The U.S. Missile Defense Agency has had to scrap 40 percent of the tests for a multibillion-dollar program that is supposed to protect the country from long-range ballistic missile threats like North Korea and Iran, according to a new federal watchdog report.

The Government Accountability Office’s latest annual audit of the Ballistic Missile Defense System (BMDS) found that risky purchasing practices, including simultaneous development and production of new anti-ballistic missiles (a.k.a. interceptors), and the use of unproven targets mean the MDA might have trouble meeting the Obama Administration’s goal of fielding the system by 2018.

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Furthermore, the “constant change to BMDS testing makes it difficult to trace progress and costs,” according to GAO.

The system, composed of a variety of sea- and ground-based interceptors and an accompanying command-and control system, has cost taxpayers $123 billion to develop since 2002. The Defense Department estimates it will spend an additional $38 billion on BMDS through 2021, for a total of $161 billion.

However, between fiscal years 2010 and 2015, the missile agency had to scuttle 40 percent of the system’s test. Unforeseen circumstances, like bad weather or test equipment malfunctions, were to blame in some instances, but others arose from high-risk acquisition strategies, that see the development and production of interceptors simultaneously and a test schedule "which leaves little to no margin to address problems that past experience has shown are likely to occur,” the audit stated.

Meanwhile, technical, funding and testing woes have forced MDA to delay 12 of the 27 capabilities planned for delivery between 2016 and 2020; some of the capabilities have been pushed back a few months or years while others have been delayed indefinitely, according to the GAO report.

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“The constant change to BMDS testing diminishes the traceability of progress and costs. The repeated flight test delays, renaming and combining tests, and removing tests, while necessary to some degree, make it difficult to determine what objectives have been met, when, and with what test,” the audit found.

MDA “is also challenged to provide the actual costs associated with testing,” the GAO added.

Since the BMDS is made up of so many platforms, it’s difficult to tell how much the umbrella effort has been or could be delayed. For instance, GAO noted, MDA delivered missile defense capabilities, as planned and other systems years ahead of schedule.