Dragon's commanded abort demonstrations, employing full burns and pulsed firings of its Draco thrusters, were completed earlier today. The craft also demonstrated its free drift mode, in which Dragon's attitude control thrusters are switched off.

NASA and SpaceX are reviewing data from the demos.

The abort and free drift functions are required for Dragon's approach to the space station. An abort may be necessary if problems develop during the final phase of rendezvous, and Dragon must go to free drift mode before being grappled by the station's robot arm.

SpaceX has thus far reported no significant issues with the spacecraft on its first day in orbit. More burns are on tap overnight and tomorrow to fine-tune its high-speed pursuit of the space station.

Dragon's laser ranging instrument, called a LIDAR system, and the capsule's thermal imager appear to working as expected, but the big test won't come until Friday's final rendezvous with the International Space Station. The sensors will then be called upon to feed precise range and relative velocity data to Dragon's flight computer and guidance system.

SpaceX's Falcon 9 rocket radiantly rose into a serene predawn sky over Florida on Tuesday, successfully launching a privately-owned capsule named Dragon into orbit on a seminal test flight to the International Space Station.

The Dragon spacecraft is on a technological shakedown mission, and if all goes according to plan, it will become the first commercial vehicle to reach the space station, a 450-ton orbiting complex staffed by six crew members from the United States, Russia and the Netherlands.

Read our full story.

The guidance, navigation, and control systems door has been secured and all systems are performing well, according to NASA. The instruments inside the navigation bay are being checked out now.

The Dragon spacecraft uses a LIDAR, or laser, instrument to attain position, range and attitude information on the space station. Dragon's rendezvous suite also includes a thermal imager.

Elon Musk just tweeted: "Navigation bay pointing to deep space and star map being generated by star tracker one. Yes!"

Elon Musk just tweeted: "Dragon spaceship opens the navigation pod bay door without hesitation. So much nicer than HAL9000 :)"

The opening of the bay door was a significant milestone in the mission. Inside the bay are Dragon's navigation sensors and grapple fixture required for the mission to the space station.

Officials expect the door to the bay containing Dragon's navigation sensors to open soon. They will be watching to make sure the door locks in place, a prerequisite for berthing with the space station.

SpaceX CEO and chief designer Elon Musk on the moment of this morning's launch:

"Every bit of adrenaline in my body released at that point," Musk said. "It's obviously an extremely intense moment. The main thing I was wondering was would we have a valve-related issue on launch and would the first stage perform nominally. Actually, it worked perfectly, so I was really glad to see that."

"We've never actually had solar arrays deploy in space, so it's the first time we've done solar arrays. A number of things could have gone wrong, but everything went right."
"Most of the company gathered around mission control," Musk said. "They're seeing the fruit of the labors and seeing if it's going to work. There's so much hope riding on that rocket, so when it worked and Dragon worked and solar arrays deployed, and people saw their handiwork in space and operating as it should, it was tremendous elation. For us, it's like winning the Super Bowl."

"Although there's a lot ahead to complete this mission, we're certainly off to a good start," NASA Administrator Charlie Bolden just told reporters.

"It's a great day for America," Bolden said. "It's actually a great day for the world. There are people who thought we had gone away, but we haven't gone away at all."

"Today was the first in a number of milestones in this mission. What a spectacular start. It was a picture-perfect launch. Everything on the vehicle is working well," Bolden said. "The next big milestone for us is about an hour for now when the GNC door on the DRagon module opens. That exposes the star trackers and the navigation instruments on the Dragon to deep space. More importantly, when that door opens, it has the grapple fixture on the inside of the door. We have to have the door locked in place in order to berth with the International Space Station."

The exact time of the bay door opening is 6:11 a.m. EDT (1011 GMT).

The White House has issued a statement from John Holdren, assistant to the president for science and technology.

"Congratulations to the teams at SpaceX and NASA for this morning’s successful launch of the Falcon 9 rocket from Cape Canaveral Air Force Station in Florida. Every launch into space is a thrilling event, but this one is especially exciting because it represents the potential of a new era in American spaceflight." "

Partnering with U.S. companies such as SpaceX to provide cargo and eventually crew service to the International Space Station is a cornerstone of the President’s plan for maintaining America’s leadership in space. This expanded role for the private sector will free up more of NASA’s resources to do what NASA does best -- tackle the most demanding technological challenges in space, including those of human space flight beyond low Earth orbit. I could not be more proud of our NASA and SpaceX scientists and engineers, and I look forward to following this and many more missions like it."

SpaceX founder Elon Musk tweeted: "Falcon flew perfectly!! Dragon in orbit, comm locked and solar arrays active!! Feels like a giant weight just came off my back :)"

A series of important test objectives remain to be achieved today, beginning with an absolute GPS navigation demo at 4:39 a.m. EDT (0839 GMT). The bay door containing the compartment with Dragon's navigation sensors and grapple fixture will open at 6:11 a.m. EDT (1011 GMT) for a series of functional checks.

View a schedule of the big events left for today.

And check out our story on the new systems Dragon will test in orbit beginning today.

T+plus 15 minutes. The Falcon 9 rocket and Dragon spacecraft reached an orbit with a high point of 346 kilometers, or 215 miles, a low point of 297 kilometers, or 184 miles, and an inclination of 51.66 degrees to the equator. All the values are close to prelaunch predictions.

Launch team members exchanged jubilent high-fives and officials at SpaceX headquarters in California let out raucous cheers when Dragon separated.

T+plus 12 minutes. The solar panels have fully extended, setting the stage for a two-day pursuit of the International Space Station!

T+plus 10 minutes, 35 seconds. SpaceX confirms the Dragon spacecraft is in orbit.

Dragon's two solar arrays should begin their deployment steps in less than two minutes. It takes about two minutes for the two solar panel wings to fully extend to their 54-foot length tip-to-tip.

DRAGON SEPARATION!

T+plus 9 minutes, 27 seconds. Second stage engine cutoff.

T+plus 8 minutes. One minute until the second stage Merlin engine is supposed to shut down as the vehicle reaches orbit. The rocket is aiming for an orbit with a low point of 192 miles, a high point of 211 miles, and an inclination of 51.6 degrees.

T+plus 7 minutes, 30 seconds. Falcon 9 is now traveling 1,000 kilometers northeast of Cape Canaveral at an altitude of 300 kilometers. All systems normal.

T+plus 6 minutes. No problems with the launch so far. The second stage is stable and a rocketcam is returning live video from the rocket.

T+plus 5 minutes, 30 seconds. Altitude is 220 kilometers, velocity is 3.4 kilometers per second, and downrange distance is 470 kilometers.

T+plus 5 minutes. Everything reported to be going well with this second stage engine firing. The Merlin vacuum engine uses an ultra-thin niobium nozzle extension for greater efficiency in the upper atmosphere.

The rocket's flight path is taking it northeast parallel to the U.S. East Coast.

T+plus 4 minutes. The Dragon's nose cone has been jettisoned, and SpaceX reports the Merlin vacuum engine is firing well.

T+plus 3 minutes, 15 seconds. The Falcon 9 first stage engines have cut off, the stages have separated, and the rocket's second stage Merlin vacuum engine has ignited for its nearly six-minute firing to reach orbital velocity.

T+plus 2 minutes. Now soaring at an altitude of more than 20 miles, the Falcon 9 rocket's first stage will shut down and jettison in about one minute. Two engines will be turned off first, followed a few moments later by the remaining seven engines.

T+plus 1 minute, 30 seconds. The Falcon 9 has surpassed Mach 1 as it soars into the upper atmosphere, trailing an orange flame high above Cape Canaveral.

T+plus 1 minute. The Falcon 9 rocket is approaching the speed of sound and the phase of maximum aerodynamic pressure.

LIFTOFF! The Falcon 9 rocket is soaring into space, sending Dragon on an historic commercial voyage to the International Space Station. The vehicle has cleared the tower.

T-minus 60 seconds and counting. In the final minute of the countdown, the flight computer will command checks of the first stage Merlin engine steering system and the Falcon 9 propellant tanks will be pressurized for flight. Thousands of gallons of water will also be dumped onto the launch pad deck to suppress the sound and acoustics of liftoff.

The command to start the ignition sequence for the first stage will be issued at T-minus 3 seconds, triggering the Merlin engines' TEA-TEB ignitor source moments before the powerplants actually ramp up to full power.

T-minus 2 minutes and counting. The SpaceX launch director and the Air Force Eastern Range have given their final approvals for liftoff of the Falcon 9 rocket at 3:44:38 a.m. EDT (0744:38 GMT).

T-minus 3 minutes and counting. The rocket's destruct system is on internal power and being armed, the ground TEA-TEB first ignition system is ready for launch, and liquid oxygen topping is being terminated.

T-minus 4 minutes, 30 seconds. Both stages of the Falcon 9 rocket are now running on internal power.

T-minus 5 minutes and counting. The Falcon 9 computer is aligned for flight. The automated sequence is now terminating loading of gaseous nitrogen into the second stage attitude control system.

T-minus 6 minutes and counting. The Falcon 9 computer and navigation system is configuring for flight and all nine Merlin engine pumps on the first stage are chilled in for ignition. The first stage fuel bleed sequence has started.

T-minus 7 minutes and counting. Within the next minute, the Falcon 9's flight computer will be commanded to its alignment state. The Merlin engine pumps are continuing to chill down.

T-minus 8 minutes and counting. The Dragon spacecraft is being placed on internal power.

T-minus 9 minutes and counting. Prevalves leading to the Falcon 9's Merlin first stage engines are opening, permitting super-cold liquid oxygen to flow into the engines to condition them for ignition.

T-minus 10 minutes and counting. The terminal countdown autosequence has started. Any hold after this point will result in an automatic abort.

T-minus 12 minutes. The launch team has verified all consoles are go for liftoff at 3:44:38 a.m. EDT (0744:38 GMT).

T-minus 15 minutes and counting. The final poll of the SpaceX launch team will begin in two minutes.

T-minus 20 minutes and counting. Track today's launch with this timeline of the Falcon 9's ascent into orbit.

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

With a half-hour left in this morning's countdown, all systems are currently in good shape for liftoff.

The International Space Station will be about 249 miles above Earth over the North Atlantic east of St. John's, Newfoundland. The Expedition 31 crew aboard the complex will be watching the Falcon 9 launch live.

T-minus 37 minutes. All downrange tracking stations report they are ready to support today's launch, and winds aloft are acceptable for liftoff.

T-minus 40 minute and counting. The Falcon 9 launch team is placing the vehicle in final setup.

The International Space Station flight control team just issued their "go" for launch.

Air Force weather officer Mike McAleenan reports there is "not really a threat at all for any kind of launch constraint" this morning. He forecasts a few clouds at 2,500 feet, a few clouds at 25,000 feet, southwest winds at 10 knots, and a temperature of 72 degrees Fahrenheit at launch time.

Overall, there is a less than 10 percent chance of weather violating launch constraints this morning.

T-minus 1 hour and counting. The launch team is standing by for a weather briefing from Air Force meteorologists. Final setup of the Falcon 9 rocket for launch begins in 20 minutes.

T-minus 1 hour, 20 minutes. All steps in the countdown up to this point, including fueling, C-band beacon tests, and checks of the Falcon 9's destruct system, have been completed.

The next event in the countdown comes at T-minus 40 minutes, when preps for liftoff will resume.

In addition to the standard optical and radar tracking systems stationed up and down Florida's Space Coast, a team from NASA's Langley Research Center has deployed a network of highly-calibrated cameras and thermal imagers to observe this morning's launch.

The Scientifically Calibrated In Flight Imagery, or SCIFLI, team has positioned an optical tracking system near Daytona Beach, Fla., north of the launch pad for the SpaceX Falcon 9 launch.

NASA's two former shuttle solid rocket booster recovery ships, the Freedom Star and Liberty Star, are sailing off the northeast coast of the United States.

X-band radar, long-range optical and thermal tracking systems mounted on ships will capture the Falcon 9 launch in its last phase of ascent, including second stage engine cutoff, Dragon separation, and deployment of the craft's solar arrays.

"We're looking at getting very high spatial resolution, high definition quality visual imaging during the launch as well as high spatial resolution thermal imaging from an infrared camera," said Tom Horvath, SCIFLI principal investigator at NASA Langley.

Celestial Computing Inc. of Boston equipped Freedom Star with a gyro-stabilized tracking mount and hardware.

"This ship-based imaging capability is unique," said Horvath. "NASA does not possess a shipboard gyro-stabilized tracker with the large aperture/long focal length optics coupled to state-of-the-art detectors."

T-minus 1 hour, 52 minutes. The strongback has been lowered to the launch position. All systems are go for launch at this point.

SpaceX says the countdown is proceeding on schedule this morning. Liftoff remains set for 3:44:38 a.m. EDT (0744:38 GMT).

T-minus 2 hours and counting. The strongback structure, a support structure that supports umbilical connections to the rocket, will soon be retracted about 15 degrees away from the Falcon 9.

The strongback is part of the Falcon 9's transporter-erector, a device which holds the rocket during rollout to the launch pad and erecting it vertical.

Follow along in the countdown with this timeline of the steps before liftoff.

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

T-minus 2 hours, 30 minutes. The Falcon 9 rocket should be undergoing communications checks with the Air Force Eastern Range now, and coming up should be testing of the booster's destruct system. But unlike other countdowns at the Cape, we have no way of confirming the completion of these key steps.

We have received confirmation that fueling is complete.

T-minus 3 hours and counting. Fueling of the Falcon 9 rocket should be nearing an end. Vapors streaming from the Falcon 9 rocket's first and second stage come from the boil-off of cryogenic liquid oxygen, which is kept at Minus-298 degrees Fahrenheit.

The liquid oxygen is stored in a 125,000-gallon sphere in the southeast quadrant of Complex 40. The rocket's RP-1 kerosene fuel is kept in cylinder-shaped tanks on the west side of the pad.

Engineers planned to load nearly 39,000 gallons of super cold liquid oxygen and almost 25,000 gallons of kerosene fuel into the first stage tanks. About 7,300 gallons of liquid oxygen and 4,600 gallons of kerosene will go into the second stage.

The propellants flow into the first stage through the launch mount at the base of the rocket. Kerosene and liquid oxygen will be pumped up the strongback umbilical tower to enter the second stage.

A few dozen engineers and managers are stationed inside the SpaceX Launch Control Center near the south gate to Cape Canaveral Air Force Station.

The weather outlook has improved to a 90 percent chance of acceptable weather for this morning's launch attempt. The forecast calls for a few clouds at 2,500 feet and scattered clouds at 25,000 feet with a temperature of 75 degrees Fahrenheit.

The countdown is underway at Cape Canaveral, and the launch team is getting ready to start loading the two-stage Falcon 9 rocket with refined kerosene and liquid oxygen propellants.

The process should begin at about 11:54 p.m. EDT (0354 GMT) with the loading of liquid oxygen into both stages of the booster. A highly-refinded kerosene - known as RP-1 - will be pumped into the rocket beginning 10 minutes later.

Follow along in the countdown with this timeline of the steps before liftoff.

SpaceX's Dragon spacecraft will almost immediately begin testing new components following its deployment in orbit, extending solar arrays, demonstrating sensors for precise navigation, and proving its safety procedures during the capsule's first day in space.

SpaceX founder Elon Musk is quick to note the Dragon spacecraft has flown in orbit before. An earlier Dragon capsule circled Earth twice and returned to Earth in December 2010, becoming the first privately-owned craft to reach orbit and land.

But that mission used battery power, and it did not approach the International Space Station. Both the 2010 flight and the upcoming mission are aimed at fulfilling NASA demonstration objectives for subsequent flights to deliver cargo to the complex.

The spacecraft on SpaceX's next flight will test new systems minutes after launch, beginning with deployment of two solar array wings vital to generating electricity for the mission.

Read our full story on the changes to the Dragon spacecraft for this mission.

SpaceX is pressing ahead with another launch attempt early Tuesday, aiming for liftoff at 3:44 a.m. EDT (0744 GMT) after engineers made repairs to an engine on the Falcon 9 rocket over the weekend.

Computers aborted the countdown a half-second before launch Saturday due to high pressure in one of the Falcon 9 rocket's nine first stage engines. Engineers opted to replace a check valve in the first stage's center engine - engine No. 5 - after inspections.

"The failed valve was replaced on Saturday and after thorough analysis the vehicle has been cleared for launch," SpaceX said in a statement.

Liftoff is targeted for 3:44:38 a.m. EDT (0744:38 GMT), the moment Earth's rotation brings Cape Canaveral's Complex 40 launch pad in the path of the space station's orbit.

The Dragon spacecraft should be captured by the International Space Station's robotic arm at about 8:06 a.m. EDT (1206 GMT) on Friday.

The weather forecast shows an 80 percent chance of acceptable conditions for Tuesday's launch opportunity. The primary threat is violating the cumulus cloud rule.

Tropical Storm Alberto, now located about 175 miles east of Jacksonville, Fla., will not be a factor for Tuesday morning's launch, according to U.S. Air Force forecasters.

Dry air is prevalent over the Space Coast, and light winds are expected from the west-northwest today becoming variable overnight," Air Force meteorologists wrote in a forecast synopsis. "Given this favorable weather pattern, there is only a slight chance of isolated showers which will likely be offshore along the Gulf Stream."

The outlook calls for a few clouds at 3,000 feet and scattered clouds at 12,000 feet. The temperature is forecast to be 75 degrees Fahrenheit, and winds will be variable at 5 knots.

Countdown procedures tonight should begin around 8:30 p.m. EDT (0030 GMT) with the power-up of the Falcon 9 rocket and Dragon spacecraft.

SpaceX has replaced a check valve in one of the Falcon 9 rocket's first stage engines, but analysis continues before the company proceeds with another launch attempt Tuesday morning.

Engineers traced the cause of Saturday's aborted launch countdown to a valve in engine No. 5, the center engine in the Falcon 9 rocket's tic-tac-toe layout of Merlin engines on the first stage.

Computers detected higher-than-allowable pressures inside the engine's thrust chamber after ignition and ordered a cutoff of the countdown a half-second before liftoff.

The launch opportunity Tuesday comes at 3:44:38 a.m. EDT (0744:38 GMT).

The Falcon 9 rocket remains vertical at Cape Canaveral's Complex 40 launch pad.

"Today's launch was aborted when the flight computer detected slightly high pressure in the engine 5 combustion chamber," SpaceX said in a statement. "We have discovered root cause and repairs are underway."

"During rigorous inspections of the engine, SpaceX engineers discovered a faulty check valve on the Merlin engine. We are now in the process of replacing the failed valve. Those repairs should be complete tonight. We will continue to review data on Sunday. If things look good, we will be ready to attempt to launch on Tuesday, May 22, at 3:44 a.m. Eastern."

SpaceX has traced this morning's pressure spike in Engine No. 5 to a turbopump valve, according to a tweet by company founder and CEO Elon Musk.

"Engine pressure anomaly traced to turbopump valve," Musk wrote. "Replacing on engine 5 and verifying no common mode."

The next launch attempt is expected Tuesday at 3:44 a.m. EDT (0744 GMT) after this morning's countdown was aborted a half-second before liftoff.

The plan is for technicians to visually inspect the exterior of Engine No. 5, which is in the center position in the tic-tac-toe pattern of engines on the Falcon 9 first stage.

The team may also conduct boroscope inspections of the thrust chamber, but Shotwell said SpaceX is hopeful the engine will not need disassembly or replacement. Removing the engine adding a fresh unit would take several days.

The plan is for today's inspections to be conducted while the 15-story booster remains in a vertical position atop the launch pad. The Falcon 9 may be lowered and moved back into the hangar later to protect the rocket and Dragon spacecraft from weather conditions until another launch opportunity nears.

Gwynne Shotwell, president of SpaceX, says technicians will inspect Engine No. 5 on the Falcon 9's first stage beginning at around 12 p.m. EDT (1600 GMT). Further data analysis of readings from the engine during the ignition prompted SpaceX to order hands-on inspections before clearing the rocket for liftoff.

The next launch opportunity is still Tuesday at 3:44 a.m. EDT (0744 GMT), but that is pending the outcome of today's inspection.

Shotwell said the abort was triggered one-half second before liftoff after all nine engines were ignited.

Alan Lindenmoyer, NASA's commercial crew and cargo manager, said there should be another launch opportunity May 23 at 3:22 a.m. EDT (0722 GMT).

The target launch time for Tuesday is 3:44:34 a.m. EDT (0744:34 GMT).

The rocket is being drained of propellant as the launch team backs out of this morning's countdown following an abort moments before liftoff at 4:55 a.m. EDT (0855 GMT).

NASA plans a post-scrub news conference at 6:30 a.m. EDT (1030 GMT). We'll carry it live.

The strongback has been returned to a position next to the Falcon 9.

SpaceX reports the problem triggering the abort was a high chamber pressure reading on Engine No. 5 of the first stage. Aborts are common in Falcon 9 countdowns when computers recognize a limit out of a predefined range. Sometimes the fix is as simple as adjusting the acceptable range in the computer, but with a one-second launch window this morning, there will no opportunity to resolve the issue today.

The launch team is safing the rocket and Dragon spacecraft after this morning's abort. The next launch opportunity is Tuesday morning at 3:44 a.m. EDT (0744 GMT), assuming whatever issue caused the abort is resolved in time.

It's not clear what caused the abort just before liftoff, but this will mean Falcon 9 will not launch today. The rocket had a near-instantaneous launch window in which it could fly today.

ABORT. The ignition sequence started, but there was a countdown cutoff before launch.

T-minus 60 seconds and counting. In the final minute of the countdown, the flight computer will command checks of the first stage Merlin engine steering system and the Falcon 9 propellant tanks will be pressurized for flight. Thousands of gallons of water will also be dumped onto the launch pad deck to suppress the sound and acoustics of liftoff.

The command to start the ignition sequence for the first stage will be issued at T-minus 3 seconds, triggering the Merlin engines' TEA-TEB ignitor source moments before the powerplants actually ramp up to full power.

T-minus 2 minutes and counting. The SpaceX launch director and the Air Force Eastern Range have given their final approvals for liftoff of the Falcon 9 rocket at 4:55 a.m. EDT (0855 GMT).

T-minus 3 minutes and counting. The rocket's destruct system is on internal power and being armed, the ground TEA-TEB first ignition system is ready for launch, and liquid oxygen topping is being terminated.

T-minus 4 minutes, 30 seconds. Both stages of the Falcon 9 rocket are now running on internal power.

T-minus 5 minutes and counting. The Falcon 9 computer is aligned for flight. The automated sequence is now terminating loading of gaseous nitrogen into the second stage attitude control system.

T-minus 6 minutes and counting. The Falcon 9 computer and navigation system is configuring for flight and all nine Merlin engine pumps on the first stage are chilled in for ignition. The first stage fuel bleed sequence has started.

T-minus 7 minutes and counting. Within the next minute, the Falcon 9's flight computer will be commanded to its alignment state. The Merlin engine pumps are continuing to chill down.

T-minus 8 minutes and counting. The Dragon spacecraft is being placed on internal power.

T-minus 9 minutes and counting. Prevalves leading to the Falcon 9's Merlin first stage engines are opening, permitting super-cold liquid oxygen to flow into the engines to condition them for ignition.

T-minus 10 minutes and counting. The terminal countdown autosequence has started. Any hold after this point will result in an automatic abort.

T-minus 12 minutes. The launch team has verified all consoles are go for liftoff at 4:55:26 a.m. EDT (0855:26 GMT). The International Space Station is now flying just east of the launch site, visible in the skies over Cape Canaveral.

T-minus 15 minutes. The final poll of the SpaceX launch team occurs in two minutes.

T-minus 28 minutes. At the time of launch, the International Space Station will be over the Atlantic Ocean between Ireland and Canadian maritime island of Labrador.

T-minus 37 minutes. All downrange tracking stations report they are ready to support today's launch, and winds aloft are acceptable for liftoff.

T-minus 40 minutes. Countdown activities have resumed after a lull following fueling.

T-minus 42 minutes. The precise launch time targeted this morning is 4:55:26 a.m. EDT (0855:26 GMT), the moment Earth's rotation brings the plane of the space station's orbit over the Complex 40 launch pad.

T-minus 53 minutes. All weather conditions are currently go for launch.

T-minus 60 minutes and counting. There continue to be no problems in the countdown.

SpaceX's Dragon spacecraft has flown in orbit before, but the capsule on the company's next flight will start testing new systems almost immediately after launch, beginning with deployment of two solar array wings vital to generating electricity for the mission.

Check out a fact sheet on the Dragon spacecraft.

The Dragon which flew on SpaceX's first NASA-sponsored demonstration mission in December 2010 looks a lot like the craft launching Saturday. But the new Dragon is much more sophisticated, incorporating solar energy, advanced navigation sensors, and a communications link with the International Space Station.

A new flight computer will also debut on the mission.

"This time, it is much more complicated," said Alan Lindenmoyer, manager of NASA's commercial crew and cargo program. "This Dragon is going to be the first spacecraft that SpaceX has developed with a heat rejection and power generation system. That means it's the first time you will see the spacecraft deploy its solar arrays in order to generate power."

Aerodynamic shields covering the solar arrays will be jettisoned moments after the Dragon spacecraft deploys from the upper stage of the Falcon 9 rocket at T+plus 9 minutes, 49 seconds.

The solar array extension sequence will begin at T+plus 11 minutes, 53 seconds. Once fully unfurled, the panels will stretch 54 feet tip-to-tip.

"It's a greatly enhanced Dragon," said Gwynne Shotwell, SpaceX president. "It has a power generation system, meaning solar arrays. We did not have those on the first mission."

"The other difference is less physical, but the software is dramatically more complex this mission. And we're also dual redundant on every system."

A test of the Dragon's absolute GPS navigation system begins less than an hour into the mission.

Dragon will open its instrument bay door at T+plus 2 hours, 26 minutes, exposing the craft's grapple fixture, LIDAR laser rendezvous sensor, and thermal imaging camera.

"This Dragon will be the first time you'll see a flight of the rendezvous proximity systems, the LIDARs and the imagers that are requirement to give the proper range and range rate information that is required as the vehicle gets closer to the station," Lindenmoyer said.

All of those systems will be required for Dragon's rendezvous and berthing with the space station, which is scheduled for three days after launch. A close flyby of the complex is planned two days into the mission to test the systems thoroughly.

"[This Dragon] has a charging system, cooling pumps, a thermal radiator, cabin circulation fans, and all this equipment that's necessary for meeting the requirements for mating with the International Space Station," Lindenmoyer said.

Another important change on this flight is the addition of a berthing port at the forward end of the spacecraft. It will be the attach point with the space station's Harmony module.

"This is the first time that SpaceX is flying a common berthing mechanism," Lindenmoyer said. "This is the mechanism used on all vehicles that are berthed to the space station. That is assembled onto this Dragon, as well as a hatch that SpaceX developed so we can enter the vehicle."

"It's also the first time we'll see the operation of the new flight computers on the vehicle," Lindenmoyer said. "These are specially redundant computers in order to meet the requirements for the space station. Those will be demonstrated on this mission."

"This will also be the first time we will see the trunk separated from the Dragon spacecraft. On the previous mission, Dragon was separated right at the trunk plane. This time, the trunk and the Dragon will be separated from the second stage, fly all the way to station, and then it will be separated before the re-entry," Lindenmoyer said.

"We'll also see the first-time operation of the SpaceX-developed COTS UHF Communications Unit, which is a radio that SpaceX built which has already been flown and pre-positioned on the space station," Lindenmoyer said. "This is going to allow communications between the station and the Dragon during the mission, as well as a crew command panel that can give specific commands to the vehicle. There are a lot of firsts."

"The first flight was tough," he said. "This is even more complex."

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T-minus 1 hour, 28 minutes. All steps in the countdown up to this point, including fueling, C-band beacon tests, and checks of the Falcon 9's destruct system, have been completed.

The next event in the countdown comes at T-minus 40 minutes, when preps for liftoff will resume.

T-minus 1 hour, 52 minutes. The strongback has been lowered to the launch position. All systems are go for launch at this point.

T-minus 2 hours and counting. The strongback structure, a support structure that supports umbilical connections to the rocket, will soon be retracted about 15 degrees away from the Falcon 9.

The strongback is part of the Falcon 9's transporter-erector, a device which holds the rocket during rollout to the launch pad and erecting it vertical.

All of Falcon 9's propellant tanks are full, and liquid oxygen will continue to be slowly pumped aboard to replenish the rocket's supply as the cryogenic liquid boils off into the warm atmosphere.

Liquid oxygen loading will be terminated at T-minus 3 minutes.

Loading of RP-1 kerosene fuel into the Falcon 9 rocket is complete.

Fueling of the Falcon 9 rocket has begun at Cape Canaveral's Complex 40.

The liquid oxygen is stored in a 125,000-gallon sphere in the southeast quadrant of Complex 40. The rocket's RP-1 kerosene fuel is kept in cylinder-shaped tanks on the west side of the pad.

Engineers plan to load nearly 39,000 gallons of super cold liquid oxygen and almost 25,000 gallons of kerosene fuel into the first stage tanks. About 7,300 gallons of liquid oxygen and 4,600 gallons of kerosene will go into the second stage.

The propellants flow into the first stage through the launch mount at the base of the rocket. Kerosene and liquid oxygen will be pumped up the strongback umbilical tower to enter the second stage.

A few dozen engineers and managers are stationed inside the SpaceX Launch Control Center near the south gate to Cape Canaveral Air Force Station.

The weather forecast still shows a 70 percent chance of acceptable weather for liftoff this morning. All weather rules are currently green.

The countdown is underway at Cape Canaveral, and the launch team is getting ready to start loading the two-stage Falcon 9 rocket with refined kerosene and liquid oxygen propellants.

The process should begin at about 1:05 a.m. EDT (0505 GMT) with the loading of liquid oxygen into both stages of the booster. A highly-refinded kerosene - known as RP-1 - will be pumped into the rocket beginning 10 minutes later.

Follow along in the countdown with this timeline of the steps before liftoff.

Observers from Cape Canaveral to Capitol Hill will be keenly watching SpaceX's commercial voyage to the International Space Station launching Saturday, and although officials bill the mission as a test flight, its outcome could buoy or blunt support for a private space race in human spaceflight.

Read our full story on the political stakes in SpaceX's test flight to the International Space Station.

Also read excerpts from our interview with SpaceX founder, CEO and chief designer Elon Musk.

Check out photos from today of the Falcon 9 rocket poised on the launch pad.

Also view photos from last night's rollout of the rocket.

The weather outlook continues to be favorable for liftoff tomorrow, according to Air Force forecasters.

There remains a 70 percent chance of acceptable weather conditions for the instantaneous launch opportunity at 4:55 a.m. EDT (0855 GMT).

"Diffluent flow and cold temperatures aloft coupled with a weak surface boundary over Central Florida will result in one more day of thunderstorms including a risk for severe winds and hail with all the dynamics in place," meteorologists wrote in a forecast synposis.

"The surface boundary is forecast to move through Friday turning our winds northerly then easterly, reducing the rain and lightning threat tomorrow and Saturday. Persistent onshore flow from the east favors a chance of light rain showers, generally in the morning hours, but lightning is rare in this weather regime. In the event of a launch delay, the winds gradually become southeasterly. Southeast flow results in similar morning rain shower conditions, but the showers are typically a little heavier as conditions are more unstable."

"The primary concern for launch day and in the event of a delay is the cumulus cloud rule."

Scattered clouds are forecast at 2,500 feet, with light easterly winds and a temperature between 73 degrees and 75 degrees Fahrenheit.

SpaceX moved the Falcon 9 rocket to the launch pad overnight and hydraulically hoisted the 15-story rocket vertical, setting the stage for final preps for liftoff Saturday at 4:55 a.m. EDT (0855 GMT).

After launch officials gave the green light for continuing with launch preparations yesterday, workers at Complex 40 last night opened the doors of SpaceX's hangar and towed the Falcon 9 and its mobile transporter the 600-foot distance to the pad.

The two-stage booster was attached to a transporter-erector, a specially-built apparatus that doubles as a device to lift the rocket vertical and provide umbilical connections to the rocket during the countdown.

The strongback also provides structural support to the rocket from high winds and other weather.

Once in place at the launch pad, the transporter-erector was to be structurally mounted to the pad and plugged into communications, electrical, fueling and pressurization systems through a series of pins and flanges.

The flanges link the rocket with ground storage tanks containing liquid oxygen, kerosene fuel, helium, gaserous nitrogen and the first stage ignitor source called triethylaluminum-triethylborane, better known as TEA-TEB.

The Falcon 9 rocket is integrated with the transporter inside the SpaceX hangar, including attachments to the launcher's propellant plumbing and avionics systems.

Comparatively simple connections are made in the outdoors once the rocket is at the pad. It's part of the Falcon 9's streamlined processing infrastructure, which is designed to ultimately support transportation to the launch pad within a few hours before blastoff.

SpaceX and NASA officials will hold a 1 p.m. EDT (1700 GMT) news conference today to review the status of launch preparations.

The launch readiness review has concluded with a "go" for launch of SpaceX's pivotal test flight to the International Space Station, according to company founder, CEO and chief designer Elon Musk.

SpaceX will hold a final launch readiness review today to clear the Falcon 9 rocket and Dragon spacecraft for liftoff Saturday on a long-awaited test flight to the International Space Station.

Engineers plan to roll the 157-foot-long Falcon booster to the launch pad at Cape Canaveral following the review this evening. The two-stage rocket is now inside the company's hangar at Complex 40, a seaside launch facility at Cape Canaveral Air Force Station.

SpaceX is also expected to release a public statement after the conclusion of the launch readiness review.

NASA managers gave their go-ahead for the mission Tuesday after a readiness review in Houston, deeming SpaceX's Dragon capsule ready to attempt to become the first commercial craft to visit the space station.

About 1,146 pounds of cargo packed inside the Dragon spacecraft will be unloaded by the station's six-person crew after the vehicle arrives May 22. The flight plan calls for the Dragon to stay attached to the space station nine days.

The first official outlook issued by the U.S. Air Force's 45th Weather Squadron calls for a 30 percent chance of violating weather constraints for the Falcon 9 rocket's launch Saturday.

The primary concern is for cumulus clouds near the launch site.

"Diffluent flow aloft coupled with a weak surface boundary is slowly migrating through the peninsula, creating a wet pattern with widespread thunderstorms for the next few days," Air Force forecasters wrote in the outlook. "As the boundary moves through central Florida, our winds will become northerly then easterly, reducing the rain and lightning threat for Friday and Saturday."

"Persistent onshore flow from the east favors a chance of light rain showers, generally in the morning hours, but lightning is rare in this weather regime. In the event of a launch delay, the winds gradually become southeasterly. Southeast flow results in similar morning rain shower conditions, but the showers are typically a little heavier as conditions are more unstable."

For SpaceX's backup launch opportunity May 22, there is a 40 percent chance of weather violating launch criteria.

The outlook for Saturday calls for scattered cumulus clouds at 2,500 feet, isolated rain showers, a visibility of 7 miles, winds out of the east at 10 knots, and a temperature between 73 degrees and 75 degrees Fahrenheit.

Launch on Saturday is scheduled for 0855 GMT (4:55 a.m. EDT) from Cape Canaveral Air Force Station.

NASA's space station management team on Tuesday approved plans to launch SpaceX's first commercial flight to the complex Saturday, signing off on software changes governing the privately-built capsule's final approach to the orbiting outpost.

The 157-foot-long rocket will roll out from its hangar at Cape Canaveral Air Force Station on Thursday to be rotated vertical on top of its seaside launch pad.

Liftoff is set for Saturday at 0855 GMT (4:55 a.m. EDT). The Dragon spacecraft should be grappled by the space station's robot arm at about 1211 GMT (8:11 a.m. EDT) on May 22.

Officials have delayed the launch from the beginning of 2012 to review the Dragon spacecraft's software, particularly the code which controls the capsule's final rendezvous with the space station and detects and responds to system failures during the Dragon's delicate maneuvers near the complex.

Tuesday's Flight Readiness Review evaluated results from software tests over the last few weeks.

"The teams reported all remaining work had been completed and everyone is 'go' for launch," NASA posted in a statement on its website. "SpaceX reported that the Dragon spacecraft and its systems are ready for the mission."

Mike Horkachuck, NASA's project executive for SpaceX, said the space agency is comfortable with SpaceX's readiness for the flight.

"We closed out a lot of work over the last couple of weeks," Horkachuck said. "There was a lot of software that's been rechecked and validated by the space station program. They did an end-to-end test of the data system, as well as another stage test, checking all the software functionality as it relates to the space station. We looked at a lot of the changes that have been done on the software and gotten comfortable that all those changes were acceptable."

Astronauts Don Pettit and Andre Kuipers replaced a GPS navigation unit aboard the space station Monday after ground controllers noticed a failure last week. The GPS instrumentation is mandatory for Dragon's navigation system during the craft's final approach to the complex.

SpaceX plans an internal Launch Readiness Review on Thursday.

The flight is a crucial test for SpaceX, which holds a $1.6 billion contract for 12 cargo delivery flights to the space station. SpaceX could begin executing operational flights under the contract later this year if the upcoming test mission goes as planned.

The need for additional software assurance testing has again delayed the launch of SpaceX's commercial demonstration flight to the International Space Station, the company announced Friday, until at least May 19.

"SpaceX and NASA are nearing completion of the software assurance process, and SpaceX is submitting a request to the Cape Canaveral Air Force Station for a May 19 launch target with a backup on May 22," the company said in a statement released Friday.

"Thus far, no issues have been uncovered during this process, but with a mission of this complexity we want to be extremely diligent," the statement said.

Launch of the Falcon 9 rocket on May 19 would occur at approximately 4:55 a.m. EDT (0855 GMT).

Read our full story.

The long-awaited launch of a commercial cargo ship bound for the International Space Station almost certainly will be delayed from May 7 to at least May 10 and possibly longer, sources said late Tuesday, to give company engineers additional time to complete pre-flight tests and checkout.

SpaceX founder and chief designer Elon Musk discussed the results of an engine test firing Monday and the overall status of launch processing with senior NASA managers in a Tuesday afternoon teleconference that included Bill Gerstenmaier, NASA's director of space operations, and Mike Suffredini, the NASA space station program manager at the Johnson Space Center in Houston.

Multiple NASA sources said the current May 7 target had been ruled out, although there was confusion in some quarters as to whether there might be a slim chance of keeping on schedule if additional analyses could be completed in time.

Read our full story.

Nine kerosene-fueled engines mounted on the bottom of SpaceX's Falcon 9 rocket fired at full power for two seconds Monday, completing a key test on a Cape Canaveral launch pad before departing to the International Space Station as soon as May 7.

The engines ignited at 4:15 p.m. EDT (2015 GMT), firing for about three-and-a-half seconds and reaching full thrust for about two seconds. Called a static fire, or hotfire, the test verifies the engines, rocket and ground systems are ready for launch.

Read our full story.

SpaceX founder, CEO and chief designer just tweeted: "Woohoo, rocket hold down firing completed and all looks good!!"

SpaceX reports the Falcon 9 engines reached full thrust for two seconds, deeming today's static fire a success.

Engineers will carefully review all data from the hotfire in the next few days before the mission's scheduled May 7 launch to the International Space Station.

Fire and smoke erupted around the base of the Falcon 9 rocket as expected, and we're awaiting results of the test.

T-minus 1 minute and counting. A steering check of the first stage engines will occur at T-minus 50 seconds, and propellant tanks will pressurize for ignition at T-minus 40 seconds.

T-minus 3 minutes and counting. The launch team is configuring the first stage's TEA-TEB ignitor system and has stopped loading liquid oxygen into the rocket in preparation for ignition.

T-minus 4 minutes and counting. All systems remain go for the hotfire test.

T-minus 5 minutes and counting. The launch vehicle is being transitioned to internal power. And Dragon is configured for battery charging via its solar arrays as if it were in space.

T-minus 6 minutes and counting. Launch controllers report the Dragon spacecraft is operating on internal power. The first stage fuel bleed cycle is underway and the navigation system is being aligned for flight.

T-minus 8 minutes and counting. The pumps of the Falcon's nine Merlin first stage engines are being conditioned for ignition. And the Dragon is being prepared for transitioning onto internal power supplies.

T-minus 10 minutes and counting. The terminal countdown has started for the second time today.

T-minus 14 minutes and counting. The countdown has resumed for a static fire attempt at about 4:15 p.m. EDT (2015 GMT).

The launch team reports a limit was "improperly set" that caused the hold in the last countdown. SpaceX says the limit is being reset for another static fire attempt today.

SpaceX says they "may recycle and try again today" after the last-minute hold in the countdown.

The countdown clock has been recycled to T-minus 13 minutes and holding. This is the standard point at which the countdown can resume before going into the terminal phase.

If this problem had occurred on launch day, the mission would be scrubbed for the day because the Falcon 9 rocket has only an instantaneous window to reach the International Space Station.

And SpaceX can only launch every three days or so to ensure enough propellant is aboard the Dragon spacecraft for a series of demonstration maneuvers and tests on the way to rendezvousing with the space station.

SpaceX has tweeted they are "reviewing data" following this hold in the countdown.

Launch controllers are studying the cause of the hold.

HOLD. The countdown was halted at T-minus 47 seconds.

T-minus 1 minute and counting. A steering check of the first stage engines will occur at T-minus 50 seconds, and propellant tanks will pressurize for ignition at T-minus 40 seconds.

T-minus 3 minutes and counting. The launch team is configuring the first stage's TEA-TEB ignitor system and has stopped loading liquid oxygen into the rocket in preparation for ignition.

T-minus 5 minutes and counting. The launch vehicle is being transitioned to internal power.

T-minus 5 minutes, 30 seconds. Dragon is being configured to battery charging via its solar arrays once in space.

T-minus 6 minutes and counting. The first stage fuel bleed cycle is underway and the navigation system is being aligned for flight.

T-minus 7 minutes and counting. Launch controllers report the Dragon spacecraft is operating on internal power.

T-minus 8 minutes and counting. The pumps of the Falcon's nine Merlin first stage engines are being conditioned for ignition. And the Dragon is being prepared for transitioning onto internal power supplies.

T-minus 10 minutes and counting. The countdown has entered the terminal phase.

T-minus 11 minutes and counting. The launch director has given approval to enter the terminal countdown at T-minus 10 minutes.

T-minus 13 minutes and counting. Managers are polling the launch team to verify their readiness to enter the terminal countdown.

A readiness poll at the T-minus 20 minute mark indicated all systems are go for today's static fire.

T-minus 20 minutes and counting. The Falcon 9 rocket's nine Merlin first stage engines will be ignited today for 3.5 seconds, enough time to verify systems on the ground and the launch vehicle responsible for the final few seconds of the countdown.

The engines will be at full power for approximately 2 seconds.

The engines will produce nearly 1 million pounds of total sea level thrust, more than four times the power of a 747 jumbo jet at full throttle. The engines consume about 3,200 pounds of kerosene and liquid oxygen propellants per second, according to SpaceX.

The 157-foot-tall Falcon 9 rocket is fueled and the strongback support structure is retracted, NASA's Kennedy Space Center just tweeted. There are no issues reported for an on-time static firing of the rocket's nine Merlin 1C engines at 3 p.m. EDT (1900 GMT).

SpaceX reports everything is on schedule for today's static fire, which is set for 3 p.m. EDT (1900 GMT).

Engineers will fill propellant tanks in both stages of the Falcon 9 rocket this afternoon. The stages are powered by Merlin engines burning refined kerosene fuel and liquid oxygen.

The liquid oxygen is stored in a 125,000-gallon sphere in the southeast quadrant of Complex 40. The rocket's RP-1 kerosene fuel is kept in cylinder-shaped tanks on the west side of the pad.

Engineers plan to load nearly 39,000 gallons of super cold liquid oxygen and almost 25,000 gallons of kerosene fuel into the first stage tanks. About 7,300 gallons of liquid oxygen and around 4,600 gallons of kerosene will go into the second stage.

The propellants flow into the first stage through the launch mount at the base of the rocket. Kerosene and liquid oxygen will be pumped up the strongback umbilical tower to enter the second stage.

On the eve of a crucial preflight firing of its nine first stage engines, SpaceX's Falcon 9 rocket was towed to the launch pad at Cape Canaveral on Sunday.

Mounted on a transporter-erector device, the 157-foot-long booster was transported 600 feet from the hangar to the launch pad at Complex 40. A SpaceX spokesperson confirmed the rollout Sunday.

The transporter-erector was pulled along rail tracks. Once in place at the launch pad, the transporter-erector structurally mounted to the pad and plugged into communications, electrical, fueling and pressurization systems through a series of pins and flanges.

The flanges link the rocket with ground storage tanks containing liquid oxygen, kerosene fuel, helium, gaserous nitrogen and the first stage ignitor source called triethylaluminum-triethylborane, better known as TEA-TEB.

The Falcon 9 rocket is integrated with the transporter inside the SpaceX hangar, including attachments to the launcher's propellant plumbing and avionics systems.

Comparatively simple connections are made in the outdoors once the rocket is at the pad. It's part of the Falcon 9's streamlined processing infrastructure, which is designed to ultimately support transportation to the launch pad within a few hours before blastoff.

Hydraulic pistons lifted the rocket to a vertical position on the pad later Sunday. It was not clear whether the Falcon 9 would be lowered again before Monday's practice countdown and engine firing.

The rehearsal is the final prelaunch simulation of the countdown, and the SpaceX team will conduct the test from the company's Launch Control Center at the south end of Cape Canaveral Air Force Station near Port Canaveral.

The countdown will start in the late morning at Cape Canaveral, leading to a two-second ignition of the Falcon 9's Merlin 1C first stage engines at 3 p.m. EDT (1900 GMT).

Check out photos of the Dragon spacecraft being connected with the Falcon 9 rocket.

Engineers connected SpaceX's Dragon spacecraft to its Falcon 9 launcher Thursday, setting the stage for a busy weekend of preparations for a brief firing of the rocket's nine main engines on the launch pad Monday.

The Falcon 9 rocket's Merlin 1C first stage engines will fire for about two seconds in a standard pre-launch test of the fully assembled rocket before liftoff.

The hotfire is scheduled for the conclusion of a practice countdown at 3 p.m. EDT (1900 GMT) on launch pad 40 at Cape Canaveral Air Force Station, Fla. SpaceX plans to webcast the event starting at 2:30 p.m. EDT.

Read our full story.

SpaceX engineers have completed loading hypergolic maneuvering propellant into the Dragon capsule at Cape Canaveral, according to a company spokesperson.

The propellant will fuel the Dragon spacecraft's 18 Draco thrusters during the ship's flight to the International Space Station. The thrusters, which each generate up to 90 pounds of thrust, will regulate the craft's approach to the space station.

The Draco thrusters also control the Dragon's attitude, or orientation, throughout the mission. They can fire in bursts as short as a few milliseconds or for many minutes to adjust the capsule's orbit and return it to Earth, according to SpaceX.

SpaceX designed and built the Dragon thrusters in-house.

The Draco thrusters burn monomethyl hydrazine fuel and nitrogen tetroxide oxidizer. The Dragon capsule contains eight propellant tanks - four each for the fuel and oxidizer.

Launch preparations are continuing while SpaceX and NASA officials review the results of hardware-in-the-loop testing, which validates the performance of the Dragon spacecraft during the final phase of its approach to the space station.

Launch remains set for May 7 at 9:38 a.m. EDT (1338 GMT).

The gumdrop-shaped capsule and its unpressurized trunk section will be rotated from a vertical stand to a horizontal position Thursday for attachment to the Falcon 9 rocket's second stage inside SpaceX's hangar at Cape Canaveral's launch pad 40, said Kirstin Brost Grantham, a company spokesperson.

An aerodynamic nose cone will be added Friday to the tip of the Dragon spacecraft.

The Falcon 9 rocket will then be towed to the launch pad for a countdown rehearsal and hotfire of the nine Merlin 1C first stage engines, which will ignite for several seconds Monday to check their health and the entire launcher's readiness for flight.

The test flight will attempt to berth with the space station, making the Dragon the first commercial vehicle to visit the $100 billion complex. Future Dragon missions will deliver vital supplies to the space station and return equipment to Earth.

While engineers analyze and tweak software coding, SpaceX will continue making physical preparations to the Dragon spacecraft and Falcon 9 booster for a commercial launch to the International Space Station as soon as May 7, officials said Tuesday.

Managers on Tuesday officially reset the flight's target launch date to May 7. The precise launch opportunity will be at 9:38 a.m. EDT (1338 GMT). SpaceX could make a second launch attempt May 10 if officials are comfortable with flying the Dragon mission before the arrival of three astronauts aboard a Russian Soyuz spacecraft.

SpaceX decided Monday to delay the launch from April 30 to allow for more hardware-in-the-loop testing and proper data reviews. The company has conducted extensive software testing since last year to meet NASA's stringent safety requirements for approaching the space station.

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Launch of SpaceX's commercial Dragon cargo freighter to the International Space Station will be delayed until at least early May to give engineers more time to wring out the craft's software, the company announced Monday.

The launch of Dragon, aiming to become the first commercial spacecraft to reach the space station, was scheduled from Cape Canaveral on April 30, following three days later by ship's planned approach and berthing with the orbiting laboratory.

A review of comprehensive testing of the spacecraft Monday prompted officials to order a delay in the flight by about one week. A potential launch opportunity is available as soon as May 7, according to a NASA source familiar with the mission.

Read our full story.

NASA and SpaceX officials will proceed with preparations for the scheduled April 30 launch of a commercial demo flight to the International Space Station, managers decided during a Flight Readiness Review on Monday. But more software verifications are needed before officials give final approval for the mission.

The Flight Readiness Review concluded SpaceX's launch date target of April 30 is realistic, but more discussions are planned in the coming days before final launch preparations begin.

An internal SpaceX readiness review is on tap for April 22, followed by a meet-up between SpaceX and NASA officials April 23.

Bill Gerstenmaier, NASA's associate administrator for human exploration and operations, said more "in-the-loop" testing between hardware and software is scheduled over the next few days.

There are also several software validation steps remaining before flight, according to Mike Suffredini, NASA's space station program manager.

Liftoff on April 30 would occur in a brief launch opportunity at 12:22 p.m. EDT (1622 GMT). The Dragon spacecraft will berth with the space station May 3.

Engineers are meeting today to set an official launch date for the demonstration flight of SpaceX's Dragon cargo capsule, which is set to fly to the International Space Station after blasting off as soon as April 30.

NASA will hold a press conference this afternoon to discuss the outcome of the Flight Readiness Review, which will assess the mission's preparations and open issues before deciding whether the flight can go forward.

SpaceX is targeting launch at 12:22 p.m. EDT (1622 GMT) on April 30 from Cape Canaveral, Fla.

Workers will roll the Falcon 9 rocket to the launch pad a few days before the flight for a brief hotfire of the booster's nine Merlin first stage engines, verifying they are ready for liftoff.

The upcoming flight is a demonstration mission under NASA's Commercial Orbital Transportation Services, or COTS, program. The COTS program was conceived to foster private development of vehicles to supply the International Space Station. With the retirement of the space shuttle, SpaceX and Orbital Sciences Corp., NASA's other COTS partner, will be the only U.S. vehicles flying to the space station.

NASA has agreed to pay $396 million to SpaceX in the COTS program. The space agency issues monetary awards upon the completion of major milestones, with the final payments due following the upcoming mission, assuming the flight goes as planned.

If the SpaceX launch occurs April 30, the Dragon spacecraft will reach the vicinity of the space station May 2 for a flyby, in which controllers will test the capsule's communications, navigation and control systems. If the flyby goes well, officials will give the green light for the Dragon to approach the complex May 3 for grappling by the lab's robot arm.

Technicians will load more than 1,000 pounds of food and clothing into SpaceX's Dragon capsule next month for delivery to the International Space Station on the commercial craft's first flight to the outpost.

Working inside the company's hangar adjacent to the Falcon 9 launch pad, technicians will carefully stow approximately 530 kilograms, or 1,168 pounds, of station-bound cargo inside the Dragon's pressurized section. Most of the supplies are currently scheduled to be loaded inside Dragon in mid-April.

Josh Byerly, a NASA spokesperson, said the cargo is comprised of mostly low-value items such as food, water, and clothing to supplement supplies delivered this week aboard Europe's Automated Transfer Vehicle.

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The Falcon 9 rocket and Dragon spacecraft rolled back into their hangar in Florida on Friday, a day after completing a launch exercise in which cryogenic propellants were pumped inside the privately-developed booster.

Technicians hydraulically lowered the two-stage rocket from its launch mount Thursday night, and engineers planned to practice cargo loading techniques with the Dragon capsule Friday.

SpaceX constructed a mobile access room, giving specialists a clean pathway into the Dragon spacecraft outside its pristine processing facility. The apparatus will permit SpaceX and NASA place items into the Dragon when the spacecraft is attached to its booster, just before the rocket is erected on the launch pad.

Late cargo could include urgent spare parts, fresh food, or time-sensitive experiments.

With the vehicle back inside the hangar at Cape Canaveral's Complex 40, SpaceX will unbolt the Dragon spacecraft, move it to a work stand and begin final processing before its scheduled launch to the International Space Station in late April.

Upcoming steps in Falcon and Dragon flight preparations include filling the capsule's pressurized compartment with cargo, installing pyrotechnic ordnance, and fueling the spacecraft with propellant for its maneuvering jets.

The Dragon spacecraft will fly to the space station on a demonstration mission, aiming to become the first commercial vehicle to visit the complex.

SpaceX has a $1.6 billion contract with NASA for 12 commercial resupply flights to the space station.

SpaceX loaded more than 75,000 gallons of liquid propellant into the Falcon 9 rocket Thursday for a pre-launch countdown test designed to wring out any issues with the launcher, ground systems and support teams before the mission blasts off to the International Space Station as soon as late April.

Thursday's countdown served as practice for SpaceX and an exercise of all the equipment needed on launch day. A brief ignition of the Falcon's nine first stage Merlin engines is scheduled a few days before liftoff.

The privately-developed rocket was capped with a Dragon spacecraft. The capsule is heading to the International Space Station on a NASA-sponsored test flight, aiming to become the first commercial mission to visit the complex.

Read our full story.

We have posted photos of the Falcon 9 rocket on the launch pad in Florida this afternoon.

A test firing of all nine Merlin first stage engines is planned several days before the Falcon 9 blasts off to check their health. It also serves as a last tune-up for the launch team.

Launch is currently scheduled for late April, but SpaceX and NASA have not set an official date.

SpaceX has released the following statement:

"Today SpaceX held a launch readiness test including the full countdown procedures for the upcoming test flight.

We ran down the countdown clock to a planned abort at T - 5 seconds at 12:18 PM eastern. The test went well.

Over the coming days we will continue to review the data as we prepare for our upcoming mission."

The countdown successfully reached the planned cutoff point at T-minus 5 seconds, moments before the Falcon 9 rocket's nine main engines would ignite during a real launch.

The countdown concluded at about 12:19 p.m. EST (1719 GMT), and the launch vehicle has been safed and is on external power.

Draining of the rocket's two stages will begin soon, according to NASA. Minor troubleshooting and testing with powerful Air Force range radars are also planned this afternoon.

Mounted on top of the 15-story rocket, SpaceX's Dragon spacecraft will be unbolted from the Falcon 9 in the coming days for final checks, loading of about 1,000 pounds of food and clothing for the space station crew, then filled with toxic hypergolic propellant to maneuver the capsule in space.

Launch of the SpaceX demonstration mission to the space station is set for late April. When the mission does launch, it will have just an instant to lift off each day, the moment when the space station's orbital plane passes over the launch pad.

SpaceX holds a $1.6 billion commercial resupply services contract for 12 cargo delivery flights to the space station. The April test flight is designed to prove the Dragon spacecraft can safely approach and berth with the complex before operational missions commence later this year.

NASA reports the countdown has recycled and is now setting up for a simulated launch time in a few minutes. The countdown will proceed to a cutoff at T-minus 5 seconds.

SpaceX loaded more than 75,000 gallons of liquid propellant into the Falcon 9 rocket Thursday for a pre-launch countdown test designed to wring out any issues with the launcher, ground systems and engineering teams.

According to NASA's Kennedy Space Center, the test was aiming for a simulated launch time of 11:23 a.m. EST (1623 GMT).

Vapors from the Falcon 9 rocket's cryogenic liquid oxygen are streaming away from the rocket at launch pad 40 at Cape Canaveral Air Force Station.

There is no word from SpaceX or NASA yet if the countdown test, called a wet dress rehearsal, successfully reached the mock launch time a few minutes ago.

Riding a specially-built transporter, the Falcon 9 rocket was towed to the launch pad at Complex 40 this morning, almost ready to begin a crucial countdown and fueling test later this week.

The slender white rocket, crowned by a Dragon spaceship, was lifted vertical on top of the launch mount after a 600-foot trip from an integration hangar on the south side of the pad.

The plan calls for the rocket be lowered again Wednesday for more checks, then the final countdown and fueling are planned Thursday morning.

The wet dress rehearsal, a pre-launch check of hardware and the launch team, is scheduled for Thursday. During the test, engineers will load rocket-grade kerosene and liquid oxygen propellants into the two-stage launcher.

The countdown will be aborted in the final seconds before the Falcon 9's first stage engines would ignite.

The target launch date for SpaceX's first test flight to the International Space Station is now no earlier than late April, the company announced Thursday, as the California-based firm and NASA continue extensive software testing to prove the Dragon spacecraft can safely approach the 450-ton orbiting complex.

"SpaceX is continuing to work with NASA to set a new target date for launch, expected to be late April," SpaceX said in a statement. "The primary driver for the schedule continues to be the need to conduct extensive software testing. This is a challenging mission, and we intend to take every necessary precaution in order to improve the likelihood of success."

NASA and SpaceX officials previously selected March 20 as a "placeholder" date on the Air Force range, but SpaceX CEO Elon Musk said last week he expected the flight would slip until April.

Sources said SpaceX has reserved April 20 for the launch on the Air Force Eastern Range, which manages a network of tracking and communications assets for rocket launchings from Cape Canaveral, Fla.

A launch in late April is contingent upon not only the completion of software testing, inspections and reviews of SpaceX's Falcon 9 rocket and Dragon capsule, but also on finding a place for the mission in the space station's traffic pattern.

A Russian Progress resupply craft is due to lift off April 20 from the Baikonur Cosmodrome in Kazakhstan. The Progress and Dragon missions will use different docking ports on separate ends of the space station, but officials prefer several days between arrivals and departures of visiting spacecraft to give crews time to prepare for the next vehicle.

The space station's robot arm is expected to grapple the Dragon about three days after its launch, but after the capsule makes a practice rendezvous with the outpost to check guidance, navigation and abort functions.

Josh Byerly, a NASA spokesperson, said last week the Dragon mission would have until about the third week of April to lift off or else be delayed to May.

The SpaceX flight, which is carried out under a developmental agreement with NASA, will try to demonstrate the Dragon spacecraft can deliver cargo to the space station on operational flights beginning later this year.

SpaceX founder and chief executive Elon Musk said Friday the first Dragon spacecraft will likely reach the International Space Station in April, echoing a senior NASA manager's comments earlier this week as engineers update the spaceship's operating software after problems surfaced during a simulation in January.

Officials last month postponed the mission's launch from Feb. 7, citing the need for additional work to optimize for the safety and success of the flight.

Describing an "insane amount of testing" on the Dragon's control software, Musk said a sizable chunk of the work in the weeks ahead will wring out the capsule's fault-tolerance capabilities, which are designed to respond to system failures without jeopardizing the space station astronauts or the spacecraft.

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Launch of a SpaceX commercial cargo ship on an initial test flight to the International Space Station, originally planned for Feb. 7, is expected to slip to at least the end of March, officials said Friday, to give engineers time to complete additional hardware and software testing in the wake of a recent simulation, software analysis and work in Florida to close out the craft for flight.

The company has not set an official target launch date for its Dragon cargo carrier, but the long-awaited mission is not expected to fly before March 20 and it could slip to early-to-mid April depending on what it takes to shoehorn the flight into an already busy space station schedule and to book a slot with the Air Force Eastern Range, which orchestrates all East Coast launches. Three of the station's six crew members plan to return to Earth March 16, a European cargo ship is expected to arrive March 19 and three fresh crew members are scheduled to dock on March 31.

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