A better strategy is to start mining asteroids. Sell mined stuff on earth to make enough money to build ships and lift necessary bootstrapping material out of t

A better strategy is to start mining asteroids. Sell mined stuff on earth to make enough money to build ships and lift necessary bootstrapping material out of the gravity well. Also plenty of water out there and sunshine, burning hydrogen as fuel should be possible. Now you will have enough machinery in space to mine building stuff and enough fuel. Gradually build permanent colonies in space. It won't get all humans out of earth soon or ever. But it will put some humans permanently out of earth. Then keep building colonies as population in space grows. Once you have done this you have done two things: 1. Eliminate a existential threat to humanity more or less permanently. 2. A slow but steadily growing population of humans in space. Several decades/centuries after that you could have humans permanently living and working in space. From there its a matter of population growth, resources are a plenty and you have a universe for the taking. Once you have a massive presence in space interstellar travel by definition is a question of acquiring real estate to keep up with population growth.