L ASER WEAPONS orbiting in space and warplanes that shoot down rockets sound like the doodlings of a teenage boy. Both appear in the Trump administration’s missile-defence review, published on January 17th. It lays out a celestial vision of homeland defence that looks cosmically expensive and technologically dubious.

America does not skimp on shooting missiles out of the sky. Its 2018 budget allocated $19.3bn to the task—roughly equivalent to the entire defence budget of Canada or Turkey. Since 2001 it has splashed out over $130bn. Some of that is spent on ship-based Aegis and land-based Patriot and Terminal High Altitude Area Defence ( THAAD ) systems, which are aimed at short or medium-range missiles. Intercontinental ballistic missiles ( ICBM s) fly higher and faster.

For those, America has built a sprawling “ground-based midcourse defence” ( GMD ) directed at North Korea and Iran. At $67bn and rising, it is the Pentagon’s fourth-most-expensive weapon system. Launches are spotted by infrared satellites and a radar network stretching from Cape Cod to Japan, and then—in theory—struck by one of 44 interceptors in Alaska and California.

Though GMD was declared ready in 2004, it was not tested against an ICBM -type target until 2017 and then under generous conditions. Using four interceptors against one warhead is assumed to give a 97% chance of a hit. That sounds promising. But if merely a dozen missiles were volleyed at America, not only would it soak up more than $3bn of interceptors but a single warhead would still have a 30% chance of getting acquainted with an American city. The average revolver offers better odds for a game of Russian roulette.

The Trump administration has been adding interceptors, beefing up radars and conducting new tests. But the latest missile-defence review, the first in nine years, makes some more radical proposals.

One is to shoot down missiles in their “boost phase” as they take off, when they are slower and harder to disguise, rather than above the atmosphere as GMD aims to do. Since the boost phase lasts for only a few minutes, that requires spotting launches and pouncing quickly. The suggestion is that fighter jets like the F -35 or even drones could be “surged” towards enemy launchpads in a crisis, armed with new interceptor missiles or compact lasers. That carries obvious risks.

So the second strategy is to do more sensing and shooting from space. This fits with Mr Trump’s galactic proclivities. In December he ordered the creation of a new Space Command to run military operations in space. A new Space Force and Space Development Agency are in the works.

The Pentagon is especially keen to put larger numbers of smaller and cheaper satellites into lower orbit for “birth to death tracking”: from detecting tell-tale plumes at launch to establishing whether an intercept is successful. Officials are also beginning a six-month study into the feasibility of putting the interceptors themselves, whether rockets or lasers, into space.

Few of these ideas are new. An airborne laser was successfully tested against missiles in 2010. The Obama administration poured hundreds of millions of dollars into space sensors. The vision of orbiting lasers harks back to the Reagan administration’s Strategic Defence Initiative, widely dubbed “Star Wars”.

In 2012 the National Research Council published a detailed and scathing judgment of such methods. Boost-phase defence, it said, “is not practical or cost effective under real-world conditions for the foreseeable future”. It pointed out that rocket motors burn out so quickly that interceptors would have to get unfeasibly close to the launch-pad.

Space-based interceptors might deal with that problem, but would require a preposterously large constellation of satellites costing hundreds of billions of dollars. The Pentagon insists that new, commercially available technology will bring down costs. Its task is to persuade Congress that the budget, at least, is not headed to infinity and beyond.