NASA has chosen the air-launched Pegasus rocket to deploy its solar observatory designed to study how the Sun's atmosphere is energized.

File image of Pegasus rocket launch. Credit: NASA TV

The Interface Region Imaging Spectrograph spacecraft, or IRIS, is under development by the Lockheed Martin Advanced Technology Center in Palo Alto, Calif. The Earth-orbiting craft will be outfitted with an ultraviolet telescope and spectrograph to help scientists understand a mystery about the Sun's working forces.

Project officials say IRIS, which is a highly-focused science mission, will close the knowledge gap by following the flow of energy and plasma between the Sun's surface and the solar corona.

"With IRIS we have a unique opportunity to provide significant missing pieces in our understanding of energy transport on the Sun," Alan Title, IRIS principal investigator, said last summer when NASA authorized the mission. "The complex processes and enormous contrasts of density, temperature and magnetic field within this interface region require instrument and modeling capabilities that are only now within our reach."

Launch is targeted for December 2012.

The Orbital Sciences Pegasus booster has been used extensively over the past two decades to carry NASA's small science satellites into orbit. IRIS will ride aboard the Pegasus' 42nd flight since debuting in April 1990.

Our Pegasus coverage archive can be viewed here.

A previous Pegasus rocket undergoes launch preparations inside a Vandenberg hangar in 2008. Credit: Chris Miller/Spaceflight Now See more panoramas and photos here

The deal announced Tuesday awards $40 million to the commercial firm for building the Pegasus XL rocket and all the associated support costs required to perform the launch.

After departing from the runway at California's Vandenberg Air Force Base, an L-1011 carrier jet will haul the winged rocket over the Pacific Ocean and release it. The solid-fueled booster will fly southward into a Sun-synchronous orbit.

IRIS is part of NASA's Small Explorer Program. The satellite's development and operations cannot exceed $105 million.

NASA has another Pegasus launch planned for February 2012 with NuSTAR, the Nuclear Spectroscopic Telescope Array spacecraft. This astrophysics observatory will use the U.S. military's Kwajalein Atoll launch range in the Central Pacific.

Pegasus has delivered more than 70 satellites to orbit in its 37 good flights out of 40 thus far, including the current string of 26 consecutive successful launches.