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VANDENBERG AIR FORCE BASE, Calif. – An Orbital Sciences L-1011 aircraft takes off from Vandenberg Air Force Base in California at 9:30 p.m. EDT, headed over the Pacific Ocean to release the Pegasus XL rocket carrying NASA's Interface Region Imaging Spectrograph, or IRIS, solar observatory. Release of the rocket from under the wing of the aircraft is scheduled for 10:27 p.m. EDT. IRIS will open a new window of discovery using spectrometry and imaging to trace the flow of energy and plasma through the chromospheres and transition region into the sun’s corona. The spacecraft will observe how solar material moves, gathers energy and heats up as it travels through a largely unexplored region of the solar atmosphere. This interface region, located between the sun's visible surface and its upper atmosphere, is where most of its ultraviolet emission is generated. These emissions impact the near-Earth space environment and Earth's climate. NASA's Launch Services Program at the agency's Kennedy Space Center in Florida is managing the countdown and launch. For more information, visit http://www.nasa.gov/iris. Photo credit: NASA/Daniel Casper