A new report from the space agency's inspector general concludes that NASA has no easy choice when it comes to the future of the massive International Space Station that orbits about 400km above the Earth. The station, which has been continually inhabited by humans for nearly two decades, is presently the only destination in outer space for astronauts and cosmonauts to visit.

Recently, the White House announced that it wanted to end direct NASA funding for the space station in the year 2025 and transfer operations to the private sector. Key Senators who oversee NASA's activities have pushed back against this idea in a bipartisan way, saying that the station should be funded through 2028 in order to complete its mission of scientific research. That extension would also help development and testing of new technologies needed for deep space exploration by humans—such as toilets that don't break every six months.

In the report published Monday, NASA Inspector General Paul Martin found that both of these choices have some significant downsides. "Each of the options for transitioning or retiring the ISS present NASA with distinct challenges and associated cost," the report states.

Option 1: Privatize the station

NASA spent more than a decade, and nearly $100 billion, to construct the station along with international partners in Russia, Europe, Canada, and Japan during the 2000s. NASA now spends $3 to $4 billion a year to maintain the station, including an ongoing maintenance contract with Boeing and commercial cargo and crew services. This is about half of NASA's total budget for human exploration activities.

The White House would like to hand the station over to a private entity in 2025, with NASA assuming "tenant" status in low-Earth orbit, and then spend those extra billions of dollars to give its notional plans to send humans to the Moon and Mars some basis in reality. However, since the transition proposal was made, it has not been clear whether any private entity could profitably operate the station. The inspector general sounds the alarm about this as well.

"Based on our audit work, we question the viability of NASA’s plans," the report states. "Specifically, we question whether a sufficient business case exists under which private companies will be able to develop a self-sustaining and profit-making business independent of significant federal funding. In particular, it is unlikely that a private entity or entities would assume the Station’s annual operating costs, currently projected at $1.2 billion in 2024."

In short, the market beyond existing microgravity research—which might include space tourism, satellite servicing, manufacturing of goods, and research and development—simply does not yet exist. Claims that such markets will exist six years from now are "overly optimistic," the report finds. And without the station after 2024, the report notes that NASA would not complete a number of astronaut health and technology development projects, including environmental control and life support systems.

Option 2: Fly it through the 2020s

Alternatively, Congress could get its way, and the station would fly through 2028. The station's prime contractor, Boeing, has recently certified that the major components of the orbiting laboratory will remain in good working condition through then. This would provide NASA and its academic partners the certainty they need for long-duration missions, and it would allow the space agency to prove out the technology it needs to safely send humans deeper into space.

However, the price is high: a continued expenditure of $3 to $4 billion a year, plus further delays of a human return to the Moon or eventual trips to Mars in a few decades.

"We found that, assuming funding for NASA’s human exploration program remains constant, a continuation of ISS funding through 2028 will require either increased funding in the 2020s to develop exploration systems needed for Mars missions or will require the Agency to push out the timeline for its Mars exploration plans," the report states.

Continuing to fly the station could also stunt the nascent efforts of companies like Bigelow and Axiom to develop lower-cost, smaller private stations in low-Earth orbit.

The bottom line is that one of the most consequential space disagreements between the Trump White House and Congress concerns the future of the International Space Station. From the standpoint of advancing human activity into the final frontier, neither side's position seems like a surefire winner right now.