Updated 1:20 p.m. EST

KENNEDY SPACE CENTER, Fla. — Gusty winds and a problem with valves on the first stage of United Launch Alliance’s Delta 4 Heavy rocket postponed the scheduled Dec. 4 launch of NASA’s Orion spacecraft on a critical early test flight by a day.

The Delta 4 Heavy was scheduled to lift off from Space Launch Complex 37 at Cape Canaveral Air Force Station, Florida, at 7:05 a.m. EST, placing the uncrewed Orion spacecraft into Earth orbit for a four-and-a-half-hour test flight.

However, the countdown was stopped twice with less than four minutes before liftoff when winds exceeded preset limits. The countdown was also briefly delayed when a boat entered restricted waters off the coast from the launch site.

A third launch attempt, about halfway into the 2-hour, 39-minute launch window, also was halted with just over three minutes before launch. Propellant valves on two of the three booster cores that comprise the Delta 4 Heavy’s first stage failed to close, and the problem could not be resolved before the launch window closed at 9:44 a.m. EST.

“We had some fuel valves on the common booster cores that had gotten cold and a little sluggish in their performance,” Dan Collins, United Launch Alliance chief operating officer, said at a briefing here several hours after the launch scrub. He said engineers saw a similar problem on a previous Delta 4 Heavy flight with a long launch window.

“We’re very confident we’re going to be able to exonerate the hardware and make another attempt” on Friday, Collins said.

NASA has rescheduled the launch for 7:05 a.m. EST Dec. 5, if the valve problem can be resolved. Forecasts call for only a 40 percent chance of acceptable weather that day, down from 70 percent chance Dec. 4.

Collins said that, given the capacity of the tanks on the pad that store the Delta’s liquid hydrogen and liquid oxygen propellants, they can accommodate launch attempts on two days out of every three. Thus, if a Dec. 5 launch attempt is scrubbed, they would not attempt a launch on Dec. 6 even though the range is reserved for that day.

“We will work with the range” if the launch is delayed past Dec. 6, Collins said. One constraint is a planned Atlas 5 launch of a classified satellite from Vandenberg Air Force Base, California, on Dec. 11. If the Orion launch slips past Dec. 7, “we would have to probably work with our customers to figure out how to move around to accommodate that.”

The mission, designated Exploration Flight Test 1 (EFT-1), is the first flight test of the Orion spacecraft. It is intended to test several key systems on Orion, including its heat shield and parachutes. Engineers will also measure the susceptibility of the spacecraft’s electronics to radiation as Orion passes through the lower portions of the Van Allen belts.

“It’s a test flight,” said Mark Geyer, NASA Orion program manager, during a pre-launch briefing here Dec. 3. “We are pushing the systems to make sure they work as we expect.”

Those tests will support further development of Orion, which is now expected to fly again in 2018 on the first launch of the Space Launch System heavy-lift rocket. That will be followed by first crewed Orion launch, also on the SLS, in 2021.

“It’s a huge day for us,” said NASA Administrator Charles Bolden at a Dec. 3 media event in front of the Delta 4 Heavy rocket, noting that the mission was a test flight that carried with it risks. “In order to have a big day, you have to take big risks.”

The Dec. 4 launch attempt attracted an estimated 30,000 people to KSC, the largest crowd for a launch since the end of the space shuttle program in 2011. It also generated excitement among those in the agency working on the program.

“It’s just a great time to be here,” said Mike Sarafin, Orion Flight director, describing the mood at mission control at the Johnson Space Center. “We haven’t had this feeling in a while, since the end of the shuttle program.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...