The National Academy of Sciences is casting more doubt on whether the Obama administration's European-based missile defense shield can protect the United States and recommends scrapping key parts of the system.

The academy's assessment could complicate White House efforts to persuade Congress to fund the still-developing program. Though the academy says the plans would protect Europe effectively, some lawmakers already are asking why the U.S., at a time of tight budgets, should spend billions of dollars on a system that provides limited homeland defense.

The conclusions from the academy, which advises the government on science and technology, are contained in a letter to lawmakers obtained by The Associated Press.

The academy's letter bolsters two earlier reports by Defense Department advisers and congressional investigators that said the European system faced significant delays, cost overruns and technology problems.

The letter is dated April 30 and addressed to the chairman of the House Armed Services strategic forces subcommittee, Rep. Michael Turner, R-Ohio, and the panel's top Democrat, California Rep. Loretta Sanchez. It is based on unclassified parts of a broad academy report on U.S. missile defense capabilities not yet released.

Rick Lehner, a spokesman for the Missile Defense Agency, said he was unaware of the academy's report and declined to comment.

Republicans, who have been questioning President Barack Obama's national security credentials ahead of the November elections, are likely to seize on the letter to bolster their argument that the European plans were poorly thought through and designed to appease Russia.

The defense shield is one of Obama's top military programs. Soon after he took office in 2009, he revamped a Bush administration missile defense plan that had been a chief source of tension with Russia. The Russian government believed the program is aimed at its missiles, while the U.S. said the system was designed to counter any Iranian missile threat.

While Russia initially welcomed the Obama administration's changes, it since has ramped up its criticism. On Thursday, Russia's top military officer went so far as to threaten pre-emptive military action on missile defense facilities in Eastern Europe if the U.S. goes ahead with its plans.

Obama's plan calls for slower interceptors than the earlier plan that could address Iran's medium-range missiles. The interceptors would be upgraded gradually over four phases, culminating in 2020 with newer versions, still in development, that the administration says will protect Europe and the United States. The early phases call for using Aegis radars on ships and a more powerful radar based in Turkey. Later phases call for moving Aegis radars to Romania and Poland.

The academy says the proposed system could effectively defend Europe and U.S. troops based there against short- and medium-range missiles from Iran if the system uses an interceptor that is fast enough. But it dismisses the administration's claims that the system eventually will offer protection to the United States as well. It says the system is "at best less than optimal for homeland defense."

It recommends eliminating the last phase of the Obama plan because it says the interceptors planned for that phase will not be fast enough to take down intercontinental missiles launched from Iran. It says the Bush administration plan would have faced the same problem.

It also recommends abandoning a satellite tracking system now in development that the administration has argued could solve weaknesses in the system's radars. A report by the Defense Science Board, a Pentagon advisory group, argued that the radars planned for the shield were too weak to track missiles effectively. The administration has denied that and said its satellite system would bolster the missile shield's capabilities.

But in blunt language, the academy rejects that claim, saying the satellites would be too far away from the threat to provide useful data. It also says the system would cost up to three times the administration's estimates.

According to a congressional aide who has seen the academy's study, it estimates the satellite system would cost $27.7 billion. The aide spoke on condition of anonymity because he was not authorized to comment publicly.

The report recommends "terminating all effort" on the satellite project.

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Online:

The National Academy of Science's letter: http://apne.ws/JuTlSn

Missile Defense Agency: http://www.mda.mil/system/system.html

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