One struck the word "limited" from language describing the mission of the country's homeland missile defense system. The system is designed to thwart a small-scale attack by a non-superpower such as North Korea or Iran.
A related provision calls for the Pentagon to start "research, development, test and evaluation" of space-based systems for missile defense.
Together, the provisions signal that the U.S. will seek to use advanced technology to defeat both small-scale and large-scale nuclear attacks. That could unsettle the decades-old balance of power among the major nuclear states.
Huge bipartisan majorities in both houses of Congress approved the policy changes over the past month, with virtually no public debate.
Although the White House had earlier criticized the changes, it stopped short of threatening a veto.On Friday, Obama signed the legislation.
In a four-page signing statement, the president criticized various aspects of the bill, including the structure of a cyber-security command and limits on administrative leave for employees, but said nothing about the changes in nuclear defense policy.
Before Obama's action, proponents and opponents of the policy changes agreed that they could have dramatic effects.
Leading defense scientists said the idea that a space-based system could provide security against nuclear attack is a fantasy.
"It defies the laws of physics and is not based on science of any kind," said 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 at the request of Congress.
"Even if we darken the sky with hundreds or thousands of satellites and interceptors, there's no way to ensure against a dedicated attack," Montague said in an interview. "So it's an opportunity to waste a prodigious amount of money."
He called the provisions passed by Congress "insanity, pure and simple."
Franks said that striking the word "limited" from the homeland defense system's mission, and at the same time pursuing a space-based system, would put the U.S. on a path to better safeguard its security. He said the new approach would protect both U.S. territory and surveillance satellites.
"I hope that the day will come when we could have solid-state lasers in space that can defeat any missile attack," said Franks, who represents suburbs north and west of Phoenix. "That day is a long ways off. But fortunately, it's a little closer, and a little more certain, with the passage of these amendments."
The new policy Franks championed says America "should maintain and improve a robust layered missile defense system capable of defending the territory of the United States and its allies against the developing and increasingly complex ballistic missile threat."
A space-based defense system would hinge on annual congressional appropriations and decisions by the incoming Trump administration.
The National Academy study, released in 2012, concluded that even a bare-bones space-based system would cost about $200 billion to put in place, and hundreds of billions to operate in subsequent years.
Franks, asked whether the country could afford it, replied: "What is national security worth? It's priceless."
Philip E. Coyle III, a former assistant secretary of Defense who headed the Pentagon office responsible for testing and evaluating weapon systems, described the idea of a space-based nuclear shield as "a sham."
"To do this would cost just gazillions and gazillions," Coyle said. "The technology isn't at hand — nor is the money.It's unfortunate from my point of view that the Congress doesn't see that."
He added: "Both Russia and China will use it as an excuse to do something that they want to do."
The word "limited" has guided U.S. policy since the National Missile Defense Act of 1999. The qualifier reflects, in part, the reality that intercepting and destroying incoming warheads is supremely difficult, and that it would be impractical to field enough interceptors to counter a large-scale attack.
The current homeland anti-missile system — the Ground-based Midcourse Defense system, or GMD — relies on interceptors at Vandenberg Air Force Base in California and Ft. Greely, Alaska. In flight tests, the system, which has cost taxpayers more than $40 billion, has managed to destroy mock enemy warheads only about half the time.
The first of Franks' amendments — to eliminate "limited" from U.S. policy — was approved in April by the House Armed Services Committee with no debate and without a recorded roll-call vote.
At a committee hearing May 17, a senior Democrat on the panel, Rep. Jim Cooper of Tennessee, offered mild protest.
"I think it was a mistake to mandate a poorly thought out, unaffordable and unrealistic missile defense policy, including plans for a space-based missile deterrent," Cooper said.
But neither Cooper nor any other House Democrat sought to overturn the provisions, and he was among those who voted to pass the overall bill the next day.
Franks' Republican partner on the legislation, Sen. Ted Cruz of Texas, enjoyed a similarly smooth path.
Deliberations of the Senate Armed Services Committee were closed, forestalling public debate. The legislation was approved by a roll call vote of 16-10, with two Democrats, Sens. Joe Manchin of West Virginia and Tim Kaine of Virginia, the party's eventual vice presidential nominee, joining the Republican majority.
In June, Sen. Ed Markey (D-Mass.) sought to restore "limited," saying that the change in U.S. policy would create "the impetus for a new arms race" with Russia and China. Markey offered an amendment on the Senate floor but could not muster enough support to bring it to vote.
The policy changes were greeted with opposition from another quarter as well. At a congressional hearing in April, Franks pressed Vice Adm. James D. Syring, director of the U.S. Missile Defense Agency, for his stance on expanding American capability into space.
Syring replied: "I have serious concerns about the technical feasibility of the interceptors in space, and I have serious concerns about the long-term affordability of a program like that."