Weapons!

It turns out if you want to blow up an asteroid coming your way, or at least nudge it off its course, lobbing some megatons of TNTs in its general direction is a lot harder than sending over some nukes.

Also, things put on top of nukes when they blow up go really high, really fast, even if they're heavy. So if you add a nuke chute to the back of your spaceship, and some mechanism for dampening the blow you can build some really awesome interplanetary transports. With enough nukes you can even go to other stars in more or less reasonable time frames!

The only realistic plan to go to the stars and return within, say, 40 years without using techno-magic is to use lasers [1]. Nukes just cannot do that at all, they are only good for efficient travel within solar system.

[1] - https://www.academia.edu/1884741/Roundtrip_interstellar_trav...

The Project Orion people had extensive calculations on how to send a city-sized ship to another star on a ~100 years trip with nukes.

It is laser propulsion that is techno-magic. Even today. I won't register on whatever site to read yet another article on that, but I do bet neither the lasers nor the sail on this one could be made today.

Project Orion was believed to work with technology from the fifties too. That makes me confident that we could build one today, if we wanted to.

What happened to the plan to use a laser to push it of course?