The newest spacecraft to launch in NASA's oldest program -- the Explorer project that dates back to America's first satellite -- was propelled into Earth orbit Thursday night by an air-launched rocket off the coast of California.

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NASA has confirmed a good deployment of the IRIS satellite's solar arrays.

"We're thrilled," says NASA launch director Tim Dunn. "We're very excited!"

Dunn said NASA has made initial contact with IRIS through its satellite tracking network, and he confirmed the start of deployment of the spacecraft's power-generating solar arrays.

"We've got a very happy spacecraft on orbit and we've got a thrilled launch team on the ground," Dunn said.

The official drop time was 0227:46 GMT (10:27:46 p.m. EDT; 7:27:46 p.m. PDT).

IRIS separation confirmed! The signal from the small satellite has been received by NASA's network of tracking and data relay satellites, verifying the spacecraft has deployed from the Pegasus XL's third stage.

Before the loss of telemetry, data indicated the Pegasus rocket achieved the correct orbit with an apogee of 670 kilometers, a perigee of 623 kilometers, and an inclination of 97.9 degrees.

T+plus 13 minutes, 15 seconds. The signal from the Pegasus XL rocket has been lost due to a problem with tracking from an NASA DC-9 tracking plane. This is not indicative of a problem with the rocket.

T+plus 11 minutes. Quick-look data shows the rocket has achieved the proper orbit.

T+plus 10 minutes, 41 seconds. Burnout of the third stage motor has been confirmed. Standing by for release of the satellite about three minutes from now.

T+plus 9 minutes, 36 seconds. IGNITION. The solid-fueled third stage motor has been lit, accelerating the IRIS spacecraft the rest of the way into orbit! The third stage is 4.4 feet long and 3.2 feet in diameter with a thrust of 8,000 pounds.

T+plus 9 minutes, 25 seconds. STAGING. The spent second stage has separated.

T+plus 8 minutes, 40 seconds. The vehicle is now reorienting in preparation for stage separation and third stage burn.

T+plus 7 minutes. The Interface Region Imaging Spectrograph spacecraft, or IRIS, is embarking on a $181 million mission to peer into the mysterious region around the sun where the temperatures dramatically escalate from the surface to the tumultuous atmosphere.

T+plus 6 minutes, 40 seconds. Pegasus is 329 miles in altitude.

T+plus 5 minutes, 40 seconds. The rocket is in a good orientation and the power system is strong.

T+plus 4 minutes. No problems have been reported in today's flight.

T+plus 3 minutes. The flight path, altitude and all systems are nominal aboard the Pegasus.

T+plus 2 minutes, 40 seconds. The solid-fueled second stage has burned out. The Pegasus rocket is now in a coast period for the next few of minutes as it ascends toward the high point of its trajectory achieved by the first two stages. During this time the rocket will compute the performance of the flight thus far and adjust the third stage ignition time if necessary.

T+plus 2 minute, 15 seconds. The two halves of the payload fairing enclosing the IRIS satellite on the front end of the Pegasus rocket have jettisoned. Second stage still burning as the vehicle climbs into space.

T+plus 1 minute, 35 seconds. STAGING. The first stage has jettisoned, allowing the Pegasus' second stage to ignite!

The first stage was 33.7 feet long, 4.2 feet in diameter, a wingspan of 22 feet and produced 163,000 pounds of thrust. The second stage is 4.2 feet long and 4.2 feet wide for 44,000 pounds of thrust.

T+plus 1 minute, 18 seconds. The solid-propellant first stage has burned out. The vehicle is now in a ballistic coast for a few seconds before the spent stage is jettisoned and the second stage ignites.

T+plus 1 minute. The rocket is more than 100,000 feet in altitude with a velocity now exceeding 2,500 mph.

T+plus 40 seconds. Passing through the region of maximum aerodynamic pressure.

T+plus 30 seconds. The Pegasus rocket is pitched up 35 degrees as it climbs on the power of its solid-fuel first stage motor at over 1,500 mph.

T+plus 20 seconds. Pegasus has gone supersonic as the winged first stage propels the 55-foot-long rocket through the evening sky.

IGNITION of the Pegasus rocket launching NASA's IRIS spacecraft to observe the the physics of the sun.

DROP. Pegasus is away! The Orbital Sciences Pegasus XL rocket has been released from the L-1011 aircraft over the Pacific Ocean about 100 miles northwest of Vandenberg.

The batteries for the first stage flight control fins are being activated, allowing the fins to undergo a sweep test prior to launch. The fins are used to steer the rocket during its initial climb to space.

With the batteries activated there is just 90 seconds to launch today or else an abort will be called. That is due to the limited life of the batteries.

In the final moments prior to release of Pegasus, the L-1011 carrier aircraft crew will oversee the last seconds of the countdown and push the button that will drop the air-launched vehicle, with the IRIS spacecraft aboard, from the belly of the jet.

T-minus 1 minute. The L-1011 is adjusting its course to acquire the proper heading. One of the key duties of the aircraft crew is releasing Pegasus on the correct trajectory so that the rocket doesn't have to perform any abrupt steering after ignition.

T-minus 2 minutes, 30 seconds. The drop time has been adjusted to reflect the L-1011's flight course.

T-minus 3 minutes. The rocket's SIGI guidance computer is being configured for flight.

The launch team has been given a "go" to enter the final phase of today's countdown.

T-minus 5 minutes. The Range is confirmed clear for launch.

T-minus 6 minutes, 30 seconds. NASA Launch Manager Tim Dunn has polled the space agency team to verify all is in readiness for launch.

T-minus 7 minutes, 30 seconds. The Pegasus rocket's avionics have switched from power provided by the L-1011 to internal battery power.

T-minus 8 minutes, 30 seconds. The L-1011 is now headed south on the trajectory to the launch box.

Weather conditions are reported "go" for launch.

T-minus 10 minutes and counting.

T-minus 11 minutes. Final checks of the Pegasus rocket's destruct system are complete.

T-minus 12 minutes. The aircraft has begun the 180-degree left-overhead turn on the course headed back toward the launch point.

T-minus 13 minutes. The spacecraft is in good shape following the transition to internal power.

T-minus 14 minutes. The IRIS spacecraft is switching to internal power for launch.

T-minus 15 minutes. No issues are being worked in the countdown.

T-minus 18 minutes. Further checks on the rocket's safety system have occurred successfully.

T-minus 20 minutes and counting. Although the countdown is managed by the ground-based team, the aircraft crew actually pushes the button to launch Pegasus on its journey into space.

The circuitry for the release system has been armed by the launch panel operator aboard the aircraft. Later, a switch will be flipped in the cockpit by the co-pilot. This switch, located on the right-hand portion of the center console between the captain and pilot, "enables" the release to be become active.

In the final seconds of the countdown the Orbital Sciences launch conductor on the ground will call out "drop on my mark...3, 2, 1, drop." At that point, the co-pilot will push a button next to the enable switch, releasing the Pegasus rocket and IRIS to fall away from the L-1011 aircraft. See a photo of the drop button taken during a tour of the L-1011.

"It takes a couple seconds and then it releases," Bill Weaver, the long-time captain explained during a previous interview. "There is no doubt about it that the rocket has released. There is a tremendous reaction throughout the airplane. It weighs 52,000 pounds, so we experience an instantaneous weight loss of 52,000 pounds and the center of gravity shifts aft 10 percent, so the nose comes up in a pretty pronounced fashion, which is good because we like that for separation.

"We drop it at 39,000 feet and after the drop we end up eventually around 41,000, we gain a couple thousand feet altitude or separation and also we do about a 10 degree heading change to get out of the rocket exhaust.

"Five seconds after we drop it, (Pegasus) is about 500 feet below drop altitude and the first stage lights off and it pulls up. In the meantime, we have turned 10 degrees off the heading. By the time we roll out we can see it. We can hear it. When that rocket motor lights off it sounds like a freight train roaring underneath the plane. It is a pretty impressive event.

"We don't really see till we get out of the bank, then we have a really good view. We can see it all the way through first stage burn out, second stage ignition. We can't normally see the stage 3. One time we did at Vandenberg. Conditions were just right -- perfect sun, perfect atmosphere."

T-minus 22 minutes and counting. The release mechanism that will drop the Pegasus rocket from the L-1011 carrier jet was just armed. This hydraulic system involves four main hooks holding the Pegasus to the aircraft as well as a nose hook. Co-pilot Ebb Harris will command the actual drop event by pushing a button in the cockpit.

Don Walter is at the controls of the L-1011 airplane.

T-minus 23 minutes. The L-1011 just crossed the "PREV" reverse waypoint on the "racetrack pattern" on the lap before launch.

T-minus 25 minutes. The aircraft crew reports no turbulence or clouds in the drop box.

T-minus 26 minutes. The carrier jet is passing through the launch box west of Big Sur, providing a check of the weather before the aircraft makes a lap around "the racetrack pattern" for return to the rocket drop zone at 7:27 p.m. PDT.

T-minus 28 minutes. Countdown checks on the rocket's safety system have been completed with the Vandenberg range.

Now 30 minutes from the planned launch time. Today marks the 42nd flight of the air-launched Pegasus rocket and the 32nd using the XL version. The rocket weighs 51,000 pounds, stretches 55 feet long and is comprised of three solid-fueled stages for boosting small satellites into space.

The first stage is 33.7 feet long, 4.2 feet in diameter, has a wingspan of 22 feet and produces 163,000 pounds of thrust. The second stage is 4.2 feet long and 4.2 feet wide for 44,000 pounds of thrust. The third stage is 4.4 feet long and 3.2 feet in diameter with a thrust of 8,000 pounds.

T-minus 35 minutes and counting. The IRIS spacecraft was manufactured by Lockheed Martin for NASA. Once fully deployed in space with its solar arrays unfolded, the craft will measure 23 feet wing to wing and 7 feet long.

IRIS is a NASA Small Explorer Mission to observe how solar material moves, gathers energy and heats up as it travels through a little-understood region in the sun's lower atmosphere. This interface region between the sun's photosphere and corona powers its dynamic million-degree atmosphere and drives the solar wind.

T-minus 40 minutes and counting. This launch originates from the belly of the L-1011, named Stargazer. Originally delivered to Air Canada in March 1974, it was purchased by Orbital Sciences in 1992 and modified as the Pegasus launch platform.

T-minus 46 minutes and counting. The aircraft is passing the peak-power waypoint on its journey to the launch area.

T-minus 50 minutes and counting. Stargazer is following a predetermined course commonly called "the racetrack pattern" to reach the precise point where the Pegasus can be released for launch. This ferryflight lasts about an hour, flying north from Vandenberg, making a U-turn and then coming back around to launch the rocket.

T-minus 56 minutes, 30 seconds. WHEELS UP. The "Stargazer" carrier aircraft with the Orbital Sciences Pegasus XL rocket has departed Vandenberg Air Force Base in California for today's launch for NASA's IRIS spacecraft. The booster will be released from the jet over the Pacific Ocean around 7:27 p.m. PDT to propel the satellite into a sun-synchronous polar orbit.

T-minus 57 minutes. Stargazer has begun its takeoff roll down Vandenberg's three-mile-long runway.

T-minus 59 minutes. The Orbital Sciences launch conductor Adam Lewis just completed a poll of his ground launch team. And all systems are reported ready for the L-1011's takeoff.

T-minus 1 hour, 7 minutes. NASA Launch Manager Tim Dunn has polled the space agency team to verify all is in readiness for departure of the L-1011 aircraft.

T-minus 1 hour, 13 minutes. The L-1011 is taxiing to the runway from the staging area, called the Hot Pad, where the carrier aircraft has been parked for the past two weeks.

T-minus 1 hour, 15 minutes. The "go" has been given for taxi.

The bag covering the front end of the Pegasus rocket - providing protection over the launcher's fairing - has been removed in one of the final steps before the L-1011 carrier plane begins to taxi for takeoff.

T-minus 1 hour, 20 minutes. The L-1011 aircraft has started all three of its engines, which are newly installed, fuel efficient powerplants making their second mission today, and the "Hot Pad" staging area equipment has been disconnected.

Live streaming video and commentary coverage of today's launch begins now. Hit reload to this page to auto-load the webcast.

Drop of the Pegasus rocket from the belly of its L-1011 carrier airplane is targeted for 7:27:34 p.m. PDT (10:27:34 p.m. EDT; 0227:34 GMT).

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T-minus 1 hour, 55 minutes and counting. To recap, the weather is 100 percent favorable for the Pegasus rocket today. An earlier aircraft issue was examined and resolved, clearing the way for launch at 7:27 p.m. PDT.

T-minus 2 hours, 15 minutes. The L-1011 aircraft's engines are being started in preparation for takeoff.

T-minus 2 hours, 30 minutes and counting. The weather outlook calls for a zero chance of violation of takeoff and launch criteria.

The launch team is opening its countdown checklist and kicking off today's operations to send NASA's IRIS sun-studying satellite into orbit using the Pegasus XL rocket. The air-launched booster carrying the satellite will be released from the belly of an L-1011 carrier aircraft over the Pacific Ocean just off the coast of the California to fire into a polar orbit.

The aircraft will take off from the runway at Vandenberg Air Force Base around 6:30 p.m. in preparation for the launch at 7:27 p.m. PDT.

Watch this page for updates throughout the evening and a live streaming video broadcast starting at 6 p.m. PDT.

After a one-day delay to complete repairs to the electrical system feeding key facilities at the Western Range, officials today cleared the Orbital Sciences Pegasus XL rocket to launch NASA's IRIS sun-studying satellite on Thursday evening off the coast of California.

Senior mission managers held the Launch Readiness Review, confirmed the Range would be up to support and granted approval to enter into the countdown tomorrow afternoon at Vandenberg Air Force Base. Launch is scheduled for 7:27 p.m. PDT (10:27 p.m. EDT).

When power was restored to the base after a region-wide power outage late Sunday, a fire broke out and damaged a piece of hardware in the electrical grid that supplies several buildings. A replacement part was being fabricated in Los Angeles for delivery to Vandenberg late Tuesday. Checks of the Range were being performed today.

Under the control of pilot Don Walter, the modified L-1011 carrier aircraft with Pegasus hooked to its belly should be airborne about an hour before launch for the trip to the rocket's pre-set drop point over the Pacific Ocean about 100 miles northwest of Vandenberg, west of Monterey. The launch box is 10 miles wide and 40 miles long and the targeted drop point located at 36 degrees North latitude and 123 degrees West longitude.

The available launch window extends from 7:25 to 7:30 p.m. local (10:25-10:30 p.m. EDT; 0225-0230 GMT).

The rocket was assembled at Vandenberg and the Western Range will track its ascent for telemetry-relay and safety services.

Weather forecasters predict an 80 percent chance of favorable conditions. Cloud ceilings and reduced visibility pose only slight concerns.

With the push of a button in the Stargazer's cockpit by co-pilot Ebb Harris, the 51,000-pound Pegasus rocket is cast free to fall for five seconds, dropping 300 feet below the aircraft while traveling at Mach 0.82. During the plunge, the onboard flight computer will sense the rocket's separation from the carrier jet and issue a command to release the safety inhibits in preparation for ignition.

The first stage solid-fueled motor of Pegasus is lit at T+plus 5 seconds to begin the powered journey to orbit on a southerly heading into a sun-synchronous polar orbit desired by IRIS.

At T+plus 1 minute, 18 seconds, the Orion 50S XL first stage motor consumes all of its solid-fuel propellant and burns out at an altitude of 33 miles. A short ballistic coast period begins before the spent first stage, including the wing structure, is separated at T+plus 1 minute, 33 seconds to fall into the Pacific.

The Pegasus rocket's Orion 50 XL second stage begins firing at T+plus 1 minute, 34 seconds to continue the trek to orbit. During the firing, at T+plus 2 minutes, 11 seconds, the payload fairing nose cone that protected the satellite during atmospheric ascent is jettisoned at an altitude of 73 miles.

Having consumed its supply of solid-fuel propellant, the second stage motor burns out at T+plus 2 minutes, 48 seconds some 117 miles in altitude. The rocket will coast for a few minutes toward the high point of its trajectory before releasing the spent stage at T+plus 8 minutes, 47 seconds.

The solid-fueled Orion 38 third stage ignites at T+plus 8 minutes, 58 seconds to deliver the IRIS spacecraft into a 420-by-385-mile-high orbit around Earth. That orbit is achieved with burnout of the third stage at T+plus 10 minutes, 6 seconds, completing the powered phase of the Pegasus rocket's 42nd launch for the winged booster since 1990.

Deployment of the 403-pound satellite occurs at T+plus 13 minutes, 8 seconds.

DELAY. Launch of the Orbital Sciences Pegasus XL rocket carrying NASA's IRIS solar satellite has been postponed at least 24 hours while the Western Range make repairs to power systems at Vandenberg Air Force Base, Calif.

Those repairs are expected to be completed later today, allowing the Launch-minus 1 day checks to be performed tomorrow.

Officials will hold the Launch Readiness Review tomorrow, and the confirmation of Range readiness to support will be verified.

The launch is reset for Thursday evening at 7:27 p.m. local time.

The Orbital Sciences Pegasus XL rocket was attached to the L-1011 carrier jet that will ferry the launcher off the coast of California next Wednesday evening and release the 55-foot-long booster to fire into orbit.

Technicians rolled the rocket horizontally from its assembly hangar today at 6 a.m., arriving at 7:20 a.m. on the "Hot Pad" staging area next to the runway at Vandenberg Air Force Base, Calif. There, the 51,000-pound rocket was secured to the belly of the L-1011 by hydraulic hooks, an operation that was completed at 1:30 p.m. local time.

Senior mission managers held the Flight Readiness Review on Tuesday and gave approval for the rollout and continued pre-launch preparations.

The Combined Systems Test to verify the entire launch configuration will be run on Thursday. The checkout will confirm the Pegasus, its payload and the L-1011 are flight-ready.

Next Wednesday's launch of the Pegasus will be possible during a precise window extending from 7:25:04 to 7:30:04 p.m. local (10:25:04-10:30:04 p.m. EDT).

The launch team is targeting the middle of the window -- 7:27 p.m. -- for the drop and ignition to put NASA's IRIS solar observatory into its desired sun-synchronous polar orbit around Earth.

The L-1011 flight crew will receive a verbal "go" command from the ground, then push the button on the center console in the cockpit that releases the three-stage, all-solid rocket to free-fall for five seconds before the first stage motor lights.

The preliminary weather outlook for next week is favorable.

The winged Pegasus XL rocket will be rolled from its hangar and mated to the belly of the L-1011 carrier aircraft this week at Vandenberg Air Force Base in California in preparation for a dramatic mid-air launch over the Pacific on June 26.

The three-stage, all-solid booster will propel NASA's Interface Region Imaging Spectrograph spacecraft, or IRIS, into orbit to study how the sun's atmosphere is energized.

Assembly and testing of the Orbital Sciences rocket was conducted in Building 1555, a large hangar on North Base at Vandenberg. IRIS arrived from Lockheed Martin in mid-April and underwent its final processing within a tented area in front of the launcher.

The 7-foot-long spacecraft was mated to the Pegasus at the end of May and then encapsulated by the two halves of the rocket's nose cone last week.

The finished product -- a 51,000-pound rocket with its iconic wing, rudder and fins -- will be rolled out Wednesday for the 3.8-mile drive to Vandenberg's runway to join the L-1011. The carrier jet flew in to pick up Pegasus last week.

With the aircraft jacked up, the trailer hauling the rocket will be able to slide underneath. Ground crews then hoist Pegasus and firmly lock it into place with hydraulic hooks that release the vehicle at launch.

As the final days before flight progress, a comprehensive combined systems test between all of the elements will be run, readiness reviews held and engineers button up the various vehicle compartments.

On June 26, launch day, the L-1011 will depart Vandenberg's runway around 6:30 p.m. local (9:30 p.m. EDT) and fly a pre-determine "race track" pattern northward over the open ocean before making a U-turn and achieving a southerly heading for drop and ignition of the Pegasus at approximately 7:27 p.m. PDT (10:27 p.m. EDT).

It will take approximately 13 minutes to inject IRIS into the desired sun-synchronous polar orbit and release the craft from the launch vehicle.

Weighing 403 pounds, the spacecraft is ideally sized for the Pegasus launch that will be making its 42nd flight. Its heritage includes deploying over 70 satellites since 1990 for NASA, commercial customers and the U.S. military.

IRIS will unfold its twin power-generating solar panels and begin the on-orbit checkout and commissioning period. The two-year mission goes into service about 60 days after launch to observe a mysterious region around the sun, between its surface and the thousand-times-hotter upper atmosphere, known as the corna.

"IRIS will extend our observations of the sun to a region that has historically been difficult to study," said Joe Davila, IRIS project scientist at NASA's Goddard Space Flight Center. "Understanding the interface region better improves our understanding of the whole corona and, in turn, how it affects the solar system."

A solar observatory designed to study how the Sun's atmosphere is energized has been trucked to Vandenberg Air Force Base in California, the ground base where the air-launched Pegasus rocket booster will be readied to propel the NASA satellite into orbit in June.

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