The detonation engine still uses both fuel and oxidizer. It isn't nuclear. The nuclear engine was just mentioned at the end as another engine being worked on.

Start at the moon, mine it to build a large-ish space habitat that can grow food and perfect closed loop life support systems. Build a second one and park it in orbit around Mars. Then be able to land on Mars and return at will. Profit.

Hypothetically speaking, you could put solar panels on a helium balloon and float it above the horizon at the north pole and gain the advantage of being in sunlight 24/7 and not have to convert it to microwaves to beam the energy back to earth.

electric to microwave is in the ball park of 80% and microwave to electric is about 85%, so 68% or so total. Though there may be atmospheric attenuation.

Earth's population growth is slowing. As standard of living increases people have fewer kids. if the population stays relatively stable, we'll be fine. Climate change is an issue but it can be managed. Nuclear war can hopefully be diverted. There are several paths to the end of the world as we know it that are completely out of our hands. Super volcanoes, pandemics maybe, I'm sure there are others... Those happen rarely.

In our solar system we have pretty much unlimited growth, unlimited energy. If we go to space we can have a society that dwarfs any scifi society out there. We don't have to go. We'll be fine here on Earth for probably hundreds of thousands of years. But society can be much more if we choose to go to space.

Expanding to other stars is unnecessary and difficult if we never develop ftl travel. But even then, we still can go if we so choose. But not in our life time.

Tbh I doubt we'll land humans on mars this century. Pretty sure we'll see orbital habitats first. We need to figure out long-term life support without resupply.