Landing on an asteroid and mining it for resources might sound far out, but NASA is very serious about it.

So serious, in fact, that the organization's Jet Propulsion Laboratory has put out a call for design proposals of the spacecraft that will eventually land on an asteroid, then send a huge chunk of it over to humans for experimentation. The proposal isn't open to the public, though--only to the four space companies (Lockheed Martin, SSL, Orbital ATK, and Boeing) that already submitted conceptual designs.

The plan entails landing on the asteroid with an unmanned spacecraft, then corralling a several-ton boulder and sending it into orbit around the moon. Once there, crews would be able to access it, testing it and drilling for natural resources.

As the Jet Propulsion Lab explains in a letter on NASA's site, mining asteroids has several purposes. It could help scientists find answers to the very big, very unanswered questions about the origins of the solar system.

It could also serve to aid future space travel, including planned missions to Mars. Elements and compounds below the surface could be used to create fuel, including water, which can be split into useful hydrogen and oxygen. Fuel that originates in space would conceivably be cheaper to use, and would extend the distance that spacecrafts could travel before returning to earth.

With that in mind, there's a lot of money to be made by whoever is able to access these resources. In 2015, President Obama signed into law a bill allowing U.S. citizens ownership of whatever resources they're able to extract from an asteroid.

The four companies NASA has partnered with might be the biggest firms trying to pull off that futuristic feat, but they're certainly not the only ones. Planetary Resources, a Redmond, Washington-based company founded by Peter Diamandis and commercial spaceflight pioneer Eric C. Anderson, is currently working on its own asteroid mining technology. While it does so, the startup is creating high-power telescopes to sell to space exploration companies--which it then plans to use to scope out asteroids during its own missions.

Silicon Valley-based Made In Space, which makes 3-D printers for use outside of the earth's atmosphere--and in 2014 made headlines for creating a usable wrench in the International Space Station--has an especially innovative plan. Instead of sending large spacecraft capable of moving asteroids, the startup wants to send a small craft that could land on the asteroid and 3-D print a propulsion system and guidance mechanisms--essentially turning the asteroid into its own spaceship that would then drive itself to wherever a crew was located.

And Deep Space Industries, another Silicon Valley startup, thinks it can land robotic landers on an asteroid within three years--and wants to build an entire space city within 30. "You need a lot of raw materials from asteroids to enable that," CEO Daniel Faber told Gizmodo in August.

Even if these companies aren't in the running to partner with NASA for its asteroid mission--yet--it's clear the Jet Propulsion Lab will have company.

NASA plans to choose the specific asteroid it will land on in 2020. It already has one in mind but is seeking alternative options in case that one doesn't pan out.

The four proposals are due on October 24, and NASA plans to announce which one it'll be going with sometime next year.