An aircraft flies by a launch pad as it departs from Vandenberg Air Force Base in California.

An F-18 aircraft flies by a launch pad as it departs from Vandenberg Air Force Base in California. The plane will serve as the "chase plane" accompanying the Orbital Sciences L-1011 aircraft as it transports the Pegasus XL rocket carrying NASA's Interface Region Imaging Spectrograph, or IRIS, solar observatory over the Pacific Ocean. Release of the rocket from under the wing of the L-1011 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