A solar observatory built by Lockheed Martin Space Systems’ Advanced Technology Center in Palo Alto, Calif., arrived at Vandenberg Air Force Base, Calif., April 16 for integration with an Orbital Sciences Corp. Pegasus XL air-launched rocket.

The Interface Region Imaging Spectrograph (IRIS), a NASA Astrophysics Small Explorer-class mission, is scheduled to launch no earlier than May 28 on a two-year mission to study the region of the sun where the star’s inner and outer regions — its photosphere and super-hot corona, respectively — meet. The ultraviolet telescope’s observations could help improve space weather forecasts, NASA said in an April 17 press release.

On launch day, a Lockheed Martin L-1011 carrier aircraft will fly IRIS and its Pegasus XL launcher about 161 kilometers northwest from Vandenberg to a drop-off spot 11.9 kilometers above the Pacific Ocean.

IRIS is the only spacecraft currently manifested for launch on the reliable Pegasus XL, which can boost about 450 kilograms to low Earth orbit. There is a dearth of payloads small enough for the rocket to launch, and Orbital has said it might have to shut down the Pegasus assembly line if it cannot find another payload to launch before the middle of this decade.

Meanwhile, the price of a Pegasus ride is inching upward. IRIS is costing NASA about $40 million to launch; the most recent Pegasus XL launch, which took place back in June when the rocket carried the Nuclear Spectroscopic Telescope Array to orbit, cost the agency about $36 million.

Prospects worsened for Pegasus XL last year when NASA pulled the plug on the Gravity and Extreme Magnetism Small Explorer (GEMS) X-ray observatory. GEMS, which was to be launched by a Pegasus XL in 2014, was canceled after an external review found it was likely to bust a revised $135 million cost cap NASA approved in January 2013, and miss its launch date.

IRIS and GEMS both had their costs capped at $105 million in 2008, back when they were selected as Astrophysics Small Explorer missions. IRIS missions scientists have said their mission remains below the cost cap.