“In other words, conventional invasion has an expiration date attached. I would guess it’s about 5 years after cheap launch becomes a reality.”
This prompted reader David Davenport to ask what was meant by “cheap launch”.
Generally speaking, the biggest expense for a satellite is getting it up there. Rockets are terribly inefficient and costly, but so far no one has built a viable alternative. But there have been some ideas proposed to get around this problem.
One of the easiest is to build a big gun and shoot small payloads into orbit. This would work to get the packages up into Low Earth Orbit (LEO), but the acceleration the payload would be subjected to would mean that it would have to be rather overbuilt. This would increase the cost per, and the size of the gun would mean that it would have to be fixed in place. Your options for the orbital path of the satellite would be limited. Still, this is the cheapest and most practical method for launching orbital weapons, and its well within the technical capability that we have now.
(As an aside, famed weapons designer Gerald Bull was very enthusiastic about this idea. He was so taken with the concept of using a giant cannon to launch satellites that he agreed to work for Saddam Hussein in the 1980’s. In return for helping the Iraqis with their missile and artillery projects, Saddam was going to fund Bull’s space-launch supergun.)
Another idea is to use ground based lasers to boost craft up to the edge of the atmosphere, where conventional rockets could take over. High intensity lasers would be pulsed into the underside of a vehicle and focused by mirrors into an air-filled chamber. The intense heat created would cause the air to heat up and explosively expand. The craft would act like a rocket without having to carry all of that heavy fuel, at least until it travels so high that the atmosphere runs out.
The most popular proposal is for a space elevator, which is a giant cable tens of thousands of miles long that stretches out into space. In simple terms, use an elevator to crawl up the cable and then simply push the satellite out the door when you get high enough.
The non-trivial technical problems that keep the space elevator from being built are legion. First and foremost is developing a material both strong enough to withstand the stress and light enough to make it possible to haul a strand about 65,000 miles straight up. Until I see bulletproof T-shirts for sale, or buy a car that weighs 2 or 3 pounds but can still withstand a 40 MPH collision with a brick wall, I’m afraid that it’s a pipe dream.
So TMLutas is very correct in saying that any large invasion force is impossible a few years after cheap ways of launching space-based defenses are developed. But, since it doesn’t look like they’re going to be developed any time soon, we’ll just have to keep on muddling along the way we have been.