The Orion lunar test flight was originally supposed to happen in 2017, but delays with both the service module and SLS pushed that back to mid-2020. NASA’s Office of Inspector General says that date may slip even further. The first Orion flight with crew aboard, a lap around the Moon and back without stopping in lunar orbit, is not expected until 2023 — 51 years after humans last left low-Earth orbit during Apollo 17.

Orion and SLS have origins in the George W. Bush administration, which initiated a program called Constellation to land humans on the Moon by 2020. In 2010, the Obama administration redirected NASA to an asteroid and Mars, and under a deal struck with Congress to preserve space shuttle-era jobs and facilities, Orion and SLS evolved into their present forms. The Trump administration shifted NASA’s human spaceflight goal back to the Moon in late 2017, directing the agency to “enable human expansion across the solar system.”

In September, NASA released a 21-page plan called the National Space Exploration Campaign to meet that directive. The plan centers around SLS, Orion, and commercial rockets constructing a small space station called the Gateway in lunar orbit that would serve as a waypoint for missions to and from the lunar surface. As for surface missions, rather than going straight to large, Apollo-style landers, NASA plans to start with small robotic missions and scale up from there.

The plan also says American astronauts will be back on the Moon by 2030, and more recent statements by NASA officials have pledged an earlier goal: 2028. Mary Lynne Dittmar, the president and CEO of the Coalition for Deep Space Exploration, which represents the aerospace industry, supports the strategy.

“The current approach, which involves a broad based coalition of industry, international partners and government, can establish U.S. leadership in space for decades to come, unleashing the best of innovation, entrepreneurial spirit, government-funded infrastructure, and expanding international relationship and U.S. soft power – all while also supporting the aerospace and defense sector at home,” she said.

But many experts say a Moon landing by 2028 is impossible without a major funding boost to NASA’s human spaceflight program. Approximately half of the agency’s $21 billion current annual budget directly or indirectly supports human spaceflight activities. Most of that is consumed by International Space Station operations and transportation, SLS, and Orion, leaving little wiggle room to build hardware in support of a return to the Moon.

“Extraordinary plans require extraordinary budgets, which NASA has not seen since the Apollo missions,” said Laura Seward Forczyk, founder of Atlanta-based space consulting firm Astralytical. “NASA has been locked in a cycle of different but similar Moon-to-Mars or asteroid-to-Mars or LEO-to-Mars human spaceflight architectures for decades.”

To shift more funds to deep space exploration, the Trump administration has proposed to stop directly funding the ISS in 2025 and turn the space laboratory’s operations over to the commercial sector. The National Space Exploration Campaign claims that once commercial astronaut providers like SpaceX and Boeing are up and running, a new market will flourish for in-space activities that include tourism and manufacturing.

Matthew Hersch, a science and technology historian and assistant professor at Harvard University, says that premise is flawed.

“It imagines a new program of exploration to be paid for by convincing private industry to pay for NASA’s existing human spaceflight operations, though it describes no mechanism for creating such a market,” he said.

Congress has already signaled it is unlikely to support a withdrawal of ISS funding in 2025. Dittmar notes establishing the infrastructure, suppliers and investment pipelines to support commercial activities currently happening on the International Space Station took years to develop.

“Personally I think [repeating the model for a standalone commercial market] will happen, but I don’t think it will happen by 2025,” she said. “The focus of both government and private efforts needs to be on establishing demand, which is the crimp in the economic development pipeline."

With the fate of the ISS up in the air and no funding increase on the horizon — on the contrary, the Trump administration is considering a 5 percent cut to agencies like NASA — critics say the agency should rethink its deep space exploration plans, starting with the Gateway.