WASHINGTON — NASA formally issued a call for proposals to provide cargo transportation services for its lunar Gateway Aug. 16, offering up to $7 billion in contracts to support operations of the human-tended facility.

The Gateway Logistics Services request for proposals (RFP) is similar to a draft version of the RFP issued by the agency in June. Proposals are due to NASA Oct. 1, with the agency expected to award one or more contracts before the end of the calendar year.

Under the program, companies would deliver at least 3,400 kilograms of pressurized cargo and 1,000 kilograms of unpressurized cargo to the Gateway on each mission. The vehicle would also be required to dispose of at least as much pressurized and unpressurized cargo as it delivers to the Gateway.

One change in the requirements for such cargo spacecraft from the draft RFP is mission duration. The draft RFP stated that cargo vehicles should be designed to remain attached to the Gateway for three years, and that the capability for even longer missions “should be considered.” The final RFP requires vehicles to be designed for only one-year stays at the Gateway, but with the option for potentially longer missions. “The nominal mission docked duration is expected to be six months,” the final RFP notes.

The program will use fixed firm price contracts with milestone payment schedules, like that used for cargo transportation services to and from the International Space Station. The payment schedule included in the RFP anticipates providing 75 percent of the payment of each mission prior to launch.

The RFP is the latest step in NASA’s development of the lunar Gateway, which emphasizes the use of commercial services and partnerships. In May, NASA awarded a $375 million contract to Maxar for the Gateway’s first module, the Power and Propulsion Element. The element will be owned by Maxar throughout the contract, at the end of which NASA will have the option to purchase it for use on the Gateway.

In a separate procurement filing in July, NASA announced its intent to award a sole-source contract to Northrop Grumman to build a “mini-hab” module for the Gateway that will serve as an initial habitation module. Northrop Grumman was one of six companies working on habitation module designs under the Next Space Technologies for Exploration Partnerships 2 program, but the only one NASA concluded could deliver a module in time to support a 2024 human mission to the lunar surface.

NASA plans to use the Gateway as a staging zone for human lunar landings. A human lander, likely consisting of two or three modules, will be delivered to the Gateway on commercial launch vehicles. A crewed Orion spacecraft, launched on a Space Launch System rocket, will then travel to and dock with the Gateway. Astronauts will transfer to the lander for a mission to the south polar region of the moon.

The agency’s current plans call for then expanding the Gateway with the addition of new habitation, utilization and other modules, many of which will be provided by international partners. Exactly when and how the Gateway will be expanded has yet to be determined.

NASA Administrator Jim Bridenstine has, in recent months, likened the Gateway to the command and service modules of the Apollo spacecraft of a half-century ago. “Think of a small space station in orbit around the moon, a reusable command and service module that will be in orbit around the moon for 15 years, at least,” he said in an Aug. 16 speech at the Marshall Space Flight Center to announce that center would manage the human lunar lander effort.