An assembler is a theoretical nanomachine that can take raw materials and use them to build or assemble desirable products -- one molecule at a time. Each assembler would only be the size of a few atoms. Ideally, scientists could use assemblers to build devices, structures or perhaps even food from the molecular level using trillions of these machines. Because the proposed molecular assemblers are so small, they must work in large groups in order to build any object of useful size or capacity -- working alone, it would take a single molecular assembler millions of years to do its job.

Of course, the need to build trillions of assemblers seems like a difficult obstacle to the development of nanomachines. For that reason, one of the primary tasks of a molecular assembler will be to build other molecular assemblers: The first assembler builds a second, then both assemblers each build another, and so on, leading to a pattern of exponential population growth. One major concern facing potential nanotechnology developers is how to safely control and contain populations of self-copying assemblers -- if an unmanaged culture of assemblers were to enter a world ecosystem, it's possible that the machines could inflict severe environmental damage.

If used correctly, however, molecular assemblers and the nanorobots that they build could have several extremely beneficial effects on the environment. For example:

• Nanorobots could be launched into the atmosphere to rebuild damaged portions of the ozone layer.
• Nanorobots could be used to purify the oceans, removing contaminants and cleaning up oil spills.
• Manufacturing materials using nanoparticles would waste less energy and produce less pollution.

Assemblers could also offer a potential solution to the world's energy crisis. Our natural resources wouldn't be endangered, since we wouldn't have the same need to burn wood, coal or oil for fuel. In any case, molecular assemblers could replicate these substances, if necessary. Though the possible benefits of nanotechnology are very exciting, we're years away from being able to design an assembler.