WASHINGTON (Reuters) -President Donald Trump said on Tuesday he had selected a design for the $175-billion Golden Dome missile defense shield and named a Space Force general to head the ambitious program aimed at blocking threats from China and Russia.

The program, first ordered by Trump in January, aims to create a network of satellites, perhaps numbering in the hundreds, to detect, track and potentially intercept incoming missiles.

Trump told a White House press conference that U.S. Space Force General Michael Guetlein would be the lead program manager for an effort widely viewed as the keystone to Trump's military planning.

Golden Dome will "protect our homeland," Trump said, adding that Canada had said it wanted to be part of it.

In a statement, the office of Canadian Prime Minister Mark Carney said he and his ministers were discussing a new security and economic relationship with their American counterparts.

"These discussions naturally include strengthening NORAD and related initiatives such as the Golden Dome," it added.

Trump said the defense shield, which would cost some $175 billion, should be operational by the end of his term in January 2029, but industry experts were less certain of that timeframe and the cost.

   "Ronald Reagan wanted it many years ago, but they didn't have the technology," Trump said, referring to the space-based missile defense system, popularly called "Star Wars", that Reagan proposed.

The Golden Dome program faces both political scrutiny and funding uncertainty.

"The new datapoint is the $175 billion, but the question remains, over what period of time. It's probably 10 years," said Tom Karako of the Center for Strategic and International Studies.

Silicon Valley and U.S. software expertise can be leveraged to bring advances, while also using existing missile defense systems, he added.

This month, the Congressional Budget Office estimated that Golden Dome could cost as much as $831 billion over two decades.

Democratic lawmakers have voiced concern about the procurement process and involvement of Trump ally Elon Musk's SpaceX, which has emerged as a frontrunner alongside Palantir and Anduril to build key components of the system.

"The new autonomous space-age defense ecosystem is more about Silicon Valley than it is about 'big metal,'" Senator Kevin Cramer of North Dakota said at the White House event.

"So what's exciting about this is it makes it available to everybody to participate, to compete." 

"Big metal" refers to legacy defense contractors. 

The Golden Dome idea was inspired by Israel's land-based Iron Dome defense shield that protects it from missiles and rockets.

Trump's Golden Dome is much more extensive, including a massive array of surveillance satellites and a separate fleet of attacking satellites that would shoot down offensive missiles soon after lift-off.

Tuesday's announcement kicks off the Pentagon's effort to test and ultimately buy the missiles, systems, sensors and satellites that will constitute Golden Dome.

Trump said Alaska would be a big part of the program, while Florida, Georgia and Indiana would also benefit.

Many of the early systems are expected to come from existing production lines. Attendees at the press conference named L3Harris Technologies, Lockheed Martin and RTX Corp as potential contractors for the massive project.

L3 has invested $150 million in building out its new facility in Fort Wayne, Indiana, where it makes the Hypersonic and Ballistic Tracking Space Sensor satellites that are part of a Pentagon effort to better detect and track hypersonic weapons with space-based sensors and could be adapted for Golden Dome.

Golden Dome's funding remains uncertain. Republican lawmakers have proposed a $25-billion initial investment for Golden Dome as part of a broader $150-billion defense package, but this funding is tied to a contentious reconciliation bill that faces significant hurdles in Congress.

"Unless reconciliation passes, the funds for Golden Dome may not materialize," said an industry executive following the program, who spoke on condition of anonymity. "This puts the entire project timeline in jeopardy."