KENNEDY SPACE CENTER, Fla. — Poor weather postponed the Atlas 5 launch of an Orbital ATK Cygnus cargo spacecraft Dec. 3, delaying for at least a day the resumption of deliveries of essential supplies and experiments to the International Space Station by U.S. companies.

Clouds and showers at Cape Canaveral, Florida, kept the Atlas from launching when the 30-minute window opened at 5:55 p.m. Eastern time. With weather conditions showing no sign of changing, controllers scrubbed the launch at 6:11 p.m. NASA has rescheduled the launch for 5:33 p.m. Eastern Dec. 4, although forecasts predict only a 30-percent chance of acceptable weather at the scheduled launch time because of clouds, rain and wind.

Cygnus, carrying more than 3,500 kilograms of supplies, equipment and experiments for the ISS, will arrive at the station early Dec. 7 if it launches Dec. 4. The station’s robotic arm will grapple the spacecraft and berth it to the Earth-facing port on the Node 1, or Unity, module, the first time that docking port has been used by a cargo spacecraft.

The launch, when it does occur, will be the first for the Cygnus since the October 2014 failure of an Antares rocket shortly after liftoff from Wallops Island, Virginia. Shortly after that failure, Orbital purchased an Atlas 5 launch from ULA to fulfill its contractual requirements to NASA to deliver cargo to the ISS.

Orbital and ULA went contract to launch in about one year, a much faster schedule than for a typical satellite launch. Frank DeMauro, a vice president of Orbital ATK’s Space Systems Group who manages the Cygnus program, said in a Dec. 3 interview here that the flight-proven design of the Cygnus, and the fact that the launch environment of the Atlas is similar to the Antares, allowed the companies to accelerate integration work.

“Nothing was cut out,” he said of that integration work. “We’re confident it was a full effort, we just did it in a 12-month period.”

The use of the Atlas for this Cygnus mission, and a second planned for March 2016, is a stopgap measure by Orbital ATK as it upgrades its Antares rocket. The company is replacing the AJ-26 engines implicated in the 2014 failure with RD-181 engines from Russian company NPO Energomash.

Orbital is currently installing the first set of RD-181 engines in an Antares first stage. That work has required only minor modifications to the stage itself, said Mike Pinkston, vice president and general manager of the Antares program at Orbital ATK, in a Dec. 3 interview.

“It’s probably as close to a drop-in replacement as you could find to the AJ-26,” he said. “There are a number of changes, but in the grand scheme of things they’re relatively minor compared to what they could be.”

Pinkston said work on Antares is on schedule to perform a static fire test on the pad at Wallops in early March. A successful test would allow Orbital to resume Antares launches of Cygnus as soon as late May.

The Atlas launch of Cygnus will be the first U.S. cargo mission to the station since the failed launch of a SpaceX Falcon 9 carrying a Dragon spacecraft in June. The last successful U.S. cargo mission to the station was the previous Dragon mission in April.

“2015 has been a difficult year for ISS,” said said NASA ISS program manager Kirk Shireman at a Dec. 2 briefing here, referring to those two failures as well as the loss of a Russian Progress cargo spacecraft in April. “So we’re looking forward to those supplies being replenished by the Cygnus.”

Shireman added that NASA currently expects the first Dragon flight since the June failure no earlier than Jan. 8. That launch will be the second or third for Falcon 9 since the accident, after a launch of 11 Orbcomm satellites tentatively scheduled for mid-December and, possibly, the SES-9 satellite for SES.

“Of course, there’s still technical issues because they’re recovering from an accident as well,” Shireman said. “So there’s a chance that date won’t hold.”

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...