President Obama has just signed legislation that may heighten tensions with Russia and China and potentially launch the country on an expensive and ambitious effort to build space-based defense systems.

The National Defense Authorization Act is a year-end policy bill that encompasses virtually every aspect of the US military. It has two provisions with potentially momentous consequences.

According to the LA Times, the word "limited" is from language describing the mission of the country's homeland missile defense system. It's designed to thwart a small-scale attack by a non-superpower such as North Korea or Iran. A related provision has called for the Pentagon to begin "research, development, test and evaluation" of space-based systems for missile defense.

Together, these provisions signal that the US will seek to use advanced technology to defeat both small-scale and large-scale nuclear attacks. This could unsettle the decades-old balance of power among major nuclear states.

Huge bipartisan majorities in both houses of Congress have approved the policy changes over the past month with virtually no public debate. However, although the White House had earlier criticized the changes, it stopped short of threatening a veto. On Friday, Obama signed the legislation.

LA Times notes that the president criticized various aspects of the bill, including the structure of a cyber-security command and limits on administrative leave for employees -- but they didn't say anything about the changes in nuclear defense policy.

Leading defense scientists said the idea that a space-based system could provide security against nuclear attack is a fantasy.

L. David Montague, a retired president of missile systems for Lockheed Corp. and co-chair of a National Academy of Sciences panel that studied missile defense technologies, said it "defies" the laws of physics and is not based on science of any kind.

Rep. Trent Franks, who introduced the policy changes in the House, said he drew inspiration from President Reagan's Strategic Defense Initiative of the 1980s. The initiative intended to use lasers and other space-based weaponry to render nuclear weapons "important and obsolete."

According to LA Times, this was known as "Star Wars" and the initiative cost taxpayers $30 billion, but no such system was developed.

Franks said the country should maintain and improve a robust layered missile defense system that's capable of defending the US territory and its allies against the developing and increasingly-l complex ballistic missile threat.