WASHINGTON — Having delayed the award of follow-on Commercial Resupply Services (CRS) contracts until at least November, NASA is considering ordering more International Space Station cargo deliveries from Orbital ATK and SpaceX, both of which had already quietly hauled in additional orders under CRS deals signed in 2008.

After contract modifications initiated late last year and finalized this summer, SpaceX is on the hook for a total of 15 flights to the space station, up from the 12 NASA ordered in 2008. Orbital ATK wound up with 10 flights, up from eight, following the latest round of contract modifications, NASA spokeswoman Stephanie Schierholz wrote in an Aug. 20 email.

“NASA is discussing additional modifications with both companies,” Schierholz wrote.

Schierholz declined to disclose the value of NASA’s latest CRS orders. “[W]e are in an active procurement for CRS-2 [and] to maintain fair competition under the CRS contract, it is essential that NASA protect the commercial pricing aspects under the contracts,” she wrote.

NASA’s original CRS order obligated SpaceX and Orbital ATK to deliver 20 metric tons of cargo each to ISS. At the time, that worked out to 12 flights for SpaceX totaling $1.6 billion and eight flights for Orbital ATK totaling $1.9 billion. Each company’s indefinite quantity, indefinite delivery CRS contract is good through 2018 and has a maximum value of $3.1 billion.

Orbital ATK and SpaceX now face competition for a follow-on CRS contract from at least three companies: Boeing, Sierra Nevada and Lockheed Martin. Those companies, and the incumbents, have all confirmed they bid on CRS-2 when NASA solicited offers in September.

In the CRS contract mods already completed, Orbital ATK originally got three additional flights, Schierholz said. However, two of Orbital ATK’s missions were combined after the company decided to enlarge its Cygnus space tug and launch it aboard a United Launch Alliance Atlas 5 from Cape Canaveral Air Force Station, Florida. Atlas 5 is more powerful than Orbital ATK’s Antares vehicle, which is out of commission until early next year following an Oct. 28 launch failure at the Mid-Atlantic Regional Spaceport in Wallops Island, Virginia.

Orbital ATK is using Atlas 5 while it replaces Antares’ main-stage engine with new Russian-made RD-181s. Antares was using AJ-26 engines, Soviet-vintage NK-33s refurbished and rebranded by Aerojet Rocketdyne of Sacramento, California. After the AJ-26 was blamed for the Antares failure, Orbital ATK decided to switch engines and ordered one ULA Atlas 5 through Lockheed Martin Commercial Launch Services for a Cygnus launch slated for December.

Orbital ATK disclosed Aug. 12 that it ordered a second Atlas 5 for a Cygnus mission in early 2016.

“We do not have any other Atlas launches planned for our CRS contract,” Orbital ATK spokeswoman Vicki Cox wrote in an Aug. 13 email. She would not say whether the company held options to purchase more Atlas 5s.

Both Atlas 5-launched Cygnus tugs will carry their maximum load of 3,500 kilograms of pressurized cargo. Orbital ATK will launch at least three, and possibly four, CRS missions in 2016, Frank DeMauro, the company’s Cygnus program manager, said in an Aug. 12 phone interview.

Like Orbital ATK, SpaceX is also recovering from a launch failure during a CRS mission. The company’s Falcon 9 rocket disintegrated about two minutes after launch on what was supposed to be its seventh paid cargo run for NASA.

SpaceX founder and Chief Executive Elon Musk implicated a faulty upper-stage strut in the failure, although the Hawthorne, California-based company has yet to publish an official report on the accident.

Musk has also said SpaceX could resume launching as soon as September, but an industry source said Aug. 19 the company is now planning a November return to flight. That launch would be a commercial mission for SES of Luxembourg using the newly upgraded Falcon 9 rocket.

SpaceX’s next cargo run for NASA, its eighth, would also happen in November, this person said.

SpaceX spokesman John Taylor declined to comment about the company’s return-to-flight timetable and referred questions about its CRS contract to NASA.