HOUSTON — In order to add Monday’s spacewalk to send shuttle astronauts to subdue a pesky solar array outside the International Space Station (ISS) without giving up the mission’s late inspection of Discovery’s heat shield, mission managers have sacrificed one of two optional days typically reserved at the end of each spaceflight.

Discovery spacewalkers Robert Curbeam and Christer Fuglesang are due to help shake and shimmy the stubborn portside solar array, one of two extending from the station’s mast-like Port 6 (P6) truss, at 1:47 p.m. EST (1847 GMT).

NASA’s mission operations representative Phil Engelauf told reporters here at the Johnson Space Center that in the end, it was safety versus schedule and safety wins out.

NASA has been committed to ensuring the health of its spacecraft since the 2003 Columbia accident, and has used late inspections on the past two shuttle flights to scan for signs of damage from micrometeorites or orbital debris. During September’s STS-115 flight, a piece of orbital debris apparently pierced a radiator panel along one of the Atlantis orbiter’s payload bay doors, causing minor damage but no risk to the shuttle’s crew.

“The ultimate conclusion was, we wanted to protect the safety issue of late inspection over the schedule issue,” Engelauf said late Saturday.

That schedule issue revolves around where Discovery will land, and the potential ripple effect pushing the solar array spacewalk into an already EVA heavy ISS Expedition 14 mission or tacking it onto a subsequent shuttle flight would have on efforts to complete the station’s assembly by NASA’s September 2010 mandate.

NASA typically reserves two days (for example, Discovery carries supplies for a 12-day mission, plus two more) in the case of bad weather at its Kennedy Space Center landing site in Florida or a systems glitch aboard the spacecraft.

The shuttle can land on three sites in the continental U.S.: KSC’s Shuttle Landing Facility, a desert landing strip at Edwards Air Force Base in California or the White Sands Space Harbor at White Sands Missile Range in New Mexico.

Since NASA launches shuttles out of KSC, landing there is always preferable. An Edwards landing typically adds a full week and $1 million to the turnaround costs of one more readying a shuttle for launch, and a White Sands touchdown - a shuttle last landed there during STS-3 in 1982 - carries its own schedule hit.

Discovery was originally scheduled to land on Thursday, Dec. 21, with Friday and Saturday as backups. With the landing now set to Friday with only one backup, Engelauf said that Discovery’s crew will land Friday barring poor weather at all three shuttle touchdown sites or some unforeseen spacecraft glitch.

“We have incurred, I think, some programmatic schedule risk that we’re going to put the orbiter at a landing strip that might cost us a little bit more turnaround time,” Engelauf said. “But we think that the bigger picture trade warrants that given the significance of getting this array retracted means for the station and what it means for the rest of the [assembly] sequence.”

 Click here for SPACE.com’s shuttle mission coverage, and here for an archive of Discovery’s STS-116 mission stories.