Landing the rocket Monday was a giant step toward making spacecraft reusable – a longtime quest of [Elon] Musk and aerospace engineers.

Their goal is to recycle the first stage, including its expensive engines, so it can be used repeatedly, lowering the cost of space travel.

The main problem with cost is that each rocket flies exactly once. Imagine if airplanes only flew once—a plane ticket would cost $1.5 million. The air travel industry is a big thing only because they can reuse the same plane again and again and again. In order to create a genuinely reusable rocket that revolutionizes the cost of space travel, you have to figure out how to land the giant first stage of the rocket vertically. For decades, space agencies around the world have put billions of dollars into figuring out how to land a rocket successfully, and no one has done it.