NASA cost estimates for operating the International Space Station through 2024 are "overly optimistic," the agency's inspector general reported Thursday, adding that the price of new U.S.-built space taxis likely will be higher than currently projected, exceeding the cost of flying aboard Russian Soyuz spacecraft.

NASA Inspector General Paul Martin also raised questions about NASA's ability to safely operate the lab complex through 2024, the current goal, unless engineers can develop ways to offset age-related solar array degradation; minimize equipment failures and get large replacement components to the lab in the absence of the space shuttle.

"While the ISS program is actively working to mitigate these risks, anticipating the correct amount of replacement parts and transporting them to the ISS present major challenges to extending station operations 10 or more years beyond its original expected service life," Martin concluded.

More troubling, perhaps, the OIG found that the "assumptions underlying the agency's budget projections for the ISS are overly optimistic and that its actual costs may be higher."

The report said NASA projects the space station budget will grow from $3 billion a year to nearly $4 billion by fiscal 2020. But the OIG found station costs rose 26 percent between fiscal 2011 and 2013 "and an average of 8 percent annually over the life of the program."

Much of the projected cost increase, the report said, was due to higher transportation costs.

"NASA's estimates for the cost of commercial crew transportation services expected to replace the Russian Soyuz are based on the cost of a Soyuz seat in FY 2016 -- $70.7 million -- per seat for a total cost of $283 million per mission for transporting four astronauts," the report said.

"However, the program's independent government cost estimates project significantly higher transportation costs when the agency transitions to contracts with commercial spaceflight companies."

NASA has relied on the three-seat Russian Soyuz to ferry astronauts to and from the space station since even before the shuttle's retirement in 2011. While the cost per seat is significant, it is far less than the cost of a seat on the much more powerful, and more expensive, space shuttle.

Even so, the lack of a U.S.-built ferry craft has rankled lawmakers and NASA managers alike.

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Space station cost projections questioned