NASA's NuSTAR X-ray astrophysics observatory, grounded in March by concerns with its Pegasus rocket, will have an opportunity to launch in June from a remote Pacific military base, space agency officials said Tuesday.

The Pegasus XL rocket fully integrated for the NuSTAR mission. Credit: NASA/Randy Beaudoin
 
Engineering reviews of the air-launched Orbital Sciences Corp. Pegasus rocket continue, according to NASA, with officials focusing on software to be used by a new computer flying on the Pegasus for the first time.

The reviews were not finished in time for the $165 million mission to be ready before the end of a launch window in late March.

"We're still working on the flight software program," said George Diller, a spokesperson at the Kennedy Space Center.

The clamshell-like nose cone on the Pegasus rocket has been cleared for launch. Engineers were studying commonality between the Pegasus payload fairing and the shroud on the ground-launched Taurus rocket, another Orbital Sciences vehicle, which failed due to fairing separation problems on back-to-back flights in 2009 and 2011.

The Pegasus booster has flown 40 times, and the upcoming launch will mark the 25th mission of the Pegasus XL configuration, which features more powerful rocket motors than earlier versions.

Workers inspect the Pegasus payload fairing. Credit: NASA/Randy Beaudoin
 
The Pegasus XL rocket is fully assembled inside a hangar at Vandenberg Air Force Base, Calif. Technicians will tow the rocket to the base's runway a few weeks before launch and bolt it to an L-1011 carrier aircraft, which will fly the Pegasus and NuSTAR payload to Kwajalein Atoll in the Marshall Islands.

The L-1011 jumbo jet will drop the rocket about 40,000 feet over the Pacific Ocean, and the three-stage, winged Pegasus will light to propel the 800-pound NuSTAR satellite into a 340-mile-high orbit.

The U.S. Army's Reagan Test Site should give NASA a specific launch date in a few days. The space agency has requested to launch in the first week of June if possible.

While officials wait for the next launch opportunity, workers will remove the Pegasus payload fairing next week for testing of the NuSTAR spacecraft. Engineers will install an updated flight program into the Pegasus computer when the software analysis is finished, according to Diller.

The NuSTAR mission will be the first flight of the computer on a Pegasus rocket.

NuSTAR stands for the Nuclear Spectroscopic Telescope Array.

The spacecraft will deploy a 33-foot boom with X-ray optics a week after launch and begin its science mission a few weeks later.

The mission will explore the high-energy X-ray universe for two years, probing the supermassive black hole at the center of the Milky Way galaxy, surveying for collapsed dead stars, and studying the exchange of heat between the sun and its million-degree atmosphere.