Under pressure from Congress and the Trump administration to bolster its military presence in space, the U.S. Defense Department is once again studying ways of mounting missile or lasers onto satellites in order to shoot down ballistic missiles before they can strike U.S. soil.

But experts say there's no way to build an effective orbital missile shield that doesn't cost hundreds of billions of dollars. And even if Congress somehow found the money, there's no guarantee space-based missile defenses would work when it matters most—when a North Korean or Iranian rocket is arcing toward an American city.

The 2019 National Defense Authorization Act, which President Donald Trump signed into law in mid-August, directs the U.S. Missile Defense Agency to begin work on an orbital system that can hit enemy rockets while they're still climbing into space—the phase of flight where they're slowest and most vulnerable to interception.

The NDAA instructs the Missile Defense Agency to initially design the orbital shield around "kinetic interceptors"—that is, missiles that destroy other missiles. At the same time, the act requires the agency to begin looking at high-power lasers that might eventually replace the kinetic interceptors aboard the missile-shield satellites.

“If you deploy a space-based interceptor constellation, which is something we’ve studied in excess of 30 years, I think the effectivity is beyond doubt; it’s not technically hard to do,” Michael Griffin, the undersecretary of defense for research and engineering, told reporters at an August trade event.

But “it does represent a substantial policy shift,” Griffin added. “It’s a new cost not presently in the budget.”

An orbital missile-defense system "is not against the laws of physics but might as well be in terms of difficulty and possibility of success," Victoria Samson, a space expert with the Secure World Foundation in Colorado, told The Daily Beast.

While the push for space-based weaponry coincides with Trump's own, bizarre efforts to establish a "Space Force," in fact the Congressional mandate, primarily backed by Sen. Ted Cruz of Texas, was in the works long before Trump first mentioned Space Force back in March.

Indeed, Congress and the military for decades have been trying to develop missiles and lasers for shooting down enemy ballistic missiles, with limited success. Despite spending around $10 billion a year for a generation, the Missile Defense Agency has managed to build only a handful of working systems, including sea- and ground-launched missiles. And even these missile-defenses have fared poorly in testing.

The Missile Defense Agency was not able to respond to inquiries before this story’s deadline.

An orbital missile shield is, in some ways, the Holy Grail of American missile-defense efforts. In the 1980s, President Ronald Reagan's Strategic Defense Initiative—a.k.a. "Star Wars"—fronted a plan to deploy hundreds of satellites packing small rocket-destroying missiles.

These so-called "Brilliant Pebbles" satellites would have orbited over the Soviet Union, primed to fire their missiles the moment the Soviets launched nuclear-tipped rockets toward the United States. Leaving aside the potential for Brilliant Pebbles to spark a dangerous arms race, the concept far exceeded the 1980s technology and space-launch capacity.

"This proposal has a remarkable number of problems," Charles Bennett, then a Florida Congressman, wrote in The New York Times. "The system requires technological breakthroughs in dozens of areas." In the 1990s, Brilliant Pebbles faded from Pentagon budgets.

Now, thanks to Congress, a version of that system is back in military plans. And its prospects have not improved.

The main problem is coverage. While the military and the defense industry possess the know-how to build a missile-armed satellite, the sheer number of satellites and missiles that a reliable shield would require makes it impractical.

A satellite in low orbit is constantly moving relative to Earth. "This means an interceptor that is within range of a missile launch site at one moment will quickly move out of range," David Wright, a physicist with the Massachusetts-based Union of Concerned Scientists, explained in a series of blog posts.

Round-the-clock coverage of all the potential launch sites in North Korea alone could require hundreds, possibly thousands, of armed satellites, Wright estimated. A 2012 study by the National Academies of Science and Engineering projected that such as system would cost at least $300 billion, or roughly half of the military's entire annual budget.

At present, the United States possesses around 860 satellites. Building an orbital missile shield could easily double that number. Launching and controlling all of those extra satellites would stretch U.S. space infrastructure to the limit. "It is a hugely difficult thing to do," Samson warned.

Moreover, even an orbital missile shield boasting hundreds of satellites would ensure that just a few satellites would be in position to defend against an enemy attack. It wouldn't be hard for an adversary to "punch a hole" in such a flimsy shield, Wright warned.

Existing satellite designs would probably max out at four missiles, owing to the weight of the munitions and their fuel. An attacker could overwhelm the missile shield by launching more rockets than the U.S. system has interceptors. Or the attacker could launch a few rockets at the satellites themselves, forcing them to use up their missile defending themselves.

It's not for no reason that Congress directed the Missile Defense Agency to study lasers as possible future armament for an orbital missile shield. A laser-armed satellite could fire more shots than could a missile-armed satellite.

But there's a catch. "We don’t have lasers that could do this," Wright said. "You would need to have a powerful enough laser that it could destroy a missile from hundreds of kilometers away. And you would need to make it small and light enough that you could launch large numbers of them into space."

In the 1980s, the Brilliant Pebbles missile-shield concept exceeded the capabilities of current technology. Thirty years later, not much has changed. A marginally-effective orbital missile shield would cost a fortune and might never work in actual combat.

"The idea of an orbital missile shield keeps on resurfacing because people want it to work, not necessarily because it can work," Samson said.