WASHINGTON — Orbital Sciences Corp.’s first contracted cargo run to the international space station has been postponed until at least Jan. 7 so spacewalking astronauts can replace a faulty valve in the outpost’s cooling system.

It will take astronauts Rick Mastracchio and Mike Hopkins three spacewalks to replace the flow-control valve on one of the station’s two ammonia-filled coolant lines, which began acting up Dec. 11. The station relies on liquid ammonia to cool its internal and external systems.

Spacewalks are scheduled for Dec. 21, Dec. 23, and Dec. 25, NASA said in a Dec. 17 press release. The broken valve is inside of a pump module that will have to replaced entirely.

NASA tried to fix the cooling system with a software patch, but the effort evidently failed. The patch, uploaded to station Dec. 17, attempted to regulate the temperature of the ammonia flowing through the cooling system by commanding one of station’s computers to loosen and tighten a radiator return isolation valve — a different piece of hardware from the failed flow-control valve.

The attempted fix required new software because the isolation valve was designed to be either fully open or fully closed during normal station operations, Judd Frieling, lead flight director for Expedition 38, said in a Dec. 17 NASA TV briefing. Expedition 38 is the name of the current mission aboard the space station.

Dulles, Va.-based Orbital owes NASA eight cargo-delivery and trash-disposal missions under a $1.9 billion Commercial Resupply Services contract signed in 2008. The company used advances on that contract — plus $288 million from a 2007 Commercial Orbital Transportation Services contract — to design and build its Antares rocket and expendable Cygnus cargo carrier spacecraft. Orbital collects a portion of the remainder of its fees after each successful mission.

Orbital’s space-station resupply missions launch from Pad 0A at the Mid-Atlantic Regional Spaceport, a Virginia-owned launch complex located at NASA’s Wallops Flight Facility on Wallops Island, Va. For the mission now slated to launch in mid-January, the company will deliver 1,465 kilograms of cargo.

Cygnus will be berthed to the station’s Harmony module, where it will remain for 42 days while astronauts unload cargo and then fill it with trash. The vessel along with the refuse will be burned up when it re-enters the atmosphere.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.