will probe the interior of the Moon and map its gravity field a hundred to a thousand times better than ever before. We will learn more about the interior of the Moon with GRAIL than all previous lunar missions combined," said Ed Weiler, NASA's associate administrator for the Science Mission Directorate.

"We crashed into the surface of the Moon with Ranger several times, we landed on the Moon several times with Surveyor and scratched the surface, lunar orbiters photographed its surface, then with Apollo humans walked on the surface and drove dune-buggies across the surface. But GRAIL is the first mission to look below that surface.

"How will it do that? It's actually two small satellites that will fly in tandem about 200 km apart about 50 km high. The satellites will monitor the distance between each other, and as the first satellite goes over a higher mass concentration -- or higher gravity -- it will speed up slightly. That will increase the distance. And then as the second satellite goes over, that distance will close again. That's how it actually maps the gravitational field of the Moon, and will do that over the course of three months and do a complete map three times."

Weiler says the legacy of GRAIL's mapping will guide the next robotic and human voyages to the Moon.

"We don't really have a good feel for lunar gravity. The fact that this mission is going improve our understanding of the gravity field of the Moon by factors of a hundred to a thousand is incredible. That can be very critical to future landing of robotic spacecraft or human spacecraft, precisely knowing what the gravity fields are you're going to have to deal with," he said.

"The Moon is not a nice solid sphere, solid rock. It's got lunar mascons (mass concentrations). It's actually quite a dicy thing to fly orbits around the Moon because the Moon is not very uniform. This mission's goal is to really, really pin that down -- what is the gravity field of the Moon. And we won't do it once, we'll do it three times. So we'll have some redundancy there."

Scientists expect voluminous amounts of data from the GRAIL spacecraft that supercomputers will crunch to actually generate the gravity maps.

"I like to call it a real physics mission. This is physics at its best," said Weiler. "You have to get a feel for the incredible amount of data that's going to come out of this, and if it weren't for computers I don't know how we'd ever do a mission like this because if you had to use a slide-rule it would be a long, long time getting the data out!"

Watch this page for continuing live updates and streaming video throughout the countdown and GRAIL's departure from Earth aboard the Delta rocket.

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The pre-launch news conference is coming up in a few minutes from the Kennedy Space Center. You can watch it live in our streaming video.

The Launch Readiness Review has concluded and given the "go" to press ahead with Thursday morning's liftoff as planned. This morning's meeting examined the status of the Delta rocket, the GRAIL spacecraft, the network of ground support and the weather forecast. The review culminated with official consensus to enter into the countdown operations Wednesday night.

The weather outlook for Thursday's launch opportunity of the Moon-bound GRAIL mission has worsened a bit in the latest forecast issued by Air Force meteorologists this morning.

"A deep unstable southwesterly flow pattern is established across the region allowing warm, moist tropical air to flow across central Florida. A surface trough continues to move into the area from the west and will become stationary on Wednesday stretching from the eastern Gulf of Mexico through central and northern Florida. This pattern combined with the warm moist air already in place will keep the weather pattern active and unstable for the remainder of the week. Frequent scattered showers and thunderstorms are likely during the afternoon/evening hours Tuesday through Friday.

"For (pad gantry) roll, southwest winds gusting 20-25 knots are expected during roll. Additionally, late evening thunderstorm/lightning activity may be a threat to delay beginning of the roll.

"On launch day, winds at Complex 17 are expected from the West gusting in the upper teens during the count and through the window. The primary concern for launch is anvil clouds moving over the Cape from persistent thunderstorm/lightning activity in the eastern Gulf and Florida west coast. Cumulus clouds and precipitation may also be a concern associated with the surface boundary draped across the region."

The conditions at the 8:37 a.m. EDT launch time are predicted to include scattered low- and mid-level clouds, a broken layer of high clouds, isolated showers in the area, good visibility, westerly winds from 270 degrees at 10 to 18 knots and a temperature of 81 degrees F.

"In the event of a 24-hour delay, similar conditions are expected (Friday) with winds from the southwest gusting in the upper teens. The primary concerns for a 24-hour delay are anvil clouds, cumulus clouds and precipitation," forecasters say.

Launch technicians and flight managers are gearing up for a busy day of rocket testing and mission assessments at pad 17B and around the Florida spaceport.

After enjoying the Labor Day holiday break, everyone is back to work today for the final round of testing on the Delta 2 vehicle with the Eastern Range while the Launch Readiness Review is conducted.

"The most important part is it's the final major electrical test of the rocket. We call it F3, Task 3. It's Range Safety beacon checks," NASA launch director Tim Dunn said in a recent interview.

The C-band link used for tracking the rocket as it flies downrange and the command receiver decoders in the vehicle's safety system will be verified.

"And then we'll do that final azimuth alignment where we look at the flight computer RIFCA and look at the Porro prism on the side of the rocket one final time to make sure now that we have mated the spacecraft and we have put the fairing on that you haven't shifted. We are looking at arc-second errors there. So very, very minor things," said Dunn.

Meanwhile, the LRR is scheduled for this morning to confirm all systems are ready to enter into the countdown for Thursday's liftoff.

"Simultaneously, we'll be over in the O&C Building at the mission briefing room doing the Launch Readiness Review," said Dunn.

The pre-launch press conference is planned for 1 p.m. EDT with Dunn, GRAIL project officials and representatives from NASA Headquarters, United Launch Alliance and the 45th Weather Squadron.

After getting through today's activities, most of Wednesday will be quiet before launch operations commence late in the evening.

"Because we're launching at 8:30 in the morning on Thursday, we'll have a crew sync day on Wednesday. That will be another off day for most of the crew. There will be a subset of the crew that has to come in about 7 or 8 o'clock (p.m.) and prep the launch pad for the tower roll," Dunn said.

The gantry should be retracted a little before midnight EDT, allowing technicians to finish configuring the launch complex in the overnight hours early Thursday. The Terminal Countdown will start ticking at 5:57 a.m. EDT, leading to liftoff at 8:37 a.m. EDT.

Watch this page for continuing live mission updates and streaming video!

Weather forecasters are predicting a 60 percent chance of acceptable conditions for launch Thursday morning of the Delta 2-Heavy rocket carrying the Moon-bound GRAIL twins.

"The low pressure over Louisiana associated with remnants of Tropical Storm Lee will move northeastward and away from the southeastern states Monday-Wednesday and remain northwest of east-central Florida," the launch weather team reported today.

"A deep unstable southwesterly flow pattern will set up across the Central Florida region and usher in warm and moist air leading to scattered showers and thunderstorms possible during the afternoon/evening hours for the remainder of the week. Additionally, a surface trough will stall across the area stretching from eastern Gulf of Mexico to central and northern Florida providing additional instability.

"For (pad gantry) roll, light southwest winds are expected during roll. Late evening thunderstorm/lightning activity may be a threat and possibly delay beginning of the roll.

"On launch day, winds at Complex 17 are expected from the southwest gusting in the low-mid teens during the count and through the window. The primary concern for launch is anvil clouds advecting over the Cape from persistent thunderstorm/lightning activity in the rastern Gulf and Florida west coast. Cumulus clouds may also be a concern associated with the surface boundary draped across the region."

The specifics for launch time include scattered and broken levels of clouds, isolated showers in the area, good visibility, southwesterly winds from 250 degrees at 10 to 15 knots and a temperature of 81 degrees F.

"In the event of a 24-hour delay, similar conditions are expected (Friday) with winds from the southwest gusting in the low-mid teens. The primary concerns for a 24-hour delay are anvil clouds and cumulus clouds," forecasters say.

Men have walked on it and satellites have surveyed it from every angle, but another mission to the Moon launching this week seeks profound new insights about our nearest neighbor in the night sky.

"There have been many missions that have gone to the Moon, orbited the Moon, landed on the Moon, brought back samples of the Moon. But the missing piece of the puzzle in trying to understand the Moon is what the deep interior is like," said Maria Zuber, principal investigator of the upcoming GRAIL mission.

"What was going on in the interior? How did it melt? Is there a core? How did the core form? How did the interior convect? Why are the impact basins on the near side flooded with magma and give us this man-in-the-moon shape whereas the back side of the Moon doesn't have any of this? These are all mysteries that despite the fact we've studied the Moon before we don't understand how that has happened. GRAIL is a mission that is going to tell us that."

Read our preview story.

With all of the second stage fueling activities successfully accomplished Friday, the Delta 2 rocket team is enjoying a breather over the holiday weekend before getting into the GRAIL countdown operations.

"The only thing that separates GRAIL from being an absolute traditional Delta 2 flow is the Labor Day weekend. We wanted to keep that as a three-day holiday for the crew," NASA launch director Tim Dunn said in a recent interview.

"Obviously, the spacecraft is mated and we will be fueled, so there's going to be some prop-watch activity for a limited number of crew members, but for the most part everyone gets a nice three-day weekend."

Given where the holiday fell on the calendar and the desire to launch at the opening of GRAIL's lunar window on Thursday, schedulers opted to load the rocket's storable hypergolic propellants a few days earlier than normal for a Delta 2, then take the weekend break before resuming the pre-launch timeline.

When the full team returns on Tuesday, the final electrical testing of the rocket will be performed while managers hold their Launch Readiness Review that gives authority to enter into the countdown sequence.

The pre-launch news conference is scheduled for Tuesday at 1 p.m. EDT (1700 GMT). You can watch it live in our streaming video on this page.

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Storable hypergolic propellants are being loaded into the Delta 2 rocket's second stage today in preparation for next Thursday's lunar launch.

Technicians at Cape Canaveral Air Force Station's Complex 17 began filling the stage with nitrogen tetroxide oxidizer this morning. The hydrazine propellant mixture will be placed aboard tomorrow, a NASA spokesperson says.

The fuels will power the stage's Aerojet-made main engine during the two firings needed to propel the GRAIL satellites out of Earth orbit to begin a 2.6-million-mile roundabout trek to the Moon.

Commencing the second stage fueling is a major milestone in preparing for launch. And the decision to start the loading isn't taken lightly.

"One of the major constraints for a launch period for Delta 2 is how much time can that second stage propellant be on board. That's a 37-day clock from the day you load," said NASA launch director Tim Dunn.

The "clock" is based on how long the stage remains certified to fly after the storable propellants begin flowing into the vehicle. Exposure to the fuels limits the lifetime for seals and other parts of the stage.

Originally planned to start yesterday, the fueling operations were pushed back 24 hours to sort out a software issue. But that schedule change will have no impact on the launch date since there's still slack left in the timelines.

Meanwhile, members of the launch and management teams held a countdown dress rehearsal yesterday to practice countdown scripts, problem solving and procedures. Officials said the event went well.

The Delta 2 rocket and the GRAIL lunar gravity spacecraft underwent the Flight Readiness Review today to assess the progress of work and any technical issues being addressed.

Although three open items exist, a NASA spokesperson says none are likely showstoppers and the tentative approval was given to proceed with loading the rocket's second stage with storable propellants beginning Thursday as planned.

The long-range weather look appears generally favorable for blastoff next Thursday at the opening of GRAIL's 42-day Moon launch period, which is driven by the mission duration and avoiding lunar eclipses while in mapping mode.

The two satellites were powered up today for final pre-flight testing at the launch pad.

With Hurricane Irene no longer a concern for Cape Canaveral, preparations continue smoothly for the launch of GRAIL.

"Yesterday's final encapsulation of the spacecraft is an important mission milestone," said David Lehman, GRAIL's project manager from NASA's Jet Propulsion Laboratory. "Our two spacecraft are now sitting comfortably inside the payload fairing which will protect them during ascent. Next time the GRAIL twins will see the light of day, they will be about 95 miles up and accelerating."

Installation of the Delta 2 rocket's nose cone began last night and was completed this morning at launch pad 17B. The 10-foot-diameter composite fairing shrouds the GRAIL spacecraft during the climb through Earth's atmosphere in the initial five minutes of flight.

The payload fairing attachment work was accelerated in the overall pre-launch schedule due to concerns with Hurricane Irene. Getting the shroud installed provides an extra level of protection to the satellites at the pad.

The Integrated Systems Test for the United Launch Alliance Delta 2 rocket and the Moon-bound GRAIL spacecraft is being run today to confirm the combined booster and payload are ready for their departure from Earth next month.

This thorough electrical test simulates the countdown and the rocket's trek from liftoff through spacecraft deployment that sends the dual satellites on escape trajectories.

"What distinguishes this test is it's near real-time, same-as-launch-day where we'll turn on the rocket, slew the engines, we do some checkouts with the Range, then we do a simulated count down to T-0. Then from T-0 we'll run the full real-time mission duration out through roughly 4,300 seconds for this mission," said NASA launch director Tim Dunn.

"Formerly in the Delta 2 world it's called Flight Program Verification. Now that ULA is trying to standardize terms between Atlas, Delta 2 and Delta 4, it's IST."

GRAIL and the Delta have been operating on separate paths during manufacturing and assembly. But they finally met Thursday, getting bolted together in preparation for the crucial ascent on September 8 to propel the spacecraft off the ground and out of Earth orbit.

The spacecraft, built by Lockheed Martin, underwent the on-pad functional test that wrapped up Saturday. That check was designed to confirm the satellites remained healthy following their recent fueling operations and subsequent transport to Complex 17.

All of the rigorous testing ensures the $496 million mission will proceed without a hitch during the three-and-a-half-month circuitous route to the Moon, then gather the unprecedented lunar gravity maps next year.

Meanwhile, meteorologists are monitoring the progress of Hurricane Irene. The projected track takes it east of Cape Canaveral as a 'major hurricane' on Friday. NASA declared its lowest-level HURCON 4 hurricane alert at 12 noon EDT today. The Hurricane Condition 4 means personnel should start reviewing procedures that would be used to secure facilities for possible arrival of a storm.

After 55 years of launches, the historic Complex 17 at the Cape Canaveral received what could be its final satellite payload this morning when NASA's twin Moon-mapper probes arrived for mounting atop the Delta 2 rocket.

The $496 million Gravity Recovery and Interior Laboratory mission is scheduled for blastoff September 8 to send the tandem satellites on their lunar voyage that seeks answers to scientists' lingering questions about the Moon.

Once maneuvered into lunar orbits, the mirror-image craft will bounce radio signals off each other to measure subtle changes in the distance between the two formation-flying satellites caused by the Moon's gravity. Such data about the gravity field can be used to infer the Moon's origins and composition of the lunar's deep interior structure.

The venerable booster dispatching the spacecraft is United Launch Alliance's Delta 2, a workhorse rocket that will be making its 110th flight from Complex 17.

But after sending GRAIL to the Moon, no further Florida launches are scheduled for the rocket to perform. Long-time users of the vehicle such the Global Positioning System and NASA science projects have moved to the larger Atlas 5 and Delta 4 systems, leaving the Delta 2 with an uncertain future.

The California launch site for sending Delta 2 rockets into polar orbits from Vandenberg Air Force Base has one more mission firmly planned this October carrying a weather satellite. That pad's long-term viability depends on the potential for NASA or commercial customers to purchase five unsold Delta 2 vehicles for satellites being designed in the coming years that require West Coast launches.

However, the prospects for Delta 2-class launch services needing an East Coast pad to reach equatorial orbits or Earth-escape trajectories appear virtually non-existant in the next few years.

That means GRAIL could be Complex 17's grand finale unless the future outlook changes -- and soon.

Complex 17 was built by the Air Force for the Thor intermediate range ballistic missile test program. Featuring two launch pads and a central blockhouse, construction of the site began in April 1956.

Pad 17B hosted its first Thor launch in January 1957 and pad 17A supported its initial flight in August 1957.

The complex, located on the southern end of Cape Canaveral Air Force Station, has undergone numerous upgrades and modifications over the decades as the Delta family was born and the rockets grew larger.

By the late 1980s, the Delta 2 vehicle was created to deploy the Global Positioning System via dozens of launches to construct the orbiting constellation and sustain it.

The dependable rocket also attracted customers to launch communications satellites and science spacecraft, flying multiple missions per year in a mixture of military, civil and commercial business.

But when the Air Force steered its next-generation GPS satellites to the new Atlas 5 and Delta 4 vehicles, the Delta 2 lost its anchor tenant. Launch pricing and the capabilities offered by the rockets combined with a changing landscape of the medium-size satellite market dealt tough blows to the Delta 2's future.

Since its maiden voyage in February 1989, the Delta 2 rocket has launched successfully 147 times in 149 attempts. The remarkable record has only one out-right failure and one partial mishap when a lower-than-planned orbit was reached.

That enviable track record launched the twin Mars rovers Spirit and Opportunity, Mars Global Surveyor and Odyssey, the Mars Pathfinder lander, the Spitzer Space Telescope, the Mercury-orbiting MESSENGER, asteroid explorers NEAR and Dawn, comet-smashing Deep Impact, sampling probes Stardust and Genesis, four dozen GPS satellites and countless telecommunications birds.

It's been a good life for the Delta 2 rocket. What the future holds remains to be written.

NASA confirms GRAIL has been successfully attached to the Delta rocket.

Launch is exactly three weeks away from right now. Liftoff is targeted for Thursday, September 8 at 8:37 a.m. EDT.

GRAIL has been maneuvered into the cleanroom area in the upper levels of pad 17B where the spacecraft will be attached to the Delta 2 rocket later today.

Hoisting of GRAIL into the Delta 2 rocket's launch pad has begun. The overhead crane is pulling the payload up the rail tracks on the south-side of the mobile service gantry.

GRAIL has been raised off the trailer and maneuvered to the stabilization bracket on the launch pad tower that will help guide the payload during this morning's hoisting operation.

The crane has been hooked up to GRAIL as the sun starts to rise over Complex 17.

Using a ladder propped up against the payload canister, a worker has climbed atop GRAIL to attach the lifting crane.

The GRAIL twins have completed their middle-of-the-night road trip from Titusville to the Complex 17 launch site at Cape Canaveral.

The convoy traveled from the commercially-run Astrotech satellite processing campus, crossing the river into the spaceport and down to the Delta 2 rocket's launch pad.

The truck pulling the GRAIL spacecraft arrived at the base of pad 17B around 4:20 a.m. The satellites, packed inside a metal shipping container, rode aboard a small trailer-like contraption during this morning's trek.

The GRAIL satellite tandem that will map the Moon's gravity like never before have been cleared to make an overnight trek to Cape Canaveral's Complex 17 launch pad where the Delta 2 rocket awaits its passengers.

NASA's Gravity Recovery and Interior Laboratory spacecraft have spent the past three months at the Astrotech processing facility near Kennedy Space Center getting ready to depart Earth on the lunar voyage.

Launch remains targeted for September 8 at 8:37 a.m. EDT (1237 GMT).

Since arriving in Florida from Lockheed Martin's factory in Colorado on May 20, the craft have been thoroughly tested, communications checked through the Deep Space Network, the power-generating solar arrays and internal batteries installed, and the maneuvering propellants loaded aboard.

Just last week, the two satellites were placed atop the deployer system that will release the craft to begin their independent flights to the Moon. They were bagged in protective coverings on Thursday, then encapsulated in the transportation canister on Friday.

After final assessments were completed today, managers gave approval to send GRAIL to the launch pad tonight. The satellites will be hoisted atop the Delta on Thursday.

No matter when they launch during the upcoming 42-day window, the satellites will arrive at the Moon on December 31 and January 1, entering preliminary elliptical orbits before eventually circularizing into precise polar orbits.

The $496 million mission will see the GRAIL duo bounce radio signals off each other in a method of surveying the lunar gravity field. The data should allow scientists to determine the Moon's interior structure from crust-to-core.

NASA's twin Gravity Recovery and Interior Laboratory satellites are in launch configuration, ready to move to the pad and meet a Delta 2 rocket for liftoff toward the moon. These photos show the spacecraft today during a media photo opportunity.

Two small spacecraft flew to the Kennedy Space Center in the belly of an Air Force cargo plane Friday, ready to start final preparations for launch to the Moon in September.

The Gravity Recovery and Interior Laboratory, or GRAIL, mission is scheduled to blast off Sept. 8 on a Delta 2 rocket from Cape Canaveral, Fla.

GRAIL's two nearly identical spacecraft flew on a U.S. Air Force cargo plane from Lockheed Martin Corp. in Denver to Kennedy Space Center's space shuttle landing strip, then the probes were moved to a nearby cleanroom facility.

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