3-D printers can already self-replicate, sort of. They can make many of the parts, just not the electronics and other key elements [see article here]. These would have to be supplied from Earth.
But there are already self-replicating machines that are microscopically small, called nanobots (Nano for "very small" and bot for "robot"). These nanobots create a variety of tiny structures that can be assembled into larger machines that can perform almost any task. For example, these machines can search for the raw materials they need by:
- Digging in the ground
- Autonomously roaming around on the ground
- Self-propelling in water
- Flying in the air
They use the raw materials they collect for energy or to make other machines (via the nanobots). It's amazing that these nanobots already exist and scientists have absolutely no idea where they came from. What are they? They're called "DNA". DNA is a microscopically small "machine" that makes cells (tiny structures) that together form various living things (plants, animals and us). The living things can dig, walk, swim, fly. And they can make copies of themselves with no extra spare parts required.
Once again, scientists have no idea where DNA came from, but many do know one thing for sure: there is no God. I'm not sure how they know that when they have no idea where something as amazing as DNA came from. But they do.