WASHINGTON — Citing a desire to both maximize the cargo delivered to the International Space Station and ensure it stays on schedule, Orbital ATK said Nov. 4 it will launch its next Cygnus mission on an Atlas V rather than its own Antares rocket.

Orbital ATK said that the OA-7 Cygnus mission, previously planned to launch on an Antares rocket from Wallops Island, Virginia, will instead launch on a United Launch Alliance Atlas V from Cape Canaveral, Florida, in the spring of 2017. The company said this is a one-time arrangement, with future Cygnus launches returning to the Antares.

The shift in launch vehicles for OA-7 was a "collaborative effort" between NASA and Orbital ATK, said Frank DeMauro, vice president of human space systems in the company’s Space Systems Group. "We jointly realized that getting a little more cargo on OA-7 in the spring was in NASA's interest," he said in a Nov. 4 interview. "We jointly realized that having the highest assurance that we could meet the schedule was also in NASA's interest." [Launch Photos: Orbital ATK's Antares Rocket Returns to Flight]

Launching on an Atlas, DeMauro said, allows the Cygnus to carry more than 300 kilograms of additional cargo versus using an Antares. The OA-7 mission will be able to accommodate more than 3,500 kilograms of cargo in its pressurized module, as well as an external cubesat deployer from NanoRacks, bringing its total capacity to about 3,600 kilograms.

DeMauro said there were no issues with the upgraded version of the Antares that launched for the first time Oct. 17 on the OA-5 Cygnus mission. However, moving the launch to the Atlas does give the company's workforce "margin flexibility" to prepare for the next launch, which now is likely to take place in the middle of 2017. "It is fair to say that the Atlas will have a higher probability of supporting that mission in the spring, even though we do believe Antares would have done that," he said.

That additional performance and schedule assurance comes as two other cargo spacecraft that support the ISS suffer delays. Launches of SpaceX Dragon spacecraft are on hold as the company continues an investigation into a Sept. 1 explosion of a Falcon 9 during preparations for a static-fire test. The next Dragon mission, previously scheduled for November, is now expected to take place no earlier than January, depending on when the Falcon 9 returns to service.

A Japanese H-2 Transfer Vehicle mission also experienced delays because of a spacecraft leak found during launch preparations. That mission, originally planned to launch in early October, is now scheduled for Dec. 9.

An Orbital ATK Cygnus spacecraft is encapsulated in a payload fairing ahead of a launch atop a United Launch Alliance Atlas V rocket in March 2016. (Image credit: United Launch Alliance)

DeMauro said Orbital ATK's Commercial Resupply Services (CRS) contract with NASA gives the company flexibility to switch launch vehicles for Cygnus missions. He declined, though, to discuss any increased costs to Orbital ATK by switching launch vehicles or if NASA will pay more for this mission as a result, citing proprietary company financial information.

"We are still working to finalize the details of this change," he said. "While we expect it to go forward, we're still working through the final details."

From an operational standpoint, DeMauro said there will be little difference between OA-7 and two previous Cygnus missions that launched on Atlas 5 vehicles in December 2015 and March 2016. Orbital ATK ordered those launches to continue cargo services after the failure of an Antares rocket carrying another Cygnus spacecraft in October 2014.

DeMauro said the company will use the same payload processing facility at NASA's Kennedy Space Center for the OA-7 mission as it did for the two previous ones, and follow similar procedures to prepare the spacecraft for launch. "That's one of the things that enabled such a quick turnaround," he said.

After OA-7, DeMauro said Orbital ATK will resume using Antares vehicles for Cygnus mission for the remainder of its current CRS contract, likely to run through 2018. That includes two Antares launches in the second half of 2017 and two in 2018. The exact schedule of missions will depend on the overall ISS schedule and when NASA needs those missions to fly, he said.

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