The report is titled "Space Station: Cost to Operate After Assembly Is Uncertain" and was written by the General Accounting Office, the investigative arm of Congress. A copy was obtained for space.com by States News Service Friday. It says that NASA cost estimates for the station "does not include all funding requirements related to space station operations."

The biggest threat to the stations operations, the GAO report says, is that Russia may not fulfill its obligations to the Space Station, as it has already failed to supply what it promised for the construction phase.

NASA has not estimated the cost of providing another propulsion module for re-boosting the complex if the Russian model fails. Other partners in the station may also default, the report suggests.

The report also notes that NASA does not fully include in its projections those costs that fall under other NASA budget items, such as space shuttle flights and space communications.

"There is no full-cost accounting system in place at NASA yet," the report said, and there are no budget estimates for the station past 2004, the year the ISS is scheduled to begin full operations.

The GAO says there is nothing in the operational budget to cover the replacement of obsolete parts or upgrading working parts.

The report quoted "space station officials" as estimating the cost of a "robust enhancement program" at $100 million. Such costs have not yet been budgeted.

NASA could not be reached for comment late Friday.

The upcoming report is the second GAO study of the station in less than a month. Last month the office released a study that said NASA efforts at commercialization of the station were uncertain and had been met with skepticism by the aerospace industry.